fancy-regex 0.11.0

// Copyright 2016 The Fancy Regex Authors.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

//! Backtracking VM for implementing fancy regexes.
//!
//! Read <https://swtch.com/~rsc/regexp/regexp2.html> for a good introduction for how this works.
//!
//! The VM executes a sequence of instructions (a program) against an input string. It keeps track
//! of a program counter (PC) and an index into the string (IX). Execution can have one or more
//! threads.
//!
//! One of the basic instructions is `Lit`, which matches a string against the input. If it matches,
//! the PC advances to the next instruction and the IX to the position after the matched string.
//! If not, the current thread is stopped because it failed.
//!
//! If execution reaches an `End` instruction, the program is successful because a match was found.
//! If there are no more threads to execute, the program has failed to match.
//!
//! A very simple program for the regex `a`:
//!
//! ```text
//! 0: Lit("a")
//! 1: End
//! ```
//!
//! The `Split` instruction causes execution to split into two threads. The first thread is executed
//! with the current string index. If it fails, we reset the string index and resume execution with
//! the second thread. That is what "backtracking" refers to. In order to do that, we keep a stack
//! of threads (PC and IX) to try.
//!
//! Example program for the regex `ab|ac`:
//!
//! ```text
//! 0: Split(1, 4)
//! 1: Lit("a")
//! 2: Lit("b")
//! 3: Jmp(6)
//! 4: Lit("a")
//! 5: Lit("c")
//! 6: End
//! ```
//!
//! The `Jmp` instruction causes execution to jump to the specified instruction. In the example it
//! is needed to separate the two threads.
//!
//! Let's step through execution with that program for the input `ac`:
//!
//! 1. We're at PC 0 and IX 0
//! 2. `Split(1, 4)` means we save a thread with PC 4 and IX 0 for trying later
//! 3. Continue at `Lit("a")` which matches, so we advance IX to 1
//! 4. `Lit("b")` doesn't match at IX 1 (`"b" != "c"`), so the thread fails
//! 5. We continue with the previously saved thread at PC 4 and IX 0 (backtracking)
//! 6. Both `Lit("a")` and `Lit("c")` match and we reach `End` -> successful match (index 0 to 2)

use regex::Regex;
use std::collections::BTreeSet;
use std::usize;

use crate::error::RuntimeError;
use crate::prev_codepoint_ix;
use crate::Error;
use crate::Result;
use crate::{codepoint_len, RegexOptions};

/// Enable tracing of VM execution. Only for debugging/investigating.
const OPTION_TRACE: u32 = 1 << 0;
/// When iterating over all matches within a text (e.g. with `find_iter`), empty matches need to be
/// handled specially. If we kept matching at the same position, we'd never stop. So what we do
/// after we've had an empty match, is to advance the position where matching is attempted.
/// If `\G` is used in the pattern, that means it no longer matches. If we didn't tell the VM about
/// the fact that we skipped because of an empty match, it would still treat `\G` as matching. So
/// this option is for communicating that to the VM. Phew.
pub(crate) const OPTION_SKIPPED_EMPTY_MATCH: u32 = 1 << 1;

// TODO: make configurable
const MAX_STACK: usize = 1_000_000;

