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    // Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    // This is the backtracking matching engine. It has the same exact capability
    // as the full NFA simulation, except it is artificially restricted to small
    // regexes on small inputs because of its memory requirements.
    //
    // In particular, this is a *bounded* backtracking engine. It retains worst
    // case linear time by keeping track of the states that is has visited (using a
    // bitmap). Namely, once a state is visited, it is never visited again. Since a
    // state is keyed by `(instruction index, input index)`, we have that its time
    // complexity is `O(mn)` (i.e., linear in the size of the search text).
    //
    // The backtracking engine can beat out the NFA simulation on small
    // regexes/inputs because it doesn't have to keep track of multiple copies of
    // the capture groups. In benchmarks, the backtracking engine is roughly twice
    // as fast as the full NFA simulation. Note though that its performance doesn't
    // scale, even if you're willing to live with the memory requirements. Namely,
    // the bitset has to be zeroed on each execution, which becomes quite expensive
    // on large bitsets.

    use exec::ProgramCache;
    use input::{Input, InputAt};
    use prog::{Program, InstPtr};
    use re_trait::Slot;

    type Bits = u32;

    const BIT_SIZE: usize = 32;
    const MAX_SIZE_BYTES: usize = 256 * (1 << 10); // 256 KB

    /// Returns true iff the given regex and input should be executed by this
    /// engine with reasonable memory usage.
    pub fn should_exec(num_insts: usize, text_len: usize) -> bool {
        // Total memory usage in bytes is determined by:
        //
        //   ((len(insts) * (len(input) + 1) + bits - 1) / bits) * (size_of(u32))
        //
        // The actual limit picked is pretty much a heuristic.
        // See: https://github.com/rust-lang-nursery/regex/issues/215
        let size = ((num_insts * (text_len + 1) + BIT_SIZE - 1) / BIT_SIZE) * 4;
        size <= MAX_SIZE_BYTES
    }

    /// A backtracking matching engine.
    #[derive(Debug)]
    pub struct Bounded<'a, 'm, 'r, 's, I> {
        prog: &'r Program,
        input: I,
        matches: &'m mut [bool],
        slots: &'s mut [Slot],
        m: &'a mut Cache,
    }

    /// Shared cached state between multiple invocations of a backtracking engine
    /// in the same thread.
    #[derive(Clone, Debug)]
    pub struct Cache {
        jobs: Vec<Job>,
        visited: Vec<Bits>,
    }

    impl Cache {
        /// Create new empty cache for the backtracking engine.
        pub fn new(_prog: &Program) -> Self {
            Cache { jobs: vec![], visited: vec![] }
        }
    }

    /// A job is an explicit unit of stack space in the backtracking engine.
    ///
    /// The "normal" representation is a single state transition, which corresponds
    /// to an NFA state and a character in the input. However, the backtracking
    /// engine must keep track of old capture group values. We use the explicit
    /// stack to do it.
    #[derive(Clone, Copy, Debug)]
    enum Job {
        Inst { ip: InstPtr, at: InputAt },
        SaveRestore { slot: usize, old_pos: Option<usize> },
    }

    impl<'a, 'm, 'r, 's, I: Input> Bounded<'a, 'm, 'r, 's, I> {
        /// Execute the backtracking matching engine.
        ///
        /// If there's a match, `exec` returns `true` and populates the given
        /// captures accordingly.
        pub fn exec(
            prog: &'r Program,
            cache: &ProgramCache,
            matches: &'m mut [bool],
            slots: &'s mut [Slot],
            input: I,
            start: usize,
        ) -> bool {
            let mut cache = cache.borrow_mut();
            let mut cache = &mut cache.backtrack;
            let start = input.at(start);
            let mut b = Bounded {
                prog: prog,
                input: input,
                matches: matches,
                slots: slots,
                m: cache,
            };
            b.exec_(start)
        }

        /// Clears the cache such that the backtracking engine can be executed
        /// on some input of fixed length.
        fn clear(&mut self) {
            // Reset the job memory so that we start fresh.
            self.m.jobs.clear();

            // Now we need to clear the bit state set.
            // We do this by figuring out how much space we need to keep track
            // of the states we've visited.
            // Then we reset all existing allocated space to 0.
            // Finally, we request more space if we need it.
            //
            // This is all a little circuitous, but doing this unsafely
            // doesn't seem to have a measurable impact on performance.
            // (Probably because backtracking is limited to such small
            // inputs/regexes in the first place.)
            let visited_len =
                (self.prog.len() * (self.input.len() + 1) + BIT_SIZE - 1)
                /
                BIT_SIZE;
            self.m.visited.truncate(visited_len);
            for v in &mut self.m.visited {
                *v = 0;
            }
            if visited_len > self.m.visited.len() {
                let len = self.m.visited.len();
                self.m.visited.reserve_exact(visited_len - len);
                for _ in 0..(visited_len - len) {
                    self.m.visited.push(0);
                }
            }
        }

