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    // Copyright 2014-2016 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.

    use std::collections::HashMap;
    use std::iter;
    use std::result;
    use std::sync::Arc;

    use syntax::is_word_byte;
    use syntax::hir::{self, Hir};
    use utf8_ranges::{Utf8Range, Utf8Sequence, Utf8Sequences};

    use prog::{
        Program, Inst, InstPtr, EmptyLook,
        InstSave, InstSplit, InstEmptyLook, InstChar, InstRanges, InstBytes,
    };

    use Error;

    type Result = result::Result<Patch, Error>;

    #[derive(Debug)]
    struct Patch {
        hole: Hole,
        entry: InstPtr,
    }

    /// A compiler translates a regular expression AST to a sequence of
    /// instructions. The sequence of instructions represents an NFA.
    pub struct Compiler {
        insts: Vec<MaybeInst>,
        compiled: Program,
        capture_name_idx: HashMap<String, usize>,
        num_exprs: usize,
        size_limit: usize,
        suffix_cache: SuffixCache,
        utf8_seqs: Option<Utf8Sequences>,
        byte_classes: ByteClassSet,
    }

    impl Compiler {
        /// Create a new regular expression compiler.
        ///
        /// Various options can be set before calling `compile` on an expression.
        pub fn new() -> Self {
            Compiler {
                insts: vec![],
                compiled: Program::new(),
                capture_name_idx: HashMap::new(),
                num_exprs: 0,
                size_limit: 10 * (1 << 20),
                suffix_cache: SuffixCache::new(1000),
                utf8_seqs: Some(Utf8Sequences::new('\x00', '\x00')),
                byte_classes: ByteClassSet::new(),
            }
        }

        /// The size of the resulting program is limited by size_limit. If
        /// the program approximately exceeds the given size (in bytes), then
        /// compilation will stop and return an error.
        pub fn size_limit(mut self, size_limit: usize) -> Self {
            self.size_limit = size_limit;
            self
        }

        /// If bytes is true, then the program is compiled as a byte based
        /// automaton, which incorporates UTF-8 decoding into the machine. If it's
        /// false, then the automaton is Unicode scalar value based, e.g., an
        /// engine utilizing such an automaton is responsible for UTF-8 decoding.
        ///
        /// The specific invariant is that when returning a byte based machine,
        /// the neither the `Char` nor `Ranges` instructions are produced.
        /// Conversely, when producing a Unicode scalar value machine, the `Bytes`
        /// instruction is never produced.
        ///
        /// Note that `dfa(true)` implies `bytes(true)`.
        pub fn bytes(mut self, yes: bool) -> Self {
            self.compiled.is_bytes = yes;
            self
        }

        /// When disabled, the program compiled may match arbitrary bytes.
        ///
        /// When enabled (the default), all compiled programs exclusively match
        /// valid UTF-8 bytes.
        pub fn only_utf8(mut self, yes: bool) -> Self {
            self.compiled.only_utf8 = yes;
            self
        }

        /// When set, the machine returned is suitable for use in the DFA matching
        /// engine.
        ///
        /// In particular, this ensures that if the regex is not anchored in the
        /// beginning, then a preceding `.*?` is included in the program. (The NFA
        /// based engines handle the preceding `.*?` explicitly, which is difficult
        /// or impossible in the DFA engine.)
        pub fn dfa(mut self, yes: bool) -> Self {
            self.compiled.is_dfa = yes;
            self
        }

        /// When set, the machine returned is suitable for matching text in
        /// reverse. In particular, all concatenations are flipped.
        pub fn reverse(mut self, yes: bool) -> Self {
            self.compiled.is_reverse = yes;
            self
        }

        /// Compile a regular expression given its AST.
        ///
        /// The compiler is guaranteed to succeed unless the program exceeds the
        /// specified size limit. If the size limit is exceeded, then compilation
        /// stops and returns an error.
        pub fn compile(
            mut self,
            exprs: &[Hir],
        ) -> result::Result<Program, Error> {
            debug_assert!(exprs.len() >= 1);
            self.num_exprs = exprs.len();
            if exprs.len() == 1 {
                self.compile_one(&exprs[0])
            } else {
                self.compile_many(exprs)
            }
        }