/// Instruction of the VM.
#[derive(Debug, Clone)]
pub enum Insn {
    /// Successful end of program
    End,
    /// Match any character (including newline)
    Any,
    /// Match any character (not including newline)
    AnyNoNL,
    /// Match the literal string at the current index
    Lit(String), // should be cow?
    /// Split execution into two threads. The two fields are positions of instructions. Execution
    /// first tries the first thread. If that fails, the second position is tried.
    Split(usize, usize),
    /// Jump to instruction at position
    Jmp(usize),
    /// Save the current string index into the specified slot
    Save(usize),
    /// Save `0` into the specified slot
    Save0(usize),
    /// Set the string index to the value that was saved in the specified slot
    Restore(usize),
    /// Repeat greedily (match as much as possible)
    RepeatGr {
        /// Minimum number of matches
        lo: usize,
        /// Maximum number of matches
        hi: usize,
        /// The instruction after the repeat
        next: usize,
        /// The slot for keeping track of the number of repetitions
        repeat: usize,
    },
    /// Repeat non-greedily (prefer matching as little as possible)
    RepeatNg {
        /// Minimum number of matches
        lo: usize,
        /// Maximum number of matches
        hi: usize,
        /// The instruction after the repeat
        next: usize,
        /// The slot for keeping track of the number of repetitions
        repeat: usize,
    },
    /// Repeat greedily and prevent infinite loops from empty matches
    RepeatEpsilonGr {
        /// Minimum number of matches
        lo: usize,
        /// The instruction after the repeat
        next: usize,
        /// The slot for keeping track of the number of repetitions
        repeat: usize,
        /// The slot for saving the previous IX to check if we had an empty match
        check: usize,
    },
    /// Repeat non-greedily and prevent infinite loops from empty matches
    RepeatEpsilonNg {
        /// Minimum number of matches
        lo: usize,
        /// The instruction after the repeat
        next: usize,
        /// The slot for keeping track of the number of repetitions
        repeat: usize,
        /// The slot for saving the previous IX to check if we had an empty match
        check: usize,
    },
    /// Negative look-around failed
    FailNegativeLookAround,
    /// Set IX back by the specified number of characters
    GoBack(usize),
    /// Back reference to a group number to check
    Backref(usize),
    /// Begin of atomic group
    BeginAtomic,
    /// End of atomic group
    EndAtomic,
    /// Delegate matching to the regex crate for a fixed size
    DelegateSized(Box<Regex>, usize),
    /// Delegate matching to the regex crate
    Delegate {
        /// The regex
        inner: Box<Regex>,
        /// The same regex but matching an additional character on the left.
        ///
        /// E.g. if `inner` is `^\b`, `inner1` is `^(?s:.)\b`. Why do we need this? Because `\b`
        /// needs to know the previous character to work correctly. Let's say we're currently at the
        /// second character of the string `xy`. Should `\b` match there? No. But if we'd run `^\b`
        /// against `y`, it would match (incorrect). To do the right thing, we run `^(?s:.)\b`
        /// against `xy`, which does not match.
        ///
        /// We only need this for regexes that "look left", i.e. need to know what the previous
        /// character was.
        inner1: Option<Box<Regex>>,
        /// The first group number that this regex captures (if it contains groups)
        start_group: usize,
        /// The last group number
        end_group: usize,
    },
    /// Anchor to match at the position where the previous match ended
    ContinueFromPreviousMatchEnd,
    /// Continue only if the specified capture group has already been populated as part of the match
    BackrefExistsCondition(usize),
}

/// Sequence of instructions for the VM to execute.
#[derive(Debug, Clone)]
pub struct Prog {
    /// Instructions of the program
    pub body: Vec<Insn>,
    n_saves: usize,
}

impl Prog {
    pub(crate) fn new(body: Vec<Insn>, n_saves: usize) -> Prog {
        Prog { body, n_saves }
    }

    #[doc(hidden)]
    pub(crate) fn debug_print(&self) {
        for (i, insn) in self.body.iter().enumerate() {
            println!("{:3}: {:?}", i, insn);
        }
    }
}

#[derive(Debug)]
struct Branch {
    pc: usize,
    ix: usize,
    nsave: usize,
}

#[derive(Debug)]
struct Save {
    slot: usize,
    value: usize,
}

struct State {
    /// Saved values indexed by slot. Mostly indices to s, but can be repeat values etc.
    /// Always contains the saves of the current state.
    saves: Vec<usize>,
    /// Stack of backtrack branches.
    stack: Vec<Branch>,
    /// Old saves (slot, value)
    oldsave: Vec<Save>,
    /// Number of saves at the end of `oldsave` that need to be restored to `saves` on pop
    nsave: usize,
    explicit_sp: usize,
    /// Maximum size of the stack. If the size would be exceeded during execution, a `StackOverflow`
    /// error is raised.
    max_stack: usize,
    options: u32,
}

