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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 module implements the Pike VM. That is, it guarantees linear time
    // search of a regex on any text with memory use proportional to the size of
    // the regex.
    //
    // It is equal in power to the backtracking engine in this crate, except the
    // backtracking engine is typically faster on small regexes/texts at the
    // expense of a bigger memory footprint.
    //
    // It can do more than the DFA can (specifically, record capture locations
    // and execute Unicode word boundary assertions), but at a slower speed.
    // Specifically, the Pike VM exectues a DFA implicitly by repeatedly expanding
    // epsilon transitions. That is, the Pike VM engine can be in multiple states
    // at once where as the DFA is only ever in one state at a time.
    //
    // Therefore, the Pike VM is generally treated as the fallback when the other
    // matching engines either aren't feasible to run or are insufficient.

    use std::mem;

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

    /// An NFA simulation matching engine.
    #[derive(Debug)]
    pub struct Fsm<'r, I> {
        /// The sequence of opcodes (among other things) that is actually executed.
        ///
        /// The program may be byte oriented or Unicode codepoint oriented.
        prog: &'r Program,
        /// An explicit stack used for following epsilon transitions. (This is
        /// borrowed from the cache.)
        stack: &'r mut Vec<FollowEpsilon>,
        /// The input to search.
        input: I,
    }

    /// A cached allocation that can be reused on each execution.
    #[derive(Clone, Debug)]
    pub struct Cache {
        /// A pair of ordered sets for tracking NFA states.
        clist: Threads,
        nlist: Threads,
        /// An explicit stack used for following epsilon transitions.
        stack: Vec<FollowEpsilon>,
    }

    /// An ordered set of NFA states and their captures.
    #[derive(Clone, Debug)]
    struct Threads {
        /// An ordered set of opcodes (each opcode is an NFA state).
        set: SparseSet,
        /// Captures for every NFA state.
        ///
        /// It is stored in row-major order, where the columns are the capture
        /// slots and the rows are the states.
        caps: Vec<Slot>,
        /// The number of capture slots stored per thread. (Every capture has
        /// two slots.)
        slots_per_thread: usize,
    }

    /// A representation of an explicit stack frame when following epsilon
    /// transitions. This is used to avoid recursion.
    #[derive(Clone, Debug)]
    enum FollowEpsilon {
        /// Follow transitions at the given instruction pointer.
        IP(InstPtr),
        /// Restore the capture slot with the given position in the input.
        Capture { slot: usize, pos: Slot },
    }

    impl Cache {
        /// Create a new allocation used by the NFA machine to record execution
        /// and captures.
        pub fn new(_prog: &Program) -> Self {
            Cache {
                clist: Threads::new(),
                nlist: Threads::new(),
                stack: vec![],
            }
        }
    }

    impl<'r, I: Input> Fsm<'r, I> {
        /// Execute the NFA 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: &mut [bool],
            slots: &mut [Slot],
            quit_after_match: bool,
            input: I,
            start: usize,
        ) -> bool {
            let mut cache = cache.borrow_mut();
            let mut cache = &mut cache.pikevm;
            cache.clist.resize(prog.len(), prog.captures.len());
            cache.nlist.resize(prog.len(), prog.captures.len());
            let at = input.at(start);
            Fsm {
                prog: prog,
                stack: &mut cache.stack,
                input: input,
            }.exec_(
                &mut cache.clist,
                &mut cache.nlist,
                matches,
                slots,
                quit_after_match,
                at,
            )
        }

        fn exec_(
            &mut self,
            mut clist: &mut Threads,
            mut nlist: &mut Threads,
            matches: &mut [bool],
            slots: &mut [Slot],
            quit_after_match: bool,
            mut at: InputAt,
        ) -> bool {
            let mut matched = false;
            let mut all_matched = false;
            clist.set.clear();
            nlist.set.clear();
    'LOOP:  loop {
                if clist.set.is_empty() {
                    // Three ways to bail out when our current set of threads is
                    // empty.
                    //
                    // 1. We have a match---so we're done exploring any possible
                    //    alternatives. Time to quit. (We can't do this if we're
                    //    looking for matches for multiple regexes, unless we know
                    //    they all matched.)
                    //
                    // 2. If the expression starts with a '^' we can terminate as
                    //    soon as the last thread dies.
                    if (matched && matches.len() <= 1)
                        || all_matched
                        || (!at.is_start() && self.prog.is_anchored_start) {
                        break;
                    }

                    // 3. If there's a literal prefix for the program, try to
                    //    jump ahead quickly. If it can't be found, then we can
                    //    bail out early.
                    if !self.prog.prefixes.is_empty() {
                        at = match self.input.prefix_at(&self.prog.prefixes, at) {
                            None => break,
                            Some(at) => at,
                        };
                    }
                }