        fn compile_one(mut self, expr: &Hir) -> result::Result<Program, Error> {
            // If we're compiling a forward DFA and we aren't anchored, then
            // add a `.*?` before the first capture group.
            // Other matching engines handle this by baking the logic into the
            // matching engine itself.
            let mut dotstar_patch = Patch { hole: Hole::None, entry: 0 };
            self.compiled.is_anchored_start = expr.is_anchored_start();
            self.compiled.is_anchored_end = expr.is_anchored_end();
            if self.compiled.needs_dotstar() {
                dotstar_patch = self.c_dotstar()?;
                self.compiled.start = dotstar_patch.entry;
            }
            self.compiled.captures = vec![None];
            let patch = self.c_capture(0, expr)?;
            if self.compiled.needs_dotstar() {
                self.fill(dotstar_patch.hole, patch.entry);
            } else {
                self.compiled.start = patch.entry;
            }
            self.fill_to_next(patch.hole);
            self.compiled.matches = vec![self.insts.len()];
            self.push_compiled(Inst::Match(0));
            self.compile_finish()
        }

        fn compile_many(
            mut self,
            exprs: &[Hir],
        ) -> result::Result<Program, Error> {
            debug_assert!(exprs.len() > 1);

            self.compiled.is_anchored_start =
                exprs.iter().all(|e| e.is_anchored_start());
            self.compiled.is_anchored_end =
                exprs.iter().all(|e| e.is_anchored_end());
            let mut dotstar_patch = Patch { hole: Hole::None, entry: 0 };
            if self.compiled.needs_dotstar() {
                dotstar_patch = self.c_dotstar()?;
                self.compiled.start = dotstar_patch.entry;
            } else {
                self.compiled.start = 0; // first instruction is always split
            }
            self.fill_to_next(dotstar_patch.hole);

            let mut prev_hole = Hole::None;
            for (i, expr) in exprs[0..exprs.len() - 1].iter().enumerate() {
                self.fill_to_next(prev_hole);
                let split = self.push_split_hole();
                let Patch { hole, entry } = self.c_capture(0, expr)?;
                self.fill_to_next(hole);
                self.compiled.matches.push(self.insts.len());
                self.push_compiled(Inst::Match(i));
                prev_hole = self.fill_split(split, Some(entry), None);
            }
            let i = exprs.len() - 1;
            let Patch { hole, entry } = self.c_capture(0, &exprs[i])?;
            self.fill(prev_hole, entry);
            self.fill_to_next(hole);
            self.compiled.matches.push(self.insts.len());
            self.push_compiled(Inst::Match(i));
            self.compile_finish()
        }

        fn compile_finish(mut self) -> result::Result<Program, Error> {
            self.compiled.insts =
                self.insts.into_iter().map(|inst| inst.unwrap()).collect();
            self.compiled.byte_classes = self.byte_classes.byte_classes();
            self.compiled.capture_name_idx = Arc::new(self.capture_name_idx);
            Ok(self.compiled)
        }

        /// Compile expr into self.insts, returning a patch on success,
        /// or an error if we run out of memory.
        ///
        /// All of the c_* methods of the compiler share the contract outlined
        /// here.
        ///
        /// The main thing that a c_* method does is mutate `self.insts`
        /// to add a list of mostly compiled instructions required to execute
        /// the given expression. `self.insts` contains MaybeInsts rather than
        /// Insts because there is some backpatching required.
        ///
        /// The `Patch` value returned by each c_* method provides metadata
        /// about the compiled instructions emitted to `self.insts`. The
        /// `entry` member of the patch refers to the first instruction
        /// (the entry point), while the `hole` member contains zero or
        /// more offsets to partial instructions that need to be backpatched.
        /// The c_* routine can't know where its list of instructions are going to
        /// jump to after execution, so it is up to the caller to patch
        /// these jumps to point to the right place. So compiling some
        /// expression, e, we would end up with a situation that looked like:
        ///
        /// ```text
        /// self.insts = [ ..., i1, i2, ..., iexit1, ..., iexitn, ...]
        ///                     ^              ^             ^
        ///                     |                \         /
        ///                   entry                \     /
        ///                                         hole
        /// ```
        ///
        /// To compile two expressions, e1 and e2, concatinated together we
        /// would do:
        ///
        /// ```ignore
        /// let patch1 = self.c(e1);
        /// let patch2 = self.c(e2);
        /// ```
        ///
        /// while leaves us with a situation that looks like
        ///
        /// ```text
        /// self.insts = [ ..., i1, ..., iexit1, ..., i2, ..., iexit2 ]
        ///                     ^        ^            ^        ^
        ///                     |        |            |        |
        ///                entry1        hole1   entry2        hole2
        /// ```
        ///
        /// Then to merge the two patches together into one we would backpatch
        /// hole1 with entry2 and return a new patch that enters at entry1
        /// and has hole2 for a hole. In fact, if you look at the c_concat
        /// method you will see that it does exactly this, though it handles
        /// a list of expressions rather than just the two that we use for
        /// an example.
        fn c(&mut self, expr: &Hir) -> Result {
            use prog;
            use syntax::hir::HirKind::*;