// Each element in the stack conceptually represents the entire state
// of the machine: the pc (index into prog), the index into the
// string, and the entire vector of saves. However, copying the save
// vector on every push/pop would be inefficient, so instead we use a
// copy-on-write approach for each slot within the save vector. The
// top `nsave` elements in `oldsave` represent the delta from the
// current machine state to the top of stack.

impl State {
    fn new(n_saves: usize, max_stack: usize, options: u32) -> State {
        State {
            saves: vec![usize::MAX; n_saves],
            stack: Vec::new(),
            oldsave: Vec::new(),
            nsave: 0,
            explicit_sp: n_saves,
            max_stack,
            options,
        }
    }

    // push a backtrack branch
    fn push(&mut self, pc: usize, ix: usize) -> Result<()> {
        if self.stack.len() < self.max_stack {
            let nsave = self.nsave;
            self.stack.push(Branch { pc, ix, nsave });
            self.nsave = 0;
            self.trace_stack("push");
            Ok(())
        } else {
            Err(Error::RuntimeError(RuntimeError::StackOverflow))
        }
    }

    // pop a backtrack branch
    fn pop(&mut self) -> (usize, usize) {
        for _ in 0..self.nsave {
            let Save { slot, value } = self.oldsave.pop().unwrap();
            self.saves[slot] = value;
        }
        let Branch { pc, ix, nsave } = self.stack.pop().unwrap();
        self.nsave = nsave;
        self.trace_stack("pop");
        (pc, ix)
    }

    fn save(&mut self, slot: usize, val: usize) {
        for i in 0..self.nsave {
            // could avoid this iteration with some overhead; worth it?
            if self.oldsave[self.oldsave.len() - i - 1].slot == slot {
                // already saved, just update
                self.saves[slot] = val;
                return;
            }
        }
        self.oldsave.push(Save {
            slot,
            value: self.saves[slot],
        });
        self.nsave += 1;
        self.saves[slot] = val;

        if self.options & OPTION_TRACE != 0 {
            println!("saves: {:?}", self.saves);
        }
    }

    fn get(&self, slot: usize) -> usize {
        self.saves[slot]
    }

    // push a value onto the explicit stack; note: the entire contents of
    // the explicit stack is saved and restored on backtrack.
    fn stack_push(&mut self, val: usize) {
        if self.saves.len() == self.explicit_sp {
            self.saves.push(self.explicit_sp + 1);
        }
        let explicit_sp = self.explicit_sp;
        let sp = self.get(explicit_sp);
        if self.saves.len() == sp {
            self.saves.push(val);
        } else {
            self.save(sp, val);
        }
        self.save(explicit_sp, sp + 1);
    }

    // pop a value from the explicit stack
    fn stack_pop(&mut self) -> usize {
        let explicit_sp = self.explicit_sp;
        let sp = self.get(explicit_sp) - 1;
        let result = self.get(sp);
        self.save(explicit_sp, sp);
        result
    }

    /// Get the current number of backtrack branches
    fn backtrack_count(&self) -> usize {
        self.stack.len()
    }

    /// Discard backtrack branches that were pushed since the call to `backtrack_count`.
    ///
    /// What we want:
    /// * Keep the current `saves` as they are
    /// * Only keep `count` backtrack branches on `stack`, discard the rest
    /// * Keep the first `oldsave` for each slot, discard the rest (multiple pushes might have
    ///   happened with saves to the same slot)
    fn backtrack_cut(&mut self, count: usize) {
        if self.stack.len() == count {
            // no backtrack branches to discard, all good
            return;
        }
        // start and end indexes of old saves for the branch we're cutting to
        let (oldsave_start, oldsave_end) = {
            let mut end = self.oldsave.len() - self.nsave;
            for &Branch { nsave, .. } in &self.stack[count + 1..] {
                end -= nsave;
            }
            let start = end - self.stack[count].nsave;
            (start, end)
        };
        let mut saved = BTreeSet::new();
        // keep all the old saves of our branch (they're all for different slots)
        for &Save { slot, .. } in &self.oldsave[oldsave_start..oldsave_end] {
            saved.insert(slot);
        }
        let mut oldsave_ix = oldsave_end;
        // for other old saves, keep them only if they're for a slot that we haven't saved yet
        for ix in oldsave_end..self.oldsave.len() {
            let Save { slot, .. } = self.oldsave[ix];
            let new_slot = saved.insert(slot);
            if new_slot {
                // put the save we want to keep (ix) after the ones we already have (oldsave_ix)
                // note that it's fine if the indexes are the same (then swapping is a no-op)
                self.oldsave.swap(oldsave_ix, ix);
                oldsave_ix += 1;
            }
        }
        self.stack.truncate(count);
        self.oldsave.truncate(oldsave_ix);
        self.nsave = oldsave_ix - oldsave_start;
    }

