scryer-prolog/src/machine/system_calls.rs

use crate::parser::ast::*;
use crate::parser::parser::*;

use dashu::integer::Sign;
use dashu::integer::UBig;
use lazy_static::lazy_static;
use num_order::NumOrd;

use crate::arena::*;
use crate::atom_table::*;
#[cfg(feature = "ffi")]
use crate::ffi::*;
use crate::forms::*;
use crate::heap_iter::*;
use crate::heap_print::*;
#[cfg(feature = "http")]
use crate::http::{HttpListener, HttpResponse, HttpService};
use crate::instructions::*;
use crate::machine;
use crate::machine::code_walker::*;
use crate::machine::copier::*;
use crate::machine::heap::*;
use crate::machine::machine_errors::*;
use crate::machine::machine_indices::*;
use crate::machine::machine_state::*;
use crate::machine::partial_string::*;
use crate::machine::preprocessor::to_op_decl;
use crate::machine::stack::*;
use crate::machine::streams::*;
use crate::machine::{get_structure_index, Machine, VERIFY_ATTR_INTERRUPT_LOC};
use crate::parser::char_reader::*;
use crate::parser::dashu::Integer;
use crate::read::*;
use crate::types::*;
use rand::rngs::StdRng;
use rand::{Rng, SeedableRng};

use ordered_float::OrderedFloat;

use fxhash::{FxBuildHasher, FxHasher};
use indexmap::IndexSet;

pub(crate) use ref_thread_local::RefThreadLocal;

use std::cell::Cell;
use std::cmp::Ordering;
use std::collections::BTreeSet;
use std::convert::TryFrom;
use std::env;
#[cfg(feature = "ffi")]
use std::ffi::CString;
use std::fs;
use std::hash::{BuildHasher, BuildHasherDefault};
use std::io::{ErrorKind, Read, Write};
use std::iter::{once, FromIterator};
use std::mem;
use std::net::{SocketAddr, TcpListener, TcpStream, ToSocketAddrs};
use std::num::NonZeroU32;
use std::ops::Sub;
use std::process;
use std::str::FromStr;

use chrono::{offset::Local, DateTime};
#[cfg(not(target_arch = "wasm32"))]
use cpu_time::ProcessTime;
use std::time::{Duration, SystemTime};

#[cfg(feature = "repl")]
use crossterm::event::{read, Event, KeyCode, KeyEvent, KeyModifiers};
#[cfg(feature = "repl")]
use crossterm::terminal::{disable_raw_mode, enable_raw_mode};

use blake2::{Blake2b, Blake2s};
use ring::rand::{SecureRandom, SystemRandom};
use ring::{digest, hkdf, pbkdf2};

#[cfg(feature = "crypto-full")]
use ring::{
    aead,
    signature::{self, KeyPair},
};
use ripemd160::{Digest, Ripemd160};
use sha3::{Sha3_224, Sha3_256, Sha3_384, Sha3_512};

use crrl::{secp256k1, x25519};

#[cfg(feature = "tls")]
use native_tls::{Identity, TlsAcceptor, TlsConnector};

use base64;
use roxmltree;
use select;

use bytes::Buf;
use http_body_util::BodyExt;
#[cfg(feature = "http")]
use hyper::header::{HeaderName, HeaderValue};
#[cfg(feature = "http")]
use hyper::server::conn::http1;
#[cfg(feature = "http")]
use hyper::{HeaderMap, Method};
#[cfg(feature = "http")]
use reqwest::Url;

#[cfg(feature = "repl")]
pub(crate) fn get_key() -> KeyEvent {
    let key;
    enable_raw_mode().expect("failed to enable raw mode");
    loop {
        let key_ = read();
        if let Ok(key_) = key_ {
            if let Event::Key(key_) = key_ {
                match key_.code {
                    KeyCode::Char(_) | KeyCode::Enter | KeyCode::Tab => {
                        key = key_;
                        break;
                    }
                    _ => (),
                }
            }
        }
    }
    disable_raw_mode().expect("failed to disable raw mode");
    key
}

#[derive(Debug, Clone, Copy)]
pub struct BrentAlgState {
    pub hare: usize,
    pub tortoise: usize,
    pub power: usize,
    pub lam: usize,
    pub pstr_chars: usize,
    max_steps: i64,
}

impl BrentAlgState {
    pub fn new(hare: usize) -> Self {
        Self {
            hare,
            tortoise: hare,
            power: 1,
            lam: 0,
            pstr_chars: 0,
            max_steps: -1,
        }
    }

    #[inline(always)]
    pub fn teleport_tortoise(&mut self) {
        if self.lam == self.power {
            self.tortoise = self.hare;
            self.power <<= 1;
            self.lam = 0;
        }
    }

    #[inline(always)]
    pub fn step(&mut self, hare: usize) -> Option<CycleSearchResult> {
        self.hare = hare;
        self.lam += 1;

        if self.tortoise == self.hare {
            return Some(CycleSearchResult::Cyclic(self.lam));
        } else {
            self.teleport_tortoise();
        }

        None
    }

    #[inline(always)]
    pub fn num_steps(&self) -> usize {
        return self.lam + self.pstr_chars + self.power - 1;
    }

    #[inline(always)]
    pub fn exhausted_max_steps(&self) -> bool {
        self.max_steps > -1 && self.num_steps() as i64 >= self.max_steps
    }

    pub fn to_result(mut self, heap: &[HeapCellValue]) -> CycleSearchResult {
        loop {
            read_heap_cell!(heap[self.hare],
                (HeapCellValueTag::PStrOffset) => {
                    let (pstr_loc, offset) = pstr_loc_and_offset(heap, self.hare);
                    let offset = offset.get_num() as usize;

                    let pstr = cell_as_string!(heap[self.hare]);
                    self.pstr_chars += pstr.as_str_from(offset).chars().count();

                    return CycleSearchResult::PStrLocation(self.num_steps(), pstr_loc, offset);
                }
                (HeapCellValueTag::PStrLoc, l) => {
                    let (_pstr_loc, offset) = pstr_loc_and_offset(heap, l);
                    let offset = offset.get_num() as usize;
                    return CycleSearchResult::PStrLocation(self.num_steps(), l, offset);
                }
                (HeapCellValueTag::Atom, (name, arity)) => {
                    return if name == atom!("[]") && arity == 0 {
                        CycleSearchResult::ProperList(self.num_steps())
                    } else {
                        CycleSearchResult::NotList(self.num_steps(), heap[self.hare])
                    };
                }
                (HeapCellValueTag::Str, s) => {
                    let (name, arity) = cell_as_atom_cell!(heap[s])
                        .get_name_and_arity();

                    return if name == atom!("[]") && arity == 0 {
                        CycleSearchResult::ProperList(self.num_steps())
                    } else {
                        CycleSearchResult::NotList(self.num_steps(), heap[self.hare])
                    };
                }
                (HeapCellValueTag::Lis, l) => {
                    return CycleSearchResult::UntouchedList(self.num_steps(), l);
                }
                (HeapCellValueTag::AttrVar | HeapCellValueTag::Var, h) => {
                    if h == self.hare {
                        let var = heap[self.hare].as_var().unwrap();
                        return CycleSearchResult::PartialList(self.num_steps(), var);
                    } else {
                        self.hare = h;
                    }
                }
                _ => {
                    return CycleSearchResult::NotList(self.num_steps(), heap[self.hare]);
                }
            );
        }
    }

    fn add_pstr_offset_chars(
        &mut self,
        heap: &[HeapCellValue],
        h: usize,
        offset: usize,
    ) -> Option<CycleSearchResult> {
        read_heap_cell!(heap[h],
            (HeapCellValueTag::CStr, cstr_atom) => {
                let cstr = PartialString::from(cstr_atom);
                let num_chars = cstr.as_str_from(offset).chars().count();

                if self.max_steps == -1 || self.num_steps() + num_chars <= self.max_steps as usize {
                    self.pstr_chars += num_chars;
                    Some(CycleSearchResult::ProperList(self.num_steps()))
                } else {
                    let offset = self.num_steps() + num_chars - self.max_steps as usize;
                    self.pstr_chars += offset;
                    Some(CycleSearchResult::PStrLocation(self.max_steps as usize, h, offset))
                }
            }
            (HeapCellValueTag::PStr, pstr_atom) => {
                let pstr = PartialString::from(pstr_atom);
                let num_chars = pstr.as_str_from(offset).chars().count();

                if self.max_steps == -1 || self.num_steps() + num_chars <= self.max_steps as usize {
                    self.pstr_chars += num_chars - 1;
                    self.step(h+1)
                } else {
                    let offset = self.num_steps() + num_chars - self.max_steps as usize;
                    self.pstr_chars += offset;
                    Some(CycleSearchResult::PStrLocation(self.max_steps as usize, h, offset))
                }
            }
            _ => {
                unreachable!()
            }
        )
    }

    fn add_pstr_chars_and_step(
        &mut self,
        heap: &[HeapCellValue],
        h: usize,
    ) -> Option<CycleSearchResult> {
        read_heap_cell!(heap[h],
            (HeapCellValueTag::PStrOffset, l) => {
                let (pstr_loc, offset) = pstr_loc_and_offset(heap, l);
                self.add_pstr_offset_chars(heap, pstr_loc, offset.get_num() as usize)
            }
            _ => {
                self.add_pstr_offset_chars(heap, h, 0)
            }
        )
    }

    #[inline(always)]
    fn cycle_step(&mut self, heap: &[HeapCellValue]) -> Option<CycleSearchResult> {
        loop {
            let value = heap[self.hare];

            read_heap_cell!(value,
                (HeapCellValueTag::PStrLoc, h) => {
                    return self.add_pstr_chars_and_step(&heap, h);
                }
                (HeapCellValueTag::CStr | HeapCellValueTag::PStrOffset) => {
                    return self.add_pstr_chars_and_step(&heap, self.hare);
                }
                (HeapCellValueTag::Lis, h) => {
                    return self.step(h+1);
                }
                (HeapCellValueTag::Str, s) => {
                    let (name, arity) = cell_as_atom_cell!(heap[s]).get_name_and_arity();

                    return if name == atom!(".") && arity == 2 {
                        self.step(s+2)
                    } else {
                        Some(CycleSearchResult::NotList(self.num_steps(), value))
                    };
                }
                (HeapCellValueTag::Atom, (name, arity)) => {
                    debug_assert!(arity == 0);

                    return if name == atom!("[]") {
                        Some(CycleSearchResult::ProperList(self.num_steps()))
                    } else {
                        Some(CycleSearchResult::NotList(self.num_steps(), value))
                    };
                }
                (HeapCellValueTag::AttrVar | HeapCellValueTag::Var, h) => {
                    if self.hare == h {
                        let r = value.as_var().unwrap();
                        return Some(CycleSearchResult::PartialList(self.num_steps(), r));
                    }

                    self.hare = h;
                }
                _ => {
                    return Some(CycleSearchResult::NotList(self.num_steps(), value));
                }
            );
        }
    }

    pub fn detect_cycles(heap: &[HeapCellValue], value: HeapCellValue) -> CycleSearchResult {
        let mut pstr_chars = 0;

        let hare = read_heap_cell!(value,
            (HeapCellValueTag::Lis, offset) => {
                offset+1
            }
            (HeapCellValueTag::PStrLoc, h) => {
                let (h_offset, n) = pstr_loc_and_offset(&heap, h);
                let n = n.get_num() as usize;
                let pstr = cell_as_string!(heap[h_offset]);

                pstr_chars = pstr.as_str_from(n).chars().count() - 1;

                if heap[h].get_tag() == HeapCellValueTag::PStrOffset {
                    debug_assert!(heap[h].get_tag() == HeapCellValueTag::PStrOffset);

                    if heap[h_offset].get_tag() == HeapCellValueTag::CStr {
                        return CycleSearchResult::ProperList(pstr_chars + 1);
                    }
                }

                h_offset+1
            }
            (HeapCellValueTag::PStrOffset) => {
                unreachable!()
            }
            (HeapCellValueTag::CStr, cstr_atom) => {
                let cstr = PartialString::from(cstr_atom);
                return CycleSearchResult::ProperList(cstr.as_str_from(0).chars().count());
            }
            (HeapCellValueTag::Str, s) => {
                let (name, arity) = cell_as_atom_cell!(heap[s])
                    .get_name_and_arity();

                if name == atom!("[]") && arity == 0 {
                    return CycleSearchResult::EmptyList;
                } else if name == atom!(".") && arity == 2 {
                    s + 2
                } else {
                    return CycleSearchResult::NotList(0, value);
                }
            }
            (HeapCellValueTag::Atom, (name, arity)) => {
                return if name == atom!("[]") && arity == 0 {
                    CycleSearchResult::EmptyList
                } else {
                    CycleSearchResult::NotList(0, value)
                };
            }
            (HeapCellValueTag::AttrVar | HeapCellValueTag::StackVar | HeapCellValueTag::Var) => {
                return CycleSearchResult::PartialList(0, value.as_var().unwrap());
            }
            _ => {
                return CycleSearchResult::NotList(0, value);
            }
        );

        let mut brent_st = BrentAlgState::new(hare);

        brent_st.power += 1; // advance a step.
        brent_st.pstr_chars = pstr_chars;

        loop {
            if let Some(result) = brent_st.cycle_step(heap) {
                return result;
            }
        }
    }

    pub fn detect_cycles_with_max(
        heap: &[HeapCellValue],
        max_steps: usize,
        value: HeapCellValue,
    ) -> CycleSearchResult {
        let mut pstr_chars = 0;

        let hare = read_heap_cell!(value,
            (HeapCellValueTag::Lis, offset) => {
                if max_steps > 0 {
                    offset+1
                } else {
                    return CycleSearchResult::UntouchedList(0, offset);
                }
            }
            (HeapCellValueTag::PStrLoc, h) => {
                let (h_offset, n) = pstr_loc_and_offset(&heap, h);
                let n = n.get_num() as usize;
                let pstr = cell_as_string!(heap[h_offset]);

                pstr_chars = pstr.as_str_from(n).chars().count() - 1;

                if heap[h].get_tag() == HeapCellValueTag::PStrOffset {
                    debug_assert!(heap[h].get_tag() == HeapCellValueTag::PStrOffset);

                    if heap[h_offset].get_tag() == HeapCellValueTag::CStr {
                        return if pstr_chars + 1 <= max_steps {
                            CycleSearchResult::ProperList(pstr_chars + 1)
                        } else {
                            CycleSearchResult::UntouchedCStr(pstr.into(), max_steps)
                        };
                    }
                }

                if pstr_chars + 1 > max_steps {
                    return CycleSearchResult::PStrLocation(max_steps, h_offset, max_steps);
                }

                h_offset+1
            }
            (HeapCellValueTag::PStrOffset) => {
                unreachable!()
            }
            (HeapCellValueTag::CStr, cstr_atom) => {
                return if max_steps > 0 {
                    let cstr = PartialString::from(cstr_atom);
                    let pstr_chars = cstr.as_str_from(0).chars().count();

                    if pstr_chars <= max_steps {
                        CycleSearchResult::ProperList(pstr_chars)
                    } else {
                        CycleSearchResult::UntouchedCStr(cstr_atom, max_steps)
                    }
                } else {
                    CycleSearchResult::UntouchedCStr(cstr_atom, 0)
                };
            }
            (HeapCellValueTag::Str, s) => {
                let (name, arity) = cell_as_atom_cell!(heap[s]).get_name_and_arity();

                if name == atom!("[]") && arity == 0 {
                    return CycleSearchResult::EmptyList;
                } else if name == atom!(".") && arity == 2 {
                    if max_steps > 0 {
                        s + 2
                    } else {
                        return CycleSearchResult::UntouchedList(0, s + 1);
                    }
                } else {
                    return CycleSearchResult::NotList(0, value);
                }
            }
            (HeapCellValueTag::Atom, (name, arity)) => {
                return if name == atom!("[]") && arity == 0 {
                    CycleSearchResult::EmptyList
                } else {
                    CycleSearchResult::NotList(0, value)
                };
            }
            (HeapCellValueTag::AttrVar | HeapCellValueTag::StackVar | HeapCellValueTag::Var) => {
                return CycleSearchResult::PartialList(0, value.as_var().unwrap());
            }
            _ => {
                return CycleSearchResult::NotList(0, value);
            }
        );

        let mut brent_st = BrentAlgState::new(hare);

        brent_st.power += 1; // advance a step.
        brent_st.pstr_chars = pstr_chars;
        brent_st.max_steps = max_steps as i64;

        loop {
            if brent_st.exhausted_max_steps() {
                return brent_st.to_result(&heap);
            }

            if let Some(result) = brent_st.cycle_step(heap) {
                return result;
            }
        }
    }
}

#[derive(Debug)]
enum MatchSite {
    NoMatchVarTail(usize), // no match, we refer to the location of the uninstantiated tail instead.
    Match(usize),          // a match
}

#[derive(Debug)]
struct AttrListMatch {
    match_site: MatchSite,
    prev_tail: Option<usize>,
}

impl MachineState {
    #[inline(always)]
    pub(crate) fn unattributed_var(&mut self) {
        let attr_var = self.store(self.deref(self.registers[1]));

        if !attr_var.is_var() {
            self.fail = true;
            return;
        }

        read_heap_cell!(attr_var,
            (HeapCellValueTag::AttrVar, h) => {
                let list_cell = self.store(self.deref(self.heap[h+1]));
                self.fail = list_cell.get_tag() == HeapCellValueTag::Lis;
            }
            _ => {
            }
        );
    }

    pub(crate) fn get_attr_var_list(&mut self, attr_var: HeapCellValue) -> Option<usize> {
        read_heap_cell!(attr_var,
            (HeapCellValueTag::AttrVar, h) => {
                Some(h + 1)
            }
            (HeapCellValueTag::Var | HeapCellValueTag::StackVar) => {
                // create an AttrVar in the heap.
                let h = self.heap.len();

                self.heap.push(attr_var_as_cell!(h));
                self.heap.push(heap_loc_as_cell!(h+1));

                self.bind(Ref::attr_var(h), attr_var);

                Some(h + 1)
            }
            _ => {
                None
            }
        )
    }

    pub(crate) fn name_and_arity_from_heap(&self, cell: HeapCellValue) -> Option<PredicateKey> {
        read_heap_cell!(self.store(self.deref(cell)),
            (HeapCellValueTag::Str, s) => {
                Some(cell_as_atom_cell!(self.heap[s]).get_name_and_arity())
            }
            (HeapCellValueTag::Atom, (name, _arity)) => {
                Some((name, 0))
            }
            _ => {
                None
            }
        )
    }

    #[inline]
    pub(crate) fn variable_set<S: BuildHasher>(
        &mut self,
        seen_set: &mut IndexSet<HeapCellValue, S>,
        value: HeapCellValue,
    ) {
        let mut iter = stackful_preorder_iter(&mut self.heap, &mut self.stack, value);

        while let Some(value) = iter.next() {
            let value = unmark_cell_bits!(value);

            if value.is_var() {
                let value = unmark_cell_bits!(heap_bound_store(
                    iter.heap,
                    heap_bound_deref(iter.heap, value)
                ));

                if value.is_var() {
                    seen_set.insert(value);
                }
            }
        }
    }

    fn skip_max_list_cycle(&mut self, lam: usize) {
        fn step(heap: &[HeapCellValue], mut value: HeapCellValue) -> usize {
            loop {
                read_heap_cell!(value,
                    (HeapCellValueTag::PStrLoc, h) => {
                        let (h_offset, _) = pstr_loc_and_offset(&heap, h);
                        return h_offset+1;
                    }
                    (HeapCellValueTag::Lis, h) => {
                        return h+1;
                    }
                    (HeapCellValueTag::Str, s) => {
                        return s+2;
                    }
                    (HeapCellValueTag::AttrVar | HeapCellValueTag::Var, h) => {
                        value = heap[h];
                    }
                    _ => {
                        unreachable!();
                    }
                );
            }
        }

        let h = self.heap.len();
        self.heap.push(self.registers[3]);

        let mut hare = h;
        let mut tortoise = hare;

        for _ in 0..lam {
            hare = step(&self.heap, self.heap[hare]);
        }

        let mut prev_hare = hare;

        while hare != tortoise {
            prev_hare = hare;
            hare = step(&self.heap, self.heap[hare]);
            tortoise = step(&self.heap, self.heap[tortoise]);
        }

        // now compute the num_steps of the list prefix until hare is
        // reached in the fashion of a C do-while loop since hare
        // may point to the beginning of a cycle.

        let mut brent_st = BrentAlgState::new(h);

        brent_st.cycle_step(&self.heap);

        while prev_hare != brent_st.hare {
            brent_st.cycle_step(&self.heap);
        }

        self.heap.pop();

        let target_n = self.store(self.deref(self.registers[1]));
        self.unify_fixnum(Fixnum::build_with(brent_st.num_steps() as i64), target_n);

        if !self.fail {
            unify!(self, self.registers[4], self.heap[prev_hare]);
        }
    }

    fn finalize_skip_max_list(&mut self, n: i64, value: HeapCellValue) {
        let target_n = self.store(self.deref(self.registers[1]));
        self.unify_fixnum(Fixnum::build_with(n), target_n);

        if !self.fail {
            let xs = self.registers[4];
            unify!(self, value, xs);
        }
    }

    fn skip_max_list_result(&mut self, max_steps: i64) {
        let search_result = if max_steps == -1 {
            BrentAlgState::detect_cycles(&self.heap, self.store(self.deref(self.registers[3])))
        } else {
            BrentAlgState::detect_cycles_with_max(
                &self.heap,
                max_steps as usize,
                self.store(self.deref(self.registers[3])),
            )
        };

        match search_result {
            CycleSearchResult::PStrLocation(steps, pstr_loc, offset) => {
                let steps = if max_steps > -1 {
                    std::cmp::min(max_steps, steps as i64)
                } else {
                    steps as i64
                };

                let cell = if offset > 0 {
                    let h = self.heap.len();

                    self.heap.push(pstr_offset_as_cell!(pstr_loc));
                    self.heap
                        .push(fixnum_as_cell!(Fixnum::build_with(offset as i64)));

                    pstr_loc_as_cell!(h)
                } else {
                    pstr_loc_as_cell!(pstr_loc)
                };

                self.finalize_skip_max_list(steps, cell);
            }
            CycleSearchResult::UntouchedList(n, l) => {
                self.finalize_skip_max_list(n as i64, list_loc_as_cell!(l));
            }
            CycleSearchResult::UntouchedCStr(cstr_atom, n) => {
                let cell = if n > 0 {
                    let h = self.heap.len();

                    self.heap.push(string_as_cstr_cell!(cstr_atom));
                    self.heap.push(pstr_offset_as_cell!(h));
                    self.heap
                        .push(fixnum_as_cell!(Fixnum::build_with(n as i64)));

                    pstr_loc_as_cell!(h + 1)
                } else {
                    string_as_cstr_cell!(cstr_atom)
                };

                self.finalize_skip_max_list(n as i64, cell);
            }
            CycleSearchResult::EmptyList => {
                self.finalize_skip_max_list(0, empty_list_as_cell!());
            }
            CycleSearchResult::PartialList(n, r) => {
                self.finalize_skip_max_list(n as i64, r.as_heap_cell_value());
            }
            CycleSearchResult::ProperList(steps) => {
                self.finalize_skip_max_list(steps as i64, empty_list_as_cell!())
            }
            CycleSearchResult::NotList(n, value) => {
                self.finalize_skip_max_list(n as i64, value);
            }
            CycleSearchResult::Cyclic(lam) => {
                self.skip_max_list_cycle(lam);
            }
        };
    }

    pub fn skip_max_list(&mut self) -> CallResult {
        let max_steps = self.store(self.deref(self.registers[2]));
        let mut max_old = -1i64;

        if !max_steps.is_var() {
            let max_steps = Number::try_from(max_steps);

            let max_steps_n = match max_steps {
                Ok(Number::Fixnum(n)) => Some(n.get_num()),
                Ok(Number::Integer(n)) => {
                    let value: i64 = (&*n).try_into().unwrap();

                    Some(value)
                },
                _ => None,
            };

            if let Some(max_steps) = max_steps_n {
                if max_steps.abs() as u64 <= 1 << 63 {
                    if max_steps >= 0 {
                        max_old = max_steps;
                    } else {
                        self.fail = true;
                        return Ok(());
                    }
                } else if max_steps < 0 {
                    self.fail = true;
                    return Ok(());
                }
            } else if !max_steps.map(|n| n.is_integer()).unwrap_or(false) {
                self.fail = true;
                return Ok(());
            }
        }

        self.skip_max_list_result(max_old);
        Ok(())
    }

    fn term_variables_under_max_depth(
        &mut self,
        term: HeapCellValue,
        max_depth: usize,
        list_of_vars: HeapCellValue,
    ) {
        let mut seen_set = IndexSet::new();

        {
            let mut iter = stackful_post_order_iter(&mut self.heap, &mut self.stack, term);

            while let Some(value) = iter.next() {
                if iter.parent_stack_len() >= max_depth {
                    iter.pop_stack();
                    continue;
                }

                let value = unmark_cell_bits!(value);

                if value.is_var() {
                    seen_set.insert(value);
                }
            }
        }

        let outcome = heap_loc_as_cell!(iter_to_heap_list(&mut self.heap, seen_set.into_iter(),));

        unify_fn!(*self, list_of_vars, outcome);
    }

    #[inline(always)]
    pub(crate) fn install_new_block(&mut self, value: HeapCellValue) -> usize {
        let value = self.store(self.deref(value));

        self.block = self.b;
        self.unify_fixnum(Fixnum::build_with(self.block as i64), value);

        self.block
    }

    pub(crate) fn copy_findall_solution(
        &mut self,
        lh_offset: usize,
        copy_target: HeapCellValue,
    ) -> usize {
        let threshold = self.lifted_heap.len() - lh_offset;

        let mut copy_ball_term =
            CopyBallTerm::new(&mut self.stack, &mut self.heap, &mut self.lifted_heap);

        copy_ball_term.push(list_loc_as_cell!(threshold + 1));
        copy_ball_term.push(heap_loc_as_cell!(threshold + 3));
        copy_ball_term.push(heap_loc_as_cell!(threshold + 2));

        copy_term(copy_ball_term, copy_target, AttrVarPolicy::DeepCopy);

        threshold + lh_offset + 2
    }

    #[inline(always)]
    pub(crate) fn truncate_if_no_lifted_heap_diff(
        &mut self,
        addr_constr: impl Fn(usize) -> HeapCellValue,
    ) {
        read_heap_cell!(self.store(self.deref(self.registers[1])),
            (HeapCellValueTag::Fixnum, n) => {
                let lh_offset = n.get_num() as usize;

                if lh_offset >= self.lifted_heap.len() {
                    self.lifted_heap.truncate(lh_offset);
                } else {
                    let threshold = self.lifted_heap.len() - lh_offset;
                    self.lifted_heap.push(addr_constr(threshold));
                }
            }
            _ => {
                self.fail = true;
            }
        );
    }

    pub(crate) fn parse_number_from_string(
        &mut self,
        string: &str,
        indices: &IndexStore,
        stub_gen: impl Fn() -> FunctorStub,
    ) -> CallResult {
        use crate::parser::lexer::*;

        let nx = self.store(self.deref(self.registers[2]));
        let add_dot = !string.ends_with(".");
        let cursor = std::io::Cursor::new(string);

        let iter = std::io::Read::chain(cursor, {
            let mut dot_buf: [u8; '.'.len_utf8()] = [0u8];

            if add_dot {
                '.'.encode_utf8(&mut dot_buf);
            }

            std::io::Cursor::new(dot_buf)
        });

        let mut lexer = Lexer::new(CharReader::new(iter), self);
        let mut tokens = vec![];

        match lexer.next_token() {
            Ok(token @ Token::Literal(Literal::Atom(atom!("-")) | Literal::Char('-'))) => {
                tokens.push(token);

                if let Ok(token) = lexer.next_token() {
                    tokens.push(token);
                }
            }
            Ok(token) => {
                tokens.push(token);
            }
            Err(err) => {
                let err = self.syntax_error(err);
                return Err(self.error_form(err, stub_gen()));
            }
        }

        loop {
            match lexer.lookahead_char() {
                Err(e) if e.is_unexpected_eof() => {
                    let mut parser = Parser::from_lexer(lexer);
                    let op_dir = CompositeOpDir::new(&indices.op_dir, None);

                    tokens.reverse();

                    match parser.read_term(&op_dir, Tokens::Provided(tokens)) {
                        Err(err) => {
                            let err = self.syntax_error(err);
                            return Err(self.error_form(err, stub_gen()));
                        }
                        Ok(Term::Literal(_, Literal::Rational(n))) => {
                            self.unify_rational(n, nx);
                        }
                        Ok(Term::Literal(_, Literal::Float(n))) => {
                            self.unify_f64(n.as_ptr(), nx);
                        }
                        Ok(Term::Literal(_, Literal::Integer(n))) => {
                            self.unify_big_int(n, nx);
                        }
                        Ok(Term::Literal(_, Literal::Fixnum(n))) => {
                            self.unify_fixnum(n, nx);
                        }
                        _ => {
                            let err = ParserError::ParseBigInt(0, 0);
                            let err = self.syntax_error(err);