            self.check_size()?;
            match *expr.kind() {
                Empty => Ok(Patch { hole: Hole::None, entry: self.insts.len() }),
                Literal(hir::Literal::Unicode(c)) => {
                    self.c_char(c)
                }
                Literal(hir::Literal::Byte(b)) => {
                    assert!(self.compiled.uses_bytes());
                    self.c_byte(b)
                }
                Class(hir::Class::Unicode(ref cls)) => {
                    self.c_class(cls.ranges())
                }
                Class(hir::Class::Bytes(ref cls)) => {
                    if self.compiled.uses_bytes() {
                        self.c_class_bytes(cls.ranges())
                    } else {
                        assert!(cls.is_all_ascii());
                        let mut char_ranges = vec![];
                        for r in cls.iter() {
                            let (s, e) = (r.start() as char, r.end() as char);
                            char_ranges.push(hir::ClassUnicodeRange::new(s, e));
                        }
                        self.c_class(&char_ranges)
                    }
                }
                Anchor(hir::Anchor::StartLine) if self.compiled.is_reverse => {
                    self.byte_classes.set_range(b'\n', b'\n');
                    self.c_empty_look(prog::EmptyLook::EndLine)
                }
                Anchor(hir::Anchor::StartLine) => {
                    self.byte_classes.set_range(b'\n', b'\n');
                    self.c_empty_look(prog::EmptyLook::StartLine)
                }
                Anchor(hir::Anchor::EndLine) if self.compiled.is_reverse => {
                    self.byte_classes.set_range(b'\n', b'\n');
                    self.c_empty_look(prog::EmptyLook::StartLine)
                }
                Anchor(hir::Anchor::EndLine) => {
                    self.byte_classes.set_range(b'\n', b'\n');
                    self.c_empty_look(prog::EmptyLook::EndLine)
                }
                Anchor(hir::Anchor::StartText) if self.compiled.is_reverse => {
                    self.c_empty_look(prog::EmptyLook::EndText)
                }
                Anchor(hir::Anchor::StartText) => {
                    self.c_empty_look(prog::EmptyLook::StartText)
                }
                Anchor(hir::Anchor::EndText) if self.compiled.is_reverse => {
                    self.c_empty_look(prog::EmptyLook::StartText)
                }
                Anchor(hir::Anchor::EndText) => {
                    self.c_empty_look(prog::EmptyLook::EndText)
                }
                WordBoundary(hir::WordBoundary::Unicode) => {
                    self.compiled.has_unicode_word_boundary = true;
                    self.byte_classes.set_word_boundary();
                    self.c_empty_look(prog::EmptyLook::WordBoundary)
                }
                WordBoundary(hir::WordBoundary::UnicodeNegate) => {
                    self.compiled.has_unicode_word_boundary = true;
                    self.byte_classes.set_word_boundary();
                    self.c_empty_look(prog::EmptyLook::NotWordBoundary)
                }
                WordBoundary(hir::WordBoundary::Ascii) => {
                    self.byte_classes.set_word_boundary();
                    self.c_empty_look(prog::EmptyLook::WordBoundaryAscii)
                }
                WordBoundary(hir::WordBoundary::AsciiNegate) => {
                    self.byte_classes.set_word_boundary();
                    self.c_empty_look(prog::EmptyLook::NotWordBoundaryAscii)
                }
                Group(ref g) => {
                    match g.kind {
                        hir::GroupKind::NonCapturing => self.c(&g.hir),
                        hir::GroupKind::CaptureIndex(index) => {
                            if index as usize >= self.compiled.captures.len() {
                                self.compiled.captures.push(None);
                            }
                            self.c_capture(2 * index as usize, &g.hir)
                        }
                        hir::GroupKind::CaptureName { index, ref name } => {
                            if index as usize >= self.compiled.captures.len() {
                                let n = name.to_string();
                                self.compiled.captures.push(Some(n.clone()));
                                self.capture_name_idx.insert(n, index as usize);
                            }
                            self.c_capture(2 * index as usize, &g.hir)
                        }
                    }
                }
                Concat(ref es) => {
                    if self.compiled.is_reverse {
                        self.c_concat(es.iter().rev())
                    } else {
                        self.c_concat(es)
                    }
                }
                Alternation(ref es) => self.c_alternate(&**es),
                Repetition(ref rep) => self.c_repeat(rep),
            }
        }