    #[inline]
    fn trace_stack(&self, operation: &str) {
        if self.options & OPTION_TRACE != 0 {
            println!("stack after {}: {:?}", operation, self.stack);
        }
    }
}

fn codepoint_len_at(s: &str, ix: usize) -> usize {
    codepoint_len(s.as_bytes()[ix])
}

#[inline]
fn matches_literal(s: &str, ix: usize, end: usize, literal: &str) -> bool {
    // Compare as bytes because the literal might be a single byte char whereas ix
    // points to a multibyte char. Comparing with str would result in an error like
    // "byte index N is not a char boundary".
    end <= s.len() && &s.as_bytes()[ix..end] == literal.as_bytes()
}

/// Run the program with trace printing for debugging.
pub fn run_trace(prog: &Prog, s: &str, pos: usize) -> Result<Option<Vec<usize>>> {
    run(prog, s, pos, OPTION_TRACE, &RegexOptions::default())
}

/// Run the program with default options.
pub fn run_default(prog: &Prog, s: &str, pos: usize) -> Result<Option<Vec<usize>>> {
    run(prog, s, pos, 0, &RegexOptions::default())
}

/// Run the program with options.
#[allow(clippy::cognitive_complexity)]
pub(crate) fn run(
    prog: &Prog,
    s: &str,
    pos: usize,
    option_flags: u32,
    options: &RegexOptions,
) -> Result<Option<Vec<usize>>> {
    let mut state = State::new(prog.n_saves, MAX_STACK, option_flags);
    if option_flags & OPTION_TRACE != 0 {
        println!("pos\tinstruction");
    }
    let mut backtrack_count = 0;
    let mut pc = 0;
    let mut ix = pos;
    loop {
        // break from this loop to fail, causes stack to pop
        'fail: loop {
            if option_flags & OPTION_TRACE != 0 {
                println!("{}\t{} {:?}", ix, pc, prog.body[pc]);
            }
            match prog.body[pc] {
                Insn::End => {
                    // save of end position into slot 1 is now done
                    // with an explicit group; we might want to
                    // optimize that.
                    //state.saves[1] = ix;
                    if option_flags & OPTION_TRACE != 0 {
                        println!("saves: {:?}", state.saves);
                    }
                    if let Some(&slot1) = state.saves.get(1) {
                        // With some features like keep out (\K), the match start can be after
                        // the match end. Cap the start to <= end.
                        if state.get(0) > slot1 {
                            state.save(0, slot1);
                        }
                    }
                    return Ok(Some(state.saves));
                }
                Insn::Any => {
                    if ix < s.len() {
                        ix += codepoint_len_at(s, ix);
                    } else {
                        break 'fail;
                    }
                }
                Insn::AnyNoNL => {
                    if ix < s.len() && s.as_bytes()[ix] != b'\n' {
                        ix += codepoint_len_at(s, ix);
                    } else {
                        break 'fail;
                    }
                }
                Insn::Lit(ref val) => {
                    let ix_end = ix + val.len();
                    if !matches_literal(s, ix, ix_end, val) {
                        break 'fail;
                    }
                    ix = ix_end;
                }
                Insn::Split(x, y) => {
                    state.push(y, ix)?;
                    pc = x;
                    continue;
                }
                Insn::Jmp(target) => {
                    pc = target;
                    continue;
                }
                Insn::Save(slot) => state.save(slot, ix),
                Insn::Save0(slot) => state.save(slot, 0),
                Insn::Restore(slot) => ix = state.get(slot),
                Insn::RepeatGr {
                    lo,
                    hi,
                    next,
                    repeat,
                } => {
                    let repcount = state.get(repeat);
                    if repcount == hi {
                        pc = next;
                        continue;
                    }
                    state.save(repeat, repcount + 1);
                    if repcount >= lo {
                        state.push(next, ix)?;
                    }
                }
                Insn::RepeatNg {
                    lo,
                    hi,
                    next,
                    repeat,
                } => {
                    let repcount = state.get(repeat);
                    if repcount == hi {
                        pc = next;
                        continue;
                    }
                    state.save(repeat, repcount + 1);
                    if repcount >= lo {
                        state.push(pc + 1, ix)?;
                        pc = next;
                        continue;
                    }
                }
                Insn::RepeatEpsilonGr {
                    lo,
                    next,
                    repeat,
                    check,
                } => {
                    let repcount = state.get(repeat);
                    if repcount > lo && state.get(check) == ix {
                        // prevent zero-length match on repeat
                        break 'fail;
                    }
                    state.save(repeat, repcount + 1);
                    if repcount >= lo {
                        state.save(check, ix);
                        state.push(next, ix)?;
                    }
                }
                Insn::RepeatEpsilonNg {
                    lo,
                    next,
                    repeat,
                    check,
                } => {
                    let repcount = state.get(repeat);
                    if repcount > lo && state.get(check) == ix {
                        // prevent zero-length match on repeat
                        break 'fail;
                    }
                    state.save(repeat, repcount + 1);
                    if repcount >= lo {
                        state.save(check, ix);
