scryer-prolog/src/prolog/machine/copier.rs

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

use std::ops::IndexMut;

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

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

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

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);
}

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: vec![],
            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 copied_list(&mut self, addr: usize) -> bool {
        match &self.target[addr] {
            HeapCellValue::Addr(Addr::Lis(addr)) | HeapCellValue::Addr(Addr::HeapCell(addr)) => {
                if *addr >= self.old_h {
                    *self.value_at_scan() = HeapCellValue::Addr(Addr::Lis(*addr));
                    self.scan += 1;
                    return true;
                }
            }
            _ => {}
        };

        false
    }

    fn copied_partial_string(&mut self, addr: usize) -> bool {
        if let HeapCellValue::PartialString(ref pstr) = &self.target[addr] {
            if let Addr::PStrLocation(h, n) = pstr.tail_addr() {
                if *h >= self.old_h {
                    *self.value_at_scan() = HeapCellValue::Addr(Addr::PStrLocation(*h, *n));
                    self.scan += 1;
                    return true;
                }
            }
        }

        false
    }

    fn copy_list(&mut self, addr: usize) {
        if self.copied_list(addr) {
            return;
        }

        let threshold = self.target.threshold();

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

        let ra = self.target[addr].as_addr(threshold);
        let rd = self.target.store(self.target.deref(ra.clone()));

        self.target.push(HeapCellValue::Addr(ra.clone()));

        let hcv = HeapCellValue::Addr(self.target[addr + 1].as_addr(addr + 1));

        self.target.push(hcv);

        match rd.clone() {
            Addr::AttrVar(h) | Addr::HeapCell(h) | Addr::PStrTail(h, _)
                if h >= self.old_h => {
                    self.target[threshold] = HeapCellValue::Addr(rd)
                }
            var @ Addr::AttrVar(_)
          | var @ Addr::HeapCell(..)
          | var @ Addr::StackCell(..)
          | var @ Addr::PStrTail(..) => {
                if ra == rd {
                    self.reinstantiate_var(var, threshold);

                    if let AttrVarPolicy::StripAttributes = self.attr_var_policy {
                        self.trail.push((Ref::HeapCell(addr), HeapCellValue::Addr(ra)));
                        self.target[addr] = HeapCellValue::Addr(Addr::HeapCell(threshold));
                    }
                } else {
                    self.target[threshold] = HeapCellValue::Addr(ra);
                }
            }
            _ => {
                self.trail.push((
                    Ref::HeapCell(addr),
                    HeapCellValue::Addr(self.target[addr].as_addr(addr)),
                ));

                self.target[addr] = HeapCellValue::Addr(Addr::Lis(threshold))
            }
        };

        self.scan += 1;
    }


    fn copy_partial_string(&mut self, addr: usize, n: usize) {
        let threshold = self.target.threshold();

        let tail_addr =
            match &self.target[addr] {
                HeapCellValue::PartialString(ref pstr) => {
                    self.trail.push((
                        Ref::PStrTail(addr, 0),
                        HeapCellValue::Addr(pstr.tail.clone()),
                    ));

                    self.target.store(self.target.deref(pstr.tail.clone()))
                }
                _ => {
                    unreachable!()
                }
            };

        let pstr =
            match &mut self.target[addr] {
                HeapCellValue::PartialString(ref mut pstr) => {
                    let mut new_pstr = pstr.clone_from_offset(n);

                    if let Addr::PStrTail(h, n) = &tail_addr {
                        new_pstr.tail = if *h == addr {
                            Addr::PStrTail(threshold, *n)
                        } else {
                            Addr::HeapCell(threshold + 1)
                        };
                    } else {
                        new_pstr.tail = Addr::HeapCell(threshold + 1);
                    }

                    pstr.tail = Addr::PStrLocation(threshold, 0);
                    new_pstr
                }
                _ => {
                    unreachable!()
                }
            };

        match tail_addr {
            Addr::PStrTail(h, _) if h == addr => {
                self.target.push(HeapCellValue::PartialString(pstr));
            }
            addr => {
                self.target.push(HeapCellValue::PartialString(pstr));
                self.target.push(HeapCellValue::Addr(addr));
            }
        }
    }

    fn copy_partial_string_from(&mut self, addr: usize, n: usize) {
        if self.copied_partial_string(addr) {
            return;
        }

        let threshold = self.target.threshold();

        self.target[self.scan] =
            HeapCellValue::Addr(Addr::PStrLocation(threshold, 0));

        self.scan += 1;

        self.copy_partial_string(addr, n);
    }

    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::PStrTail(h, n) => {
                match &mut self.target[h] {
                    HeapCellValue::PartialString(ref mut pstr) => {
                        pstr.tail = Addr::PStrTail(frontier, n);
                    }
                    _ => {
                        unreachable!()
                    }
                }

                self.target[frontier] = HeapCellValue::Addr(Addr::PStrTail(frontier, n));
                self.trail.push((
                    Ref::PStrTail(h, n),
                    HeapCellValue::Addr(Addr::PStrTail(h, n))
                ));
            }
            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].clone();
                    self.target.push(list_val);
                }
            }
            _ => unreachable!()
        }
    }

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

        match rd.clone() {
            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].clone() {
            HeapCellValue::NamedStr(arity, name, fixity) => {
                let threshold = self.target.threshold();

                *self.value_at_scan() = HeapCellValue::Addr(Addr::Str(threshold));
                self.target[addr] = HeapCellValue::Addr(Addr::Str(threshold));

                self.trail.push((
                    Ref::HeapCell(addr),
                    HeapCellValue::NamedStr(arity, name.clone(), fixity.clone()),
                ));

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

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

        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() {
                HeapCellValue::NamedStr(..) => {
                    self.scan += 1;
                }
                HeapCellValue::Addr(ref addr) => {
                    match addr.clone() {
                        Addr::Lis(addr) => {
                            self.copy_list(addr);
                        }
                        addr @ Addr::AttrVar(_)
                      | addr @ Addr::HeapCell(_)
                      | addr @ Addr::StackCell(..)
                      | addr @ Addr::PStrTail(..) => {
                            self.copy_var(addr);
                        }
                        Addr::Str(addr) => {
                            self.copy_structure(addr);
                        }
                        Addr::PStrLocation(addr, n) => {
                            self.copy_partial_string_from(addr, n);
                        }
                        Addr::Con(_) | Addr::DBRef(_) | Addr::Stream(_) => {
                            self.scan += 1;
                        }
                    }
                }
                HeapCellValue::PartialString(_) => {
                    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::PStrTail(h, _) =>
                    if let HeapCellValue::PartialString(ref mut pstr) = &mut self.target[h] {
                        pstr.tail = value.as_addr(0);
                    },
                Ref::StackCell(fr, sc) =>
                    self.target.stack().index_and_frame_mut(fr)[sc] = value.as_addr(0),
            }
        }
    }
}