        fn c_capture(&mut self, first_slot: usize, expr: &Hir) -> Result {
            if self.num_exprs > 1 || self.compiled.is_dfa {
                // Don't ever compile Save instructions for regex sets because
                // they are never used. They are also never used in DFA programs
                // because DFAs can't handle captures.
                self.c(expr)
            } else {
                let entry = self.insts.len();
                let hole = self.push_hole(InstHole::Save { slot: first_slot });
                let patch = self.c(expr)?;
                self.fill(hole, patch.entry);
                self.fill_to_next(patch.hole);
                let hole = self.push_hole(InstHole::Save { slot: first_slot + 1 });
                Ok(Patch { hole: hole, entry: entry })
            }
        }

        fn c_dotstar(&mut self) -> Result {
            Ok(if !self.compiled.only_utf8() {
                self.c(&Hir::repetition(hir::Repetition {
                    kind: hir::RepetitionKind::ZeroOrMore,
                    greedy: false,
                    hir: Box::new(Hir::any(true)),
                }))?
            } else {
                self.c(&Hir::repetition(hir::Repetition {
                    kind: hir::RepetitionKind::ZeroOrMore,
                    greedy: false,
                    hir: Box::new(Hir::any(false)),
                }))?
            })
        }

        fn c_char(&mut self, c: char) -> Result {
            self.c_class(&[hir::ClassUnicodeRange::new(c, c)])
        }

        fn c_class(&mut self, ranges: &[hir::ClassUnicodeRange]) -> Result {
            assert!(!ranges.is_empty());
            if self.compiled.uses_bytes() {
                CompileClass {
                    c: self,
                    ranges: ranges,
                }.compile()
            } else {
                let ranges: Vec<(char, char)> =
                    ranges.iter().map(|r| (r.start(), r.end())).collect();
                let hole = if ranges.len() == 1 && ranges[0].0 == ranges[0].1 {
                    self.push_hole(InstHole::Char { c: ranges[0].0 })
                } else {
                    self.push_hole(InstHole::Ranges { ranges: ranges })
                };
                Ok(Patch { hole: hole, entry: self.insts.len() - 1 })
            }
        }

        fn c_byte(&mut self, b: u8) -> Result {
            self.c_class_bytes(&[hir::ClassBytesRange::new(b, b)])
        }

        fn c_class_bytes(&mut self, ranges: &[hir::ClassBytesRange]) -> Result {
            debug_assert!(!ranges.is_empty());

            let first_split_entry = self.insts.len();
            let mut holes = vec![];
            let mut prev_hole = Hole::None;
            for r in &ranges[0..ranges.len() - 1] {
                self.fill_to_next(prev_hole);
                let split = self.push_split_hole();
                let next = self.insts.len();
                self.byte_classes.set_range(r.start(), r.end());
                holes.push(self.push_hole(InstHole::Bytes {
                    start: r.start(), end: r.end(),
                }));
                prev_hole = self.fill_split(split, Some(next), None);
            }
            let next = self.insts.len();
            let r = &ranges[ranges.len() - 1];
            self.byte_classes.set_range(r.start(), r.end());
            holes.push(self.push_hole(InstHole::Bytes {
                start: r.start(), end: r.end(),
            }));
            self.fill(prev_hole, next);
            Ok(Patch { hole: Hole::Many(holes), entry: first_split_entry })
        }

        fn c_empty_look(&mut self, look: EmptyLook) -> Result {
            let hole = self.push_hole(InstHole::EmptyLook { look: look });
            Ok(Patch { hole: hole, entry: self.insts.len() - 1 })
        }