                            return Err(self.error_form(err, stub_gen()));
                        }
                    }

                    break;
                }
                Ok('.') => {
                    lexer.skip_char('.');
                }
                Ok(c) => {
                    let (line_num, col_num) = (lexer.line_num, lexer.col_num);

                    let err = ParserError::UnexpectedChar(c, line_num, col_num);
                    let err = self.syntax_error(err);

                    return Err(self.error_form(err, stub_gen()));
                }
                Err(_) => unreachable!(),
            }
        }

        Ok(())
    }

    pub(crate) fn call_continuation_chunk(
        &mut self,
        chunk: HeapCellValue,
        return_p: usize,
    ) -> usize {
        let chunk = self.store(self.deref(chunk));

        let s = chunk.get_value() as usize;
        let arity = cell_as_atom_cell!(self.heap[s]).get_arity();

        let num_cells = arity - 1;
        let p_functor = self.heap[s + 1];

        let cp = to_local_code_ptr(&self.heap, p_functor).unwrap();
        let prev_e = self.e;

        let e = self.stack.allocate_and_frame(num_cells);
        let and_frame = self.stack.index_and_frame_mut(e);

        and_frame.prelude.e = prev_e;
        and_frame.prelude.cp = return_p;

        self.p = cp + 1;

        /*
        if num_cells > 0 {
            if let HeapCellValueTag::Fixnum = self.heap[s + 2].get_tag() {
                and_frame[1] = fixnum_as_cell!(Fixnum::build_with(self.b as i64));
            } else {
                and_frame[1] = self.heap[s + 2];
            }
        }
        */

        for index in s + 2..s + 2 + num_cells {
            if let HeapCellValueTag::CutPoint = self.heap[index].get_tag() {
                // adjust cut point to occur after call_continuation.
                and_frame[index - (s + 1)] = fixnum_as_cell!(Fixnum::as_cutpoint(self.b as i64));
            } else {
                and_frame[index - (s + 1)] = self.heap[index];
            }
        }

        self.e = e;
        self.p
    }

    pub fn value_to_str_like(&mut self, value: HeapCellValue) -> Option<AtomOrString> {
        read_heap_cell!(value,
            (HeapCellValueTag::CStr, cstr_atom) => {
                // avoid allocating a String if possible:
                // We must be careful to preserve the string "[]" as is,
                // instead of turning it into the atom [], i.e., "".
                if cstr_atom == atom!("[]") {
                    Some(AtomOrString::String("[]".to_string()))
                } else {
                    Some(AtomOrString::Atom(cstr_atom))
                }
            }
            (HeapCellValueTag::Atom, (atom, arity)) => {
                if arity == 0 {
                    // ... likewise.
                    Some(AtomOrString::Atom(atom))
                } else {
                    None
                }
            }
            (HeapCellValueTag::Str, s) => {
                let (name, arity) = cell_as_atom_cell!(self.heap[s])
                    .get_name_and_arity();

                if arity == 0 {
                    Some(AtomOrString::Atom(name))
                } else {
                    None
                }
            }
            (HeapCellValueTag::Char, c) => {
                Some(AtomOrString::String(c.to_string()))
            }
            _ => {
                if value.is_constant() {
                    return None;
                }

                let h = self.heap.len();
                self.heap.push(value);

                let mut iter = HeapPStrIter::new(&self.heap, h);
                let string = iter.to_string();
                let at_terminator = iter.at_string_terminator();

                self.heap.pop();

                // if the iteration doesn't terminate like a string
                // (i.e. with the [] atom or a CStr), it is not
                // "str_like" so return None.
                if at_terminator {
                    Some(AtomOrString::String(string))
                } else {
                    None
                }
            }
        )
    }

    pub(crate) fn codes_to_string(
        &mut self,
        addrs: impl Iterator<Item = HeapCellValue>,
        stub_gen: impl Fn() -> FunctorStub,
    ) -> Result<String, MachineStub> {
        let mut string = String::new();

        for addr in addrs {
            let addr = self.store(self.deref(addr));

            match Number::try_from(addr) {
                Ok(Number::Fixnum(n)) => match u32::try_from(n.get_num()) {
                    Ok(n) => {
                        if let Some(c) = std::char::from_u32(n) {
                            string.push(c);
                            continue;
                        }
                    }
                    _ => {}
                },
                Ok(Number::Integer(n)) => {
                    let n: u32 = (&*n).try_into().unwrap();
                    if let Some(c) = std::char::from_u32(n) {
                        string.push(c);
                        continue;
                    }
                }
                _ => {
                    let err = self.type_error(ValidType::Integer, addr);
                    return Err(self.error_form(err, stub_gen()));
                }
            }

            let err = self.representation_error(RepFlag::CharacterCode);
            return Err(self.error_form(err, stub_gen()));
        }

        Ok(string)
    }

    pub(crate) fn strip_module(
        &self,
        mut qualified_goal: HeapCellValue,
        mut module_loc: HeapCellValue,
    ) -> (HeapCellValue, HeapCellValue) {
        loop {
            read_heap_cell!(qualified_goal,
                (HeapCellValueTag::Str, s) => {
                    let (name, arity) = cell_as_atom_cell!(self.heap[s])
                        .get_name_and_arity();

                    if name == atom!(":") && arity == 2 {
                        module_loc = self.heap[s+1];
                        qualified_goal = self.heap[s+2];
                    } else {
                        break;
                    }
                }
                (HeapCellValueTag::AttrVar | HeapCellValueTag::Var, h) => {
                    if qualified_goal != self.heap[h] {
                        qualified_goal = self.heap[h];
                    } else {
                        break;
                    }
                }
                _ => {
                    break;
                }
            );
        }

        (module_loc, qualified_goal)
    }
}

impl Machine {
    #[inline(always)]
    pub(crate) fn delete_all_attributes_from_var(&mut self) {
        let attr_var = self.deref_register(1);

        if let HeapCellValueTag::AttrVar = attr_var.get_tag() {
            let attr_var_loc = attr_var.get_value() as usize;
            self.machine_st.heap[attr_var_loc] = heap_loc_as_cell!(attr_var_loc);
            self.machine_st
                .trail(TrailRef::Ref(Ref::attr_var(attr_var_loc)));
        }
    }

    #[inline(always)]
    pub(crate) fn get_clause_p(&self, module_name: Atom) -> (usize, usize) {
        use crate::machine::loader::CompilationTarget;

        let key_cell = self.machine_st.registers[1];
        let key = self.machine_st.name_and_arity_from_heap(key_cell).unwrap();

        let compilation_target = if module_name == atom!("user") {
            CompilationTarget::User
        } else {
            CompilationTarget::Module(module_name)
        };

        let skeleton = self
            .indices
            .get_predicate_skeleton(&compilation_target, &key)
            .unwrap();

        if self.machine_st.b > self.machine_st.e {
            let or_frame = self.machine_st.stack.index_or_frame(self.machine_st.b);
            let bp = or_frame.prelude.bp;

            match &self.code[bp] {
                &Instruction::IndexingCode(ref indexing_code) => {
                    match &indexing_code[or_frame.prelude.boip as usize] {
                        &IndexingLine::IndexedChoice(ref indexed_choice) => {
                            let p = or_frame.prelude.biip as usize - 1;

                            match &indexed_choice[p] {
                                &IndexedChoiceInstruction::Try(offset)
                                | &IndexedChoiceInstruction::Retry(offset)
                                | &IndexedChoiceInstruction::DefaultRetry(offset) => {
                                    let clause_clause_loc = skeleton.core.clause_clause_locs[p];
                                    (clause_clause_loc, bp + offset)
                                }
                                &IndexedChoiceInstruction::Trust(_)
                                | &IndexedChoiceInstruction::DefaultTrust(_) => {
                                    unreachable!()
                                }
                            }
                        }
                        _ => {
                            unreachable!()
                        }
                    }
                }
                _ => unreachable!(),
            }
        } else {
            let module_name = match compilation_target {
                CompilationTarget::User => atom!("builtins"),
                CompilationTarget::Module(target) => target,
            };

            let bp = self
                .indices
                .get_predicate_code_index(atom!("$clause"), 2, module_name)
                .and_then(|idx| idx.local())
                .unwrap();

            macro_rules! extract_ptr {
                ($ptr: expr) => {
                    match $ptr {
                        IndexingCodePtr::External(p) => {
                            return (
                                skeleton.core.clause_clause_locs.back().cloned().unwrap(),
                                bp + p,
                            )
                        }
                        IndexingCodePtr::Internal(boip) => boip,
                        _ => unreachable!(),
                    }
                };
            }

            match &self.code[bp] {
                &Instruction::IndexingCode(ref indexing_code) => {
                    let indexing_code_ptr = match &indexing_code[0] {
                        &IndexingLine::Indexing(IndexingInstruction::SwitchOnTerm(
                            _,
                            _,
                            c,
                            _,
                            s,
                        )) => {
                            if key.1 > 0 {
                                s
                            } else {
                                c
                            }
                        }
                        _ => {
                            unreachable!()
                        }
                    };

                    let boip = extract_ptr!(indexing_code_ptr);

                    let boip = match &indexing_code[boip] {
                        &IndexingLine::Indexing(IndexingInstruction::SwitchOnStructure(ref hm)) => {
                            boip + extract_ptr!(hm.get(&key).cloned().unwrap())
                        }
                        &IndexingLine::Indexing(IndexingInstruction::SwitchOnConstant(ref hm)) => {
                            boip + extract_ptr!(hm.get(&Literal::Atom(key.0)).cloned().unwrap())
                        }
                        _ => boip,
                    };

                    match &indexing_code[boip] {
                        &IndexingLine::IndexedChoice(ref indexed_choice) => {
                            return (
                                skeleton.core.clause_clause_locs.back().cloned().unwrap(),
                                bp + indexed_choice.back().unwrap().offset(),
                            );
                        }
                        _ => unreachable!(),
                    }
                }
                _ => {
                    return (
                        skeleton.core.clause_clause_locs.back().cloned().unwrap(),
                        bp,
                    );
                }
            }
        }
    }

    #[inline(always)]
    pub(crate) fn deref_register(&self, i: usize) -> HeapCellValue {
        self.machine_st
            .store(self.machine_st.deref(self.machine_st.registers[i]))
    }

    #[inline(always)]
    pub(crate) fn fast_call(
        &mut self,
        arity: usize,
        call_at_index: impl Fn(&mut Machine, Atom, usize, IndexPtr) -> CallResult,
    ) -> CallResult {
        let arity = arity - 1;
        let (mut module_name, mut goal) = self
            .machine_st
            .strip_module(self.machine_st.registers[1], heap_loc_as_cell!(0));

        let load_registers = |machine_st: &mut MachineState,
                              goal: HeapCellValue,
                              goal_arity: usize| {
            read_heap_cell!(goal,
                (HeapCellValueTag::Str | HeapCellValueTag::Atom, s) => {
                    if goal_arity > 1 {
                        for idx in (1 .. arity + 1).rev() {
                            machine_st.registers[idx + goal_arity] = machine_st.registers[idx + 1];
                        }
                    } else if goal_arity == 0 {
                        for idx in 1 .. arity + 1 {
                            machine_st.registers[idx] = machine_st.registers[idx + 1];
                        }
                    }

                    for idx in 1 .. goal_arity + 1 {
                        machine_st.registers[idx] = machine_st.heap[s+idx];
                    }
                }
                _ => {
                    unreachable!()
                }
            )
        };

        let (mut name, mut goal_arity, index_cell_opt) = read_heap_cell!(goal,
            (HeapCellValueTag::Str, s) => {
                let (name, arity) = cell_as_atom_cell!(self.machine_st.heap[s]).get_name_and_arity();

                (name, arity, if self.machine_st.heap.len() > s + arity + 1 {
                    get_structure_index(self.machine_st.heap[s + arity + 1])
                } else {
                    None
                })
            }
            (HeapCellValueTag::Atom, (name, arity)) => {
                debug_assert_eq!(arity, 0);
                (name, arity, None)
            }
            _ => {
                self.machine_st.fail = true;
                return Ok(());
            }
        );

        let mut arity = arity + goal_arity;

        let index_cell = index_cell_opt.or_else(|| {
            let is_internal_call = name == atom!("$call") && goal_arity > 0;

            if !is_internal_call && self.indices.goal_expansion_defined((name, arity)) {
                None
            } else {
                if is_internal_call {
                    debug_assert_eq!(goal.get_tag(), HeapCellValueTag::Str);
                    goal = self.machine_st.heap[goal.get_value() as usize + 1];
                    (module_name, goal) = self.machine_st.strip_module(goal, module_name);

                    if let Some((inner_name, inner_arity)) =
                        self.machine_st.name_and_arity_from_heap(goal)
                    {
                        arity -= goal_arity;
                        (name, goal_arity) = (inner_name, inner_arity);
                        arity += goal_arity;
                    } else {
                        return None;
                    }
                }

                let module_name = if module_name.get_tag() != HeapCellValueTag::Atom {
                    if let Some(load_context) = self.load_contexts.last() {
                        load_context.module
                    } else {
                        atom!("user")
                    }
                } else {
                    cell_as_atom!(module_name)
                };

                self.indices
                    .get_predicate_code_index(name, arity, module_name)
            }
        });

        if let Some(code_index) = index_cell {
            if !code_index.is_undefined() {
                load_registers(&mut self.machine_st, goal, goal_arity);
                self.machine_st.neck_cut();
                return call_at_index(self, name, arity, code_index.get());
            }
        }

        self.machine_st.fail = true;
        Ok(())
    }

    #[inline(always)]
    pub(crate) fn compile_inline_or_expanded_goal(&mut self) -> CallResult {
        let goal = self.deref_register(1);
        let module_name = self.deref_register(4);

        // supp_vars are the supplementary variables generated by
        // complete_partial_goal prior to goal_expansion.
        let mut supp_vars = IndexSet::with_hasher(FxBuildHasher::default());

        self.machine_st
            .variable_set(&mut supp_vars, self.machine_st.registers[2]);

        struct GoalAnalysisResult {
            is_simple_goal: bool,
            goal: HeapCellValue,
            key: PredicateKey,
            expanded_vars: IndexSet<HeapCellValue, BuildHasherDefault<FxHasher>>,
            supp_vars: IndexSet<HeapCellValue, BuildHasherDefault<FxHasher>>,
        }

        let result = read_heap_cell!(goal,
            (HeapCellValueTag::Str, s) => {
                let (name, arity) = cell_as_atom_cell!(self.machine_st.heap[s])
                    .get_name_and_arity();

                let mut expanded_vars = IndexSet::with_hasher(FxBuildHasher::default());

                // fill expanded_vars with variables of the partial
                // goal pre-completion by complete_partial_goal.
                for idx in s + 1 .. s + arity - supp_vars.len() + 1 {
                    self.machine_st.variable_set(&mut expanded_vars, self.machine_st.heap[idx]);
                }

                let is_simple_goal = if arity >= supp_vars.len() {
                    // post_supp_args are the arguments to the
                    // post-expansion complete goal gathered from the
                    // final supp_vars.len() arguments. they must
                    // agree in supp_vars in order of entry of
                    // insertion as well as the previous
                    // supp_vars.len() argument's variables being
                    // disjoint from them. if they are not, the
                    // expanded goal are not simple.

                    let post_supp_args = self.machine_st.heap[s+arity-supp_vars.len()+1 .. s+arity+1]
                        .iter()
                        .cloned();

                    post_supp_args
                        .zip(supp_vars.iter())
                        .all(|(arg_term, supp_var)| {
                            let (module_loc, arg_term) = self.machine_st.strip_module(
                                arg_term,
                                heap_loc_as_cell!(0),
                            );

                            if module_loc.is_var() || module_loc == atom_as_cell!(atom!("user")) {
                                if arg_term.is_var() && supp_var.is_var() {
                                    return arg_term == *supp_var;
                                }
                            }

                            false
                        }) && expanded_vars.intersection(&supp_vars).next().is_none()
                } else {
                    false
                };

                let goal = if is_simple_goal {
                    let h = self.machine_st.heap.len();
                    let arity = arity - supp_vars.len();

                    for idx in 0 .. arity + 1 {
                        let value = self.machine_st.heap[s + idx];
                        self.machine_st.heap.push(value);
                    }

                    self.machine_st.heap[h] = atom_as_cell!(name, arity);

                    str_loc_as_cell!(h)
                } else {
                    goal
                };

                GoalAnalysisResult {
                    is_simple_goal,
                    goal,
                    key: (name, arity),
                    expanded_vars,
                    supp_vars
                }
            }
            (HeapCellValueTag::Atom, (name, arity)) => {
                debug_assert_eq!(arity, 0);

                let h = self.machine_st.heap.len();
                self.machine_st.heap.push(goal);

                GoalAnalysisResult {
                    is_simple_goal: true,
                    goal: str_loc_as_cell!(h),
                    key: (name, 0),
                    expanded_vars: IndexSet::with_hasher(FxBuildHasher::default()),
                    supp_vars,
                }
            }
            (HeapCellValueTag::Char, c) => {
                let name = AtomTable::build_with(&self.machine_st.atom_tbl,&c.to_string());

                let h = self.machine_st.heap.len();
                self.machine_st.heap.push(atom_as_cell!(name));

                GoalAnalysisResult {
                    is_simple_goal: true,
                    goal: str_loc_as_cell!(h),
                    key: (name, 0),
                    expanded_vars: IndexSet::with_hasher(FxBuildHasher::default()),
                    supp_vars,
                }
            }
            _ => {
                self.machine_st.fail = true;
                return Ok(());
            }
        );

        if result.key.0 == atom!(":") {
            self.machine_st.fail = true;
            return Ok(());
        }

        let expanded_term = if result.is_simple_goal {
            let idx = self.get_or_insert_qualified_code_index(module_name, result.key);
            self.machine_st
                .heap
                .push(untyped_arena_ptr_as_cell!(UntypedArenaPtr::from(idx)));
            result.goal
        } else {
            // all supp_vars must appear later!
            let vars = IndexSet::<HeapCellValue, BuildHasherDefault<FxHasher>>::from_iter(
                result.expanded_vars.difference(&result.supp_vars).cloned(),
            );

            let vars: Vec<_> = vars
                .union(&result.supp_vars) // difference + union does not cancel.
                .map(|v| Term::Var(Cell::default(), VarPtr::from(format!("_{}", v.get_value()))))
                .collect();

            let helper_clause_loc = self.code.len();

            match self.compile_standalone_clause(temp_v!(1), &vars) {
                Err(e) => {
                    let err = self.machine_st.session_error(e);
                    let stub = functor_stub(atom!("call"), result.key.1);

                    return Err(self.machine_st.error_form(err, stub));
                }
                Ok(()) => {
                    let h = self.machine_st.heap.len();

                    self.machine_st.heap.push(atom_as_cell!(atom!("$aux"), 0));

                    for value in result.expanded_vars.difference(&result.supp_vars).cloned() {
                        self.machine_st.heap.push(value);
                    }

                    let anon_str_arity = self.machine_st.heap.len() - h - 1;

                    self.machine_st.heap[h] = atom_as_cell!(atom!("$aux"), anon_str_arity);

                    let idx = CodeIndex::new(
                        IndexPtr::index(helper_clause_loc),
                        &mut self.machine_st.arena,
                    );

                    self.machine_st
                        .heap
                        .push(untyped_arena_ptr_as_cell!(UntypedArenaPtr::from(idx)));

                    str_loc_as_cell!(h)
                }
            }
        };

        let truncated_goal = self.machine_st.registers[3];
        unify!(&mut self.machine_st, expanded_term, truncated_goal);

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn is_expanded_or_inlined(&self) -> bool {
        let (_module_loc, qualified_goal) = self
            .machine_st
            .strip_module(self.machine_st.registers[1], empty_list_as_cell!());

        if HeapCellValueTag::Str == qualified_goal.get_tag() {
            let s = qualified_goal.get_value() as usize;
            let (name, arity) = cell_as_atom_cell!(self.machine_st.heap[s]).get_name_and_arity();

            if name == atom!("$call") {
                return false;
            }

            if self.machine_st.heap.len() > s + 1 + arity {
                let idx_cell = self.machine_st.heap[s + 1 + arity];

                if HeapCellValueTag::Cons == idx_cell.get_tag() {
                    match_untyped_arena_ptr!(cell_as_untyped_arena_ptr!(idx_cell),
                        (ArenaHeaderTag::IndexPtr, _ip) => {
                            return true;
                        }
                        _ => {
                        }
                    );
                }
            }
        }

        false
    }

    #[inline(always)]
    pub(crate) fn strip_module(&mut self) {
        let (module_loc, qualified_goal) = self
            .machine_st
            .strip_module(self.machine_st.registers[1], self.machine_st.registers[2]);

        let target_module_loc = self.machine_st.registers[2];

        unify_fn!(&mut self.machine_st, module_loc, target_module_loc);

        let target_qualified_goal = self.machine_st.registers[3];

        unify_fn!(&mut self.machine_st, qualified_goal, target_qualified_goal);
    }

    #[inline(always)]
    pub(crate) fn prepare_call_clause(&mut self, arity: usize) -> CallResult {
        let qualified_goal = self.deref_register(2);

        // the first two arguments don't belong to the containing call/N.
        let arity = arity - 2;

        let (name, narity, s) = self
            .machine_st
            .setup_call_n_init_goal_info(qualified_goal, arity)?;

        // assemble goal from pre-loaded (narity) and supplementary
        // (arity) arguments.

        let target_goal = if arity == 0 {
            qualified_goal
        } else {
            // if narity + arity > 0 {
            let h = self.machine_st.heap.len();
            self.machine_st
                .heap
                .push(atom_as_cell!(name, narity + arity));

            for idx in 1..narity + 1 {
                self.machine_st.heap.push(self.machine_st.heap[s + idx]);
            }

            for idx in 1..arity + 1 {
                self.machine_st
                    .heap
                    .push(self.machine_st.registers[2 + idx]);
            }

            if narity + arity > 0 {
                str_loc_as_cell!(h)
            } else {
                heap_loc_as_cell!(h)
            }
        };

        let target_qualified_goal = self.machine_st.registers[1];

        unify_fn!(&mut self.machine_st, target_goal, target_qualified_goal);

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn dynamic_module_resolution(
        &mut self,
        narity: usize,
    ) -> Result<(Atom, PredicateKey), MachineStub> {
        let module_name = self.deref_register(1);

        let module_name = read_heap_cell!(module_name,
            (HeapCellValueTag::Atom, (name, _arity)) => {
                debug_assert_eq!(_arity, 0);
                name
            }
            (HeapCellValueTag::Str, s) => {
                let (module_name, _arity) = cell_as_atom_cell!(self.machine_st.heap[s])
                    .get_name_and_arity();

                debug_assert_eq!(_arity, 0);
                module_name
            }
            _ if module_name.is_var() => {
                if let Some(load_context) = self.load_contexts.last() {
                    load_context.module
                } else {
                    atom!("user")
                }
            }
            _ => {
                unreachable!()
            }
        );

        let goal = self.deref_register(2);

        let (name, arity, s) = self.machine_st.setup_call_n_init_goal_info(goal, narity)?;

        // TODO: think we just need the 'Greater' branch here.
        match arity.cmp(&2) {
            Ordering::Less => {
                for i in arity + 1..arity + narity + 1 {
                    self.machine_st.registers[i] = self.machine_st.registers[i + 2 - arity];
                }
            }
            Ordering::Greater => {
                for i in (arity + 1..arity + narity + 1).rev() {
                    self.machine_st.registers[i] = self.machine_st.registers[i + 2 - arity];
                }
            }
            Ordering::Equal => {}
        }

        let key = (name, arity + narity);

        for i in 1..arity + 1 {
            self.machine_st.registers[i] = self.machine_st.heap[s + i];
        }

        Ok((module_name, key))
    }

    #[inline(always)]
    pub(crate) fn is_reset_cont_marker(&self, p: usize) -> bool {
        match &self.code[p] {
            &Instruction::CallResetContinuationMarker
            | &Instruction::ExecuteResetContinuationMarker => true,
            _ => false,
        }
    }

    #[inline(always)]
    pub(crate) fn bind_from_register(&mut self) {
        let reg = self.deref_register(2);
        let n = match Number::try_from(reg) {
            Ok(Number::Fixnum(n)) => usize::try_from(n.get_num()).ok(),
            Ok(Number::Integer(n)) => {
                let value: usize = (&*n).try_into().unwrap();
                Some(value)
            },
            _ => {
                unreachable!()
            }
        };

        if let Some(n) = n {
            if n <= MAX_ARITY {
                let target = self.machine_st.registers[n];
                let addr = self.machine_st.registers[1];

                unify_fn!(self.machine_st, addr, target);
                return;
            }
        }

        self.machine_st.fail = true;
    }

    #[inline(always)]
    pub(crate) fn current_hostname(&mut self) {
        #[cfg(feature = "hostname")]
        match hostname::get().ok() {
            Some(host) => match host.to_str() {
                Some(host) => {
                    let hostname = AtomTable::build_with(&self.machine_st.atom_tbl, host);

                    let a1 = self.deref_register(1);
                    self.machine_st.unify_atom(hostname, a1);

                    return;
                }
                None => {}
            },
            None => {}
        }

        self.machine_st.fail = true;
    }

    #[inline(always)]
    pub(crate) fn current_input(&mut self) -> CallResult {
        let addr = self.deref_register(1);
        let stream = self.user_input;

        if let Some(var) = addr.as_var() {
            self.machine_st.bind(var, stream_as_cell!(stream));
            return Ok(());
        }

        read_heap_cell!(addr,
            (HeapCellValueTag::Cons, cons_ptr) => {
                match_untyped_arena_ptr!(cons_ptr,
                    (ArenaHeaderTag::Stream, other_stream) => {
                        self.machine_st.fail = stream != other_stream;
                    }
                    _ => {
                        let stub = functor_stub(atom!("current_input"), 1);
                        let err = self.machine_st.domain_error(DomainErrorType::Stream, addr);

                        return Err(self.machine_st.error_form(err, stub));
                    }
                );
            }
            _ => {
                let stub = functor_stub(atom!("current_input"), 1);
                let err = self.machine_st.domain_error(DomainErrorType::Stream, addr);

                return Err(self.machine_st.error_form(err, stub));
            }
        );

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn current_output(&mut self) -> CallResult {
        let addr = self.deref_register(1);
        let stream = self.user_output;

        if let Some(var) = addr.as_var() {
            self.machine_st.bind(var, stream_as_cell!(stream));
            return Ok(());
        }

        read_heap_cell!(addr,
            (HeapCellValueTag::Cons, cons_ptr) => {
                match_untyped_arena_ptr!(cons_ptr,
                    (ArenaHeaderTag::Stream, other_stream) => {
                        self.machine_st.fail = stream != other_stream;
                    }
                    _ => {
                        let stub = functor_stub(atom!("current_output"), 1);
                        let err = self.machine_st.domain_error(DomainErrorType::Stream, addr);

                        return Err(self.machine_st.error_form(err, stub));
                    }
                );
            }
            _ => {
                let stub = functor_stub(atom!("current_output"), 1);
                let err = self.machine_st.domain_error(DomainErrorType::Stream, addr);

                return Err(self.machine_st.error_form(err, stub));
            }
        );

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn directory_files(&mut self) -> CallResult {
        if let Some(dir) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            let str = dir.as_str();
            let path = std::path::Path::new(&*str);
            let mut files = Vec::new();

            if let Ok(entries) = fs::read_dir(path) {
                for entry in entries {
                    if let Ok(entry) = entry {
                        if let Some(name) = entry.file_name().to_str() {
                            let name = AtomTable::build_with(&self.machine_st.atom_tbl, name);
                            files.push(atom_as_cstr_cell!(name));

                            continue;
                        }
                    }

                    let stub = functor_stub(atom!("directory_files"), 2);
                    let err = self.machine_st.representation_error(RepFlag::Character);
                    let err = self.machine_st.error_form(err, stub);

                    return Err(err);
                }

                let files_list = heap_loc_as_cell!(iter_to_heap_list(
                    &mut self.machine_st.heap,
                    files.into_iter()
                ));

                unify!(self.machine_st, self.machine_st.registers[2], files_list);
                return Ok(());
            }
        }

        self.machine_st.fail = true;
        Ok(())
    }

    #[inline(always)]
    pub(crate) fn file_size(&mut self) {
        if let Some(file) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            let len = Number::arena_from(
                fs::metadata(&*file.as_str()).unwrap().len(),
                &mut self.machine_st.arena,
            );

            match len {
                Number::Fixnum(n) => self
                    .machine_st
                    .unify_fixnum(n, self.machine_st.registers[2]),
                Number::Integer(n) => self
                    .machine_st
                    .unify_big_int(n, self.machine_st.registers[2]),
                _ => unreachable!(),
            }
        } else {
            self.machine_st.fail = true;
        }
    }

    #[inline(always)]
    pub(crate) fn file_exists(&mut self) {
        if let Some(file) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            let file_str = file.as_str();

            if !std::path::Path::new(&*file_str).exists()
                || !fs::metadata(&*file_str).unwrap().is_file()
            {
                self.machine_st.fail = true;
            }
        } else {
            self.machine_st.fail = true;
        }
    }