        /// Start backtracking at the given position in the input, but also look
        /// for literal prefixes.
        fn exec_(&mut self, mut at: InputAt) -> bool {
            self.clear();
            // If this is an anchored regex at the beginning of the input, then
            // we're either already done or we only need to try backtracking once.
            if self.prog.is_anchored_start {
                return if !at.is_start() {
                    false
                } else {
                    self.backtrack(at)
                };
            }
            let mut matched = false;
            loop {
                if !self.prog.prefixes.is_empty() {
                    at = match self.input.prefix_at(&self.prog.prefixes, at) {
                        None => break,
                        Some(at) => at,
                    };
                }
                matched = self.backtrack(at) || matched;
                if matched && self.prog.matches.len() == 1 {
                    return true;
                }
                if at.is_end() {
                    break;
                }
                at = self.input.at(at.next_pos());
            }
            matched
        }

        /// The main backtracking loop starting at the given input position.
        fn backtrack(&mut self, start: InputAt) -> bool {
            // N.B. We use an explicit stack to avoid recursion.
            // To avoid excessive pushing and popping, most transitions are handled
            // in the `step` helper function, which only pushes to the stack when
            // there's a capture or a branch.
            let mut matched = false;
            self.m.jobs.push(Job::Inst { ip: 0, at: start });
            while let Some(job) = self.m.jobs.pop() {
                match job {
                    Job::Inst { ip, at } => {
                        if self.step(ip, at) {
                            // Only quit if we're matching one regex.
                            // If we're matching a regex set, then mush on and
                            // try to find other matches (if we want them).
                            if self.prog.matches.len() == 1 {
                                return true;
                            }
                            matched = true;
                        }
                    }
                    Job::SaveRestore { slot, old_pos } => {
                        if slot < self.slots.len() {
                            self.slots[slot] = old_pos;
                        }
                    }
                }
            }
            matched
        }

        fn step(&mut self, mut ip: InstPtr, mut at: InputAt) -> bool {
            use prog::Inst::*;
            loop {
                // This loop is an optimization to avoid constantly pushing/popping
                // from the stack. Namely, if we're pushing a job only to run it
                // next, avoid the push and just mutate `ip` (and possibly `at`)
                // in place.
                match self.prog[ip] {
                    Match(slot) => {
                        if slot < self.matches.len() {
                            self.matches[slot] = true;
                        }
                        return true;
                    }
                    Save(ref inst) => {
                        if let Some(&old_pos) = self.slots.get(inst.slot) {
                            // If this path doesn't work out, then we save the old
                            // capture index (if one exists) in an alternate
                            // job. If the next path fails, then the alternate
                            // job is popped and the old capture index is restored.
                            self.m.jobs.push(Job::SaveRestore {
                                slot: inst.slot,
                                old_pos: old_pos,
                            });
                            self.slots[inst.slot] = Some(at.pos());
                        }
                        ip = inst.goto;
                    }
                    Split(ref inst) => {
                        self.m.jobs.push(Job::Inst { ip: inst.goto2, at: at });
                        ip = inst.goto1;
                    }
                    EmptyLook(ref inst) => {
                        let prev = self.input.previous_char(at);
                        let next = self.input.next_char(at);
                        if inst.matches(prev, next) {
                            ip = inst.goto;
                        } else {
                            return false;
                        }
                    }
                    Char(ref inst) => {
                        if inst.c == at.char() {
                            ip = inst.goto;
                            at = self.input.at(at.next_pos());
                        } else {
                            return false;
                        }
                    }
                    Ranges(ref inst) => {
                        if inst.matches(at.char()) {
                            ip = inst.goto;
                            at = self.input.at(at.next_pos());
                        } else {
                            return false;
                        }
                    }
                    Bytes(ref inst) => {
                        if let Some(b) = at.byte() {
                            if inst.matches(b) {
                                ip = inst.goto;
                                at = self.input.at(at.next_pos());
                                continue;
                            }
                        }
                        return false;
                    }
                }
                if self.has_visited(ip, at) {
                    return false;
                }
            }
        }

        fn has_visited(&mut self, ip: InstPtr, at: InputAt) -> bool {
            let k = ip * (self.input.len() + 1) + at.pos();
            let k1 = k / BIT_SIZE;
            let k2 = usize_to_u32(1 << (k & (BIT_SIZE - 1)));
            if self.m.visited[k1] & k2 == 0 {
                self.m.visited[k1] |= k2;
                false
            } else {
                true
            }
        }
    }

    fn usize_to_u32(n: usize) -> u32 {
        if (n as u64) > (::std::u32::MAX as u64) {
            panic!("BUG: {} is too big to fit into u32", n)
        }
        n as u32
    }

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