        fn c_concat<'a, I>(&mut self, exprs: I) -> Result
                where I: IntoIterator<Item=&'a Hir> {
            let mut exprs = exprs.into_iter();
            let first = match exprs.next() {
                Some(expr) => expr,
                None => {
                    return Ok(Patch { hole: Hole::None, entry: self.insts.len() })
                }
            };
            let Patch { mut hole, entry } = self.c(first)?;
            for e in exprs {
                let p = self.c(e)?;
                self.fill(hole, p.entry);
                hole = p.hole;
            }
            Ok(Patch { hole: hole, entry: entry })
        }

        fn c_alternate(&mut self, exprs: &[Hir]) -> Result {
            debug_assert!(
                exprs.len() >= 2, "alternates must have at least 2 exprs");

            // Initial entry point is always the first split.
            let first_split_entry = self.insts.len();

            // Save up all of the holes from each alternate. They will all get
            // patched to point to the same location.
            let mut holes = vec![];

            let mut prev_hole = Hole::None;
            for e in &exprs[0..exprs.len() - 1] {
                self.fill_to_next(prev_hole);
                let split = self.push_split_hole();
                let prev_entry = self.insts.len();
                let Patch { hole, entry } = self.c(e)?;
                if prev_entry == self.insts.len() {
                    // TODO(burntsushi): It is kind of silly that we don't support
                    // empty-subexpressions in alternates, but it is supremely
                    // awkward to support them in the existing compiler
                    // infrastructure. This entire compiler needs to be thrown out
                    // anyway, so don't feel too bad.
                    return Err(Error::Syntax(
                        "alternations cannot currently contain \
                         empty sub-expressions".to_string()));
                }
                holes.push(hole);
                prev_hole = self.fill_split(split, Some(entry), None);
            }
            let prev_entry = self.insts.len();
            let Patch { hole, entry } = self.c(&exprs[exprs.len() - 1])?;
            if prev_entry == self.insts.len() {
                // TODO(burntsushi): See TODO above.
                return Err(Error::Syntax(
                    "alternations cannot currently contain \
                     empty sub-expressions".to_string()));
            }
            holes.push(hole);
            self.fill(prev_hole, entry);
            Ok(Patch { hole: Hole::Many(holes), entry: first_split_entry })
        }

        fn c_repeat(&mut self, rep: &hir::Repetition) -> Result {
            use syntax::hir::RepetitionKind::*;
            match rep.kind {
                ZeroOrOne => self.c_repeat_zero_or_one(&rep.hir, rep.greedy),
                ZeroOrMore => self.c_repeat_zero_or_more(&rep.hir, rep.greedy),
                OneOrMore => self.c_repeat_one_or_more(&rep.hir, rep.greedy),
                Range(hir::RepetitionRange::Exactly(min_max)) => {
                    self.c_repeat_range(&rep.hir, rep.greedy, min_max, min_max)
                }
                Range(hir::RepetitionRange::AtLeast(min)) => {
                    self.c_repeat_range_min_or_more(&rep.hir, rep.greedy, min)
                }
                Range(hir::RepetitionRange::Bounded(min, max)) => {
                    self.c_repeat_range(&rep.hir, rep.greedy, min, max)
                }
            }
        }

        fn c_repeat_zero_or_one(&mut self, expr: &Hir, greedy: bool) -> Result {
            let split_entry = self.insts.len();
            let split = self.push_split_hole();
            let Patch { hole: hole_rep, entry: entry_rep } = self.c(expr)?;

            let split_hole = if greedy {
                self.fill_split(split, Some(entry_rep), None)
            } else {
                self.fill_split(split, None, Some(entry_rep))
            };
            let holes = vec![hole_rep, split_hole];
            Ok(Patch { hole: Hole::Many(holes), entry: split_entry })
        }

        fn c_repeat_zero_or_more(&mut self, expr: &Hir, greedy: bool) -> Result {
            let split_entry = self.insts.len();
            let split = self.push_split_hole();
            let Patch { hole: hole_rep, entry: entry_rep } = self.c(expr)?;

            self.fill(hole_rep, split_entry);
            let split_hole = if greedy {
                self.fill_split(split, Some(entry_rep), None)
            } else {
                self.fill_split(split, None, Some(entry_rep))
            };
            Ok(Patch { hole: split_hole, entry: split_entry })
        }