    #[inline(always)]
    pub(crate) fn directory_exists(&mut self) {
        if let Some(dir) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            let dir_str = dir.as_str();

            if !std::path::Path::new(&*dir_str).exists()
                || !fs::metadata(&*dir_str).unwrap().is_dir()
            {
                self.machine_st.fail = true;
            }
        } else {
            self.machine_st.fail = true;
        }
    }

    #[inline(always)]
    pub(crate) fn file_time(&mut self) {
        if let Some(file) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            let which = cell_as_atom!(self.deref_register(2));

            if let Ok(md) = fs::metadata(&*file.as_str()) {
                if let Ok(time) = match which {
                    atom!("modification") => md.modified(),
                    atom!("access") => md.accessed(),
                    atom!("creation") => md.created(),
                    _ => {
                        unreachable!()
                    }
                } {
                    let chars_atom = self.systemtime_to_timestamp(time);

                    self.machine_st
                        .unify_complete_string(chars_atom, self.machine_st.registers[3]);

                    return;
                }
            }
        }

        self.machine_st.fail = true;
    }

    #[inline(always)]
    pub(crate) fn directory_separator(&mut self) {
        self.machine_st
            .unify_char(std::path::MAIN_SEPARATOR, self.machine_st.registers[1]);
    }

    #[inline(always)]
    pub(crate) fn make_directory(&mut self) {
        if let Some(dir) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            match fs::create_dir(&*dir.as_str()) {
                Ok(_) => {}
                _ => {
                    self.machine_st.fail = true;
                }
            }
        } else {
            self.machine_st.fail = true;
        }
    }

    #[inline(always)]
    pub(crate) fn make_directory_path(&mut self) {
        if let Some(dir) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            match fs::create_dir_all(&*dir.as_str()) {
                Ok(_) => {}
                _ => {
                    self.machine_st.fail = true;
                }
            }
        } else {
            self.machine_st.fail = true;
        }
    }

    #[inline(always)]
    pub(crate) fn delete_file(&mut self) {
        if let Some(file) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            match fs::remove_file(&*file.as_str()) {
                Ok(_) => {}
                _ => {
                    self.machine_st.fail = true;
                }
            }
        }
    }

    #[inline(always)]
    pub(crate) fn rename_file(&mut self) {
        if let Some(file) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            if let Some(renamed) = self
                .machine_st
                .value_to_str_like(self.machine_st.registers[2])
            {
                if fs::rename(&*file.as_str(), &*renamed.as_str()).is_ok() {
                    return;
                }
            }
        }

        self.machine_st.fail = true;
    }

    #[inline(always)]
    pub(crate) fn file_copy(&mut self) {
        if let Some(file) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            if let Some(copied) = self
                .machine_st
                .value_to_str_like(self.machine_st.registers[2])
            {
                if fs::copy(&*file.as_str(), &*copied.as_str()).is_ok() {
                    return;
                }
            }
        }

        self.machine_st.fail = true;
    }

    #[inline(always)]
    pub(crate) fn delete_directory(&mut self) {
        if let Some(dir) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            match fs::remove_dir(&*dir.as_str()) {
                Ok(_) => {}
                _ => {
                    self.machine_st.fail = true;
                }
            }
        }
    }

    #[inline(always)]
    pub(crate) fn working_directory(&mut self) -> CallResult {
        if let Ok(dir) = env::current_dir() {
            let current = match dir.to_str() {
                Some(d) => d,
                _ => {
                    let stub = functor_stub(atom!("working_directory"), 2);
                    let err = self.machine_st.representation_error(RepFlag::Character);
                    let err = self.machine_st.error_form(err, stub);

                    return Err(err);
                }
            };

            let current_atom = AtomTable::build_with(&self.machine_st.atom_tbl, &current);

            let a1 = self.deref_register(1);
            self.machine_st.unify_complete_string(current_atom, a1);

            if self.machine_st.fail {
                return Ok(());
            }

            let target = self.deref_register(2);

            if let Some(next) = self.machine_st.value_to_str_like(target) {
                if env::set_current_dir(std::path::Path::new(&*next.as_str())).is_ok() {
                    return Ok(());
                }
            }
        }

        self.machine_st.fail = true;
        Ok(())
    }

    #[inline(always)]
    pub(crate) fn path_canonical(&mut self) -> CallResult {
        if let Some(path) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            match fs::canonicalize(&*path.as_str()) {
                Ok(canonical) => {
                    let cs = match canonical.to_str() {
                        Some(s) => s,
                        _ => {
                            let stub = functor_stub(atom!("path_canonical"), 2);
                            let err = self.machine_st.representation_error(RepFlag::Character);
                            let err = self.machine_st.error_form(err, stub);

                            return Err(err);
                        }
                    };

                    let canonical_atom = AtomTable::build_with(&self.machine_st.atom_tbl, cs);

                    let a2 = self.deref_register(2);
                    self.machine_st.unify_complete_string(canonical_atom, a2);

                    return Ok(());
                }
                _ => {}
            }
        }

        self.machine_st.fail = true;
        Ok(())
    }

    #[inline(always)]
    pub(crate) fn atom_chars(&mut self) {
        let a1 = self.deref_register(1);
        let a2 = self.deref_register(2);

        read_heap_cell!(a1,
            (HeapCellValueTag::Char) => {
                let h = self.machine_st.heap.len();

                self.machine_st.heap.push(a1);
                self.machine_st.heap.push(empty_list_as_cell!());

                unify!(self.machine_st, self.machine_st.registers[2], list_loc_as_cell!(h));
            }
            (HeapCellValueTag::Str, s) => {
                let (name, arity) = cell_as_atom_cell!(self.machine_st.heap[s])
                    .get_name_and_arity();

                if arity == 0 {
                    self.machine_st.unify_complete_string(
                        name,
                        a2,
                    );
                } else {
                    self.machine_st.fail = true;
                }
            }
            (HeapCellValueTag::Atom, (name, arity)) => {
                if arity == 0 {
                    self.machine_st.unify_complete_string(
                        name,
                        a2,
                    );
                } else {
                    self.machine_st.fail = true;
                }
            }
            (HeapCellValueTag::Var | HeapCellValueTag::AttrVar | HeapCellValueTag::StackVar) => {
                let a2 = self.deref_register(2);

                if let Some(str_like) = self.machine_st.value_to_str_like(a2) {
                    let atom_cell = match str_like {
                        AtomOrString::Atom(atom) => {
                            atom_as_cell!(if atom == atom!("[]") {
                                AtomTable::build_with(&self.machine_st.atom_tbl, "")
                            } else {
                                atom
                            })
                        }
                        AtomOrString::String(string) => {
                            atom_as_cell!(AtomTable::build_with(&self.machine_st.atom_tbl, &string))
                        }
                    };

                    self.machine_st.bind(a1.as_var().unwrap(), atom_cell);
                    return;
                }

                self.machine_st.fail = true;
            }
            _ => {
                unreachable!();
            }
        );
    }

    #[inline(always)]
    pub(crate) fn atom_codes(&mut self) -> CallResult {
        let a1 = self.deref_register(1);

        read_heap_cell!(a1,
            (HeapCellValueTag::Char, c) => {
                let h = self.machine_st.heap.len();

                self.machine_st.heap.push(fixnum_as_cell!(Fixnum::build_with(c as i64)));
                self.machine_st.heap.push(empty_list_as_cell!());

                unify!(self.machine_st, list_loc_as_cell!(h), self.machine_st.registers[2]);
            }
            (HeapCellValueTag::Atom, (name, arity)) => {
                if arity == 0 {
                    let name = name.as_str();
                    let iter = name.chars()
                        .map(|c| fixnum_as_cell!(Fixnum::build_with(c as i64)));

                    let h = iter_to_heap_list(&mut self.machine_st.heap, iter);
                    unify!(self.machine_st, heap_loc_as_cell!(h), self.machine_st.registers[2]);
                } else {
                    self.machine_st.fail = true;
                }
            }
            (HeapCellValueTag::Str, s) => {
                let (name, arity) = cell_as_atom_cell!(self.machine_st.heap[s])
                    .get_name_and_arity();

                if arity == 0 {
                    let name = name.as_str();
                    let iter = name.chars()
                        .map(|c| fixnum_as_cell!(Fixnum::build_with(c as i64)));

                    let h = iter_to_heap_list(&mut self.machine_st.heap, iter);
                    unify!(self.machine_st, heap_loc_as_cell!(h), self.machine_st.registers[2]);
                } else {
                    self.machine_st.fail = true;
                }
            }
            (HeapCellValueTag::Var | HeapCellValueTag::AttrVar | HeapCellValueTag::StackVar) => {
                let stub_gen = || functor_stub(atom!("atom_codes"), 2);

                match self.machine_st.try_from_list(self.machine_st.registers[2], stub_gen) {
                    Ok(addrs) => {
                        let string = self.machine_st.codes_to_string(addrs.into_iter(), stub_gen)?;
                        let atom = AtomTable::build_with(&self.machine_st.atom_tbl, &string);

                        self.machine_st.bind(a1.as_var().unwrap(), atom_as_cell!(atom));
                    }
                    Err(e) => {
                        return Err(e);
                    }
                }
            }
            _ => {
                unreachable!();
            }
        );

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn atom_length(&mut self) {
        let a1 = self.deref_register(1);

        let len: i64 = read_heap_cell!(a1,
            (HeapCellValueTag::Str, s) => {
                let (name, arity) = cell_as_atom_cell!(self.machine_st.heap[s])
                    .get_name_and_arity();

                if arity == 0 {
                    name.as_str().chars().count() as i64
                } else {
                    self.machine_st.fail = true;
                    return;
                }
            }
            (HeapCellValueTag::Atom, (name, arity)) => {
                if arity == 0 {
                    name.as_str().chars().count() as i64
                } else {
                    self.machine_st.fail = true;
                    return;
                }
            }
            (HeapCellValueTag::Char) => {
                1
            }
            _ => {
                unreachable!()
            }
        );

        let a2 = self.deref_register(2);
        self.machine_st.unify_fixnum(Fixnum::build_with(len), a2);
    }

    #[inline(always)]
    pub(crate) fn call_continuation(&mut self, last_call: bool) -> CallResult {
        let stub_gen = || functor_stub(atom!("call_continuation"), 1);
        let a1 = self.deref_register(1);

        match self.machine_st.try_from_list(a1, stub_gen) {
            Err(e) => Err(e),
            Ok(cont_chunks) => {
                let mut return_p = if last_call {
                    self.machine_st.cp
                } else {
                    self.machine_st.p + 1
                };

                self.machine_st.p = return_p;

                for chunk in cont_chunks.into_iter().rev() {
                    return_p = self.machine_st.call_continuation_chunk(chunk, return_p);
                }

                Ok(())
            }
        }
    }

    #[inline(always)]
    pub(crate) fn chars_to_number(&mut self) -> CallResult {
        let stub_gen = || functor_stub(atom!("number_chars"), 2);
        let a1 = self.deref_register(1);
        let atom_or_string = self.machine_st.value_to_str_like(a1).unwrap();

        self.machine_st
            .parse_number_from_string(&*atom_or_string.as_str(), &self.indices, stub_gen)
    }

    #[inline(always)]
    pub(crate) fn create_partial_string(&mut self) {
        let atom = cell_as_atom!(self.deref_register(1));

        if atom == atom!("") {
            self.machine_st.fail = true;
            return;
        }

        let pstr_h = self.machine_st.heap.len();

        self.machine_st.heap.push(pstr_as_cell!(atom));
        self.machine_st.heap.push(heap_loc_as_cell!(pstr_h + 1));

        unify!(
            self.machine_st,
            self.machine_st.registers[2],
            pstr_loc_as_cell!(pstr_h)
        );

        if !self.machine_st.fail {
            let tail = self.machine_st.registers[3];
            unify!(self.machine_st, tail, heap_loc_as_cell!(pstr_h + 1));
        }
    }

    #[inline(always)]
    pub(crate) fn is_partial_string(&mut self) {
        let value = self.deref_register(1);

        let h = self.machine_st.heap.len();
        self.machine_st.heap.push(value);

        let mut iter = HeapPStrIter::new(&self.machine_st.heap, h);

        while let Some(_) = iter.next() {}

        let at_end_of_pstr = iter.focus.is_var() || iter.at_string_terminator();
        self.machine_st.fail = !at_end_of_pstr;

        self.machine_st.heap.pop();
    }

    #[inline(always)]
    pub(crate) fn partial_string_tail(&mut self) {
        let pstr = self.deref_register(1);
        let a2 = self.deref_register(2);

        read_heap_cell!(pstr,
            (HeapCellValueTag::PStrLoc, h) => {
                let (h, _) = pstr_loc_and_offset(&self.machine_st.heap, h);

                if HeapCellValueTag::CStr == self.machine_st.heap[h].get_tag() {
                    self.machine_st.unify_atom(
                        atom!("[]"),
                        a2
                    );
                } else {
                    unify_fn!(
                        self.machine_st,
                        heap_loc_as_cell!(h+1),
                        a2
                    );
                }
            }
            (HeapCellValueTag::CStr) => {
                self.machine_st.unify_atom(
                    atom!("[]"),
                    a2
                );
            }
            (HeapCellValueTag::Lis, h) => {
                unify_fn!(
                    self.machine_st,
                    heap_loc_as_cell!(h+1),
                    self.machine_st.registers[2]
                );
            }
            _ => {
                self.machine_st.fail = true;
            }
        );
    }

    #[inline(always)]
    pub(crate) fn peek_byte(&mut self) -> CallResult {
        let stub_gen = || functor_stub(atom!("peek_byte"), 2);

        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("peek_byte"),
            2,
        )?;

        self.machine_st.check_stream_properties(
            stream,
            StreamType::Binary,
            Some(self.machine_st.registers[2]),
            atom!("peek_byte"),
            2,
        )?;

        if stream.past_end_of_stream() {
            if EOFAction::Reset != stream.options().eof_action() {
                return Ok(());
            } else if self.machine_st.fail {
                return Ok(());
            }
        }

        let addr = self.deref_register(2);

        if stream.at_end_of_stream() {
            self.machine_st.unify_fixnum(Fixnum::build_with(-1), addr);

            if self.machine_st.fail {
                self.machine_st.fail = false;
            } else {
                return Ok(());
            }
        }

        let addr = match addr {
            addr if addr.is_var() => addr,
            addr => match Number::try_from(addr) {
                Ok(Number::Integer(n)) => {
                    let result: Result<u8, _> = (&*n).try_into();
                    if let Ok(value) = result {
                        fixnum_as_cell!(Fixnum::build_with(value as i64))
                    } else {
                        let err = self.machine_st.type_error(ValidType::InByte, addr);
                        return Err(self.machine_st.error_form(err, stub_gen()));
                    }
                }
                Ok(Number::Fixnum(n)) => {
                    if let Ok(nb) = u8::try_from(n.get_num()) {
                        fixnum_as_cell!(Fixnum::build_with(nb as i64))
                    } else {
                        let err = self.machine_st.type_error(ValidType::InByte, addr);
                        return Err(self.machine_st.error_form(err, stub_gen()));
                    }
                }
                _ => {
                    let err = self.machine_st.type_error(ValidType::InByte, addr);
                    return Err(self.machine_st.error_form(err, stub_gen()));
                }
            },
        };

        loop {
            match stream.peek_byte().map_err(|e| e.kind()) {
                Ok(b) => {
                    self.machine_st
                        .unify_fixnum(Fixnum::build_with(b as i64), addr);
                    break;
                }
                Err(ErrorKind::PermissionDenied) => {
                    self.machine_st.fail = true;
                    break;
                }
                _ => {
                    self.machine_st.eof_action(
                        self.machine_st.registers[2],
                        stream,
                        atom!("peek_byte"),
                        2,
                    )?;

                    if EOFAction::Reset != stream.options().eof_action() {
                        break;
                    } else if self.machine_st.fail {
                        break;
                    }
                }
            }
        }

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn peek_char(&mut self) -> CallResult {
        let stub_gen = || functor_stub(atom!("peek_char"), 2);

        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("peek_char"),
            2,
        )?;

        self.machine_st.check_stream_properties(
            stream,
            StreamType::Text,
            Some(self.machine_st.registers[2]),
            atom!("peek_char"),
            2,
        )?;

        if stream.past_end_of_stream() {
            if EOFAction::Reset != stream.options().eof_action() {
                return Ok(());
            } else if self.machine_st.fail {
                return Ok(());
            }
        }

        let a2 = self.deref_register(2);

        if stream.at_end_of_stream() {
            let end_of_file = atom!("end_of_file");

            self.machine_st.unify_atom(end_of_file, a2);

            return Ok(());
        }

        let a2 = read_heap_cell!(a2,
            (HeapCellValueTag::Char) => {
                a2
            }
            (HeapCellValueTag::Atom, (name, arity)) => {
                if arity == 0 {
                    if let Some(c) = name.as_char() {
                        char_as_cell!(c)
                    } else {
                        let err = self.machine_st.type_error(ValidType::InCharacter, a2);
                        return Err(self.machine_st.error_form(err, stub_gen()));
                    }
                } else {
                    let err = self.machine_st.type_error(ValidType::InCharacter, a2);
                    return Err(self.machine_st.error_form(err, stub_gen()));
                }
            }
            (HeapCellValueTag::Var | HeapCellValueTag::StackVar | HeapCellValueTag::AttrVar) => {
                a2
            }
            _ => {
                let err = self.machine_st.type_error(ValidType::InCharacter, a2);
                return Err(self.machine_st.error_form(err, stub_gen()));
            }
        );

        loop {
            match stream
                .peek_char()
                .map(|result| result.map_err(|e| e.kind()))
            {
                Some(Ok(d)) => {
                    self.machine_st.unify_char(d, a2);
                    break;
                }
                Some(Err(ErrorKind::PermissionDenied)) => {
                    self.machine_st.fail = true;
                    break;
                }
                _ => {
                    self.machine_st.eof_action(
                        self.machine_st.registers[2],
                        stream,
                        atom!("peek_char"),
                        2,
                    )?;

                    if EOFAction::Reset != stream.options().eof_action() {
                        break;
                    } else if self.machine_st.fail {
                        break;
                    }
                }
            }
        }

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn peek_code(&mut self) -> CallResult {
        let stub_gen = || functor_stub(atom!("peek_code"), 2);

        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("peek_code"),
            2,
        )?;

        self.machine_st.check_stream_properties(
            stream,
            StreamType::Text,
            Some(self.machine_st.registers[2]),
            atom!("peek_code"),
            2,
        )?;

        if stream.past_end_of_stream() {
            if EOFAction::Reset != stream.options().eof_action() {
                return Ok(());
            } else if self.machine_st.fail {
                return Ok(());
            }
        }

        let a2 = self.deref_register(2);

        if stream.at_end_of_stream() {
            self.machine_st.unify_fixnum(Fixnum::build_with(-1), a2);

            if self.machine_st.fail {
                self.machine_st.fail = false;
            } else {
                return Ok(());
            }
        }

        let addr = read_heap_cell!(a2,
            (HeapCellValueTag::Var | HeapCellValueTag::StackVar | HeapCellValueTag::AttrVar) => {
                a2
            }
            _ => {
                match Number::try_from(a2) {
                    Ok(Number::Integer(n)) => {
                        let n: u32 = (&*n).try_into().unwrap();

                        let n = std::char::from_u32(n).and_then(|_| Some(n));

                        if let Some(n) = n {
                            fixnum_as_cell!(Fixnum::build_with(n as i64))
                        } else {
                            let err = self.machine_st.representation_error(RepFlag::InCharacterCode);
                            return Err(self.machine_st.error_form(err, stub_gen()));
                        }
                    }
                    Ok(Number::Fixnum(n)) => {
                        let n = u32::try_from(n.get_num())
                            .ok()
                            .and_then(|n| std::char::from_u32(n).and_then(|_| Some(n)));

                        if let Some(n) = n {
                            fixnum_as_cell!(Fixnum::build_with(n as i64))
                        } else {
                            let err = self.machine_st.representation_error(RepFlag::InCharacterCode);
                            return Err(self.machine_st.error_form(err, stub_gen()));
                        }
                    }
                    _ => {
                        let err = self.machine_st.type_error(ValidType::Integer, self.machine_st.registers[2]);
                        return Err(self.machine_st.error_form(err, stub_gen()));
                    }
                }
            }
        );

        loop {
            let result = stream.peek_char();

            match result.map(|result| result.map_err(|e| e.kind())) {
                Some(Ok(c)) => {
                    self.machine_st
                        .unify_fixnum(Fixnum::build_with(c as i64), addr);
                    break;
                }
                Some(Err(ErrorKind::PermissionDenied)) => {
                    self.machine_st.fail = true;
                    break;
                }
                _ => {
                    self.machine_st.eof_action(
                        self.machine_st.registers[2],
                        stream,
                        atom!("peek_code"),
                        2,
                    )?;

                    if EOFAction::Reset != stream.options().eof_action() {
                        break;
                    } else if self.machine_st.fail {
                        break;
                    }
                }
            }
        }

        return Ok(());
    }

    #[inline(always)]
    pub(crate) fn number_to_chars(&mut self) {
        let n = self.deref_register(1);
        let chs = self.deref_register(2);

        let string = match Number::try_from(n) {
            Ok(Number::Float(OrderedFloat(n))) => fmt_float(n),
            Ok(Number::Fixnum(n)) => n.get_num().to_string(),
            Ok(Number::Integer(n)) => n.to_string(),
            Ok(Number::Rational(r)) => {
                // n has already been confirmed as an integer, and
                // internally, Rational is assumed reduced, so its denominator
                // must be 1.
                r.numerator().to_string()
            }
            _ => {
                unreachable!()
            }
        };

        let chars_atom = AtomTable::build_with(&self.machine_st.atom_tbl, &string.trim());
        self.machine_st.unify_complete_string(chars_atom, chs);
    }

    #[inline(always)]
    pub(crate) fn number_to_codes(&mut self) {
        let n = self.deref_register(1);
        let chs = self.machine_st.registers[2];

        let string = match Number::try_from(n) {
            Ok(Number::Float(OrderedFloat(n))) => {
                format!("{0:<20?}", n)
            }
            Ok(Number::Fixnum(n)) => n.get_num().to_string(),
            Ok(Number::Integer(n)) => n.to_string(),
            Ok(Number::Rational(r)) => {
                // n has already been confirmed as an integer, and
                // internally, Rational is assumed reduced, so its
                // denominator must be 1.
                r.numerator().to_string()
            }
            _ => {
                unreachable!()
            }
        };

        let codes = string
            .trim()
            .chars()
            .map(|c| fixnum_as_cell!(Fixnum::build_with(c as i64)));

        let h = iter_to_heap_list(&mut self.machine_st.heap, codes);
        unify!(self.machine_st, heap_loc_as_cell!(h), chs);
    }

    #[inline(always)]
    pub(crate) fn codes_to_number(&mut self) -> CallResult {
        let stub_gen = || functor_stub(atom!("number_codes"), 2);

        match self
            .machine_st
            .try_from_list(self.machine_st.registers[1], stub_gen)
        {
            Err(e) => {
                return Err(e);
            }
            Ok(addrs) => {
                let string = self
                    .machine_st
                    .codes_to_string(addrs.into_iter(), stub_gen)?;
                self.machine_st.parse_number_from_string(
                    string.as_str(),
                    &self.indices,
                    stub_gen,
                )?;
            }
        }

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn lifted_heap_length(&mut self) {
        let a1 = self.machine_st.registers[1];
        let lh_len = Fixnum::build_with(self.machine_st.lifted_heap.len() as i64);

        self.machine_st.unify_fixnum(lh_len, a1);
    }

    #[inline(always)]
    pub(crate) fn char_code(&mut self) -> CallResult {
        let stub_gen = || functor_stub(atom!("char_code"), 2);
        let a1 = self.deref_register(1);
        let a2 = self.deref_register(2);

        let c = read_heap_cell!(a1,
            (HeapCellValueTag::Atom, (name, _arity)) => {
                name.as_char().unwrap()
            }
            (HeapCellValueTag::Str, s) => {
                let (name, arity) = cell_as_atom_cell!(self.machine_st.heap[s])
                    .get_name_and_arity();

                debug_assert_eq!(arity, 0);
                name.as_char().unwrap()
            }
            (HeapCellValueTag::Char, c) => {
                c
            }
            _ => {
                match Number::try_from(a2) {
                    Ok(Number::Integer(n)) => {
                        let n: u32 = (&*n).try_into().unwrap();
                        let n = std::char::from_u32(n);
                        let c = match n {
                            Some(c) => c,
                            _ => {
                                let err = self.machine_st.representation_error(RepFlag::CharacterCode);
                                return Err(self.machine_st.error_form(err, stub_gen()));
                            }
                        };

                        self.machine_st.unify_char(c, a1);
                        return Ok(());
                    }
                    Ok(Number::Fixnum(n)) => {
                        match u32::try_from(n.get_num()) {
                            Ok(n) => {
                                if let Some(c) = std::char::from_u32(n) {
                                    self.machine_st.unify_char(c, a1);
                                    return Ok(());
                                }
                            }
                            _ => {}
                        }

                        let err = self.machine_st.representation_error(RepFlag::CharacterCode);
                        return Err(self.machine_st.error_form(err, stub_gen()));
                    }
                    _ => {
                        self.machine_st.fail = true;
                        return Ok(());
                    }
                }
            }
        );

        self.machine_st
            .unify_fixnum(Fixnum::build_with(c as i64), a2);

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn char_type(&mut self) {
        let a1 = self.deref_register(1);
        let a2 = self.deref_register(2);

        let c = read_heap_cell!(a1,
            (HeapCellValueTag::Char, c) => {
                c
            }
            (HeapCellValueTag::Atom, (name, _arity)) => {
                name.as_char().unwrap()
            }
            (HeapCellValueTag::Str, s) => {
                let (name, arity) = cell_as_atom_cell!(self.machine_st.heap[s])
                    .get_name_and_arity();

                debug_assert_eq!(arity, 0);
                name.as_char().unwrap()
            }
            _ => {
                unreachable!()
            }
        );

        self.machine_st.fail = true; // This predicate fails by default.

        read_heap_cell!(a2,
            (HeapCellValueTag::Atom, (chars, _arity)) => {
                    macro_rules! macro_check {
                ($id:ident, $name:expr) => {
                if $id!(c) && chars == $name {
                    self.machine_st.fail = false;
                    return;
                }
                };
            }

            macro_rules! method_check {
                ($id:ident, $name:expr) => {
                if c.$id() && chars == $name {
                    self.machine_st.fail = false;
                    return;
                }
                };
            }

            macro_check!(alpha_char, atom!("alpha"));
            method_check!(is_alphabetic, atom!("alphabetic"));
            method_check!(is_alphanumeric, atom!("alphanumeric"));
            macro_check!(alpha_numeric_char, atom!("alnum"));
            method_check!(is_ascii, atom!("ascii"));
            method_check!(is_ascii_punctuation, atom!("ascii_punctuation"));
            method_check!(is_ascii_graphic, atom!("ascii_graphic"));
            // macro_check!(backslash_char, atom!("backslash"));
            // macro_check!(back_quote_char, atom!("back_quote"));
            macro_check!(binary_digit_char, atom!("binary_digit"));
            // macro_check!(capital_letter_char, atom!("upper"));
            // macro_check!(comment_1_char, "comment_1");
            // macro_check!(comment_2_char, "comment_2");
            method_check!(is_control, atom!("control"));
            // macro_check!(cut_char, atom!("cut"));
            macro_check!(decimal_digit_char, atom!("decimal_digit"));
            // macro_check!(decimal_point_char, atom!("decimal_point"));
            // macro_check!(double_quote_char, atom!("double_quote"));
            macro_check!(exponent_char, atom!("exponent"));
            macro_check!(graphic_char, atom!("graphic"));
            macro_check!(graphic_token_char, atom!("graphic_token"));
            macro_check!(hexadecimal_digit_char, atom!("hexadecimal_digit"));
            macro_check!(layout_char, atom!("layout"));
            method_check!(is_lowercase, atom!("lower"));
            macro_check!(meta_char, atom!("meta"));
            // macro_check!(new_line_char, atom!("new_line"));
            method_check!(is_numeric, atom!("numeric"));
            macro_check!(octal_digit_char, atom!("octal_digit"));
            macro_check!(octet_char, atom!("octet"));
            macro_check!(prolog_char, atom!("prolog"));
            // macro_check!(semicolon_char, atom!("semicolon"));
            macro_check!(sign_char, atom!("sign"));
            // macro_check!(single_quote_char, atom!("single_quote"));
            // macro_check!(small_letter_char, atom!("lower"));
            macro_check!(solo_char, atom!("solo"));
            // macro_check!(space_char, atom!("space"));
            macro_check!(symbolic_hexadecimal_char, atom!("symbolic_hexadecimal"));
            macro_check!(symbolic_control_char, atom!("symbolic_control"));
            method_check!(is_uppercase, atom!("upper"));
            // macro_check!(variable_indicator_char, atom!("variable_indicator"));
            method_check!(is_whitespace, atom!("whitespace"));
            }
                (HeapCellValueTag::Str, s) => {
                let (name, arity) = cell_as_atom_cell!(self.machine_st.heap[s])
                .get_name_and_arity();

            match (name, arity) {
                (atom!("to_upper"), 1) => {
                let reg = self.machine_st.deref(self.machine_st.heap[s+1]);
                let atom = AtomTable::build_with(&self.machine_st.atom_tbl, &c.to_uppercase().to_string());
                let upper_str = string_as_cstr_cell!(atom);
                unify!(self.machine_st, reg, upper_str);
                self.machine_st.fail = false;
                }
                (atom!("to_lower"), 1) => {
                let reg = self.machine_st.deref(self.machine_st.heap[s+1]);
                let atom = AtomTable::build_with(&self.machine_st.atom_tbl, &c.to_lowercase().to_string());
                let lower_str = string_as_cstr_cell!(atom);
                unify!(self.machine_st, reg, lower_str);
                self.machine_st.fail = false;
                }
                _ => {
                unreachable!()
                }
            };
            }
                _ => {
                unreachable!()
            }
        );
    }

