scryer-prolog/src/machine/copier.rs

use crate::machine::machine_indices::*;
use crate::machine::stack::*;

use std::mem;
use std::ops::IndexMut;

type Trail = Vec<(Ref, HeapCellValue)>;

#[derive(Debug, Clone, Copy)]
pub(crate) enum AttrVarPolicy {
    DeepCopy,
    StripAttributes,
}

pub(crate) trait CopierTarget: IndexMut<usize, Output = HeapCellValue> {
    fn deref(&self, val: Addr) -> Addr;
    fn push(&mut self, val: HeapCellValue);
    fn stack(&mut self) -> &mut Stack;
    fn store(&self, val: Addr) -> Addr;
    fn threshold(&self) -> usize;
}

pub(crate) fn copy_term<T: CopierTarget>(target: T, addr: Addr, attr_var_policy: AttrVarPolicy) {
    let mut copy_term_state = CopyTermState::new(target, attr_var_policy);
    copy_term_state.copy_term_impl(addr);
}

#[derive(Debug)]
struct CopyTermState<T: CopierTarget> {
    trail: Trail,
    scan: usize,
    old_h: usize,
    target: T,
    attr_var_policy: AttrVarPolicy,
}

impl<T: CopierTarget> CopyTermState<T> {
    fn new(target: T, attr_var_policy: AttrVarPolicy) -> Self {
        CopyTermState {
            trail: Trail::new(),
            scan: 0,
            old_h: target.threshold(),
            target,
            attr_var_policy,
        }
    }

    #[inline]
    fn value_at_scan(&mut self) -> &mut HeapCellValue {
        let scan = self.scan;
        &mut self.target[scan]
    }

    fn trail_list_cell(&mut self, addr: usize, threshold: usize) {
        let trail_item = mem::replace(
            &mut self.target[addr],
            HeapCellValue::Addr(Addr::Lis(threshold)),
        );

        self.trail.push((Ref::HeapCell(addr), trail_item));
    }

    fn copy_list(&mut self, addr: usize) {
        for offset in 0..2 {
            if let Addr::Lis(h) = self.target[addr + offset].as_addr(addr + offset) {
                if h >= self.old_h {
                    *self.value_at_scan() = HeapCellValue::Addr(Addr::Lis(h));
                    self.scan += 1;

                    return;
                }
            }
        }

        let threshold = self.target.threshold();

        *self.value_at_scan() = HeapCellValue::Addr(Addr::Lis(threshold));

        for i in 0..2 {
            let hcv = self.target[addr + i].context_free_clone();
            self.target.push(hcv);
        }

        let cdr = self
            .target
            .store(self.target.deref(Addr::HeapCell(addr + 1)));

        if !cdr.is_ref() {
            self.trail_list_cell(addr + 1, threshold);
        } else {
            let car = self.target.store(self.target.deref(Addr::HeapCell(addr)));

            if !car.is_ref() {
                self.trail_list_cell(addr, threshold);
            }
        }

        self.scan += 1;
    }

    fn copy_partial_string(&mut self, addr: usize, n: usize) {
        if let &HeapCellValue::Addr(Addr::PStrLocation(h, _)) = &self.target[addr] {
            if h >= self.old_h {
                *self.value_at_scan() = HeapCellValue::Addr(Addr::PStrLocation(h, n));
                self.scan += 1;

                return;
            }
        }

        let threshold = self.target.threshold();

        *self.value_at_scan() = HeapCellValue::Addr(Addr::PStrLocation(threshold, n));

        self.scan += 1;

        let (pstr, has_tail) = match &self.target[addr] {
            &HeapCellValue::PartialString(ref pstr, has_tail) => {
                (pstr.clone_from_offset(0), has_tail)
            }
            _ => {
                unreachable!()
            }
        };

        self.target
            .push(HeapCellValue::PartialString(pstr, has_tail));

        let replacement = HeapCellValue::Addr(Addr::PStrLocation(threshold, n));

        let trail_item = mem::replace(&mut self.target[addr], replacement);

        self.trail.push((Ref::HeapCell(addr), trail_item));

        if has_tail {
            let tail_addr = self.target[addr + 1].as_addr(addr + 1);
            self.target.push(HeapCellValue::Addr(tail_addr));
        }
    }

    fn reinstantiate_var(&mut self, addr: Addr, frontier: usize) {
        match addr {
            Addr::HeapCell(h) => {
                self.target[frontier] = HeapCellValue::Addr(Addr::HeapCell(frontier));
                self.target[h] = HeapCellValue::Addr(Addr::HeapCell(frontier));