                        state.push(pc + 1, ix)?;
                        pc = next;
                        continue;
                    }
                }
                Insn::GoBack(count) => {
                    for _ in 0..count {
                        if ix == 0 {
                            break 'fail;
                        }
                        ix = prev_codepoint_ix(s, ix);
                    }
                }
                Insn::FailNegativeLookAround => {
                    // Reaching this instruction means that the body of the
                    // look-around matched. Because it's a *negative* look-around,
                    // that means the look-around itself should fail (not match).
                    // But before, we need to discard all the states that have
                    // been pushed with the look-around, because we don't want to
                    // explore them.
                    loop {
                        let (popped_pc, _) = state.pop();
                        if popped_pc == pc + 1 {
                            // We've reached the state that would jump us to
                            // after the look-around (in case the look-around
                            // succeeded). That means we popped enough states.
                            break;
                        }
                    }
                    break 'fail;
                }
                Insn::Backref(slot) => {
                    let lo = state.get(slot);
                    if lo == usize::MAX {
                        // Referenced group hasn't matched, so the backref doesn't match either
                        break 'fail;
                    }
                    let hi = state.get(slot + 1);
                    if hi == usize::MAX {
                        // Referenced group hasn't matched, so the backref doesn't match either
                        break 'fail;
                    }
                    let ref_text = &s[lo..hi];
                    let ix_end = ix + ref_text.len();
                    if !matches_literal(s, ix, ix_end, ref_text) {
                        break 'fail;
                    }
                    ix = ix_end;
                }
                Insn::BackrefExistsCondition(group) => {
                    let lo = state.get(group * 2);
                    if lo == usize::MAX {
                        // Referenced group hasn't matched, so the backref doesn't match either
                        break 'fail;
                    }
                }
                Insn::BeginAtomic => {
                    let count = state.backtrack_count();
                    state.stack_push(count);
                }
                Insn::EndAtomic => {
                    let count = state.stack_pop();
                    state.backtrack_cut(count);
                }
                Insn::DelegateSized(ref inner, size) => {
                    if inner.is_match(&s[ix..]) {
                        // We could analyze for ascii-only, and ix += size in
                        // that case. Unlikely to be speed-limiting though.
                        for _ in 0..size {
                            ix += codepoint_len_at(s, ix);
                        }
                    } else {
                        break 'fail;
                    }
                }
                Insn::Delegate {
                    ref inner,
                    ref inner1,
                    start_group,
                    end_group,
                } => {
                    // Note: Why can't we use `find_at` or `captures_read_at` here instead of the
                    // `inner1` regex? We only want to match at the current location, so our regexes
                    // need to have an anchor: `^foo` (without `^`, it would match `foo` anywhere).
                    // But regex like `^foo` won't match in `bar foo` with `find_at(s, 4)` because
                    // `^` only matches at the beginning of the text.
                    let re = match *inner1 {
                        Some(ref inner1) if ix > 0 => {
                            ix = prev_codepoint_ix(s, ix);
                            inner1
                        }
                        _ => inner,
                    };
                    if start_group == end_group {
                        // No groups, so we can use `find` which is faster than `captures_read`
                        match re.find(&s[ix..]) {
                            Some(m) => ix += m.end(),
                            _ => break 'fail,
                        }
                    } else {
                        let mut locations = re.capture_locations();
                        if let Some(m) = re.captures_read(&mut locations, &s[ix..]) {
                            for i in 0..(end_group - start_group) {
                                let slot = (start_group + i) * 2;
                                if let Some((start, end)) = locations.get(i + 1) {
                                    state.save(slot, ix + start);
                                    state.save(slot + 1, ix + end);
                                } else {
                                    state.save(slot, usize::MAX);
                                    state.save(slot + 1, usize::MAX);
                                }
                            }
                            ix += m.end();
                        } else {
                            break 'fail;
                        }
                    }
                }
                Insn::ContinueFromPreviousMatchEnd => {
                    if ix > pos || option_flags & OPTION_SKIPPED_EMPTY_MATCH != 0 {
                        break 'fail;
                    }
                }
            }
            pc += 1;
        }
        if option_flags & OPTION_TRACE != 0 {
            println!("fail");
        }
        // "break 'fail" goes here
        if state.stack.is_empty() {
            return Ok(None);
        }