    #[inline(always)]
    pub(crate) fn check_cut_point(&mut self) {
        let addr = self.deref_register(1);
        let old_b = cell_as_fixnum!(addr).get_num() as usize;

        let prev_b = self
            .machine_st
            .stack
            .index_or_frame(self.machine_st.b)
            .prelude
            .b;
        let prev_b = self.machine_st.stack.index_or_frame(prev_b).prelude.b;

        if prev_b > old_b {
            self.machine_st.fail = true;
        }
    }

    #[inline(always)]
    pub(crate) fn copy_term_without_attr_vars(&mut self) {
        self.machine_st.copy_term(AttrVarPolicy::StripAttributes);
    }

    #[inline(always)]
    pub(crate) fn fetch_global_var(&mut self) {
        let key = cell_as_atom!(self.deref_register(1));
        let addr = self.machine_st.registers[2];

        match self.indices.global_variables.get_mut(&key) {
            Some((ref ball, ref mut loc)) => match loc {
                Some(value_loc) => {
                    unify_fn!(self.machine_st, addr, *value_loc);
                }
                None if !ball.stub.is_empty() => {
                    let h = self.machine_st.heap.len();
                    let stub = ball.copy_and_align(h);

                    self.machine_st.heap.extend(stub.into_iter());

                    unify_fn!(self.machine_st, addr, heap_loc_as_cell!(h));

                    if !self.machine_st.fail {
                        *loc = Some(heap_loc_as_cell!(h));
                        self.machine_st.trail(TrailRef::BlackboardEntry(key));
                    }
                }
                _ => self.machine_st.fail = true,
            },
            None => self.machine_st.fail = true,
        };
    }

    #[inline(always)]
    pub(crate) fn put_code(&mut self) -> CallResult {
        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("put_code"),
            2,
        )?;

        self.machine_st.check_stream_properties(
            stream,
            StreamType::Text,
            None,
            atom!("put_code"),
            2,
        )?;

        let stub_gen = || functor_stub(atom!("put_code"), 2);
        let addr = self.deref_register(2);

        if addr.is_var() {
            let err = self.machine_st.instantiation_error();
            return Err(self.machine_st.error_form(err, stub_gen()));
        } else {
            match Number::try_from(addr) {
                Ok(Number::Integer(n)) => {
                    let n: u32 = (&*n).try_into().unwrap();
                    let n = char::try_from(n);
                    if let Some(c) = n.ok() {
                        write!(&mut stream, "{}", c).unwrap();
                        return Ok(());
                    }
                }
                Ok(Number::Fixnum(n)) => {
                    let n = n.get_num();

                    if let Some(c) = u32::try_from(n).ok().and_then(|c| char::from_u32(c)) {
                        write!(&mut stream, "{}", c).unwrap();
                        return Ok(());
                    }
                }
                _ => {
                    let err = self.machine_st.type_error(ValidType::Integer, addr);
                    return Err(self.machine_st.error_form(err, stub_gen()));
                }
            }

            let err = self.machine_st.representation_error(RepFlag::CharacterCode);
            return Err(self.machine_st.error_form(err, stub_gen()));
        }
    }

    #[inline(always)]
    pub(crate) fn put_char(&mut self) -> CallResult {
        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("put_char"),
            2,
        )?;

        self.machine_st.check_stream_properties(
            stream,
            StreamType::Text,
            None,
            atom!("put_char"),
            2,
        )?;

        let stub_gen = || functor_stub(atom!("put_char"), 2);
        let addr = self.deref_register(2);

        if addr.is_var() {
            let err = self.machine_st.instantiation_error();
            return Err(self.machine_st.error_form(err, stub_gen()));
        } else {
            read_heap_cell!(addr,
                (HeapCellValueTag::Atom, (name, _arity)) => {
                    if let Some(c) = name.as_char() {
                        write!(&mut stream, "{}", c).unwrap();
                        return Ok(());
                    }
                }
                (HeapCellValueTag::Char, c) => {
                    write!(&mut stream, "{}", c).unwrap();
                    return Ok(());
                }
                _ => {
                }
            );

            let err = self.machine_st.type_error(ValidType::Character, addr);
            return Err(self.machine_st.error_form(err, stub_gen()));
        }
    }

    #[inline(always)]
    pub(crate) fn put_chars(&mut self) -> CallResult {
        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("$put_chars"),
            2,
        )?;

        let mut bytes = Vec::new();
        let stub_gen = || functor_stub(atom!("$put_chars"), 2);

        if let Some(string) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[2])
        {
            if stream.options().stream_type() == StreamType::Binary {
                for c in string.as_str().chars() {
                    if c as u32 > 255 {
                        let err = self
                            .machine_st
                            .type_error(ValidType::Byte, char_as_cell!(c));
                        return Err(self.machine_st.error_form(err, stub_gen()));
                    }

                    bytes.push(c as u8);
                }
            } else {
                bytes = string.as_str().bytes().collect();
            }

            match stream.write_all(&bytes) {
                Ok(_) => {}
                _ => {
                    let addr = stream_as_cell!(stream);
                    let err = self
                        .machine_st
                        .existence_error(ExistenceError::Stream(addr));

                    return Err(self.machine_st.error_form(err, stub_gen()));
                }
            }
        } else {
            self.machine_st.fail = true;
        }

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn put_byte(&mut self) -> CallResult {
        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("put_byte"),
            2,
        )?;

        self.machine_st.check_stream_properties(
            stream,
            StreamType::Binary,
            None,
            atom!("put_byte"),
            2,
        )?;

        let stub_gen = || functor_stub(atom!("put_byte"), 2);
        let addr = self.deref_register(2);

        if addr.is_var() {
            let err = self.machine_st.instantiation_error();
            return Err(self.machine_st.error_form(err, stub_gen()));
        } else {
            match Number::try_from(addr) {
                Ok(Number::Integer(n)) => {
                    let n: u8 = (&*n).try_into().unwrap();

                    match n {
                        nb => {
                            match stream.write(&mut [nb]) {
                                Ok(1) => {
                                    return Ok(());
                                }
                                _ => {
                                    let err = self.machine_st.existence_error(
                                        ExistenceError::Stream(stream_as_cell!(stream))
                                    );

                                    return Err(self.machine_st.error_form(err, stub_gen()));
                                }
                            }
                        }
                    }
                }
                Ok(Number::Fixnum(n)) => {
                    if let Ok(nb) = u8::try_from(n.get_num()) {
                        match stream.write(&mut [nb]) {
                            Ok(1) => {
                                return Ok(());
                            }
                            _ => {
                                let err = self.machine_st.existence_error(ExistenceError::Stream(
                                    stream_as_cell!(stream),
                                ));

                                return Err(self.machine_st.error_form(err, stub_gen()));
                            }
                        }
                    }
                }
                _ => {}
            }
        }

        let err = self
            .machine_st
            .type_error(ValidType::Byte, self.machine_st.registers[2]);
        Err(self.machine_st.error_form(err, stub_gen()))
    }

    #[inline(always)]
    pub(crate) fn get_byte(&mut self) -> CallResult {
        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("get_byte"),
            2,
        )?;

        self.machine_st.check_stream_properties(
            stream,
            StreamType::Binary,
            Some(self.machine_st.registers[2]),
            atom!("get_byte"),
            2,
        )?;

        if stream.past_end_of_stream() {
            self.machine_st.eof_action(
                self.machine_st.registers[2],
                stream,
                atom!("get_byte"),
                2,
            )?;

            if EOFAction::Reset != stream.options().eof_action() {
                return Ok(());
            } else if self.machine_st.fail {
                return Ok(());
            }
        }

        let stub_gen = || functor_stub(atom!("get_byte"), 2);
        let addr = self.deref_register(2);

        let addr = if addr.is_var() {
            addr
        } else {
            match Number::try_from(addr) {
                Ok(Number::Integer(ref n)) if (**n).num_eq(&1_i64) => {
                    fixnum_as_cell!(Fixnum::build_with(-1))
                }
                Ok(Number::Fixnum(n)) if n.get_num() == -1_i64 => {
                    fixnum_as_cell!(Fixnum::build_with(-1))
                }
                Ok(Number::Integer(n)) => {
                    let n: Result<u8, _> = (&*n).try_into();

                    if let Ok(value) = n {
                        fixnum_as_cell!(Fixnum::build_with(value as i64))
                    } else {
                        let err = self.machine_st.type_error(ValidType::InByte, addr);
                        return Err(self.machine_st.error_form(err, stub_gen()));
                    }
                }
                Ok(Number::Fixnum(n)) => {
                    if let Ok(nb) = u8::try_from(n.get_num()) {
                        fixnum_as_cell!(Fixnum::build_with(nb as i64))
                    } else {
                        let err = self.machine_st.type_error(ValidType::InByte, addr);
                        return Err(self.machine_st.error_form(err, stub_gen()));
                    }
                }
                _ => {
                    let err = self.machine_st.type_error(ValidType::InByte, addr);
                    return Err(self.machine_st.error_form(err, stub_gen()));
                }
            }
        };

        loop {
            let mut b = [0u8; 1];

            match stream.read(&mut b) {
                Ok(1) => {
                    self.machine_st
                        .unify_fixnum(Fixnum::build_with(b[0] as i64), addr);
                    break;
                }
                _ => {
                    stream.set_past_end_of_stream(true);
                    self.machine_st
                        .unify_fixnum(Fixnum::build_with(-1), self.machine_st.registers[2]);
                    break;
                }
            }
        }

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn get_char(&mut self) -> CallResult {
        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("get_char"),
            2,
        )?;

        self.machine_st.check_stream_properties(
            stream,
            StreamType::Text,
            Some(self.machine_st.registers[2]),
            atom!("get_char"),
            2,
        )?;

        if stream.past_end_of_stream() {
            if EOFAction::Reset != stream.options().eof_action() {
                return Ok(());
            } else if self.machine_st.fail {
                return Ok(());
            }
        }

        let addr = self.deref_register(2);

        if stream.at_end_of_stream() {
            let end_of_file = atom!("end_of_file");
            stream.set_past_end_of_stream(true);

            self.machine_st.unify_atom(end_of_file, addr);

            return Ok(());
        }

        let stub_gen = || functor_stub(atom!("get_char"), 2);
        let result = self.machine_st.open_parsing_stream(stream);

        let addr = if addr.is_var() {
            addr
        } else {
            read_heap_cell!(addr,
                (HeapCellValueTag::Atom, (atom, _arity)) => {
                    char_as_cell!(atom.as_char().unwrap())
                }
                (HeapCellValueTag::Char) => {
                    addr
                }
                _ => {
                    let err = self.machine_st.type_error(ValidType::InCharacter, addr);
                    return Err(self.machine_st.error_form(err, stub_gen()));
                }
            )
        };

        let mut iter = match result {
            Ok(iter) => iter,
            Err(e) => {
                if e.is_unexpected_eof() {
                    return self.machine_st.eof_action(
                        self.machine_st.registers[2],
                        stream,
                        atom!("get_char"),
                        2,
                    );
                } else {
                    let err = self.machine_st.session_error(SessionError::from(e));
                    return Err(self.machine_st.error_form(err, stub_gen()));
                }
            }
        };

        loop {
            match iter.read_char() {
                Some(Ok(c)) => {
                    self.machine_st.unify_char(c, addr);
                    break;
                }
                _ => {
                    self.machine_st.eof_action(
                        self.machine_st.registers[2],
                        stream,
                        atom!("get_char"),
                        2,
                    )?;

                    if EOFAction::Reset != stream.options().eof_action() {
                        break;
                    } else if self.machine_st.fail {
                        break;
                    }
                }
            }
        }

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn get_n_chars(&mut self) -> CallResult {
        let stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("get_n_chars"),
            3,
        )?;

        let num = match Number::try_from(self.deref_register(2)) {
            Ok(Number::Fixnum(n)) => usize::try_from(n.get_num()).unwrap(),
            Ok(Number::Integer(n)) => match (&*n).try_into() as Result<usize, _> {
                Ok(u) => {
                    u
                }
                _ => {
                    self.machine_st.fail = true;
                    return Ok(());
                }
            },
            _ => {
                unreachable!()
            }
        };

        let mut string = String::new();

        if stream.options().stream_type() == StreamType::Binary {
            let mut buf = vec![];
            let mut chunk = stream.take(num as u64);

            chunk.read_to_end(&mut buf).ok();

            for c in buf {
                string.push(c as char);
            }
        } else {
            let mut iter = self.machine_st.open_parsing_stream(stream).map_err(|e| {
                let err = self.machine_st.session_error(SessionError::from(e));
                let stub = functor_stub(atom!("get_n_chars"), 2);

                self.machine_st.error_form(err, stub)
            })?;

            for _ in 0..num {
                let result = iter.read_char();

                match result {
                    Some(Ok(c)) => {
                        string.push(c);
                    }
                    _ => {
                        break;
                    }
                }
            }
        };

        let output = self.deref_register(3);
        let atom = AtomTable::build_with(&self.machine_st.atom_tbl, &string);

        self.machine_st.unify_complete_string(atom, output);
        Ok(())
    }

    #[inline(always)]
    pub(crate) fn get_code(&mut self) -> CallResult {
        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("get_code"),
            2,
        )?;

        self.machine_st.check_stream_properties(
            stream,
            StreamType::Text,
            Some(self.machine_st.registers[2]),
            atom!("get_code"),
            2,
        )?;

        if stream.past_end_of_stream() {
            if EOFAction::Reset != stream.options().eof_action() {
                return Ok(());
            } else if self.machine_st.fail {
                return Ok(());
            }
        }

        let addr = self.deref_register(2);

        if stream.at_end_of_stream() {
            stream.set_past_end_of_stream(true);

            self.machine_st.unify_fixnum(Fixnum::build_with(-1), addr);

            return Ok(());
        }

        let stub_gen = || functor_stub(atom!("get_code"), 2);

        let addr = if addr.is_var() {
            addr
        } else {
            match Number::try_from(addr) {
                Ok(Number::Integer(n)) => {
                    let n: u32 = (&*n).try_into().unwrap();
                    let n = std::char::from_u32(n);

                    if let Some(n) = n {
                        fixnum_as_cell!(Fixnum::build_with(n as i64))
                    } else {
                        let err = self
                            .machine_st
                            .representation_error(RepFlag::InCharacterCode);
                        return Err(self.machine_st.error_form(err, stub_gen()));
                    }
                }
                Ok(Number::Fixnum(n)) => {
                    let nf = u32::try_from(n.get_num())
                        .ok()
                        .and_then(|n| std::char::from_u32(n));

                    if nf.is_some() {
                        fixnum_as_cell!(n)
                    } else {
                        let err = self
                            .machine_st
                            .representation_error(RepFlag::InCharacterCode);
                        return Err(self.machine_st.error_form(err, stub_gen()));
                    }
                }
                _ => {
                    let err = self
                        .machine_st
                        .type_error(ValidType::Integer, self.machine_st.registers[2]);
                    return Err(self.machine_st.error_form(err, stub_gen()));
                }
            }
        };

        let mut iter = self.machine_st.open_parsing_stream(stream).map_err(|e| {
            let err = self.machine_st.session_error(SessionError::from(e));
            let stub = functor_stub(atom!("get_code"), 2);

            self.machine_st.error_form(err, stub)
        })?;

        loop {
            let result = iter.read_char();

            match result {
                Some(Ok(c)) => {
                    self.machine_st
                        .unify_fixnum(Fixnum::build_with(c as i64), addr);
                    break;
                }
                _ => {
                    self.machine_st.eof_action(
                        self.machine_st.registers[2],
                        stream,
                        atom!("get_code"),
                        2,
                    )?;

                    if EOFAction::Reset != stream.options().eof_action() {
                        break;
                    } else if self.machine_st.fail {
                        break;
                    }
                }
            }
        }

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn first_stream(&mut self) {
        let mut first_stream = None;
        let mut null_streams = BTreeSet::new();

        for stream in self.indices.streams.iter().cloned() {
            if !stream.is_null_stream() {
                first_stream = Some(stream);
                break;
            } else {
                null_streams.insert(stream);
            }
        }

        self.indices.streams = self.indices.streams.sub(&null_streams);

        if let Some(first_stream) = first_stream {
            let stream = stream_as_cell!(first_stream);

            let var = self.deref_register(1).as_var().unwrap();

            self.machine_st.bind(var, stream);
        } else {
            self.machine_st.fail = true;
        }
    }

    #[inline(always)]
    pub(crate) fn next_stream(&mut self) {
        let prev_stream = cell_as_stream!(self.deref_register(1));

        let mut next_stream = None;
        let mut null_streams = BTreeSet::new();

        for stream in self.indices.streams.range(prev_stream..).skip(1).cloned() {
            if !stream.is_null_stream() {
                next_stream = Some(stream);
                break;
            } else {
                null_streams.insert(stream);
            }
        }

        self.indices.streams = self.indices.streams.sub(&null_streams);

        if let Some(next_stream) = next_stream {
            let var = self.deref_register(2).as_var().unwrap();

            let next_stream = stream_as_cell!(next_stream);
            self.machine_st.bind(var, next_stream);
        } else {
            self.machine_st.fail = true;
        }
    }

    #[inline(always)]
    pub(crate) fn flush_output(&mut self) -> CallResult {
        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("flush_output"),
            1,
        )?;

        if !stream.is_output_stream() {
            let stub = functor_stub(atom!("flush_output"), 1);
            let addr = stream_as_cell!(stream);

            let err =
                self.machine_st
                    .permission_error(Permission::OutputStream, atom!("stream"), addr);

            return Err(self.machine_st.error_form(err, stub));
        }

        stream.flush().unwrap();
        Ok(())
    }

    #[cfg(feature = "repl")]
    #[inline(always)]
    pub(crate) fn get_single_char(&mut self) -> CallResult {
        let ctrl_c = KeyEvent {
            code: KeyCode::Char('c'),
            modifiers: KeyModifiers::CONTROL,
        };

        let key = get_key();

        if key == ctrl_c {
            let stub = functor_stub(atom!("get_single_char"), 1);
            let err = self.machine_st.interrupt_error();
            let err = self.machine_st.error_form(err, stub);

            return Err(err);
        }

        let c = match key.code {
            KeyCode::Enter => '\n',
            KeyCode::Tab => '\t',
            KeyCode::Char(c) => c,
            _ => unreachable!(),
        };
        let a1 = self.deref_register(1);
        self.machine_st.unify_char(c, a1);

        Ok(())
    }

    #[cfg(not(feature = "repl"))]
    #[inline(always)]
    pub(crate) fn get_single_char(&mut self) -> CallResult {
        let mut buffer = [0; 1];
        // is there a better way?
        if std::io::stdin().read(&mut buffer).is_err() {
            let stub = functor_stub(atom!("get_single_char"), 1);
            let err = self.machine_st.interrupt_error();
            let err = self.machine_st.error_form(err, stub);

            return Err(err);
        }
        let c = buffer[0] as char;
        let a1 = self.deref_register(1);
        self.machine_st.unify_char(c, a1);

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn head_is_dynamic(&mut self) {
        let module_name = cell_as_atom!(self.deref_register(1));

        match self
            .machine_st
            .name_and_arity_from_heap(self.machine_st.registers[2])
        {
            Some((name, arity)) => {
                self.machine_st.fail = !self
                    .indices
                    .is_dynamic_predicate(module_name, (name, arity));
            }
            None => {
                self.machine_st.fail = true;
            }
        }
    }

    #[inline(always)]
    pub(crate) fn close(&mut self) -> CallResult {
        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("close"),
            2,
        )?;

        if !stream.is_input_stream() {
            stream.flush().unwrap(); // 8.11.6.1b)
        }

        self.indices.streams.remove(&stream);

        if stream == self.user_input {
            self.user_input = self
                .indices
                .stream_aliases
                .get(&atom!("user_input"))
                .cloned()
                .unwrap();

            self.indices.streams.insert(self.user_input);
        } else if stream == self.user_output {
            self.user_output = self
                .indices
                .stream_aliases
                .get(&atom!("user_output"))
                .cloned()
                .unwrap();

            self.indices.streams.insert(self.user_output);
        }

        if !stream.is_stdin() && !stream.is_stdout() && !stream.is_stderr() {
            if let Some(alias) = stream.options().get_alias() {
                self.indices.stream_aliases.remove(&alias);
            }

            let close_result = stream.close();

            if let Err(_) = close_result {
                let stub = functor_stub(atom!("close"), 1);
                let addr = stream_as_cell!(stream);
                let err = self
                    .machine_st
                    .existence_error(ExistenceError::Stream(addr));

                return Err(self.machine_st.error_form(err, stub));
            }
        }

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn copy_to_lifted_heap(&mut self) {
        let lh_offset = cell_as_fixnum!(self.deref_register(1)).get_num() as usize;

        let copy_target = self.machine_st.registers[2];

        let old_threshold = self
            .machine_st
            .copy_findall_solution(lh_offset, copy_target);
        let new_threshold = self.machine_st.lifted_heap.len() - lh_offset;

        self.machine_st.lifted_heap[old_threshold] = heap_loc_as_cell!(new_threshold);

        for addr in self.machine_st.lifted_heap[old_threshold + 1..].iter_mut() {
            *addr -= self.machine_st.heap.len() + lh_offset;
        }
    }

    #[inline(always)]
    pub(crate) fn lookup_db_ref(&mut self) {
        let module_name = self.deref_register(1);
        let name = cell_as_atom!(self.deref_register(2));
        let arity = cell_as_fixnum!(self.deref_register(3)).get_num() as usize;

        let module_name = read_heap_cell!(module_name,
            (HeapCellValueTag::Atom, (module_name, _arity)) => {
                module_name
            }
            (HeapCellValueTag::AttrVar | HeapCellValueTag::Var) => {
                atom!("user")
            }
            _ => {
                unreachable!()
            }
        );

        self.machine_st.fail = self
            .indices
            .get_predicate_code_index(name, arity, module_name)
            .is_none();
    }

    #[inline(always)]
    pub(crate) fn get_db_refs(&mut self) {
        let name_match: fn(Atom, Atom) -> bool;
        let arity_match: fn(usize, usize) -> bool;

        let module_name = read_heap_cell!(self.deref_register(1),
            (HeapCellValueTag::Atom, (module_name, _arity)) => {
                module_name
            }
            (HeapCellValueTag::AttrVar | HeapCellValueTag::Var | HeapCellValueTag::StackVar) => {
                atom!("user")
            }
            _ => {
                unreachable!()
            }
        );

        let atom = self.deref_register(2);

        let pred_atom = if atom.is_var() {
            name_match = |_, _| true;
            atom!("")
        } else {
            name_match = |atom_1, atom_2| atom_1 == atom_2;
            cell_as_atom!(atom)
        };

        let arity = self.deref_register(3);

        let pred_arity = if arity.is_var() {
            arity_match = |_, _| true;
            0
        } else {
            arity_match = |arity_1, arity_2| arity_1 == arity_2;

            let arity = match Number::try_from(arity) {
                Ok(Number::Fixnum(n)) => Some(n.get_num() as usize),
                Ok(Number::Integer(n)) => {
                    let value: usize = (&*n).try_into().unwrap();
                    Some(value)
                },
                _ => None,
            };

            if let Some(arity) = arity {
                arity
            } else {
                self.machine_st.fail = true;
                return;
            }
        };

        let h = self.machine_st.heap.len();
        let mut num_functors = 0;

        let code_dir = if module_name == atom!("user") {
            &self.indices.code_dir
        } else {
            match self
                .indices
                .modules
                .get(&module_name)
                .map(|module| &module.code_dir)
            {
                Some(code_dir) => code_dir,
                None => {
                    self.machine_st.fail = true;
                    return;
                }
            }
        };

        for (name, arity) in code_dir.keys() {
            if name_match(pred_atom, *name) && arity_match(pred_arity, *arity) {
                self.machine_st.heap.extend(functor!(
                    atom!("/"),
                    [cell(atom_as_cell!(name)), fixnum(*arity)]
                ));

                num_functors += 1;
            }
        }

        if num_functors > 0 {
            let h = iter_to_heap_list(
                &mut self.machine_st.heap,
                (0..num_functors).map(|i| str_loc_as_cell!(h + 3 * i)),
            );

            unify!(
                self.machine_st,
                heap_loc_as_cell!(h),
                self.machine_st.registers[4]
            );
        } else {
            unify!(
                self.machine_st,
                empty_list_as_cell!(),
                self.machine_st.registers[4]
            );
        }
    }

    #[inline(always)]
    pub(crate) fn get_next_op_db_ref(&mut self) {
        let prec = self.deref_register(1);

        fn get_spec(op_spec: u8) -> Atom {
            match op_spec as u32 {
                XFX => atom!("xfx"),
                XFY => atom!("xfy"),
                YFX => atom!("yfx"),
                FX => atom!("fx"),
                FY => atom!("fy"),
                XF => atom!("xf"),
                YF => atom!("yf"),
                _ => unreachable!(),
            }
        }

        let h = self.machine_st.heap.len();

        fn write_op_functors_to_heap(
            heap: &mut Heap,
            op_descs: impl Iterator<Item = (Atom, OpDesc)>,
        ) -> usize {
            let mut num_functors = 0;

            for (name, op_desc) in op_descs {
                let prec = op_desc.get_prec();

                if prec == 0 {
                    // 8.14.4, note 2
                    continue;
                }

                let spec_atom = get_spec(op_desc.get_spec());

                heap.extend(functor!(
                    atom!("op"),
                    [fixnum(prec), cell(atom_as_cell!(spec_atom)), cell(atom_as_cell!(name))]
                ));

                num_functors += 1;
            }

            num_functors
        }

        if prec.is_var() {
            let spec = self.deref_register(2);
            let orig_op = self.deref_register(3);

            let spec_num = if spec.get_tag() == HeapCellValueTag::Atom {
                (match cell_as_atom!(spec) {
                    atom!("xfx") => XFX,
                    atom!("xfy") => XFY,
                    atom!("yfx") => YFX,
                    atom!("fx") => FX,
                    atom!("fy") => FY,
                    atom!("xf") => XF,
                    _ => unreachable!(),
                }) as u8
            } else {
                0
            };

            let num_functors = if !orig_op.is_var() {
                let orig_op = read_heap_cell!(orig_op,
                    (HeapCellValueTag::Atom, (name, _arity)) => {
                        name
                    }
                    (HeapCellValueTag::Str, s) => {
                        cell_as_atom!(self.machine_st.heap[s])
                    }
                    (HeapCellValueTag::Char, c) => {
                        AtomTable::build_with(&self.machine_st.atom_tbl, &c.to_string())
                    }
                    _ => {
                        unreachable!()
                    }
                );

                let op_descs = [
                    self.indices.op_dir.get(&(orig_op, Fixity::In)),
                    self.indices.op_dir.get(&(orig_op, Fixity::Pre)),
                    self.indices.op_dir.get(&(orig_op, Fixity::Post)),
                ];

                let number_of_keys = op_descs[0].is_some() as usize
                    + op_descs[1].is_some() as usize
                    + op_descs[2].is_some() as usize;

                let op_descs = op_descs.into_iter().filter_map(|op_desc| {
                    op_desc.map(|op_desc| (orig_op, *op_desc))
                });

                if number_of_keys == 0 {
                    self.machine_st.fail = true;
                } else {
                    let num_functors = write_op_functors_to_heap(
                        &mut self.machine_st.heap,
                        op_descs,
                    );

                    let h = iter_to_heap_list(
                        &mut self.machine_st.heap,
                        (0..num_functors).map(|i| str_loc_as_cell!(h + 4 * i)),
                    );

                    unify!(
                        self.machine_st,
                        heap_loc_as_cell!(h),
                        self.machine_st.registers[4]
                    );
                }

                return;
            } else {
                let op_descs = self.indices.op_dir.iter().filter_map(|(key, op_desc)| {
                    let (other_prec, other_spec) = (op_desc.get_prec(), op_desc.get_spec());
                    let name = key.0;

                    if other_prec == 0 {
                        // 8.14.4, note 2
                        return None;
                    }

                    if (!orig_op.is_var() && atom_as_cell!(name) != orig_op)
                        || (!spec.is_var() && other_spec != spec_num)
                    {
                        return None;
                    }