                self.trail
                    .push((Ref::HeapCell(h), HeapCellValue::Addr(Addr::HeapCell(h))));
            }
            Addr::StackCell(fr, sc) => {
                self.target[frontier] = HeapCellValue::Addr(Addr::HeapCell(frontier));
                self.target.stack().index_and_frame_mut(fr)[sc] = Addr::HeapCell(frontier);

                self.trail.push((
                    Ref::StackCell(fr, sc),
                    HeapCellValue::Addr(Addr::StackCell(fr, sc)),
                ));
            }
            Addr::AttrVar(h) => {
                let threshold = if let AttrVarPolicy::DeepCopy = self.attr_var_policy {
                    self.target.threshold()
                } else {
                    frontier
                };

                self.target[frontier] = HeapCellValue::Addr(Addr::HeapCell(threshold));
                self.target[h] = HeapCellValue::Addr(Addr::HeapCell(threshold));

                self.trail
                    .push((Ref::AttrVar(h), HeapCellValue::Addr(Addr::AttrVar(h))));

                if let AttrVarPolicy::DeepCopy = self.attr_var_policy {
                    self.target
                        .push(HeapCellValue::Addr(Addr::AttrVar(threshold)));

                    let list_val = self.target[h + 1].context_free_clone();
                    self.target.push(list_val);
                }
            }
            _ => {
                unreachable!()
            }
        }
    }

    fn copy_var(&mut self, addr: Addr) {
        let rd = self.target.store(self.target.deref(addr));

        match rd {
            Addr::AttrVar(h) | Addr::HeapCell(h) if h >= self.old_h => {
                *self.value_at_scan() = HeapCellValue::Addr(rd);
                self.scan += 1;
            }
            _ if addr == rd => {
                self.reinstantiate_var(addr, self.scan);
                self.scan += 1;
            }
            _ => {
                *self.value_at_scan() = HeapCellValue::Addr(rd);
            }
        }
    }

    fn copy_structure(&mut self, addr: usize) {
        match self.target[addr].context_free_clone() {
            HeapCellValue::NamedStr(arity, name, fixity) => {
                let threshold = self.target.threshold();

                *self.value_at_scan() = HeapCellValue::Addr(Addr::Str(threshold));

                let trail_item = mem::replace(
                    &mut self.target[addr],
                    HeapCellValue::Addr(Addr::Str(threshold)),
                );

                self.trail.push((Ref::HeapCell(addr), trail_item));

                self.target
                    .push(HeapCellValue::NamedStr(arity, name, fixity));

                for i in 0..arity {
                    let hcv = self.target[addr + 1 + i].context_free_clone();
                    self.target.push(hcv);
                }
            }
            HeapCellValue::Addr(Addr::Str(addr)) => {
                *self.value_at_scan() = HeapCellValue::Addr(Addr::Str(addr))
            }
            _ => {
                unreachable!()
            }
        }

        self.scan += 1;
    }

    fn copy_term_impl(&mut self, addr: Addr) {
        self.scan = self.target.threshold();
        self.target.push(HeapCellValue::Addr(addr));

        while self.scan < self.target.threshold() {
            match self.value_at_scan() {
                &mut HeapCellValue::Addr(addr) => match addr {
                    Addr::Con(h) => {
                        let addr = self.target[h].as_addr(h);

                        if addr == Addr::Con(h) {
                            *self.value_at_scan() = self.target[h].context_free_clone();
                        } else {
                            *self.value_at_scan() = HeapCellValue::Addr(addr);
                        }
                    }
                    Addr::Lis(h) => {
                        if h >= self.old_h {
                            self.scan += 1;
                        } else {
                            self.copy_list(h);
                        }
                    }
                    addr @ Addr::AttrVar(_)
                    | addr @ Addr::HeapCell(_)
                    | addr @ Addr::StackCell(..) => {
                        self.copy_var(addr);
                    }
                    Addr::Str(addr) => {
                        self.copy_structure(addr);
                    }
                    Addr::PStrLocation(addr, n) => {
                        self.copy_partial_string(addr, n);
                    }
                    Addr::Stream(h) => {
                        *self.value_at_scan() = self.target[h].context_free_clone();
                    }
                    _ => {
                        self.scan += 1;
                    }
                },
                _ => {
                    self.scan += 1;
                }
            }
        }

        self.unwind_trail();
    }

    fn unwind_trail(&mut self) {
        for (r, value) in self.trail.drain(0..) {
            match r {
                Ref::AttrVar(h) | Ref::HeapCell(h) => self.target[h] = value,
                Ref::StackCell(fr, sc) => {
                    self.target.stack().index_and_frame_mut(fr)[sc] = value.as_addr(0)
                }
            }
        }
    }
}