        fn c_repeat_one_or_more(&mut self, expr: &Hir, greedy: bool) -> Result {
            let Patch { hole: hole_rep, entry: entry_rep } = self.c(expr)?;
            self.fill_to_next(hole_rep);
            let split = self.push_split_hole();

            let split_hole = if greedy {
                self.fill_split(split, Some(entry_rep), None)
            } else {
                self.fill_split(split, None, Some(entry_rep))
            };
            Ok(Patch { hole: split_hole, entry: entry_rep })
        }

        fn c_repeat_range_min_or_more(
            &mut self,
            expr: &Hir,
            greedy: bool,
            min: u32,
        ) -> Result {
            let min = u32_to_usize(min);
            let patch_concat = self.c_concat(iter::repeat(expr).take(min))?;
            let patch_rep = self.c_repeat_zero_or_more(expr, greedy)?;
            self.fill(patch_concat.hole, patch_rep.entry);
            Ok(Patch { hole: patch_rep.hole, entry: patch_concat.entry })
        }

        fn c_repeat_range(
            &mut self,
            expr: &Hir,
            greedy: bool,
            min: u32,
            max: u32,
        ) -> Result {
            let (min, max) = (u32_to_usize(min), u32_to_usize(max));
            let patch_concat = self.c_concat(iter::repeat(expr).take(min))?;
            let initial_entry = patch_concat.entry;
            if min == max {
                return Ok(patch_concat);
            }
            // It is much simpler to compile, e.g., `a{2,5}` as:
            //
            //     aaa?a?a?
            //
            // But you end up with a sequence of instructions like this:
            //
            //     0: 'a'
            //     1: 'a',
            //     2: split(3, 4)
            //     3: 'a'
            //     4: split(5, 6)
            //     5: 'a'
            //     6: split(7, 8)
            //     7: 'a'
            //     8: MATCH
            //
            // This is *incredibly* inefficient because the splits end
            // up forming a chain, which has to be resolved everything a
            // transition is followed.
            let mut holes = vec![];
            let mut prev_hole = patch_concat.hole;
            for _ in min..max {
                self.fill_to_next(prev_hole);
                let split = self.push_split_hole();
                let Patch { hole, entry } = self.c(expr)?;
                prev_hole = hole;
                if greedy {
                    holes.push(self.fill_split(split, Some(entry), None));
                } else {
                    holes.push(self.fill_split(split, None, Some(entry)));
                }
            }
            holes.push(prev_hole);
            Ok(Patch { hole: Hole::Many(holes), entry: initial_entry })
        }

        fn fill(&mut self, hole: Hole, goto: InstPtr) {
            match hole {
                Hole::None => {}
                Hole::One(pc) => {
                    self.insts[pc].fill(goto);
                }
                Hole::Many(holes) => {
                    for hole in holes {
                        self.fill(hole, goto);
                    }
                }
            }
        }

        fn fill_to_next(&mut self, hole: Hole) {
            let next = self.insts.len();
            self.fill(hole, next);
        }

        fn fill_split(
            &mut self,
            hole: Hole,
            goto1: Option<InstPtr>,
            goto2: Option<InstPtr>,
        ) -> Hole {
            match hole {
                Hole::None => Hole::None,
                Hole::One(pc) => {
                    match (goto1, goto2) {
                        (Some(goto1), Some(goto2)) => {
                            self.insts[pc].fill_split(goto1, goto2);
                            Hole::None
                        }
                        (Some(goto1), None) => {
                            self.insts[pc].half_fill_split_goto1(goto1);
                            Hole::One(pc)
                        }
                        (None, Some(goto2)) => {
                            self.insts[pc].half_fill_split_goto2(goto2);
                            Hole::One(pc)
                        }
                        (None, None) => unreachable!("at least one of the split \
                                                      holes must be filled"),
                    }
                }
                Hole::Many(holes) => {
                    let mut new_holes = vec![];
                    for hole in holes {
                        new_holes.push(self.fill_split(hole, goto1, goto2));
                    }
                    if new_holes.is_empty() {
                        Hole::None
                    } else if new_holes.len() == 1 {
                        new_holes.pop().unwrap()
                    } else {
                        Hole::Many(new_holes)
                    }
                }
            }
        }

        fn push_compiled(&mut self, inst: Inst) {
            self.insts.push(MaybeInst::Compiled(inst));
        }