                    Some((key.0, *op_desc))
                });

                write_op_functors_to_heap(&mut self.machine_st.heap, op_descs)
            };

            if num_functors > 0 {
                let h = iter_to_heap_list(
                    &mut self.machine_st.heap,
                    (0..num_functors).map(|i| str_loc_as_cell!(h + 4 * i)),
                );

                unify!(
                    self.machine_st,
                    heap_loc_as_cell!(h),
                    self.machine_st.registers[4]
                );
            } else {
                self.machine_st.fail = true;
            }
        } else {
            let spec = cell_as_atom!(self.deref_register(2));
            let op_atom = cell_as_atom!(self.deref_register(3));

            let fixity = match spec {
                atom!("xfy") | atom!("yfx") | atom!("xfx") => Fixity::In,
                atom!("xf") | atom!("yf") => Fixity::Post,
                atom!("fx") | atom!("fy") => Fixity::Pre,
                _ => {
                    self.machine_st.fail = true;
                    return;
                }
            };

            match self.indices.op_dir.get(&(op_atom, fixity)).cloned() {
                Some(op_desc) => {
                    let num_functors = write_op_functors_to_heap(
                        &mut self.machine_st.heap,
                        std::iter::once((op_atom, op_desc)),
                    );

                    let h = iter_to_heap_list(
                        &mut self.machine_st.heap,
                        (0..num_functors).map(|i| str_loc_as_cell!(h + 4 * i)),
                    );

                    unify!(
                        self.machine_st,
                        heap_loc_as_cell!(h),
                        self.machine_st.registers[4]
                    );
                }
                _ => {
                    self.machine_st.fail = true;
                }
            }
        }
    }

    #[inline(always)]
    pub(crate) fn maybe(&mut self) {
        self.machine_st.fail = self.rng.gen();
    }

    #[cfg(not(target_arch = "wasm32"))]
    #[inline(always)]
    pub(crate) fn cpu_now(&mut self) {
        let secs = ProcessTime::now().as_duration().as_secs_f64();
        let secs = float_alloc!(secs, self.machine_st.arena);

        self.machine_st
            .unify_f64(secs, self.machine_st.registers[1]);
    }

    #[cfg(target_arch = "wasm32")]
    #[inline(always)]
    pub(crate) fn cpu_now(&mut self) {
        // TODO
    }

    #[inline(always)]
    pub(crate) fn det_length_rundown(&mut self) -> CallResult {
        let stub_gen = || functor_stub(atom!("length"), 2);
        let len = self.deref_register(2);

        let n = match Number::try_from(len) {
            Ok(Number::Fixnum(n)) => n.get_num() as usize,
            Ok(Number::Integer(n)) => match (&*n).try_into() as Result<usize, _> {
                Ok(n) => n,
                Err(_) => {
                    let err = self.machine_st.resource_error(ResourceError::FiniteMemory(len));
                    return Err(self.machine_st.error_form(err, stub_gen()));
                }
            },
            _ => {
                unreachable!()
            }
        };

        let h = self.machine_st.heap.len();

        iter_to_heap_list(
            &mut self.machine_st.heap,
            (0..n).map(|i| heap_loc_as_cell!(h + 2 * i + 1)),
        );

        let tail = self.deref_register(1);
        self.machine_st
            .bind(tail.as_var().unwrap(), heap_loc_as_cell!(h));

        Ok(())
    }

    #[cfg(feature = "http")]
    #[inline(always)]
    pub(crate) fn http_open(&mut self) -> CallResult {
        let address_sink = self.deref_register(1);
        let method = read_heap_cell!(self.deref_register(3),
            (HeapCellValueTag::Atom, (name, arity)) => {
                debug_assert_eq!(arity, 0);
                match name {
                    atom!("get") => Method::GET,
                    atom!("post") => Method::POST,
                    atom!("put") => Method::PUT,
                    atom!("delete") => Method::DELETE,
                    atom!("patch") => Method::PATCH,
                    atom!("head") => Method::HEAD,
                    _ => unreachable!(),
                }
            }
            _ => {
                unreachable!()
            }
        );
        let address_status = self.deref_register(4);
        let address_data = self.deref_register(5);
        let mut bytes: Vec<u8> = Vec::new();
        if let Some(string) = self.machine_st.value_to_str_like(address_data) {
            bytes = string.as_str().bytes().collect();
        }
        let stub_gen = || functor_stub(atom!("http_open"), 3);

        let headers = match self
            .machine_st
            .try_from_list(self.machine_st.registers[7], stub_gen)
        {
            Ok(addrs) => {
                let mut header_map = HeaderMap::new();
                for heap_cell in addrs {
                    read_heap_cell!(heap_cell,
                        (HeapCellValueTag::Str, s) => {
                            let name = cell_as_atom_cell!(self.machine_st.heap[s]).get_name();
                            let value = self.machine_st.value_to_str_like(self.machine_st.heap[s + 1]).unwrap();
                            header_map.insert(HeaderName::from_str(&*name.as_str()).unwrap(), HeaderValue::from_str(&*value.as_str()).unwrap());
                        }
                        _ => {
                            unreachable!()
                        }
                    )
                }
                header_map
            }
            Err(e) => return Err(e),
        };
        if let Some(address_sink) = self.machine_st.value_to_str_like(address_sink) {
            let address_string = address_sink.as_str(); //to_string();
            let address: Url = address_string.parse().unwrap();

            let client = reqwest::blocking::Client::builder().build().unwrap();

            // request
            let mut req = reqwest::blocking::Request::new(method, address);

            *req.headers_mut() = headers;
            if bytes.len() > 0 {
                *req.body_mut() = Some(reqwest::blocking::Body::from(bytes));
            }

            // do it!
            match client.execute(req) {
                Ok(resp) => {
                    // status code
                    let status = resp.status().as_u16();
                    self.machine_st
                        .unify_fixnum(Fixnum::build_with(status as i64), address_status);
                    // headers
                    let headers: Vec<HeapCellValue> = resp
                        .headers()
                        .iter()
                        .map(|(header_name, header_value)| {
                            let h = self.machine_st.heap.len();

                            let header_term = functor!(
                                AtomTable::build_with(
                                    &self.machine_st.atom_tbl,
                                    header_name.as_str()
                                ),
                                [cell(string_as_cstr_cell!(AtomTable::build_with(
                                    &self.machine_st.atom_tbl,
                                    header_value.to_str().unwrap()
                                )))]
                            );

                            self.machine_st.heap.extend(header_term.into_iter());
                            str_loc_as_cell!(h)
                        })
                        .collect();

                    let headers_list =
                        iter_to_heap_list(&mut self.machine_st.heap, headers.into_iter());
                    unify!(
                        self.machine_st,
                        heap_loc_as_cell!(headers_list),
                        self.machine_st.registers[6]
                    );
                    // body
                    let reader = resp.bytes().unwrap().reader();

                    let mut stream = Stream::from_http_stream(
                        AtomTable::build_with(&self.machine_st.atom_tbl, &address_string),
                        Box::new(reader),
                        &mut self.machine_st.arena,
                    );
                    *stream.options_mut() = StreamOptions::default();
                    if let Some(alias) = stream.options().get_alias() {
                        self.indices.stream_aliases.insert(alias, stream);
                    }

                    self.indices.streams.insert(stream);

                    let stream = stream_as_cell!(stream);

                    let stream_addr = self.deref_register(2);
                    self.machine_st.bind(stream_addr.as_var().unwrap(), stream);
                }
                Err(_) => {
                    self.machine_st.fail = true;
                }
            }
        } else {
            let err = self
                .machine_st
                .domain_error(DomainErrorType::SourceSink, address_sink);
            let stub = functor_stub(atom!("http_open"), 3);

            return Err(self.machine_st.error_form(err, stub));
        }

        Ok(())
    }

    #[cfg(feature = "http")]
    #[inline(always)]
    pub(crate) fn http_listen(&mut self) -> CallResult {
        let address_sink = self.deref_register(1);
        if let Some(address_str) = self.machine_st.value_to_str_like(address_sink) {
            let address_string = address_str.as_str();
            let addr: SocketAddr = match address_string
                .to_socket_addrs()
                .ok()
                .and_then(|mut s| s.next())
            {
                Some(addr) => addr,
                _ => {
                    self.machine_st.fail = true;
                    return Ok(());
                }
            };

            let (tx, rx) = std::sync::mpsc::sync_channel(1024);

            let _guard = self.runtime.enter();
            let listener = match self
                .runtime
                .block_on(async { tokio::net::TcpListener::bind(addr).await })
            {
                Ok(listener) => listener,
                Err(_) => {
                    return Err(self.machine_st.open_permission_error(
                        address_sink,
                        atom!("http_listen"),
                        2,
                    ));
                }
            };

            self.runtime.spawn(async move {
                loop {
                    let tx = tx.clone();
                    let (stream, _) = listener.accept().await.unwrap();

                    tokio::task::spawn(async move {
                        if let Err(err) = http1::Builder::new()
                            .serve_connection(stream, HttpService { tx })
                            .await
                        {
                            eprintln!("Error serving connection: {:?}", err);
                        }
                    });
                }
            });
            let http_listener = HttpListener { incoming: rx };
            let http_listener = arena_alloc!(http_listener, &mut self.machine_st.arena);
            let addr = self.deref_register(2);
            self.machine_st.bind(
                addr.as_var().unwrap(),
                typed_arena_ptr_as_cell!(http_listener),
            );
        }
        Ok(())
    }

    #[cfg(feature = "http")]
    #[inline(always)]
    pub(crate) fn http_accept(&mut self) -> CallResult {
        let culprit = self.deref_register(1);
        let method = self.deref_register(2);
        let path = self.deref_register(3);
        let query = self.deref_register(5);
        let stream_addr = self.deref_register(6);
        let handle_addr = self.deref_register(7);
        read_heap_cell!(culprit,
            (HeapCellValueTag::Cons, cons_ptr) => {
            match_untyped_arena_ptr!(cons_ptr,
                (ArenaHeaderTag::HttpListener, http_listener) => {
                    match http_listener.incoming.recv() {
                    Ok(request) => {
                        let method_atom = match *request.request.method() {
                        Method::GET => atom!("get"),
                        Method::POST => atom!("post"),
                        Method::PUT => atom!("put"),
                        Method::DELETE => atom!("delete"),
                        Method::PATCH => atom!("patch"),
                        Method::HEAD => atom!("head"),
                        _ => unreachable!(),
                    };
                    let path_atom = AtomTable::build_with(&self.machine_st.atom_tbl, request.request.uri().path());
                    let path_cell = atom_as_cstr_cell!(path_atom);
                    let headers: Vec<HeapCellValue> = request.request.headers().iter().map(|(header_name, header_value)| {
                        let h = self.machine_st.heap.len();

                        let header_term = functor!(
                            AtomTable::build_with(&self.machine_st.atom_tbl, header_name.as_str()),
                        [cell(string_as_cstr_cell!(AtomTable::build_with(&self.machine_st.atom_tbl, header_value.to_str().unwrap())))]
                        );

                        self.machine_st.heap.extend(header_term.into_iter());
                        str_loc_as_cell!(h)
                    }).collect();

                    let headers_list = iter_to_heap_list(&mut self.machine_st.heap, headers.into_iter());

                    let query_str = request.request.uri().query().unwrap_or("");
                    let query_atom = AtomTable::build_with(&self.machine_st.atom_tbl, query_str);
                    let query_cell = string_as_cstr_cell!(query_atom);

                    let hyper_req = request.request;
                    let buf = self.runtime.block_on(async {hyper_req.collect().await.unwrap().aggregate()});
                    let reader = buf.reader();

                    let mut stream = Stream::from_http_stream(
                        path_atom,
                        Box::new(reader),
                        &mut self.machine_st.arena
                    );
                    *stream.options_mut() = StreamOptions::default();
                    stream.options_mut().set_stream_type(StreamType::Binary);
                    self.indices.streams.insert(stream);
                    let stream = stream_as_cell!(stream);

                    let handle = arena_alloc!(request.response, &mut self.machine_st.arena);

                    self.machine_st.bind(method.as_var().unwrap(), atom_as_cell!(method_atom));
                    self.machine_st.bind(path.as_var().unwrap(), path_cell);
                    unify!(self.machine_st, heap_loc_as_cell!(headers_list), self.machine_st.registers[4]);
                    self.machine_st.bind(query.as_var().unwrap(), query_cell);
                    self.machine_st.bind(stream_addr.as_var().unwrap(), stream);
                    self.machine_st.bind(handle_addr.as_var().unwrap(), typed_arena_ptr_as_cell!(handle));
                    }
                    Err(_) => {
                        self.machine_st.fail = true;
                    }
                }
                }
                _ => {
                    unreachable!();
                }
            );
            }
            _ => {
                unreachable!();
            }
        );
        Ok(())
    }

    #[cfg(feature = "http")]
    #[inline(always)]
    pub(crate) fn http_answer(&mut self) -> CallResult {
	let culprit = self.deref_register(1);
	let status_code = self.deref_register(2);
	let status_code: u16 = match Number::try_from(status_code) {
	    Ok(Number::Fixnum(n)) => n.get_num() as u16,
	    Ok(Number::Integer(n)) => {
            let n: Result<u16, _> = (&*n).try_into();

            if let Ok(value) = n {
                value
            } else {
                self.machine_st.fail = true;
                return Ok(());
            }
        }
	    _ => unreachable!()
	};
	let stub_gen = || functor_stub(atom!("http_listen"), 2);
	let headers = match self.machine_st.try_from_list(self.machine_st.registers[3], stub_gen) {
            Ok(addrs) => {
                let mut header_map = HeaderMap::new();
                for heap_cell in addrs {
                    read_heap_cell!(heap_cell,
                        (HeapCellValueTag::Str, s) => {
                            let name = cell_as_atom_cell!(self.machine_st.heap[s]).get_name();
                            let value = self.machine_st.value_to_str_like(self.machine_st.heap[s + 1]).unwrap();
                            header_map.insert(HeaderName::from_str(&*name.as_str()).unwrap(), HeaderValue::from_str(&*value.as_str()).unwrap());
                        }
                        _ => {
                            unreachable!()
                        }
                    )
                }
                header_map
            }
            Err(e) => return Err(e),
        };
        let stream_addr = self.deref_register(4);

        read_heap_cell!(culprit,
            (HeapCellValueTag::Cons, cons_ptr) => {
            match_untyped_arena_ptr!(cons_ptr,
                (ArenaHeaderTag::HttpResponse, http_response) => {
                let mut stream = Stream::from_http_sender(
                    http_response,
                    status_code,
                    headers,
                    &mut self.machine_st.arena
                );
                *stream.options_mut() = StreamOptions::default();
                stream.options_mut().set_stream_type(StreamType::Binary);
                self.indices.streams.insert(stream);
                let stream = stream_as_cell!(stream);
                self.machine_st.bind(stream_addr.as_var().unwrap(), stream);
                }
                _ => {
                unreachable!();
                }
            );
            }
            _ => {
            unreachable!();
            }
        );

        Ok(())
    }

    #[cfg(feature = "ffi")]
    #[inline(always)]
    pub(crate) fn load_foreign_lib(&mut self) -> CallResult {
        let library_name = self.deref_register(1);
        let args_reg = self.deref_register(2);
        if let Some(library_name) = self.machine_st.value_to_str_like(library_name) {
            let stub_gen = || functor_stub(atom!("use_foreign_module"), 2);
            match self.machine_st.try_from_list(args_reg, stub_gen) {
                Ok(addrs) => {
                    let mut functions = Vec::new();
                    for heap_cell in addrs {
                        read_heap_cell!(heap_cell,
                            (HeapCellValueTag::Str, s) => {
                                let name = cell_as_atom_cell!(self.machine_st.heap[s]).get_name();
                            let args: Vec<Atom> = match self.machine_st.try_from_list(self.machine_st.heap[s + 1], stub_gen) {
                                Ok(addrs) => {
                                let mut args = Vec::new();
                                for heap_cell in addrs {
                                    args.push(cell_as_atom_cell!(heap_cell).get_name());
                                }
                                args
                                }
                                Err(e) => return Err(e)
                            };
                            let return_value = cell_as_atom_cell!(self.machine_st.heap[s + 2]);
                            functions.push(FunctionDefinition {
                                name: name.as_str().to_string(),
                                args,
                                return_value: return_value.get_name(),
                            });
                            }
                            _ => {
                                unreachable!()
                                }
                        )
                    }
                    if let Ok(_) = self
                        .foreign_function_table
                        .load_library(&*library_name.as_str(), &functions)
                    {
                        return Ok(());
                    }
                }
                Err(e) => return Err(e),
            };
        }
        self.machine_st.fail = true;
        Ok(())
    }

    #[cfg(feature = "ffi")]
    #[inline(always)]
    pub(crate) fn foreign_call(&mut self) -> CallResult {
        let function_name = self.deref_register(1);
        let args_reg = self.deref_register(2);
        let return_value = self.deref_register(3);
        if let Some(function_name) = self.machine_st.value_to_str_like(function_name) {
            let stub_gen = || functor_stub(atom!("foreign_call"), 3);
            fn map_arg(
                mut machine_st: &mut MachineState,
                source: HeapCellValue,
            ) -> crate::ffi::Value {
                match Number::try_from(source) {
                    Ok(Number::Fixnum(n)) => Value::Int(n.get_num()),
                    Ok(Number::Float(n)) => Value::Float(n.into_inner()),
                    _ => {
                        let stub_gen = || functor_stub(atom!("foreign_call"), 3);
                        if let Some(string) = machine_st.value_to_str_like(source) {
                            Value::CString(CString::new(&*string.as_str()).unwrap())
                        } else {
                            match machine_st.try_from_list(source, stub_gen) {
                                Ok(args) => {
                                    let mut iter = args.into_iter();
                                    if let Some(struct_name) =
                                        machine_st.value_to_str_like(iter.next().unwrap())
                                    {
                                        Value::Struct(
                                            struct_name.as_str().to_string(),
                                            iter.map(|x| map_arg(&mut machine_st, x)).collect(),
                                        )
                                    } else {
                                        unreachable!()
                                    }
                                }
                                _ => {
                                    unreachable!()
                                }
                            }
                        }
                    }
                }
            }

            match self.machine_st.try_from_list(args_reg, stub_gen) {
                Ok(args) => {
                    let args: Vec<_> = args
                        .into_iter()
                        .map(|x| map_arg(&mut self.machine_st, x))
                        .collect();
                    match self
                        .foreign_function_table
                        .exec(&*function_name.as_str(), args)
                    {
                        Ok(result) => {
                            match result {
                                Value::Int(n) => self
                                    .machine_st
                                    .unify_fixnum(Fixnum::build_with(n), return_value),
                                Value::Float(n) => {
                                    let n = float_alloc!(n, self.machine_st.arena);
                                    self.machine_st.unify_f64(n, return_value)
                                }
                                Value::Struct(name, args) => {
                                    let struct_value = self.build_struct(&name, args);
                                    unify!(self.machine_st, return_value, struct_value);
                                }
                                Value::CString(cstr) => {
                                    let cstr = AtomTable::build_with(
                                        &self.machine_st.atom_tbl,
                                        cstr.to_str().unwrap(),
                                    );
                                    self.machine_st.unify_complete_string(cstr, return_value);
                                }
                            }
                            return Ok(());
                        }
                        Err(e) => {
                            let stub = functor_stub(atom!("current_input"), 1);
                            let err = self.machine_st.ffi_error(e);

                            return Err(self.machine_st.error_form(err, stub));
                        }
                    }
                }
                Err(e) => return Err(e),
            }
        }
        self.machine_st.fail = true;
        Ok(())
    }

    #[cfg(feature = "ffi")]
    fn build_struct(&mut self, name: &str, mut args: Vec<Value>) -> HeapCellValue {
        args.insert(0, Value::CString(CString::new(name).unwrap()));
        let cells: Vec<_> = args
            .into_iter()
            .map(|val| match val {
                Value::Int(n) => fixnum_as_cell!(Fixnum::build_with(n)),
                Value::Float(n) => HeapCellValue::from(float_alloc!(n, self.machine_st.arena)),
                Value::CString(cstr) => atom_as_cell!(AtomTable::build_with(
                    &self.machine_st.atom_tbl,
                    &cstr.into_string().unwrap()
                )),
                Value::Struct(name, struct_args) => self.build_struct(&name, struct_args),
            })
            .collect();

        heap_loc_as_cell!(iter_to_heap_list(
            &mut self.machine_st.heap,
            cells.into_iter()
        ))
    }

    #[cfg(feature = "ffi")]
    #[inline(always)]
    pub(crate) fn define_foreign_struct(&mut self) -> CallResult {
        let struct_name = self.deref_register(1);
        let fields_reg = self.deref_register(2);
        if let Some(struct_name) = self.machine_st.value_to_str_like(struct_name) {
            let stub_gen = || functor_stub(atom!("define_foreign_struct"), 2);
            let fields: Vec<Atom> = match self.machine_st.try_from_list(fields_reg, stub_gen) {
                Ok(addrs) => {
                    let mut args = Vec::new();
                    for heap_cell in addrs {
                        args.push(cell_as_atom_cell!(heap_cell).get_name());
                    }
                    args
                }
                Err(e) => return Err(e),
            };
            self.foreign_function_table
                .define_struct(&*struct_name.as_str(), fields);
            return Ok(());
        }
        self.machine_st.fail = true;
        Ok(())
    }

    #[inline(always)]
    pub(crate) fn current_time(&mut self) {
        let timestamp = self.systemtime_to_timestamp(SystemTime::now());
        self.machine_st
            .unify_complete_string(timestamp, self.machine_st.registers[1]);
    }

    #[inline(always)]
    pub(crate) fn open(&mut self) -> CallResult {
        let alias = self.machine_st.registers[4];
        let eof_action = self.machine_st.registers[5];
        let reposition = self.machine_st.registers[6];
        let stream_type = self.machine_st.registers[7];

        let options = self
            .machine_st
            .to_stream_options(alias, eof_action, reposition, stream_type);
        let src_sink = self.deref_register(1);

        if let Some(file_spec) = self.machine_st.value_to_str_like(src_sink) {
            let file_spec = file_spec.as_atom(&*self.machine_st.atom_tbl);

            let mut stream =
                self.machine_st
                    .stream_from_file_spec(file_spec, &mut self.indices, &options)?;

            *stream.options_mut() = options;
            self.indices.streams.insert(stream);

            if let Some(alias) = stream.options().get_alias() {
                self.indices.stream_aliases.insert(alias, stream);
            }

            let stream_var = self.deref_register(3);
            self.machine_st
                .bind(stream_var.as_var().unwrap(), stream_as_cell!(stream));
        } else {
            let err = self
                .machine_st
                .domain_error(DomainErrorType::SourceSink, src_sink);
            let stub = functor_stub(atom!("open"), 4);

            return Err(self.machine_st.error_form(err, stub));
        }

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn op_declaration(&mut self) -> CallResult {
        let priority = self.deref_register(1);
        let specifier = cell_as_atom_cell!(self.deref_register(2)).get_name();

        let priority = match Number::try_from(priority) {
            Ok(Number::Integer(n)) => {
                let n: u16 = (&*n).try_into().unwrap();
                n
            },
            Ok(Number::Fixnum(n)) => u16::try_from(n.get_num()).unwrap(),
            _ => {
                unreachable!();
            }
        };

        let op = read_heap_cell!(self.deref_register(3),
            (HeapCellValueTag::Char, c) => {
                AtomTable::build_with(&self.machine_st.atom_tbl, &c.to_string())
            }
            (HeapCellValueTag::Atom, (name, _arity)) => {
                name
            }
            (HeapCellValueTag::Str, s) => {
                cell_as_atom!(self.machine_st.heap[s])
            }
            _ => {
                unreachable!()
            }
        );

        let result = to_op_decl(priority, specifier, op)
            .map_err(SessionError::from)
            .and_then(|mut op_decl| {
                if op_decl.op_desc.get_prec() == 0 {
                    Ok(op_decl.remove(&mut self.indices.op_dir))
                } else {
                    let spec = get_op_desc(
                        op_decl.name,
                        &CompositeOpDir::new(&self.indices.op_dir, None),
                    );

                    op_decl.submit(spec, &mut self.indices.op_dir)
                }
            });

        match result {
            Ok(()) => Ok(()),
            Err(e) => {
                // 8.14.3.3 l)
                let err = self.machine_st.session_error(e);
                let stub = functor_stub(atom!("op"), 3);

                Err(self.machine_st.error_form(err, stub))
            }
        }
    }

    #[inline(always)]
    pub(crate) fn set_stream_options(&mut self) -> CallResult {
        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("open"),
            4,
        )?;

        let alias = self.machine_st.registers[2];
        let eof_action = self.machine_st.registers[3];
        let reposition = self.machine_st.registers[4];
        let stream_type = self.machine_st.registers[5];

        let options = self
            .machine_st
            .to_stream_options(alias, eof_action, reposition, stream_type);
        *stream.options_mut() = options;

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn truncate_if_no_lifted_heap_growth_diff(&mut self) {
        self.machine_st
            .truncate_if_no_lifted_heap_diff(|h| heap_loc_as_cell!(h))
    }

    #[inline(always)]
    pub(crate) fn truncate_if_no_lifted_heap_growth(&mut self) {
        self.machine_st
            .truncate_if_no_lifted_heap_diff(|_| empty_list_as_cell!())
    }

    #[inline(always)]
    pub(crate) fn get_attributed_variable_list(&mut self) {
        let attr_var = self.deref_register(1);
        let attr_var_list = read_heap_cell!(attr_var,
            (HeapCellValueTag::AttrVar, h) => {
                h+1
            }
            (HeapCellValueTag::Var, h) => {
                h
            }
            _ => {
                self.machine_st.fail = true;
                return;
            }
        );

        let list_addr = self.deref_register(2);
        self.machine_st
            .bind(Ref::heap_cell(attr_var_list), list_addr);
    }

    #[inline(always)]
    pub(crate) fn get_from_attributed_variable_list(&mut self) {
        let attr_var = self.deref_register(1);
        let attr = self.deref_register(3);
        let attr_var_list = read_heap_cell!(attr_var,
            (HeapCellValueTag::AttrVar, h) => {
                self.machine_st.heap[h+1]
            }
            _ => {
                self.machine_st.fail = true;
                return;
            }
        );

        let module = self.deref_register(2);

        match self.match_attribute(attr_var_list, module, attr) {
            Some(AttrListMatch {
                match_site: MatchSite::Match(match_site),
                ..
            }) => {
                let list_head = self.machine_st.heap[match_site];

                if list_head.get_value() as usize == match_site {
                    // at the end of the list, no match found in this case.
                    self.machine_st.fail = true;
                } else {
                    let (_, qualified_goal) = self
                        .machine_st
                        .strip_module(list_head, empty_list_as_cell!());

                    unify!(self.machine_st, qualified_goal, attr);
                }
            }
            _ => {
                self.machine_st.fail = true;
            }
        }
    }

    #[inline(always)]
    pub(crate) fn get_attr_var_queue_delimiter(&mut self) {
        let addr = self.deref_register(1);
        let value = Fixnum::build_with(self.machine_st.attr_var_init.attr_var_queue.len() as i64);

        self.machine_st.unify_fixnum(value, addr);
    }

    #[inline(always)]
    pub(crate) fn get_attr_var_queue_beyond(&mut self) {
        let addr = self.deref_register(1);

        let b = match Number::try_from(addr) {
            Ok(Number::Integer(n)) => {
                let value: usize = (&*n).try_into().unwrap();
                Some(value)
            },
            Ok(Number::Fixnum(n)) => usize::try_from(n.get_num()).ok(),
            _ => {
                self.machine_st.fail = true;
                return;
            }
        };

        if let Some(b) = b {
            let iter = self.machine_st.gather_attr_vars_created_since(b);

            let var_list_addr =
                heap_loc_as_cell!(iter_to_heap_list(&mut self.machine_st.heap, iter));

            let list_addr = self.machine_st.registers[2];
            unify!(self.machine_st, var_list_addr, list_addr);
        }
    }

    #[inline(always)]
    pub(crate) fn delete_from_attributed_variable_list(&mut self) {
        let attr_var = self.deref_register(1);
        let attr = self.deref_register(3);
        let attr_var_list = read_heap_cell!(attr_var,
            (HeapCellValueTag::AttrVar, h) => {
                h + 1
            }
            _ => {
                return;
            }
        );

        let module = self.deref_register(2);

        match self.match_attribute(self.machine_st.heap[attr_var_list], module, attr) {
            Some(AttrListMatch {
                prev_tail,
                match_site: MatchSite::Match(match_site),
            }) => {
                let prev_tail = if let Some(prev_tail) = prev_tail {
                    // not at the head.
                    prev_tail
                } else {
                    if self.machine_st.heap[match_site + 1].is_var() {
                        let h = attr_var.get_value() as usize;

                        self.machine_st.heap[h] = heap_loc_as_cell!(h);
                        self.machine_st.trail(TrailRef::Ref(Ref::attr_var(h)));
                    }

                    // at the head.
                    attr_var_list
                };

                if self.machine_st.heap[match_site + 1].get_tag() == HeapCellValueTag::Lis {
                    let prev_tail_value = self.machine_st.heap[match_site + 1].get_value();
                    self.machine_st.heap[prev_tail].set_value(prev_tail_value);
                } else {
                    self.machine_st.heap[prev_tail] = heap_loc_as_cell!(prev_tail);
                }

                self.machine_st
                    .trail(TrailRef::AttrVarListLink(prev_tail, match_site));
            }
            _ => {}
        }
    }