        backtrack_count += 1;
        if backtrack_count > options.backtrack_limit {
            return Err(Error::RuntimeError(RuntimeError::BacktrackLimitExceeded));
        }

        let (newpc, newix) = state.pop();
        pc = newpc;
        ix = newix;
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use quickcheck::{quickcheck, Arbitrary, Gen};

    #[test]
    fn state_push_pop() {
        let mut state = State::new(1, MAX_STACK, 0);

        state.push(0, 0).unwrap();
        state.push(1, 1).unwrap();
        assert_eq!(state.pop(), (1, 1));
        assert_eq!(state.pop(), (0, 0));
        assert!(state.stack.is_empty());

        state.push(2, 2).unwrap();
        assert_eq!(state.pop(), (2, 2));
        assert!(state.stack.is_empty());
    }

    #[test]
    fn state_save_override() {
        let mut state = State::new(1, MAX_STACK, 0);
        state.save(0, 10);
        state.push(0, 0).unwrap();
        state.save(0, 20);
        assert_eq!(state.pop(), (0, 0));
        assert_eq!(state.get(0), 10);
    }

    #[test]
    fn state_save_override_twice() {
        let mut state = State::new(1, MAX_STACK, 0);
        state.save(0, 10);
        state.push(0, 0).unwrap();
        state.save(0, 20);
        state.push(1, 1).unwrap();
        state.save(0, 30);

        assert_eq!(state.get(0), 30);
        assert_eq!(state.pop(), (1, 1));
        assert_eq!(state.get(0), 20);
        assert_eq!(state.pop(), (0, 0));
        assert_eq!(state.get(0), 10);
    }