        fn push_hole(&mut self, inst: InstHole) -> Hole {
            let hole = self.insts.len();
            self.insts.push(MaybeInst::Uncompiled(inst));
            Hole::One(hole)
        }

        fn push_split_hole(&mut self) -> Hole {
            let hole = self.insts.len();
            self.insts.push(MaybeInst::Split);
            Hole::One(hole)
        }

        fn check_size(&self) -> result::Result<(), Error> {
            use std::mem::size_of;

            if self.insts.len() * size_of::<Inst>() > self.size_limit {
                Err(Error::CompiledTooBig(self.size_limit))
            } else {
                Ok(())
            }
        }
    }

    #[derive(Debug)]
    enum Hole {
        None,
        One(InstPtr),
        Many(Vec<Hole>),
    }

    #[derive(Clone, Debug)]
    enum MaybeInst {
        Compiled(Inst),
        Uncompiled(InstHole),
        Split,
        Split1(InstPtr),
        Split2(InstPtr),
    }

    impl MaybeInst {
        fn fill(&mut self, goto: InstPtr) {
            let filled = match *self {
                MaybeInst::Uncompiled(ref inst) => inst.fill(goto),
                MaybeInst::Split1(goto1) => {
                    Inst::Split(InstSplit { goto1: goto1, goto2: goto })
                }
                MaybeInst::Split2(goto2) => {
                    Inst::Split(InstSplit { goto1: goto, goto2: goto2 })
                }
                _ => unreachable!("not all instructions were compiled! \
                                   found uncompiled instruction: {:?}", self),
            };
            *self = MaybeInst::Compiled(filled);
        }

        fn fill_split(&mut self, goto1: InstPtr, goto2: InstPtr) {
            let filled = match *self {
                MaybeInst::Split => {
                    Inst::Split(InstSplit { goto1: goto1, goto2: goto2 })
                }
                _ => unreachable!("must be called on Split instruction, \
                                   instead it was called on: {:?}", self),
            };
            *self = MaybeInst::Compiled(filled);
        }

        fn half_fill_split_goto1(&mut self, goto1: InstPtr) {
            let half_filled = match *self {
                MaybeInst::Split => goto1,
                _ => unreachable!("must be called on Split instruction, \
                                   instead it was called on: {:?}", self),
            };
            *self = MaybeInst::Split1(half_filled);
        }

        fn half_fill_split_goto2(&mut self, goto2: InstPtr) {
            let half_filled = match *self {
                MaybeInst::Split => goto2,
                _ => unreachable!("must be called on Split instruction, \
                                   instead it was called on: {:?}", self),
            };
            *self = MaybeInst::Split2(half_filled);
        }

        fn unwrap(self) -> Inst {
            match self {
                MaybeInst::Compiled(inst) => inst,
                _ => unreachable!("must be called on a compiled instruction, \
                                   instead it was called on: {:?}", self),
            }
        }
    }

    #[derive(Clone, Debug)]
    enum InstHole {
        Save { slot: usize },
        EmptyLook { look: EmptyLook },
        Char { c: char },
        Ranges { ranges: Vec<(char, char)> },
        Bytes { start: u8, end: u8 },
    }

    impl InstHole {
        fn fill(&self, goto: InstPtr) -> Inst {
            match *self {
                InstHole::Save { slot } => Inst::Save(InstSave {
                    goto: goto,
                    slot: slot,
                }),
                InstHole::EmptyLook { look } => Inst::EmptyLook(InstEmptyLook {
                    goto: goto,
                    look: look,
                }),
                InstHole::Char { c } => Inst::Char(InstChar {
                    goto: goto,
                    c: c,
                }),
                InstHole::Ranges { ref ranges } => Inst::Ranges(InstRanges {
                    goto: goto,
                    ranges: ranges.clone(),
                }),
                InstHole::Bytes { start, end } => Inst::Bytes(InstBytes {
                    goto: goto,
                    start: start,
                    end: end,
                }),
            }
        }
    }

    struct CompileClass<'a, 'b> {
        c: &'a mut Compiler,
        ranges: &'b [hir::ClassUnicodeRange],
    }

    impl<'a, 'b> CompileClass<'a, 'b> {
        fn compile(mut self) -> Result {
            let mut holes = vec![];
            let mut initial_entry = None;
            let mut last_split = Hole::None;
            let mut utf8_seqs = self.c.utf8_seqs.take().unwrap();
            self.c.suffix_cache.clear();