    #[inline(always)]
    pub(crate) fn put_to_attributed_variable_list(&mut self) {
        let attr_var = self.deref_register(1);
        let attr = self.deref_register(3);
        let attr_var_list = match self.machine_st.get_attr_var_list(attr_var) {
            Some(h) => h,
            None => {
                self.machine_st.fail = true;
                return;
            }
        };

        let module = self.deref_register(2);

        /*
         * How to handle attribute trailing using just AttrVarListLink (which
         * should be re-named to something more general) in unwind_trail:
         *
         * Given AttrVarListLink(h, l):
         *
         * 1. Check cell at offset l.
         * 2. If h == l, set heap[h] = heap_loc_as_cell!(h).
         * 3. If cell is a Var, set heap[h] = list_loc_as_cell!(l).
         * 4. Otherwise, cell points to an element of the list which is therefore
         *    an atom or str. Set heap[h] accordingly.
         *
         * For this to work, all elements of attributed variable lists must be
         * heap cell locs pointing to later elements in the heap, either atoms (0-arity)
         * or str cells (> 0-arity).
         */

        let h = self.machine_st.heap.len();

        self.machine_st.heap.push(str_loc_as_cell!(h + 1));
        self.machine_st
            .heap
            .extend(functor!(atom!(":"), [cell(module), cell(attr)]));

        match self.match_attribute(self.machine_st.heap[attr_var_list], module, attr) {
            Some(AttrListMatch { match_site, .. }) => {
                let (match_site, l) = match match_site {
                    MatchSite::NoMatchVarTail(match_site) => {
                        let l = self.machine_st.heap[match_site].get_value();

                        // at the end of the (non-empty) list here.
                        self.machine_st.heap[match_site] = list_loc_as_cell!(h + 4);
                        self.machine_st.heap.push(heap_loc_as_cell!(h));
                        self.machine_st.heap.push(heap_loc_as_cell!(h + 5));

                        (match_site, l)
                    }
                    MatchSite::Match(match_site) => {
                        let l = self.machine_st.heap[match_site].get_value();
                        self.machine_st.heap[match_site].set_value(h as u64);

                        (match_site, l)
                    }
                };

                self.machine_st
                    .trail(TrailRef::AttrVarListLink(match_site, l as usize));
            }
            None => {
                // the list is empty.
                self.machine_st.heap[attr_var_list] = list_loc_as_cell!(h + 4);
                self.machine_st.heap.push(heap_loc_as_cell!(h));
                self.machine_st.heap.push(heap_loc_as_cell!(h + 5));

                self.machine_st
                    .attr_var_init
                    .attr_var_queue
                    .push(attr_var_list - 1);
                self.machine_st
                    .trail(TrailRef::AttrVarListLink(attr_var_list, attr_var_list));
            }
        }
    }

    fn match_attribute(
        &self,
        mut attrs_list: HeapCellValue,
        module: HeapCellValue,
        attr: HeapCellValue,
    ) -> Option<AttrListMatch> {
        let (name, arity) = match self.machine_st.name_and_arity_from_heap(attr) {
            Some(key) => key,
            None => {
                return None;
            }
        };

        let mut prev_tail = None;

        while let HeapCellValueTag::Lis = attrs_list.get_tag() {
            let mut list_head = self.machine_st.heap[attrs_list.get_value() as usize];

            loop {
                read_heap_cell!(list_head,
                    (HeapCellValueTag::AttrVar | HeapCellValueTag::Var, h) => {
                        debug_assert!(list_head != self.machine_st.heap[h]);
                        list_head = self.machine_st.heap[h];
                    }
                    (HeapCellValueTag::Str | HeapCellValueTag::Atom) => {
                        let (module_loc, qualified_goal) = self.machine_st.strip_module(
                            list_head,
                            empty_list_as_cell!(),
                        );

                        let (t_name, t_arity) = self.machine_st
                            .name_and_arity_from_heap(qualified_goal)
                            .unwrap();

                        if module == module_loc && name == t_name && arity == t_arity {
                            return Some(AttrListMatch {
                                match_site: MatchSite::Match(attrs_list.get_value() as usize),
                                prev_tail,
                            });
                        }

                        break;
                    }
                    _ => {
                        break;
                    }
                );
            }

            let tail_loc = attrs_list.get_value() as usize + 1;
            prev_tail = Some(tail_loc);

            // do the work of self.store(self.deref(...)) but inline it
            // for speed and simplify it.
            let mut list_tail = self.machine_st.heap[tail_loc];

            loop {
                read_heap_cell!(list_tail,
                    (HeapCellValueTag::AttrVar | HeapCellValueTag::Var, h) => {
                        if list_tail != self.machine_st.heap[h] {
                            list_tail = self.machine_st.heap[h];
                        } else {
                            return Some(AttrListMatch {
                                match_site: MatchSite::NoMatchVarTail(h),
                                prev_tail,
                            });
                        }
                    }
                    (HeapCellValueTag::Lis) => {
                        attrs_list = list_tail;
                        break;
                    }
                    _ => {
                        unreachable!()
                    }
                );
            }
        }

        None
    }

    #[inline(always)]
    pub(crate) fn get_continuation_chunk(&mut self) {
        let e = self.deref_register(1);
        let e = cell_as_fixnum!(e).get_num() as usize;

        let p_functor = self.deref_register(2);

        let num_cells = self.machine_st.stack.index_and_frame(e).prelude.num_cells;
        let mut addrs = vec![];

        for idx in 1..num_cells + 1 {
            let addr = self.machine_st.stack[stack_loc!(AndFrame, e, idx)];
            let addr = self.machine_st.store(self.machine_st.deref(addr));

            // avoid pushing stack variables to the heap where they
            // must not go.
            if addr.is_stack_var() {
                let h = self.machine_st.heap.len();

                self.machine_st.heap.push(heap_loc_as_cell!(h));
                self.machine_st.bind(Ref::heap_cell(h), addr);

                addrs.push(heap_loc_as_cell!(h));
            } else {
                addrs.push(addr);
            }
        }

        let chunk = str_loc_as_cell!(self.machine_st.heap.len());

        self.machine_st
            .heap
            .push(atom_as_cell!(atom!("cont_chunk"), 1 + num_cells));
        self.machine_st.heap.push(p_functor);
        self.machine_st.heap.extend(addrs);

        unify!(self.machine_st, self.machine_st.registers[3], chunk);
    }

    #[inline(always)]
    pub(crate) fn get_lifted_heap_from_offset_diff(&mut self) {
        let lh_offset = self.machine_st.registers[1];
        let lh_offset = cell_as_fixnum!(self.machine_st.store(self.machine_st.deref(lh_offset)))
            .get_num() as usize;

        if lh_offset >= self.machine_st.lifted_heap.len() {
            let solutions = self.machine_st.registers[2];
            let diff = self.machine_st.registers[3];

            unify_fn!(self.machine_st, solutions, diff);
        } else {
            let h = self.machine_st.heap.len();
            let mut last_index = h;

            for value in self.machine_st.lifted_heap[lh_offset..].iter().cloned() {
                last_index = self.machine_st.heap.len();
                self.machine_st.heap.push(value + h);
            }

            if last_index < self.machine_st.heap.len() {
                let diff = self.machine_st.registers[3];
                unify_fn!(self.machine_st, diff, self.machine_st.heap[last_index]);
            }

            self.machine_st.lifted_heap.truncate(lh_offset);

            let solutions = self.machine_st.registers[2];
            unify_fn!(self.machine_st, heap_loc_as_cell!(h), solutions);
        }
    }

    #[inline(always)]
    pub(crate) fn get_lifted_heap_from_offset(&mut self) {
        let lh_offset = self.machine_st.registers[1];
        let lh_offset = cell_as_fixnum!(self.machine_st.store(self.machine_st.deref(lh_offset)))
            .get_num() as usize;

        if lh_offset >= self.machine_st.lifted_heap.len() {
            let solutions = self.machine_st.registers[2];
            unify_fn!(self.machine_st, solutions, empty_list_as_cell!());
        } else {
            let h = self.machine_st.heap.len();

            for addr in self.machine_st.lifted_heap[lh_offset..].iter().cloned() {
                self.machine_st.heap.push(addr + h);
            }

            self.machine_st.lifted_heap.truncate(lh_offset);

            let solutions = self.machine_st.registers[2];
            unify_fn!(self.machine_st, heap_loc_as_cell!(h), solutions);
        }
    }

    #[inline(always)]
    pub(crate) fn get_double_quotes(&mut self) {
        let a1 = self.deref_register(1);

        self.machine_st.unify_atom(
            match self.machine_st.flags.double_quotes {
                DoubleQuotes::Chars => atom!("chars"),
                DoubleQuotes::Atom => atom!("atom"),
                DoubleQuotes::Codes => atom!("codes"),
            },
            a1,
        );
    }

    #[inline(always)]
    pub(crate) fn get_unknown(&mut self) {
        let a1 = self.deref_register(1);

        self.machine_st.unify_atom(
            match self.machine_st.flags.unknown {
                Unknown::Error => atom!("error"),
                Unknown::Fail => atom!("fail"),
                Unknown::Warn => atom!("warning"),
            },
            a1,
        );
    }

    #[inline(always)]
    pub(crate) fn get_scc_cleaner(&mut self) {
        let dest = self.machine_st.registers[1];

        if let Some((addr, b_cutoff, prev_block)) = self.machine_st.cont_pts.pop() {
            let b = self
                .machine_st
                .stack
                .index_or_frame(self.machine_st.b)
                .prelude
                .b;

            if b <= b_cutoff {
                self.machine_st.scc_block = prev_block;

                if let Some(r) = dest.as_var() {
                    self.machine_st.bind(r, addr);
                    return;
                }
            } else {
                self.machine_st.cont_pts.push((addr, b_cutoff, prev_block));
            }
        }

        self.machine_st.fail = true;
    }

    #[inline(always)]
    pub(crate) fn halt(&mut self) -> std::process::ExitCode {
        let code = self.deref_register(1);

        let code = match Number::try_from(code) {
            Ok(Number::Fixnum(n)) => u8::try_from(n.get_num()).unwrap(),
            Ok(Number::Integer(n)) => {
                let n: u8 = (&*n).try_into().unwrap();
                n
            },
            Ok(Number::Rational(r)) => {
                // n has already been confirmed as an integer, and
                // internally, Rational is assumed reduced, so its
                // denominator must be 1.
                let r = r.numerator().try_into().unwrap();
                r
            }
            _ => {
                unreachable!()
            }
        };

        std::process::ExitCode::from(code)
    }

    #[inline(always)]
    pub(crate) fn install_scc_cleaner(&mut self) {
        let addr = self.machine_st.registers[1];
        let b = self.machine_st.b;
        let prev_block = self.machine_st.scc_block;

        self.machine_st.run_cleaners_fn = Machine::run_cleaners;

        self.machine_st.scc_block = b;
        self.machine_st.cont_pts.push((addr, b, prev_block));
    }

    #[inline(always)]
    pub(crate) fn install_inference_counter(&mut self) -> CallResult {
        // A1 = B, A2 = L
        let a1 = self.deref_register(1);
        let a2 = self.deref_register(2);

        let n = match Number::try_from(a2) {
            Ok(Number::Fixnum(bp)) => bp.get_num() as usize,
            Ok(Number::Integer(n)) => {
                let value: usize = (&*n).try_into().unwrap();
                value
            },
            _ => {
                let stub = functor_stub(atom!("call_with_inference_limit"), 3);

                let err = self.machine_st.type_error(ValidType::Integer, a2);
                return Err(self.machine_st.error_form(err, stub));
            }
        };

        let bp = cell_as_fixnum!(a1).get_num() as usize;
        let a3 = self.deref_register(3);
        let count = self.machine_st.cwil.add_limit(n, bp);

        let result = count.try_into();
        if let Ok(value) = result{
            self.machine_st.unify_fixnum(Fixnum::build_with(value), a3);
        } else {
            let count = arena_alloc!(count.clone(), &mut self.machine_st.arena);
            self.machine_st.unify_big_int(count, a3);
        }

        self.machine_st.increment_call_count_fn = MachineState::increment_call_count;

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn module_exists(&mut self) {
        let module = self.deref_register(1);
        let module_name = cell_as_atom!(module);

        self.machine_st.fail = !self.indices.modules.contains_key(&module_name);
    }

    pub(crate) fn predicate_defined(&mut self) -> bool {
        let module_name = cell_as_atom!(self.deref_register(1));
        let name = cell_as_atom!(self.deref_register(2));
        let a3 = self.deref_register(3);

        let arity = match Number::try_from(a3) {
            Ok(Number::Fixnum(n)) => n.get_num() as usize,
            Ok(Number::Integer(n)) => {
                let result = (&*n).try_into();

                if let Ok(value) = result {
                    value
                } else {
                    return false;
                }
            }
            _ => {
                unreachable!()
            }
        };

        self.indices
            .get_predicate_code_index(name, arity, module_name)
            .map(|index| index.local().is_some())
            .unwrap_or(false)
    }

    #[inline(always)]
    pub(crate) fn no_such_predicate(&mut self) -> CallResult {
        let module_name = cell_as_atom!(self.deref_register(1));

        let head = self.deref_register(2);

        self.machine_st.fail = read_heap_cell!(head,
            (HeapCellValueTag::Str, s) => {
                let (name, arity) = cell_as_atom_cell!(self.machine_st.heap[s])
                    .get_name_and_arity();

                if ClauseType::is_inbuilt(name, arity) {
                    true
                } else {
                    let index = self.indices.get_predicate_code_index(
                        name,
                        arity,
                        module_name,
                    )
                        .map(|index| index.get())
                        .unwrap_or(IndexPtr::dynamic_undefined());

                    match index.tag() {
                        IndexPtrTag::DynamicUndefined | IndexPtrTag::Undefined => false,
                        _ => true,
                    }
                }
            }
            (HeapCellValueTag::Atom, (name, arity)) => {
                debug_assert_eq!(arity, 0);

                if ClauseType::is_inbuilt(name, 0) {
                    true
                } else {
                    let index = self.indices.get_predicate_code_index(
                        name,
                        0,
                        module_name,
                    )
                        .map(|index| index.get())
                        .unwrap_or(IndexPtr::dynamic_undefined());

                    match index.tag() {
                        IndexPtrTag::DynamicUndefined => false,
                        _ => true,
                    }
                }
            }
            _ => {
                let err = self.machine_st.type_error(ValidType::Callable, head);
                let stub = functor_stub(atom!("clause"), 2);

                return Err(self.machine_st.error_form(err, stub));
            }
        );

        Ok(())
    }
    #[inline(always)]
    pub(crate) fn redo_attr_var_binding(&mut self) {
        let var = self.deref_register(1);
        let value = self.deref_register(2);

        debug_assert_eq!(HeapCellValueTag::AttrVar, var.get_tag());
        self.machine_st.heap[var.get_value() as usize] = value;
    }

    #[inline(always)]
    pub(super) fn restore_instr_at_verify_attr_interrupt(&mut self) {
        match &self.code[VERIFY_ATTR_INTERRUPT_LOC] {
            &Instruction::VerifyAttrInterrupt => {}
            _ => {
                let instr = mem::replace(
                    &mut self.code[VERIFY_ATTR_INTERRUPT_LOC],
                    Instruction::VerifyAttrInterrupt,
                );

                self.code[self.machine_st.attr_var_init.cp] = instr;
            }
        }
    }

    #[inline(always)]
    pub(crate) fn reset_attr_var_state(&mut self, queue_len: usize) {
        self.restore_instr_at_verify_attr_interrupt();
        self.machine_st.attr_var_init.reset(queue_len);
    }

    #[inline(always)]
    pub(crate) fn remove_call_policy_check(&mut self) {
        let bp = cell_as_fixnum!(self.deref_register(1)).get_num() as usize;

        if bp == self.machine_st.b && self.machine_st.cwil.is_empty() {
            self.machine_st.cwil.reset();
            self.machine_st.increment_call_count_fn = |_| Ok(());
        }
    }

    #[inline(always)]
    pub(crate) fn remove_inference_counter(&mut self) {
        let a1 = self.deref_register(1);
        let a2 = self.deref_register(2);

        let bp = cell_as_fixnum!(a1).get_num() as usize;

        let count = self.machine_st.cwil.remove_limit(bp).clone();

        let result = count.clone().try_into();
        if let Ok(value) = result{
            self.machine_st.unify_fixnum(Fixnum::build_with(value), a2);
        } else {
            let count = arena_alloc!(count.clone(), &mut self.machine_st.arena);
            self.machine_st.unify_big_int(count, a2);
        }
    }

    #[inline(always)]
    pub(crate) fn return_from_verify_attr(&mut self) {
        self.restore_instr_at_verify_attr_interrupt();

        let e = self.machine_st.e;
        let frame_len = self.machine_st.stack.index_and_frame(e).prelude.num_cells;

        for i in 1..frame_len - 2 {
            self.machine_st.registers[i] = self.machine_st.stack[stack_loc!(AndFrame, e, i)];
        }

        self.machine_st.b0 = cell_as_fixnum!(
            self.machine_st.stack[stack_loc!(AndFrame, e, frame_len - 2)]
        )
        .get_num() as usize;

        self.machine_st.num_of_args = cell_as_fixnum!(
            self.machine_st.stack[stack_loc!(AndFrame, e, frame_len - 1)]
        )
        .get_num() as usize;

        let p = cell_as_fixnum!(self.machine_st.stack[stack_loc!(AndFrame, e, frame_len)]).get_num()
            as usize;

        self.machine_st.deallocate();
        self.machine_st.p = p;
    }

    #[inline(always)]
    pub(crate) fn restore_cut_policy(&mut self) {
        if self.machine_st.cont_pts.is_empty() {
            self.machine_st.run_cleaners_fn = |_| false;
        }
    }

    #[inline(always)]
    pub(crate) fn set_cut_point(&mut self, r: RegType) -> bool {
        let cp = self
            .machine_st
            .store(self.machine_st.deref(self.machine_st[r]));
        self.machine_st.cut_body(cp);

        (self.machine_st.run_cleaners_fn)(self)
    }

    #[inline(always)]
    pub(crate) fn set_cut_point_by_default(&mut self, r: RegType) {
        let cp = self
            .machine_st
            .store(self.machine_st.deref(self.machine_st[r]));
        self.machine_st.cut_body(cp);
    }

    #[inline(always)]
    pub(crate) fn set_input(&mut self) -> CallResult {
        let addr = self.deref_register(1);

        let stream = self.machine_st.get_stream_or_alias(
            addr,
            &self.indices.stream_aliases,
            atom!("set_input"),
            1,
        )?;

        if !stream.is_input_stream() {
            let stub = functor_stub(atom!("set_input"), 1);
            let user_alias = atom_as_cell!(atom!("user"));

            let err = self.machine_st.permission_error(
                Permission::InputStream,
                atom!("stream"),
                user_alias,
            );

            return Err(self.machine_st.error_form(err, stub));
        }

        self.user_input = stream;
        Ok(())
    }

    #[inline(always)]
    pub(crate) fn set_output(&mut self) -> CallResult {
        let addr = self.deref_register(1);
        let stream = self.machine_st.get_stream_or_alias(
            addr,
            &self.indices.stream_aliases,
            atom!("set_output"),
            1,
        )?;

        if !stream.is_output_stream() {
            let stub = functor_stub(atom!("set_output"), 1);

            let user_alias = atom_as_cell!(atom!("user"));
            let err = self.machine_st.permission_error(
                Permission::OutputStream,
                atom!("stream"),
                user_alias,
            );

            return Err(self.machine_st.error_form(err, stub));
        }

        self.user_output = stream;
        Ok(())
    }

    #[inline(always)]
    pub(crate) fn set_double_quotes(&mut self) {
        let atom = cell_as_atom!(self.deref_register(1));

        self.machine_st.flags.double_quotes = match atom {
            atom!("atom") => DoubleQuotes::Atom,
            atom!("chars") => DoubleQuotes::Chars,
            atom!("codes") => DoubleQuotes::Codes,
            _ => {
                self.machine_st.fail = true;
                return;
            }
        };
    }

    #[inline(always)]
    pub(crate) fn set_unknown(&mut self) {
        let atom = cell_as_atom!(self.deref_register(1));

        self.machine_st.flags.unknown = match atom {
            atom!("error") => Unknown::Error,
            atom!("fail") => Unknown::Fail,
            atom!("warning") => Unknown::Warn,
            _ => {
                self.machine_st.fail = true;
                return;
            }
        };
    }

    #[inline(always)]
    pub(crate) fn inference_level(&mut self) {
        let a1 = self.deref_register(1);
        let a2 = self.deref_register(2);

        let bp = cell_as_fixnum!(a2).get_num() as usize;
        let prev_b = self
            .machine_st
            .stack
            .index_or_frame(self.machine_st.b)
            .prelude
            .b;

        if prev_b <= bp {
            self.machine_st.unify_atom(atom!("!"), a1)
        } else {
            self.machine_st.unify_atom(atom!("true"), a1);
        }
    }

    #[inline(always)]
    pub(crate) fn clean_up_block(&mut self) {
        let nb = self.deref_register(1);
        let nb = cell_as_fixnum!(nb).get_num() as usize;

        let b = self.machine_st.b;

        if nb > 0 && self.machine_st.stack.index_or_frame(b).prelude.b == nb {
            self.machine_st.b = self.machine_st.stack.index_or_frame(nb).prelude.b;
        }
    }

    #[inline(always)]
    pub(crate) fn get_ball(&mut self) {
        let addr = self.deref_register(1);
        let h = self.machine_st.heap.len();

        if self.machine_st.ball.stub.len() > 0 {
            let stub = self.machine_st.ball.copy_and_align(h);
            self.machine_st.heap.extend(stub.into_iter());
        } else {
            self.machine_st.fail = true;
            return;
        }

        match addr.as_var() {
            Some(r) => self.machine_st.bind(r, self.machine_st.heap[h]),
            _ => self.machine_st.fail = true,
        };
    }

    #[inline(always)]
    pub(crate) fn push_ball_stack(&mut self) {
        if self.machine_st.ball.stub.len() > 0 {
            self.machine_st
                .ball_stack
                .push(mem::replace(&mut self.machine_st.ball, Ball::new()));
        } else {
            self.machine_st.fail = true;
        }
    }

    #[inline(always)]
    pub(crate) fn pop_ball_stack(&mut self) {
        self.machine_st.ball_stack.pop();
    }

    #[inline(always)]
    pub(crate) fn pop_from_ball_stack(&mut self) {
        if let Some(ball) = self.machine_st.ball_stack.pop() {
            self.machine_st.ball = ball;
        }
    }

    #[inline(always)]
    pub(crate) fn get_current_block(&mut self) {
        let addr = self.machine_st.registers[1];
        let block = Fixnum::build_with(self.machine_st.block as i64);

        self.machine_st.unify_fixnum(block, addr);
    }

    #[inline(always)]
    pub(crate) fn get_current_scc_block(&mut self) {
        let addr = self.machine_st.registers[1];
        let block = Fixnum::build_with(self.machine_st.scc_block as i64);

        self.machine_st.unify_fixnum(block, addr);
    }

    #[inline(always)]
    pub(crate) fn get_b_value(&mut self) {
        let n = Fixnum::as_cutpoint(i64::try_from(self.machine_st.b).unwrap());
        self.machine_st
            .unify_fixnum(n, self.machine_st.registers[1]);
    }

    #[inline(always)]
    pub(crate) fn get_cut_point(&mut self) {
        let n = Fixnum::as_cutpoint(i64::try_from(self.machine_st.b0).unwrap());
        self.machine_st
            .unify_fixnum(n, self.machine_st.registers[1]);
    }

    #[inline(always)]
    pub(crate) fn next_ep(&mut self) {
        let first_arg = self.deref_register(1);

        let next_ep_atom = |machine_st: &mut MachineState, name, arity| {
            debug_assert_eq!(name, atom!("first"));
            debug_assert_eq!(arity, 0);

            if machine_st.e == 0 {
                machine_st.fail = true;
                return;
            }

            let and_frame = machine_st.stack.index_and_frame(machine_st.e);
            let cp = and_frame.prelude.cp - 1;

            let e = and_frame.prelude.e;
            let e = Fixnum::build_with(i64::try_from(e).unwrap());

            let p = str_loc_as_cell!(machine_st.heap.len());

            machine_st
                .heap
                .extend(functor!(atom!("dir_entry"), [fixnum(cp)]));
            machine_st.unify_fixnum(e, machine_st.registers[2]);

            if !machine_st.fail {
                unify!(machine_st, p, machine_st.registers[3]);
            }
        };

        read_heap_cell!(first_arg,
            (HeapCellValueTag::Atom, (name, arity)) => {
                next_ep_atom(&mut self.machine_st, name, arity);
            }
            (HeapCellValueTag::Str, s) => {
                let (name, arity) = cell_as_atom_cell!(self.machine_st.heap[s])
                    .get_name_and_arity();

                next_ep_atom(&mut self.machine_st, name, arity);
            }
            (HeapCellValueTag::Fixnum, n) => {
                let e = n.get_num() as usize;

                if e == 0 {
                    self.machine_st.fail = true;
                    return;
                }

                // get the call site so that the number of
                // active permanent variables can be read from
                // it later.
                let and_frame = self.machine_st.stack.index_and_frame(e);
                let cp = and_frame.prelude.cp - 1;

                let p = str_loc_as_cell!(self.machine_st.heap.len());
                self.machine_st.heap.extend(functor!(atom!("dir_entry"), [fixnum(cp)]));

                let e = Fixnum::build_with(i64::try_from(and_frame.prelude.e).unwrap());
                self.machine_st.unify_fixnum(e, self.machine_st.registers[2]);

                if !self.machine_st.fail {
                    unify!(self.machine_st, p, self.machine_st.registers[3]);
                }
            }
            _ => {
                unreachable!();
            }
        );
    }

    #[inline(always)]
    pub(crate) fn points_to_continuation_reset_marker(&mut self) {
        let addr = self.deref_register(1);

        let p = match to_local_code_ptr(&self.machine_st.heap, addr) {
            Some(p) => p + 1,
            None => {
                self.machine_st.fail = true;
                return;
            }
        };

        if !self.is_reset_cont_marker(p) {
            self.machine_st.fail = true;
        }
    }

    #[inline(always)]
    pub(crate) fn quoted_token(&mut self) {
        let addr = self.deref_register(1);

        read_heap_cell!(addr,
            (HeapCellValueTag::Fixnum, n) => {
                let n = u32::try_from(n.get_num()).ok();
                let n = n.and_then(std::char::from_u32);

                self.machine_st.fail = match n {
                    Some(c) => non_quoted_token(once(c)),
                    None => true,
                };
            }
            (HeapCellValueTag::Char, c) => {
                self.machine_st.fail = non_quoted_token(once(c));
            }
            (HeapCellValueTag::Atom, (name, arity)) => {
                debug_assert_eq!(arity, 0);
                self.machine_st.fail = non_quoted_token(name.as_str().chars());
            }
            (HeapCellValueTag::Str, s) => {
                let (name, arity) = cell_as_atom_cell!(self.machine_st.heap[s])
                    .get_name_and_arity();

                debug_assert_eq!(arity, 0);
                self.machine_st.fail = non_quoted_token(name.as_str().chars());
            }
            _ => {
                self.machine_st.fail = true;
            }
        );
    }

    #[inline(always)]
    pub(crate) fn read_query_term(&mut self) -> CallResult {
        self.user_input.reset();

        set_prompt(true);
        // let result = self.machine_st.read_term(self.user_input, &mut self.indices);
        let result = self
            .machine_st
            .read_term_from_user_input(self.user_input, &mut self.indices);
        set_prompt(false);

        match result {
            Ok(()) => Ok(()),
            Err(e) => {
                self.user_input.reset();
                return Err(e);
            }
        }
    }

    #[inline(always)]
    pub(crate) fn read_term(&mut self) -> CallResult {
        set_prompt(false);

        let stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("read_term"),
            3,
        )?;

        if let Stream::Readline(..) = stream {
            self.machine_st.read_term(
                stream,
                &mut self.indices,
                MachineState::read_term_from_user_input_eof_handler,
            )
        } else {
            self.machine_st.read_term(
                stream,
                &mut self.indices,
                MachineState::read_term_eof_handler,
            )
        }
    }

    #[inline(always)]
    fn read_term_and_write_to_heap(
        &mut self,
        atom_or_string: AtomOrString,
    ) -> Result<Option<TermWriteResult>, MachineStub> {
        let string = match atom_or_string {
            AtomOrString::Atom(atom) if atom == atom!("[]") => "".to_owned(),
            _ => atom_or_string.to_string(),
        };

        let chars = CharReader::new(ByteStream::from_string(string));
        let mut parser = Parser::new(chars, &mut self.machine_st);
        let op_dir = CompositeOpDir::new(&self.indices.op_dir, None);

        let term_write_result = parser
            .read_term(&op_dir, Tokens::Default)
            .map_err(|err| error_after_read_term(err, 0, &parser))
            .and_then(|term| {
                write_term_to_heap(&term, &mut self.machine_st.heap, &self.machine_st.atom_tbl)
            });

        match term_write_result {
            Ok(term_write_result) => Ok(Some(term_write_result)),
            Err(CompilationError::ParserError(e)) if e.is_unexpected_eof() => {
                let value = self.machine_st.registers[2];
                self.machine_st.unify_atom(atom!("end_of_file"), value);