                // This simulates a preceding '.*?' for every regex by adding
                // a state starting at the current position in the input for the
                // beginning of the program only if we don't already have a match.
                if clist.set.is_empty()
                    || (!self.prog.is_anchored_start && !all_matched) {
                    self.add(&mut clist, slots, 0, at);
                }
                // The previous call to "add" actually inspects the position just
                // before the current character. For stepping through the machine,
                // we can to look at the current character, so we advance the
                // input.
                let at_next = self.input.at(at.next_pos());
                for i in 0..clist.set.len() {
                    let ip = clist.set[i];
                    if self.step(
                        &mut nlist,
                        matches,
                        slots,
                        clist.caps(ip),
                        ip,
                        at,
                        at_next,
                    ) {
                        matched = true;
                        all_matched = all_matched || matches.iter().all(|&b| b);
                        if quit_after_match {
                            // If we only care if a match occurs (not its
                            // position), then we can quit right now.
                            break 'LOOP;
                        }
                        if self.prog.matches.len() == 1 {
                            // We don't need to check the rest of the threads
                            // in this set because we've matched something
                            // ("leftmost-first"). However, we still need to check
                            // threads in the next set to support things like
                            // greedy matching.
                            //
                            // This is only true on normal regexes. For regex sets,
                            // we need to mush on to observe other matches.
                            break;
                        }
                    }
                }
                if at.is_end() {
                    break;
                }
                at = at_next;
                mem::swap(clist, nlist);
                nlist.set.clear();
            }
            matched
        }

        /// Step through the input, one token (byte or codepoint) at a time.
        ///
        /// nlist is the set of states that will be processed on the next token
        /// in the input.
        ///
        /// caps is the set of captures passed by the caller of the NFA. They are
        /// written to only when a match state is visited.
        ///
        /// thread_caps is the set of captures set for the current NFA state, ip.
        ///
        /// at and at_next are the current and next positions in the input. at or
        /// at_next may be EOF.
        fn step(
            &mut self,
            nlist: &mut Threads,
            matches: &mut [bool],
            slots: &mut [Slot],
            thread_caps: &mut [Option<usize>],
            ip: usize,
            at: InputAt,
            at_next: InputAt,
        ) -> bool {
            use prog::Inst::*;
            match self.prog[ip] {
                Match(match_slot) => {
                    if match_slot < matches.len() {
                        matches[match_slot] = true;
                    }
                    for (slot, val) in slots.iter_mut().zip(thread_caps.iter()) {
                        *slot = *val;
                    }
                    true
                }
                Char(ref inst) => {
                    if inst.c == at.char() {
                        self.add(nlist, thread_caps, inst.goto, at_next);
                    }
                    false
                }
                Ranges(ref inst) => {
                    if inst.matches(at.char()) {
                        self.add(nlist, thread_caps, inst.goto, at_next);
                    }
                    false
                }
                Bytes(ref inst) => {
                    if let Some(b) = at.byte() {
                        if inst.matches(b) {
                            self.add(nlist, thread_caps, inst.goto, at_next);
                        }
                    }
                    false
                }
                EmptyLook(_) | Save(_) | Split(_) => false,
            }
        }

        /// Follows epsilon transitions and adds them for processing to nlist,
        /// starting at and including ip.
        fn add(
            &mut self,
            nlist: &mut Threads,
            thread_caps: &mut [Option<usize>],
            ip: usize,
            at: InputAt,
        ) {
            self.stack.push(FollowEpsilon::IP(ip));
            while let Some(frame) = self.stack.pop() {
                match frame {
                    FollowEpsilon::IP(ip) => {
                        self.add_step(nlist, thread_caps, ip, at);
                    }
                    FollowEpsilon::Capture { slot, pos } => {
                        thread_caps[slot] = pos;
                    }
                }
            }
        }

        /// A helper function for add that avoids excessive pushing to the stack.
        fn add_step(
            &mut self,
            nlist: &mut Threads,
            thread_caps: &mut [Option<usize>],
            mut ip: usize,
            at: InputAt,
        ) {
            // Instead of pushing and popping to the stack, we mutate ip as we
            // traverse the set of states. We only push to the stack when we
            // absolutely need recursion (restoring captures or following a
            // branch).
            use prog::Inst::*;
            loop {
                // Don't visit states we've already added.
                if nlist.set.contains_ip(ip) {
                    return;
                }
                nlist.set.add(ip);
                match self.prog[ip] {
                    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;
                        }
                    }
                    Save(ref inst) => {
                        if inst.slot < thread_caps.len() {
                            self.stack.push(FollowEpsilon::Capture {
                                slot: inst.slot,
                                pos: thread_caps[inst.slot],
                            });
                            thread_caps[inst.slot] = Some(at.pos());
                        }
                        ip = inst.goto;
                    }
                    Split(ref inst) => {
                        self.stack.push(FollowEpsilon::IP(inst.goto2));
                        ip = inst.goto1;
                    }
                    Match(_) | Char(_) | Ranges(_) | Bytes(_) => {
                        let mut t = &mut nlist.caps(ip);
                        for (slot, val) in t.iter_mut().zip(thread_caps.iter()) {
                            *slot = *val;
                        }
                        return;
                    }
                }
            }
        }
    }

    impl Threads {
        fn new() -> Self {
            Threads {
                set: SparseSet::new(0),
                caps: vec![],
                slots_per_thread: 0,
            }
        }

        fn resize(&mut self, num_insts: usize, ncaps: usize) {
            if num_insts == self.set.capacity() {
                return;
            }
            self.slots_per_thread = ncaps * 2;
            self.set = SparseSet::new(num_insts);
            self.caps = vec![None; self.slots_per_thread * num_insts];
        }

        fn caps(&mut self, pc: usize) -> &mut [Option<usize>] {
            let i = pc * self.slots_per_thread;
            &mut self.caps[i..i + self.slots_per_thread]
        }
    }

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