    #[test]
    fn state_explicit_stack() {
        let mut state = State::new(1, MAX_STACK, 0);
        state.stack_push(11);
        state.stack_push(12);

        state.push(100, 101).unwrap();
        state.stack_push(13);
        assert_eq!(state.stack_pop(), 13);
        state.stack_push(14);
        assert_eq!(state.pop(), (100, 101));

        // Note: 14 is not there because it was pushed as part of the backtrack branch
        assert_eq!(state.stack_pop(), 12);
        assert_eq!(state.stack_pop(), 11);
    }

    #[test]
    fn state_backtrack_cut_simple() {
        let mut state = State::new(2, MAX_STACK, 0);
        state.save(0, 1);
        state.save(1, 2);

        let count = state.backtrack_count();

        state.push(0, 0).unwrap();
        state.save(0, 3);
        assert_eq!(state.backtrack_count(), 1);

        state.backtrack_cut(count);
        assert_eq!(state.backtrack_count(), 0);
        assert_eq!(state.get(0), 3);
        assert_eq!(state.get(1), 2);
    }

    #[test]
    fn state_backtrack_cut_complex() {
        let mut state = State::new(2, MAX_STACK, 0);
        state.save(0, 1);
        state.save(1, 2);

        state.push(0, 0).unwrap();
        state.save(0, 3);

        let count = state.backtrack_count();

        state.push(1, 1).unwrap();
        state.save(0, 4);
        state.push(2, 2).unwrap();
        state.save(1, 5);
        assert_eq!(state.backtrack_count(), 3);

        state.backtrack_cut(count);
        assert_eq!(state.backtrack_count(), 1);
        assert_eq!(state.get(0), 4);
        assert_eq!(state.get(1), 5);

        state.pop();
        assert_eq!(state.backtrack_count(), 0);
        // Check that oldsave were set correctly
        assert_eq!(state.get(0), 1);
        assert_eq!(state.get(1), 2);
    }

    #[derive(Clone, Debug)]
    enum Operation {
        Push,
        Pop,
        Save(usize, usize),
    }

    impl Arbitrary for Operation {
        fn arbitrary(g: &mut Gen) -> Self {
            match g.choose(&[0, 1, 2]) {
                Some(0) => Operation::Push,
                Some(1) => Operation::Pop,
                _ => Operation::Save(
                    *g.choose(&[0usize, 1, 2, 3, 4]).unwrap(),
                    usize::arbitrary(g),
                ),
            }
        }
    }

    fn check_saves_for_operations(operations: Vec<Operation>) -> bool {
        let slots = operations
            .iter()
            .map(|o| match o {
                &Operation::Save(slot, _) => slot + 1,
                _ => 0,
            })
            .max()
            .unwrap_or(0);
        if slots == 0 {
            // No point checking if there's no save instructions
            return true;
        }

        // Stack with the complete VM state (including saves)
        let mut stack = Vec::new();
        let mut saves = vec![usize::MAX; slots];

        let mut state = State::new(slots, MAX_STACK, 0);

        let mut expected = Vec::new();
        let mut actual = Vec::new();

        for operation in operations {
            match operation {
                Operation::Push => {
                    // We're not checking pc and ix later, so don't bother
                    // putting in random values.
                    stack.push((0, 0, saves.clone()));
                    state.push(0, 0).unwrap();
                }
                Operation::Pop => {
                    // Note that because we generate the operations randomly
                    // there might be more pops than pushes. So ignore a pop
                    // if the stack was empty.
                    if let Some((_, _, previous_saves)) = stack.pop() {
                        saves = previous_saves;
                        state.pop();
                    }
                }
                Operation::Save(slot, value) => {
                    saves[slot] = value;
                    state.save(slot, value);
                }
            }

            // Remember state of saves for checking later
            expected.push(saves.clone());
            let mut actual_saves = vec![usize::MAX; slots];
            for i in 0..slots {
                actual_saves[i] = state.get(i);
            }
            actual.push(actual_saves);
        }

        expected == actual
    }

    quickcheck! {
        fn state_save_quickcheck(operations: Vec<Operation>) -> bool {
            check_saves_for_operations(operations)
        }
    }
}