                Ok(None)
            }
            Err(e) => {
                let stub = functor_stub(atom!("read_term_from_chars"), 3);
                let e = self.machine_st.session_error(SessionError::from(e));

                Err(self.machine_st.error_form(e, stub))
            }
        }
    }

    #[inline(always)]
    pub(crate) fn read_from_chars(&mut self) -> CallResult {
        if let Some(atom_or_string) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            if let Some(term_write_result) = self.read_term_and_write_to_heap(atom_or_string)? {
                let result = heap_loc_as_cell!(term_write_result.heap_loc);
                let var = self.deref_register(2).as_var().unwrap();

                self.machine_st.bind(var, result);
            }

            Ok(())
        } else {
            unreachable!()
        }
    }

    #[inline(always)]
    pub(crate) fn read_term_from_chars(&mut self) -> CallResult {
        if let Some(atom_or_string) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            if let Some(term_write_result) = self.read_term_and_write_to_heap(atom_or_string)? {
                self.machine_st.read_term_body(term_write_result)
            } else {
                if !self.machine_st.fail {
                    // wrote end_of_file term in this case.
                    self.machine_st.write_read_term_options(vec![], vec![])?;
                }

                Ok(())
            }
        } else {
            unreachable!()
        }
    }

    #[inline(always)]
    pub(crate) fn reset_block(&mut self) {
        let addr = self.deref_register(1);

        read_heap_cell!(addr,
            (HeapCellValueTag::Fixnum, block) => {
                self.machine_st.block = block.get_num() as usize;
            }
            _ => {
                self.machine_st.fail = true;
            }
        );
    }

    #[inline(always)]
    pub(crate) fn reset_scc_block(&mut self) {
        let addr = self.deref_register(1);

        read_heap_cell!(addr,
            (HeapCellValueTag::Fixnum, block) => {
                self.machine_st.scc_block = block.get_num() as usize;
            }
            _ => {
                self.machine_st.fail = true;
            }
        );
    }

    #[inline(always)]
    pub(crate) fn reset_continuation_marker(&mut self) {
        let h = self.machine_st.heap.len();

        self.machine_st.registers[3] = atom_as_cell!(atom!("none"));
        self.machine_st.registers[4] = heap_loc_as_cell!(h);

        self.machine_st.heap.push(heap_loc_as_cell!(h));
    }

    #[inline(always)]
    pub(crate) fn set_ball(&mut self) {
        self.machine_st.set_ball();
    }

    #[inline(always)]
    pub(crate) fn set_seed(&mut self) {
        let seed = self.deref_register(1);

        match Number::try_from(seed) {
            Ok(Number::Fixnum(n)) => {
                let n: u64 = Integer::from(n).try_into().unwrap();
                let rng: StdRng = SeedableRng::seed_from_u64(n);
                self.rng = rng;
            },
            Ok(Number::Integer(n)) => {
                let n: u64 = (&*n).try_into().unwrap();
                let rng: StdRng = SeedableRng::seed_from_u64(n);
                self.rng = rng;
            },
            Ok(Number::Rational(n)) => {
                if n.denominator() == &UBig::from(1 as u32) {
                    let n: u64 = n.numerator().try_into().unwrap();
                    let rng: StdRng = SeedableRng::seed_from_u64(n);
                    self.rng = rng;
                }
            }
            _ => {
                self.machine_st.fail = true;
            }
        }
    }

    #[inline(always)]
    pub(crate) fn sleep(&mut self) {
        let time = self.deref_register(1);

        let time = match Number::try_from(time) {
            Ok(Number::Float(n)) => n.into_inner(),
            Ok(Number::Fixnum(n)) => n.get_num() as f64,
            Ok(Number::Integer(n)) => n.to_f64().value(),
            _ => {
                unreachable!()
            }
        };

        let duration = Duration::new(1, 0);
        let duration = duration.mul_f64(time);

        std::thread::sleep(duration);
    }

    #[inline(always)]
    pub(crate) fn socket_client_open(&mut self) -> CallResult {
        let addr = self.deref_register(1);
        let port = self.deref_register(2);

        let socket_atom = cell_as_atom!(addr);

        let port = read_heap_cell!(port,
            (HeapCellValueTag::Atom, (name, arity)) => {
                debug_assert_eq!(arity, 0);
                name
            }
            (HeapCellValueTag::Str, s) => {
                let (name, arity) = cell_as_atom_cell!(self.machine_st.heap[s])
                    .get_name_and_arity();

                debug_assert_eq!(arity, 0);
                name
            }
            _ => {
                AtomTable::build_with(&self.machine_st.atom_tbl, &match Number::try_from(port) {
                    Ok(Number::Fixnum(n)) => n.get_num().to_string(),
                    Ok(Number::Integer(n)) => n.to_string(),
                    _ => {
                        unreachable!()
                    }
                })
            }
        );

        let socket_addr = if socket_atom == atom!("") {
            atom!("127.0.0.1:80")
        } else {
            let buffer = format!("{}:{}", socket_atom.as_str(), port.as_str());
            AtomTable::build_with(&self.machine_st.atom_tbl, &buffer)
        };

        let alias = self.machine_st.registers[4];
        let eof_action = self.machine_st.registers[5];
        let reposition = self.machine_st.registers[6];
        let stream_type = self.machine_st.registers[7];

        let options = self
            .machine_st
            .to_stream_options(alias, eof_action, reposition, stream_type);

        if options.reposition() {
            return Err(self
                .machine_st
                .reposition_error(atom!("socket_client_open"), 3));
        }

        if let Some(alias) = options.get_alias() {
            if self.indices.stream_aliases.contains_key(&alias) {
                return Err(self.machine_st.occupied_alias_permission_error(
                    alias,
                    atom!("socket_client_open"),
                    3,
                ));
            }
        }

        let stream = match TcpStream::connect(&*socket_addr.as_str()).map_err(|e| e.kind()) {
            Ok(tcp_stream) => {
                let mut stream =
                    Stream::from_tcp_stream(socket_addr, tcp_stream, &mut self.machine_st.arena);

                *stream.options_mut() = options;

                if let Some(alias) = stream.options().get_alias() {
                    self.indices.stream_aliases.insert(alias, stream);
                }

                self.indices.streams.insert(stream);

                stream_as_cell!(stream)
            }
            Err(ErrorKind::PermissionDenied) => {
                return Err(self.machine_st.open_permission_error(
                    addr,
                    atom!("socket_client_open"),
                    3,
                ));
            }
            Err(ErrorKind::NotFound) => {
                let stub = functor_stub(atom!("socket_client_open"), 3);
                let err = self
                    .machine_st
                    .existence_error(ExistenceError::SourceSink(addr));

                return Err(self.machine_st.error_form(err, stub));
            }
            Err(_) => {
                // for now, just fail. expand to meaningful error messages later.
                self.machine_st.fail = true;
                return Ok(());
            }
        };

        let stream_addr = self.deref_register(3);
        self.machine_st.bind(stream_addr.as_var().unwrap(), stream);

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn socket_server_open(&mut self) -> CallResult {
        let addr = self.deref_register(1);
        let socket_atom = cell_as_atom_cell!(addr).get_name();

        let socket_atom = if socket_atom == atom!("[]") {
            atom!("127.0.0.1")
        } else {
            socket_atom
        };

        let port = self.deref_register(2);

        let port = if port.is_var() {
            String::from("0")
        } else {
            match Number::try_from(port) {
                Ok(Number::Fixnum(n)) => n.get_num().to_string(),
                Ok(Number::Integer(n)) => n.to_string(),
                _ => {
                    unreachable!()
                }
            }
        };

        let had_zero_port = &port == "0";

        let server_addr = if socket_atom == atom!("") {
            port
        } else {
            format!("{}:{}", socket_atom.as_str(), port)
        };

        let (tcp_listener, port) = match TcpListener::bind(server_addr).map_err(|e| e.kind()) {
            Ok(tcp_listener) => {
                let port = tcp_listener.local_addr().map(|addr| addr.port()).ok();

                if let Some(port) = port {
                    (
                        arena_alloc!(tcp_listener, &mut self.machine_st.arena),
                        port as usize,
                    )
                } else {
                    self.machine_st.fail = true;
                    return Ok(());
                }
            }
            Err(ErrorKind::PermissionDenied) => {
                return Err(self.machine_st.open_permission_error(
                    addr,
                    atom!("socket_server_open"),
                    2,
                ));
            }
            _ => {
                self.machine_st.fail = true;
                return Ok(());
            }
        };

        let addr = self.deref_register(3);
        self.machine_st.bind(
            addr.as_var().unwrap(),
            typed_arena_ptr_as_cell!(tcp_listener),
        );

        if had_zero_port {
            self.machine_st.unify_fixnum(
                Fixnum::build_with(port as i64),
                self.machine_st.registers[2],
            );
        }

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn socket_server_accept(&mut self) -> CallResult {
        let alias = self.machine_st.registers[4];
        let eof_action = self.machine_st.registers[5];
        let reposition = self.machine_st.registers[6];
        let stream_type = self.machine_st.registers[7];

        let options = self
            .machine_st
            .to_stream_options(alias, eof_action, reposition, stream_type);

        if options.reposition() {
            return Err(self
                .machine_st
                .reposition_error(atom!("socket_server_accept"), 4));
        }

        if let Some(alias) = options.get_alias() {
            if self.indices.stream_aliases.contains_key(&alias) {
                return Err(self.machine_st.occupied_alias_permission_error(
                    alias,
                    atom!("socket_server_accept"),
                    4,
                ));
            }
        }

        let culprit = self.deref_register(1);

        read_heap_cell!(culprit,
            (HeapCellValueTag::Cons, cons_ptr) => {
                match_untyped_arena_ptr!(cons_ptr,
                     (ArenaHeaderTag::TcpListener, tcp_listener) => {
                         match tcp_listener.accept().ok() {
                             Some((tcp_stream, socket_addr)) => {
                                 let client = AtomTable::build_with(&self.machine_st.atom_tbl, &socket_addr.to_string());

                                 let mut tcp_stream = Stream::from_tcp_stream(
                                     client,
                                     tcp_stream,
                                     &mut self.machine_st.arena,
                                 );

                                 *tcp_stream.options_mut() = options;

                                 if let Some(alias) = &tcp_stream.options().get_alias() {
                                     self.indices.stream_aliases.insert(*alias, tcp_stream);
                                 }

                                 self.indices.streams.insert(tcp_stream);

                                 let tcp_stream = stream_as_cell!(tcp_stream);
                                 let client = atom_as_cell!(client);

                                 let client_addr = self.deref_register(2);
                                 let stream_addr = self.deref_register(3);

                                 self.machine_st.bind(client_addr.as_var().unwrap(), client);
                                 self.machine_st.bind(stream_addr.as_var().unwrap(), tcp_stream);
                             }
                             None => {
                                 self.machine_st.fail = true;
                             }
                         }
                     }
                     _ => {
                     }
                );
            }
            _ => {
            }
        );

        Ok(())
    }

    #[cfg(feature = "tls")]
    #[inline(always)]
    pub(crate) fn tls_client_connect(&mut self) -> CallResult {
        if let Some(hostname) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            let stream0 = self.machine_st.get_stream_or_alias(
                self.machine_st.registers[2],
                &self.indices.stream_aliases,
                atom!("tls_client_negotiate"),
                3,
            )?;

            let connector = TlsConnector::new().unwrap();
            let stream = match connector.connect(&*hostname.as_str(), stream0) {
                Ok(tls_stream) => tls_stream,
                Err(_) => {
                    return Err(self.machine_st.open_permission_error(
                        self.machine_st.registers[1],
                        atom!("tls_client_negotiate"),
                        3,
                    ));
                }
            };

            let addr = atom!("TLS");
            let stream = Stream::from_tls_stream(addr, stream, &mut self.machine_st.arena);
            self.indices.streams.insert(stream);

            self.machine_st.heap.push(stream_as_cell!(stream));
            let stream_addr = self.deref_register(3);
            self.machine_st
                .bind(stream_addr.as_var().unwrap(), stream_as_cell!(stream));

            Ok(())
        } else {
            unreachable!();
        }
    }

    #[cfg(feature = "tls")]
    #[inline(always)]
    pub(crate) fn tls_accept_client(&mut self) -> CallResult {
        let pkcs12 = self.string_encoding_bytes(self.machine_st.registers[1], atom!("octet"));

        if let Some(password) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[2])
        {
            let identity = match Identity::from_pkcs12(&pkcs12, &*password.as_str()) {
                Ok(identity) => identity,
                Err(_) => {
                    return Err(self.machine_st.open_permission_error(
                        self.machine_st.registers[1],
                        atom!("tls_server_negotiate"),
                        3,
                    ));
                }
            };

            let stream0 = self.machine_st.get_stream_or_alias(
                self.machine_st.registers[3],
                &self.indices.stream_aliases,
                atom!("tls_server_negotiate"),
                3,
            )?;

            let acceptor = TlsAcceptor::new(identity).unwrap();

            let stream = match acceptor.accept(stream0) {
                Ok(tls_stream) => tls_stream,
                Err(_) => {
                    return Err(self.machine_st.open_permission_error(
                        self.machine_st.registers[3],
                        atom!("tls_server_negotiate"),
                        3,
                    ));
                }
            };

            let stream = Stream::from_tls_stream(atom!("TLS"), stream, &mut self.machine_st.arena);
            self.indices.streams.insert(stream);

            let stream_addr = self.deref_register(4);
            self.machine_st
                .bind(stream_addr.as_var().unwrap(), stream_as_cell!(stream));
        } else {
            unreachable!();
        }

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn socket_server_close(&mut self) -> CallResult {
        let culprit = self.deref_register(1);

        read_heap_cell!(culprit,
            (HeapCellValueTag::Cons, cons_ptr) => {
                match_untyped_arena_ptr!(cons_ptr,
                    (ArenaHeaderTag::TcpListener, tcp_listener) => {
                        unsafe {
                            // dropping closes the instance.
                            std::ptr::drop_in_place(&mut tcp_listener as *mut _);
                        }

                        tcp_listener.set_tag(ArenaHeaderTag::Dropped);
                        return Ok(());
                    }
                    _ => {
                    }
                );
            }
            _ => {
            }
        );

        let err = self.machine_st.type_error(ValidType::TcpListener, culprit);
        let stub = functor_stub(atom!("socket_server_close"), 1);

        return Err(self.machine_st.error_form(err, stub));
    }

    #[inline(always)]
    pub(crate) fn set_stream_position(&mut self) -> CallResult {
        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("set_stream_position"),
            2,
        )?;

        if !stream.options().reposition() {
            let stub = functor_stub(atom!("set_stream_position"), 2);

            let err = self.machine_st.permission_error(
                Permission::Reposition,
                atom!("stream"),
                stream_as_cell!(stream),
            );

            return Err(self.machine_st.error_form(err, stub));
        }

        let position = self.deref_register(2);

        let position = match Number::try_from(position) {
            Ok(Number::Fixnum(n)) => n.get_num() as u64,
            Ok(Number::Integer(n)) => {
                let n: Result<u64, _> = (&*n).try_into();
                if let Ok(n) = n {
                    n
                } else {
                    self.machine_st.fail = true;
                    return Ok(());
                }
            }
            _ => {
                unreachable!()
            }
        };

        stream.set_position(position);
        Ok(())
    }

    #[inline(always)]
    pub(crate) fn stream_property(&mut self) -> CallResult {
        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("stream_property"),
            2,
        )?;

        let atom = cell_as_atom!(self.deref_register(2));

        let property = match atom {
            atom!("file_name") => {
                atom_as_cell!(if let Some(file_name) = stream.file_name() {
                    file_name
                } else {
                    self.machine_st.fail = true;
                    return Ok(());
                })
            }
            atom!("mode") => atom_as_cell!(stream.mode()),
            atom!("direction") => {
                atom_as_cell!(if stream.is_input_stream() && stream.is_output_stream() {
                    atom!("input_output")
                } else if stream.is_input_stream() {
                    atom!("input")
                } else {
                    atom!("output")
                })
            }
            atom!("alias") => {
                atom_as_cell!(if let Some(alias) = stream.options().get_alias() {
                    alias
                } else {
                    self.machine_st.fail = true;
                    return Ok(());
                })
            }
            atom!("position") => {
                if let Some((position, lines_read)) = stream.position() {
                    let h = self.machine_st.heap.len();

                    let position_term = functor!(
                        atom!("position_and_lines_read"),
                        [
                            integer(position, &mut self.machine_st.arena),
                            integer(lines_read, &mut self.machine_st.arena)
                        ]
                    );

                    self.machine_st.heap.extend(position_term.into_iter());
                    str_loc_as_cell!(h)
                } else {
                    self.machine_st.fail = true;
                    return Ok(());
                }
            }
            atom!("end_of_stream") => {
                let end_of_stream_pos = stream.position_relative_to_end();
                atom_as_cell!(end_of_stream_pos.as_atom())
            }
            atom!("eof_action") => {
                atom_as_cell!(stream.options().eof_action().as_atom())
            }
            atom!("reposition") => atom_as_cell!(if stream.options().reposition() {
                atom!("true")
            } else {
                atom!("false")
            }),
            atom!("type") => {
                atom_as_cell!(stream.options().stream_type().as_property_atom())
            }
            _ => {
                unreachable!()
            }
        };

        unify!(self.machine_st, property, self.machine_st.registers[3]);
        Ok(())
    }

    #[inline(always)]
    pub(crate) fn store_global_var(&mut self) {
        let key = cell_as_atom!(self.deref_register(1));

        let value = self.machine_st.registers[2];
        let mut ball = Ball::new();

        ball.boundary = self.machine_st.heap.len();

        copy_term(
            CopyBallTerm::new(
                &mut self.machine_st.stack,
                &mut self.machine_st.heap,
                &mut ball.stub,
            ),
            value,
            AttrVarPolicy::DeepCopy,
        );

        self.indices.global_variables.insert(key, (ball, None));
    }

    #[inline(always)]
    pub(crate) fn store_backtrackable_global_var(&mut self) {
        let key = cell_as_atom!(self.deref_register(1));
        let new_value = self.deref_register(2);

        match self.indices.global_variables.get_mut(&key) {
            Some((_, ref mut loc)) => match loc {
                Some(ref mut value) => {
                    self.machine_st
                        .trail(TrailRef::BlackboardOffset(key, *value));
                    *value = new_value;
                }
                loc @ None => {
                    self.machine_st.trail(TrailRef::BlackboardEntry(key));
                    *loc = Some(new_value);
                }
            },
            None => {
                self.machine_st.trail(TrailRef::BlackboardEntry(key));
                self.indices
                    .global_variables
                    .insert(key, (Ball::new(), Some(new_value)));
            }
        }
    }

    #[inline(always)]
    pub(crate) fn term_attributed_variables(&mut self) {
        if self.machine_st.registers[1].is_constant() {
            let a2 = self.deref_register(2);
            self.machine_st.unify_atom(atom!("[]"), a2);

            return;
        }

        let seen_vars = self
            .machine_st
            .attr_vars_of_term(self.machine_st.registers[1]);
        let outcome = heap_loc_as_cell!(iter_to_heap_list(
            &mut self.machine_st.heap,
            seen_vars.into_iter()
        ));

        unify_fn!(self.machine_st, self.machine_st.registers[2], outcome);
    }

    #[inline(always)]
    pub(crate) fn term_variables(&mut self) {
        let stored_v = self.deref_register(1);
        let a2 = self.deref_register(2);

        if stored_v.is_constant() {
            self.machine_st.unify_atom(atom!("[]"), a2);
            return;
        }

        let mut seen_set = IndexSet::with_hasher(FxBuildHasher::default());

        self.machine_st.variable_set(&mut seen_set, stored_v);

        let outcome = heap_loc_as_cell!(iter_to_heap_list(
            &mut self.machine_st.heap,
            seen_set.into_iter()
        ));

        unify_fn!(self.machine_st, a2, outcome);
    }

    #[inline(always)]
    pub(crate) fn term_variables_under_max_depth(&mut self) {
        // Term, MaxDepth, VarList
        let max_depth = cell_as_fixnum!(self.deref_register(2)).get_num() as usize;

        self.machine_st.term_variables_under_max_depth(
            self.machine_st.registers[1],
            max_depth,
            self.machine_st.registers[3],
        );
    }

    #[inline(always)]
    pub(crate) fn truncate_lifted_heap_to(&mut self) {
        let a1 = self.deref_register(1);
        let lh_offset = cell_as_fixnum!(a1).get_num() as usize;

        self.machine_st.lifted_heap.truncate(lh_offset);
    }

    #[inline(always)]
    pub(crate) fn unify_with_occurs_check(&mut self) {
        let a1 = self.machine_st.registers[1];
        let a2 = self.machine_st.registers[2];

        unify_with_occurs_check!(&mut self.machine_st, a1, a2);
    }

    #[inline(always)]
    pub(crate) fn unwind_environments(&mut self) -> bool {
        let mut e = self.machine_st.e;
        let mut cp = self.machine_st.cp;

        while e > 0 {
            if self.is_reset_cont_marker(cp) {
                self.machine_st.e = e;
                self.machine_st.p = cp + 1; // skip the reset marker.

                return true;
            }

            let and_frame = self.machine_st.stack.index_and_frame(e);

            cp = and_frame.prelude.cp;
            e = and_frame.prelude.e;
        }

        false
    }

    fn walk_code_at_ptr(&mut self, index_ptr: usize) -> HeapCellValue {
        let mut h = self.machine_st.heap.len();

        let mut functors = vec![];
        let mut functor_list = vec![];

        walk_code(&self.code, index_ptr, |instr| {
            let old_len = functors.len();
            instr.enqueue_functors(h, &mut self.machine_st.arena, &mut functors);
            let new_len = functors.len();

            for index in old_len..new_len {
                let functor_len = functors[index].len();

                match functor_len {
                    0 => {}
                    1 => {
                        functor_list.push(heap_loc_as_cell!(h));
                        h += functor_len;
                    }
                    _ => {
                        functor_list.push(str_loc_as_cell!(h));
                        h += functor_len;
                    }
                }
            }
        });

        for functor in functors {
            self.machine_st.heap.extend(functor.into_iter());
        }

        heap_loc_as_cell!(iter_to_heap_list(
            &mut self.machine_st.heap,
            functor_list.into_iter()
        ))
    }

    #[inline(always)]
    pub(crate) fn wam_instructions(&mut self) -> CallResult {
        let module_name = cell_as_atom!(self.deref_register(1));
        let name = cell_as_atom!(self.deref_register(2));
        let arity = self.deref_register(3);

        let arity = match Number::try_from(arity) {
            Ok(Number::Fixnum(n)) => n.get_num() as usize,
            Ok(Number::Integer(n)) => {
                let value: usize = (&*n).try_into().unwrap();
                value
            },
            _ => {
                unreachable!()
            }
        };

        let key = (name, arity);

        let first_idx = match module_name {
            atom!("user") => self.indices.code_dir.get(&key),
            _ => match self.indices.modules.get(&module_name) {
                Some(module) => module.code_dir.get(&key),
                None => {
                    let stub = functor_stub(key.0, key.1);
                    let err = self.machine_st.session_error(SessionError::from(
                        CompilationError::InvalidModuleResolution(module_name),
                    ));

                    return Err(self.machine_st.error_form(err, stub));
                }
            },
        };

        let first_idx = match first_idx {
            Some(ref idx) if idx.local().is_some() => {
                if let Some(idx) = idx.local() {
                    idx
                } else {
                    unreachable!()
                }
            }
            _ => {
                let stub = functor_stub(name, arity);
                let err = self
                    .machine_st
                    .existence_error(ExistenceError::Procedure(name, arity));

                return Err(self.machine_st.error_form(err, stub));
            }
        };

        let listing = self.walk_code_at_ptr(first_idx);
        let listing_var = self.machine_st.registers[4];

        unify!(self.machine_st, listing, listing_var);
        Ok(())
    }

    #[inline(always)]
    pub(crate) fn inlined_instructions(&mut self) {
        let index_ptr = self.deref_register(1);
        let index_ptr = match Number::try_from(index_ptr) {
            Ok(Number::Fixnum(n)) => n.get_num() as usize,
            Ok(Number::Integer(n)) => {
                let value: usize = (&*n).try_into().unwrap();
                value
            },
            _ => {
                unreachable!()
            }
        };

        let listing = self.walk_code_at_ptr(index_ptr);
        let listing_var = self.machine_st.registers[2];

        unify!(self.machine_st, listing, listing_var);
    }

    #[inline(always)]
    pub(crate) fn write_term(&mut self) -> CallResult {
        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("write_term"),
            3,
        )?;

        self.machine_st.check_stream_properties(
            stream,
            StreamType::Text,
            None, // input
            atom!("write_term"),
            3,
        )?;

        let opt_err = if !stream.is_output_stream() {
            Some(atom!("stream")) // 8.14.2.3 g)
        } else if stream.options().stream_type() == StreamType::Binary {
            Some(atom!("binary_stream")) // 8.14.2.3 h)
        } else {
            None
        };

        if let Some(err_atom) = opt_err {
            return Err(self.machine_st.stream_permission_error(
                Permission::OutputStream,
                err_atom,
                stream,
                atom!("write_term"),
                3,
            ));
        }

        let printer = match self.machine_st.write_term(&self.indices.op_dir)? {
            Some(printer) => printer,
            None => {
                // this next line is executed by
                // MachineState::write_term in this case. it's
                // commented here because rustc can't prove
                // that it's no longer borrowed.

                // self.machine_st.fail = true;
                return Ok(());
            }
        };

        let output = printer.print();

        match write!(&mut stream, "{}", output.result()) {
            Ok(_) => {}
            Err(_) => {
                let stub = functor_stub(atom!("open"), 4);
                let err = self
                    .machine_st
                    .existence_error(ExistenceError::Stream(self.machine_st.registers[1]));

                return Err(self.machine_st.error_form(err, stub));
            }
        }

        stream.flush().unwrap();
        Ok(())
    }

    #[inline(always)]
    pub(crate) fn write_term_to_chars(&mut self) -> CallResult {
        let printer = match self.machine_st.write_term(&self.indices.op_dir)? {
            None => {
                // this next line is executed by
                // MachineState::write_term in this case. it's
                // commented here because rustc can't prove
                // that it's no longer borrowed.

                // self.machine_st.fail = true;
                return Ok(());
            }
            Some(printer) => printer,
        };

        let result = printer.print().result();
        let chars = put_complete_string(
            &mut self.machine_st.heap,
            &result,
            &self.machine_st.atom_tbl,
        );

        let result_addr = self.deref_register(1);

        if let Some(var) = result_addr.as_var() {
            self.machine_st.bind(var, chars);
        } else {
            unreachable!()
        }

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn scryer_prolog_version(&mut self) {
        use git_version::git_version;

        let buffer = git_version!(cargo_prefix = "cargo:", fallback = "unknown");
        let buffer_atom = AtomTable::build_with(&self.machine_st.atom_tbl, buffer);

        let a1 = self.deref_register(1);
        self.machine_st.unify_complete_string(buffer_atom, a1);
    }

    #[inline(always)]
    pub(crate) fn crypto_random_byte(&mut self) {
        let arg = self.machine_st.registers[1];
        let mut bytes: [u8; 1] = [0];

        match rng().fill(&mut bytes) {
            Ok(()) => {}
            Err(_) => {
                // the error payload here is of type 'Unspecified',
                // which contains no information whatsoever. So, for now,
                // just fail.
                self.machine_st.fail = true;
                return;
            }
        }

        let byte = Fixnum::build_with(bytes[0] as i64);
        self.machine_st.unify_fixnum(byte, arg);
    }

    #[inline(always)]
    pub(crate) fn crypto_data_hash(&mut self) {
        let encoding = cell_as_atom!(self.deref_register(2));
        let bytes = self.string_encoding_bytes(self.machine_st.registers[1], encoding);

        let algorithm = cell_as_atom!(self.deref_register(4));

        let ints_list = match algorithm {
            atom!("sha3_224") => {
                let mut context = Sha3_224::new();
                context.input(&bytes);

                heap_loc_as_cell!(iter_to_heap_list(
                    &mut self.machine_st.heap,
                    context
                        .result()
                        .as_ref()
                        .iter()
                        .map(|b| fixnum_as_cell!(Fixnum::build_with(*b as i64))),
                ))
            }
            atom!("sha3_256") => {
                let mut context = Sha3_256::new();
                context.input(&bytes);
                heap_loc_as_cell!(iter_to_heap_list(
                    &mut self.machine_st.heap,
                    context
                        .result()
                        .as_ref()
                        .iter()
                        .map(|b| fixnum_as_cell!(Fixnum::build_with(*b as i64))),
                ))
            }
            atom!("sha3_384") => {
                let mut context = Sha3_384::new();
                context.input(&bytes);

                heap_loc_as_cell!(iter_to_heap_list(
                    &mut self.machine_st.heap,
                    context
                        .result()
                        .as_ref()
                        .iter()
                        .map(|b| fixnum_as_cell!(Fixnum::build_with(*b as i64))),
                ))
            }
            atom!("sha3_512") => {
                let mut context = Sha3_512::new();
                context.input(&bytes);

                heap_loc_as_cell!(iter_to_heap_list(
                    &mut self.machine_st.heap,
                    context
                        .result()
                        .as_ref()
                        .iter()
                        .map(|b| fixnum_as_cell!(Fixnum::build_with(*b as i64))),
                ))
            }
            atom!("blake2s256") => {
                let mut context = Blake2s::new();
                context.input(&bytes);

                heap_loc_as_cell!(iter_to_heap_list(
                    &mut self.machine_st.heap,
                    context
                        .result()
                        .as_ref()
                        .iter()
                        .map(|b| fixnum_as_cell!(Fixnum::build_with(*b as i64))),
                ))
            }
            atom!("blake2b512") => {
                let mut context = Blake2b::new();
                context.input(&bytes);

                heap_loc_as_cell!(iter_to_heap_list(
                    &mut self.machine_st.heap,
                    context
                        .result()
                        .as_ref()
                        .iter()
                        .map(|b| fixnum_as_cell!(Fixnum::build_with(*b as i64))),
                ))
            }
            atom!("ripemd160") => {
                let mut context = Ripemd160::new();
                context.input(&bytes);

                heap_loc_as_cell!(iter_to_heap_list(
                    &mut self.machine_st.heap,
                    context
                        .result()
                        .as_ref()
                        .iter()
                        .map(|b| fixnum_as_cell!(Fixnum::build_with(*b as i64))),
                ))
            }
            _ => {
                let ints = digest::digest(
                    match algorithm {
                        atom!("sha256") => &digest::SHA256,
                        atom!("sha384") => &digest::SHA384,
                        atom!("sha512") => &digest::SHA512,
                        atom!("sha512_256") => &digest::SHA512_256,
                        _ => {
                            unreachable!()
                        }
                    },
                    &bytes,
                );

                heap_loc_as_cell!(iter_to_heap_list(
                    &mut self.machine_st.heap,
                    ints.as_ref()
                        .iter()
                        .map(|b| fixnum_as_cell!(Fixnum::build_with(*b as i64))),
                ))
            }
        };

        unify!(self.machine_st, self.machine_st.registers[3], ints_list);
    }

    #[inline(always)]
    pub(crate) fn crypto_data_hkdf(&mut self) {
        let encoding = cell_as_atom!(self.deref_register(2));
        let data = self.string_encoding_bytes(self.machine_st.registers[1], encoding);

        let stub1_gen = || functor_stub(atom!("crypto_data_hkdf"), 4);
        let salt = self
            .machine_st
            .integers_to_bytevec(self.machine_st.registers[3], stub1_gen);

        let stub2_gen = || functor_stub(atom!("crypto_data_hkdf"), 4);
        let info = self
            .machine_st
            .integers_to_bytevec(self.machine_st.registers[4], stub2_gen);

        let algorithm = cell_as_atom!(self.deref_register(5));

        let length = self.deref_register(6);

        let length = match Number::try_from(length) {
            Ok(Number::Fixnum(n)) => usize::try_from(n.get_num()).unwrap(),
            Ok(Number::Integer(n)) => match (&*n).try_into() as Result<usize, _> {
                Ok(u) => u,
                _ => {
                    self.machine_st.fail = true;
                    return;
                }
            },
            _ => {
                unreachable!()
            }
        };

        let ints_list = {
            let digest_alg = match algorithm {
                atom!("sha256") => hkdf::HKDF_SHA256,
                atom!("sha384") => hkdf::HKDF_SHA384,
                atom!("sha512") => hkdf::HKDF_SHA512,
                _ => {
                    self.machine_st.fail = true;
                    return;
                }
            };

            let salt = hkdf::Salt::new(digest_alg, &salt);
            let mut bytes: Vec<u8> = Vec::new();

            bytes.resize(length, 0);

            match salt.extract(&data).expand(&[&info[..]], MyKey(length)) {
                Ok(r) => {
                    r.fill(&mut bytes).unwrap();
                }
                _ => {
                    self.machine_st.fail = true;
                    return;
                }
            }

            heap_loc_as_cell!(iter_to_heap_list(
                &mut self.machine_st.heap,
                bytes
                    .iter()
                    .map(|b| fixnum_as_cell!(Fixnum::build_with(*b as i64))),
            ))
        };

        unify!(self.machine_st, self.machine_st.registers[7], ints_list);
    }

    #[inline(always)]
    pub(crate) fn crypto_password_hash(&mut self) {
        let stub1_gen = || functor_stub(atom!("crypto_password_hash"), 3);
        let data = self
            .machine_st
            .integers_to_bytevec(self.machine_st.registers[1], stub1_gen);
        let stub2_gen = || functor_stub(atom!("crypto_password_hash"), 3);
        let salt = self
            .machine_st
            .integers_to_bytevec(self.machine_st.registers[2], stub2_gen);

        let iterations = self.deref_register(3);

        let iterations = match Number::try_from(iterations) {
            Ok(Number::Fixnum(n)) => u64::try_from(n.get_num()).unwrap(),
            Ok(Number::Integer(n)) => {
                let n: Result<u64, _> = (&*n).try_into();
                match n {
                    Ok(i) => i,
                    _ => {
                        self.machine_st.fail = true;
                        return;
                    }
                }
            },
            _ => {
                unreachable!()
            }
        };

        let ints_list = {
            let mut bytes = [0u8; digest::SHA512_OUTPUT_LEN];

            pbkdf2::derive(
                pbkdf2::PBKDF2_HMAC_SHA512,
                NonZeroU32::new(iterations as u32).unwrap(),
                &salt,
                &data,
                &mut bytes,
            );

            heap_loc_as_cell!(iter_to_heap_list(
                &mut self.machine_st.heap,
                bytes
                    .iter()
                    .map(|b| fixnum_as_cell!(Fixnum::build_with(*b as i64))),
            ))
        };

        unify!(self.machine_st, self.machine_st.registers[4], ints_list);
    }

    #[cfg(feature = "crypto-full")]
    #[inline(always)]
    pub(crate) fn crypto_data_encrypt(&mut self) {
        let encoding = cell_as_atom!(self.deref_register(3));

        let data = self.string_encoding_bytes(self.machine_st.registers[1], encoding);
        let aad = self.string_encoding_bytes(self.machine_st.registers[2], encoding);

        let stub2_gen = || functor_stub(atom!("crypto_data_encrypt"), 7);
        let key = self
            .machine_st
            .integers_to_bytevec(self.machine_st.registers[4], stub2_gen);

        let stub3_gen = || functor_stub(atom!("crypto_data_encrypt"), 7);
        let iv = self
            .machine_st
            .integers_to_bytevec(self.machine_st.registers[5], stub3_gen);

        let unbound_key = aead::UnboundKey::new(&aead::CHACHA20_POLY1305, &key).unwrap();
        let nonce = aead::Nonce::try_assume_unique_for_key(&iv).unwrap();
        let key = aead::LessSafeKey::new(unbound_key);

        let mut in_out = data;

        let tag = match key.seal_in_place_separate_tag(nonce, aead::Aad::from(aad), &mut in_out) {
            Ok(d) => d,
            _ => {
                self.machine_st.fail = true;
                return;
            }
        };

        let tag_list = heap_loc_as_cell!(iter_to_heap_list(
            &mut self.machine_st.heap,
            tag.as_ref()
                .iter()
                .map(|b| fixnum_as_cell!(Fixnum::build_with(*b as i64))),
        ));

        let complete_string = self.u8s_to_string(&in_out);

        unify!(self.machine_st, self.machine_st.registers[6], tag_list);
        unify!(
            self.machine_st,
            self.machine_st.registers[7],
            complete_string
        );
    }

    #[cfg(feature = "crypto-full")]
    #[inline(always)]
    pub(crate) fn crypto_data_decrypt(&mut self) {
        let data = self.string_encoding_bytes(self.machine_st.registers[1], atom!("octet"));
        let encoding = cell_as_atom!(self.deref_register(5));

        let aad = self.string_encoding_bytes(self.machine_st.registers[2], encoding);
        let stub1_gen = || functor_stub(atom!("crypto_data_decrypt"), 7);

        let key = self
            .machine_st
            .integers_to_bytevec(self.machine_st.registers[3], stub1_gen);
        let stub2_gen = || functor_stub(atom!("crypto_data_decrypt"), 7);
        let iv = self
            .machine_st
            .integers_to_bytevec(self.machine_st.registers[4], stub2_gen);

        let unbound_key = aead::UnboundKey::new(&aead::CHACHA20_POLY1305, &key).unwrap();
        let nonce = aead::Nonce::try_assume_unique_for_key(&iv).unwrap();
        let key = aead::LessSafeKey::new(unbound_key);

        let mut in_out = data;

        let complete_string = {
            let decrypted_data = match key.open_in_place(nonce, aead::Aad::from(aad), &mut in_out) {
                Ok(d) => d,
                _ => {
                    self.machine_st.fail = true;
                    return;
                }
            };

            let buffer = match encoding {
                atom!("octet") => String::from_iter(decrypted_data.iter().map(|b| *b as char)),
                atom!("utf8") => match String::from_utf8(decrypted_data.to_vec()) {
                    Ok(str) => str,
                    _ => {
                        self.machine_st.fail = true;
                        return;
                    }
                },
                _ => {
                    unreachable!()
                }
            };

            if buffer.len() == 0 {
                empty_list_as_cell!()
            } else {
                atom_as_cstr_cell!(AtomTable::build_with(&self.machine_st.atom_tbl, &buffer))
            }
        };

        unify!(
            self.machine_st,
            self.machine_st.registers[6],
            complete_string
        );
    }

    #[inline(always)]
    pub(crate) fn crypto_curve_scalar_mult(&mut self) {
        let stub_gen = || functor_stub(atom!("crypto_curve_scalar_mult"), 4);
        let scalar_bytes = self
            .machine_st
            .integers_to_bytevec(self.machine_st.registers[2], stub_gen);
        let point_bytes = self
            .machine_st
            .integers_to_bytevec(self.machine_st.registers[3], stub_gen);

        let mut point = secp256k1::Point::decode(&point_bytes).unwrap();
        let scalar = secp256k1::Scalar::decode_reduce(&scalar_bytes);
        point *= scalar;

        let uncompressed = self.u8s_to_string(&point.encode_uncompressed());

        unify!(self.machine_st, self.machine_st.registers[4], uncompressed);
    }

    #[cfg(feature = "crypto-full")]
    #[inline(always)]
    pub(crate) fn ed25519_new_key_pair(&mut self) {
        let pkcs8_bytes = signature::Ed25519KeyPair::generate_pkcs8(rng()).unwrap();
        let complete_string = self.u8s_to_string(pkcs8_bytes.as_ref());

        unify!(
            self.machine_st,
            self.machine_st.registers[1],
            complete_string
        )
    }

    #[cfg(feature = "crypto-full")]
    #[inline(always)]
    pub(crate) fn ed25519_key_pair_public_key(&mut self) {
        let bytes = self.string_encoding_bytes(self.machine_st.registers[1], atom!("octet"));

        let key_pair = match signature::Ed25519KeyPair::from_pkcs8(&bytes) {
            Ok(kp) => kp,
            _ => {
                self.machine_st.fail = true;
                return;
            }
        };

        let complete_string = self.u8s_to_string(key_pair.public_key().as_ref());

        unify!(
            self.machine_st,
            self.machine_st.registers[2],
            complete_string
        );
    }

    #[cfg(feature = "crypto-full")]
    #[inline(always)]
    pub(crate) fn ed25519_sign(&mut self) {
        let key = self.string_encoding_bytes(self.machine_st.registers[1], atom!("octet"));
        let encoding = cell_as_atom!(self.deref_register(3));
        let data = self.string_encoding_bytes(self.machine_st.registers[2], encoding);

        let key_pair = match signature::Ed25519KeyPair::from_pkcs8(&key) {
            Ok(kp) => kp,
            _ => {
                self.machine_st.fail = true;
                return;
            }
        };

        let sig = key_pair.sign(&data);

        let sig_list = heap_loc_as_cell!(iter_to_heap_list(
            &mut self.machine_st.heap,
            sig.as_ref()
                .iter()
                .map(|b| fixnum_as_cell!(Fixnum::build_with(*b as i64))),
        ));

        unify!(self.machine_st, self.machine_st.registers[4], sig_list);
    }

    #[cfg(feature = "crypto-full")]
    #[inline(always)]
    pub(crate) fn ed25519_verify(&mut self) {
        let key = self.string_encoding_bytes(self.machine_st.registers[1], atom!("octet"));
        let encoding = cell_as_atom!(self.deref_register(3));
        let data = self.string_encoding_bytes(self.machine_st.registers[2], encoding);
        let stub_gen = || functor_stub(atom!("ed25519_verify"), 5);
        let signature = self
            .machine_st
            .integers_to_bytevec(self.machine_st.registers[4], stub_gen);

        let peer_public_key = signature::UnparsedPublicKey::new(&signature::ED25519, &key);

        match peer_public_key.verify(&data, &signature) {
            Ok(_) => {}
            _ => {
                self.machine_st.fail = true;
            }
        }
    }

    #[inline(always)]
    pub(crate) fn curve25519_scalar_mult(&mut self) {
        let stub1_gen = || functor_stub(atom!("curve25519_scalar_mult"), 3);
        let scalar_bytes = self
            .machine_st
            .integers_to_bytevec(self.machine_st.registers[1], stub1_gen);
        let stub2_gen = || functor_stub(atom!("curve25519_scalar_mult"), 3);
        let point_bytes = self
            .machine_st
            .integers_to_bytevec(self.machine_st.registers[2], stub2_gen);

        let result = x25519::x25519(
            &<[u8; 32]>::try_from(&point_bytes[..]).unwrap(),
            &<[u8; 32]>::try_from(&scalar_bytes[..]).unwrap(),
        );

        let string = self.u8s_to_string(&result[..]);

        unify!(self.machine_st, self.machine_st.registers[3], string);
    }

    #[inline(always)]
    pub(crate) fn first_non_octet(&mut self) {
        let addr = self.deref_register(1);

        if let Some(string) = self.machine_st.value_to_str_like(addr) {
            for c in string.as_str().chars() {
                if c as u32 > 255 {
                    let non_octet =
                        AtomTable::build_with(&self.machine_st.atom_tbl, &c.to_string());
                    self.machine_st
                        .unify_atom(non_octet, self.machine_st.registers[2]);
                    return;
                }
            }
        }

        self.machine_st.fail = true;
    }

    #[inline(always)]
    pub(crate) fn load_html(&mut self) {
        if let Some(string) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            let doc = select::document::Document::from_read(string.as_str().as_bytes()).unwrap();
            let result = self.html_node_to_term(doc.nth(0).unwrap());

            unify!(self.machine_st, self.machine_st.registers[2], result);
        } else {
            self.machine_st.fail = true;
        }
    }

    #[inline(always)]
    pub(crate) fn load_xml(&mut self) {
        if let Some(string) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            match roxmltree::Document::parse(&*string.as_str()) {
                Ok(doc) => {
                    let result = self.xml_node_to_term(doc.root_element());
                    unify!(self.machine_st, self.machine_st.registers[2], result);
                }
                _ => {
                    self.machine_st.fail = true;
                }
            }
        } else {
            self.machine_st.fail = true;
        }
    }

    #[inline(always)]
    pub(crate) fn get_env(&mut self) {
        if let Some(key) = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
        {
            match env::var(&*key.as_str()) {
                Ok(value) => {
                    let cstr = put_complete_string(
                        &mut self.machine_st.heap,
                        &value,
                        &self.machine_st.atom_tbl,
                    );

                    unify!(self.machine_st, self.machine_st.registers[2], cstr);
                }
                _ => {
                    self.machine_st.fail = true;
                }
            }
        } else {
            self.machine_st.fail = true;
        }
    }

    #[inline(always)]
    pub(crate) fn set_env(&mut self) {
        let key = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
            .unwrap();
        let value = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[2])
            .unwrap();

        env::set_var(&*key.as_str(), &*value.as_str());
    }

    #[inline(always)]
    pub(crate) fn unset_env(&mut self) {
        let key = self
            .machine_st
            .value_to_str_like(self.machine_st.registers[1])
            .unwrap();
        env::remove_var(&*key.as_str());
    }

    #[inline(always)]
    pub(crate) fn pid(&mut self) {
        let pid = process::id();

        match fixnum!(Number, pid as i64, &mut self.machine_st.arena) {
            Number::Fixnum(pid) => {
                self.machine_st
                    .unify_fixnum(pid, self.machine_st.registers[1]);
            }
            Number::Integer(pid) => {
                self.machine_st
                    .unify_big_int(pid, self.machine_st.registers[1]);
            }
            _ => {
                unreachable!();
            }
        }
    }

    #[inline(always)]
    pub(crate) fn shell(&mut self) {
        // shell executes a command in a system shell
        // the code looks for a SHELL env var to do it in a UNIX-style
        // if not found, the code looks for COMSPEC env var to do it in a DOS-style
        // the output is printed directly to stdout
        // the output status code is returned after finishing
        fn command_result(
            machine: &mut MachineState,
            command: std::io::Result<process::ExitStatus>,
        ) {
            match command {
                Ok(status) => match status.code() {
                    Some(code) => {
                        let code = integer_as_cell!(Number::arena_from(code, &mut machine.arena));
                        unify!(machine, code, machine.registers[2]);
                    }
                    _ => {
                        machine.fail = true;
                    }
                },
                _ => {
                    machine.fail = true;
                }
            }
        }

        let a1 = self.deref_register(1);
        let command = self.machine_st.value_to_str_like(a1).unwrap();

        match env::var("SHELL") {
            Ok(value) => {
                let command = process::Command::new(&value)
                    .arg("-c")
                    .arg(&*command.as_str())
                    .status();
                command_result(&mut self.machine_st, command);
            }
            _ => match env::var("COMSPEC") {
                Ok(value) => {
                    let command = process::Command::new(&value)
                        .arg("/C")
                        .arg(&*command.as_str())
                        .status();
                    command_result(&mut self.machine_st, command);
                }
                _ => {
                    self.machine_st.fail = true;
                }
            },
        };
    }

    #[inline(always)]
    pub(crate) fn chars_base64(&mut self) -> CallResult {
        let padding = cell_as_atom!(self.deref_register(3));
        let charset = cell_as_atom!(self.deref_register(4));

        let config = if padding == atom!("true") {
            if charset == atom!("standard") {
                base64::STANDARD
            } else {
                base64::URL_SAFE
            }
        } else {
            if charset == atom!("standard") {
                base64::STANDARD_NO_PAD
            } else {
                base64::URL_SAFE_NO_PAD
            }
        };

        if self.deref_register(1).is_var() {
            let b64 = self
                .machine_st
                .value_to_str_like(self.machine_st.registers[2])
                .unwrap();
            let bytes = base64::decode_config(&*b64.as_str(), config);

            match bytes {
                Ok(bs) => {
                    let string = self.u8s_to_string(&bs);

                    unify!(self.machine_st, self.machine_st.registers[1], string);
                }
                _ => {
                    self.machine_st.fail = true;
                    return Ok(());
                }
            }
        } else {
            let mut bytes = vec![];
            for c in self
                .machine_st
                .value_to_str_like(self.machine_st.registers[1])
                .unwrap()
                .as_str()
                .chars()
            {
                bytes.push(c as u8);
            }

            let b64 = base64::encode_config(bytes, config);
            let string = self.u8s_to_string(&b64.as_bytes());

            unify!(self.machine_st, self.machine_st.registers[2], string);
        }

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn load_library_as_stream(&mut self) -> CallResult {
        let library_name = cell_as_atom!(self.deref_register(1));

        use crate::machine::LIBRARIES;

        let lib_ref = LIBRARIES.borrow();
        let lib = lib_ref.get(&*library_name.as_str());
        match lib {
            Some(library) => {
                let lib_stream = Stream::from_static_string(library, &mut self.machine_st.arena);
                unify!(
                    self.machine_st,
                    stream_as_cell!(lib_stream),
                    self.machine_st.registers[2]
                );

                let mut path_buf = machine::current_dir();

                path_buf.push("/lib");
                path_buf.push(&*library_name.as_str());

                let library_path_str = path_buf.to_str().unwrap();
                let library_path =
                    AtomTable::build_with(&self.machine_st.atom_tbl, library_path_str);

                self.machine_st
                    .unify_atom(library_path, self.machine_st.registers[3]);
            }
            None => {
                let stub = functor_stub(atom!("load"), 1);
                let err = self
                    .machine_st
                    .existence_error(ExistenceError::ModuleSource(ModuleSource::Library(
                        library_name,
                    )));

                return Err(self.machine_st.error_form(err, stub));
            }
        }

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn devour_whitespace(&mut self) -> CallResult {
        let mut stream = self.machine_st.get_stream_or_alias(
            self.machine_st.registers[1],
            &self.indices.stream_aliases,
            atom!("$devour_whitespace"),
            1,
        )?;

        let mut parser = Parser::new(stream, &mut self.machine_st);

        match devour_whitespace(&mut parser) {
            Ok(false) => {
                // not at EOF.
                stream.add_lines_read(parser.lines_read());
            }
            Ok(true) => {
                stream.add_lines_read(parser.lines_read());
                self.machine_st.fail = true;
            }
            Err(err) => {
                let stub = functor_stub(atom!("load"), 1);
                let err = self.machine_st.syntax_error(err);

                return Err(self.machine_st.error_form(err, stub));
            }
        }

        Ok(())
    }

    #[inline(always)]
    pub(crate) fn is_sto_enabled(&mut self) {
        if self.machine_st.unify_fn as usize == MachineState::unify_with_occurs_check as usize {
            self.machine_st
                .unify_atom(atom!("true"), self.machine_st.registers[1]);
        } else if self.machine_st.unify_fn as usize
            == MachineState::unify_with_occurs_check_with_error as usize
        {
            self.machine_st
                .unify_atom(atom!("error"), self.machine_st.registers[1]);
        } else {
            self.machine_st
                .unify_atom(atom!("false"), self.machine_st.registers[1]);
        }
    }

    #[inline(always)]
    pub(crate) fn set_sto_as_unify(&mut self) {
        self.machine_st.unify_fn = MachineState::unify_with_occurs_check;
        self.machine_st.bind_fn = MachineState::bind_with_occurs_check_wrapper;
    }

    #[inline(always)]
    pub(crate) fn set_nsto_as_unify(&mut self) {
        self.machine_st.unify_fn = MachineState::unify;
        self.machine_st.bind_fn = MachineState::bind;
    }

    #[inline(always)]
    pub(crate) fn set_sto_with_error_as_unify(&mut self) {
        self.machine_st.unify_fn = MachineState::unify_with_occurs_check_with_error;
        self.machine_st.bind_fn = MachineState::bind_with_occurs_check_with_error_wrapper;
    }

    #[inline(always)]
    pub(crate) fn home_directory(&mut self) {
        let path = match dirs_next::home_dir() {
            Some(path) => path,
            None => {
                self.machine_st.fail = true;
                return;
            }
        };

        if path.is_dir() {
            if let Some(path) = path.to_str() {
                let path_string =
                    put_complete_string(&mut self.machine_st.heap, path, &self.machine_st.atom_tbl);

                unify!(self.machine_st, self.machine_st.registers[1], path_string);
                return;
            }
        }

        self.machine_st.fail = true;
    }

    pub(crate) fn debug_hook(&mut self) {}

    #[inline(always)]
    pub(crate) fn pop_count(&mut self) {
        let number = self.deref_register(1);
        let pop_count = integer_as_cell!(match Number::try_from(number) {
            Ok(Number::Fixnum(n)) => {
                Number::Fixnum(Fixnum::build_with(n.get_num().count_ones() as i64))
            }
            Ok(Number::Integer(n)) => {
                let value: usize = if n.sign() == Sign::Positive {
                    UBig::from((&*n).clone().into_parts().1).count_ones()
                } else { 0 };
                Number::arena_from(value, &mut self.machine_st.arena)
            }
            _ => {
                unreachable!()
            }
        });

        unify!(self.machine_st, self.machine_st.registers[2], pop_count);
    }

    pub(super) fn systemtime_to_timestamp(&mut self, system_time: SystemTime) -> Atom {
        let datetime: DateTime<Local> = system_time.into();

        let mut fstr = "[".to_string();
        const SPECIFIERS: [char; 19] = [
            'Y', 'm', 'd', 'H', 'M', 'S', 'y', 'b', 'B', 'a', 'A', 'w', 'u', 'U', 'W', 'j', 'D',
            'x', 'v',
        ];

        for spec in SPECIFIERS {
            fstr.push_str(&format!("'{}'=\"%{}\", ", spec, spec).to_string());
        }

        fstr.push_str("finis].");
        let s = datetime.format(&fstr).to_string();

        AtomTable::build_with(&self.machine_st.atom_tbl, &s)
    }

    pub(super) fn string_encoding_bytes(
        &mut self,
        data_arg: HeapCellValue,
        encoding: Atom,
    ) -> Vec<u8> {
        let data = self.machine_st.value_to_str_like(data_arg).unwrap();

        match encoding {
            atom!("utf8") => data.as_str().bytes().collect(),
            atom!("octet") => data.as_str().chars().map(|c| c as u8).collect(),
            _ => {
                unreachable!()
            }
        }
    }

    pub(super) fn xml_node_to_term(&mut self, node: roxmltree::Node) -> HeapCellValue {
        if node.is_text() {
            put_complete_string(
                &mut self.machine_st.heap,
                node.text().unwrap(),
                &self.machine_st.atom_tbl,
            )
        } else {
            let mut avec = Vec::new();

            for attr in node.attributes() {
                let name = AtomTable::build_with(&self.machine_st.atom_tbl, attr.name());
                let value = put_complete_string(
                    &mut self.machine_st.heap,
                    &attr.value(),
                    &self.machine_st.atom_tbl,
                );

                avec.push(str_loc_as_cell!(self.machine_st.heap.len()));

                self.machine_st.heap.push(atom_as_cell!(atom!("="), 2));
                self.machine_st.heap.push(atom_as_cell!(name));
                self.machine_st.heap.push(value);
            }

            let attrs = heap_loc_as_cell!(iter_to_heap_list(
                &mut self.machine_st.heap,
                avec.into_iter()
            ));

            let mut cvec = Vec::new();

            for child in node.children() {
                cvec.push(self.xml_node_to_term(child));
            }

            let children = heap_loc_as_cell!(iter_to_heap_list(
                &mut self.machine_st.heap,
                cvec.into_iter()
            ));

            let tag = AtomTable::build_with(&self.machine_st.atom_tbl, node.tag_name().name());

            let result = str_loc_as_cell!(self.machine_st.heap.len());

            self.machine_st
                .heap
                .push(atom_as_cell!(atom!("element"), 3));
            self.machine_st.heap.push(atom_as_cell!(tag));
            self.machine_st.heap.push(attrs);
            self.machine_st.heap.push(children);

            result
        }
    }

    pub(super) fn html_node_to_term(&mut self, node: select::node::Node) -> HeapCellValue {
        match node.name() {
            None => put_complete_string(
                &mut self.machine_st.heap,
                &node.text(),
                &self.machine_st.atom_tbl,
            ),
            Some(name) => {
                let mut avec = Vec::new();

                for attr in node.attrs() {
                    let name = AtomTable::build_with(&self.machine_st.atom_tbl, attr.0);
                    let value = put_complete_string(
                        &mut self.machine_st.heap,
                        &attr.1,
                        &self.machine_st.atom_tbl,
                    );

                    avec.push(str_loc_as_cell!(self.machine_st.heap.len()));

                    self.machine_st.heap.push(atom_as_cell!(atom!("="), 2));
                    self.machine_st.heap.push(atom_as_cell!(name));
                    self.machine_st.heap.push(value);
                }

                let attrs = heap_loc_as_cell!(iter_to_heap_list(
                    &mut self.machine_st.heap,
                    avec.into_iter()
                ));

                let mut cvec = Vec::new();

                for child in node.children() {
                    cvec.push(self.html_node_to_term(child));
                }

                let children = heap_loc_as_cell!(iter_to_heap_list(
                    &mut self.machine_st.heap,
                    cvec.into_iter()
                ));

                let tag = AtomTable::build_with(&self.machine_st.atom_tbl, name);
                let result = str_loc_as_cell!(self.machine_st.heap.len());

                self.machine_st
                    .heap
                    .push(atom_as_cell!(atom!("element"), 3));
                self.machine_st.heap.push(atom_as_cell!(tag));
                self.machine_st.heap.push(attrs);
                self.machine_st.heap.push(children);

                result
            }
        }
    }

    pub(super) fn u8s_to_string(&mut self, data: &[u8]) -> HeapCellValue {
        let buffer = String::from_iter(data.iter().map(|b| *b as char));

        if buffer.len() == 0 {
            empty_list_as_cell!()
        } else {
            atom_as_cstr_cell!(AtomTable::build_with(&self.machine_st.atom_tbl, &buffer))
        }
    }
}

fn rng() -> &'static dyn SecureRandom {
    use std::ops::Deref;

    lazy_static! {
        static ref RANDOM: SystemRandom = SystemRandom::new();
    }

    RANDOM.deref()
}

struct MyKey<T: core::fmt::Debug + PartialEq>(T);

impl hkdf::KeyType for MyKey<usize> {
    fn len(&self) -> usize {
        self.0
    }
}