            for (i, range) in self.ranges.iter().enumerate() {
                let is_last_range = i + 1 == self.ranges.len();
                utf8_seqs.reset(range.start(), range.end());
                let mut it = (&mut utf8_seqs).peekable();
                loop {
                    let utf8_seq = match it.next() {
                        None => break,
                        Some(utf8_seq) => utf8_seq,
                    };
                    if is_last_range && it.peek().is_none() {
                        let Patch { hole, entry } = self.c_utf8_seq(&utf8_seq)?;
                        holes.push(hole);
                        self.c.fill(last_split, entry);
                        last_split = Hole::None;
                        if initial_entry.is_none() {
                            initial_entry = Some(entry);
                        }
                    } else {
                        if initial_entry.is_none() {
                            initial_entry = Some(self.c.insts.len());
                        }
                        self.c.fill_to_next(last_split);
                        last_split = self.c.push_split_hole();
                        let Patch { hole, entry } = self.c_utf8_seq(&utf8_seq)?;
                        holes.push(hole);
                        last_split = self.c.fill_split(last_split, Some(entry), None);
                    }
                }
            }
            self.c.utf8_seqs = Some(utf8_seqs);
            Ok(Patch {
                hole: Hole::Many(holes),
                entry: initial_entry.unwrap(),
            })
        }

        fn c_utf8_seq(&mut self, seq: &Utf8Sequence) -> Result {
            if self.c.compiled.is_reverse {
                self.c_utf8_seq_(seq)
            } else {
                self.c_utf8_seq_(seq.into_iter().rev())
            }
        }

        fn c_utf8_seq_<'r, I>(&mut self, seq: I) -> Result
                where I: IntoIterator<Item=&'r Utf8Range> {
            // The initial instruction for each UTF-8 sequence should be the same.
            let mut from_inst = ::std::usize::MAX;
            let mut last_hole = Hole::None;
            for byte_range in seq {
                let key = SuffixCacheKey {
                    from_inst: from_inst,
                    start: byte_range.start,
                    end: byte_range.end,
                };
                {
                    let pc = self.c.insts.len();
                    if let Some(cached_pc) = self.c.suffix_cache.get(key, pc) {
                        from_inst = cached_pc;
                        continue;
                    }
                }
                self.c.byte_classes.set_range(byte_range.start, byte_range.end);
                if from_inst == ::std::usize::MAX {
                    last_hole = self.c.push_hole(InstHole::Bytes {
                        start: byte_range.start,
                        end: byte_range.end,
                    });
                } else {
                    self.c.push_compiled(Inst::Bytes(InstBytes {
                        goto: from_inst,
                        start: byte_range.start,
                        end: byte_range.end,
                    }));
                }
                from_inst = self.c.insts.len().checked_sub(1).unwrap();
                debug_assert!(from_inst < ::std::usize::MAX);
            }
            debug_assert!(from_inst < ::std::usize::MAX);
            Ok(Patch { hole: last_hole, entry: from_inst })
        }
    }

    /// `SuffixCache` is a simple bounded hash map for caching suffix entries in
    /// UTF-8 automata. For example, consider the Unicode range \u{0}-\u{FFFF}.
    /// The set of byte ranges looks like this:
    ///
    /// [0-7F]
    /// [C2-DF][80-BF]
    /// [E0][A0-BF][80-BF]
    /// [E1-EC][80-BF][80-BF]
    /// [ED][80-9F][80-BF]
    /// [EE-EF][80-BF][80-BF]
    ///
    /// Each line above translates to one alternate in the compiled regex program.
    /// However, all but one of the alternates end in the same suffix, which is
    /// a waste of an instruction. The suffix cache facilitates reusing them across
    /// alternates.
    ///
    /// Note that a HashMap could be trivially used for this, but we don't need its
    /// overhead. Some small bounded space (LRU style) is more than enough.
    ///
    /// This uses similar idea to [`SparseSet`](../sparse/struct.SparseSet.html),
    /// except it uses hashes as original indices and then compares full keys for
    /// validation against `dense` array.
    struct SuffixCache {
        sparse: Box<[usize]>,
        dense: Vec<SuffixCacheEntry>,
    }

    #[derive(Clone, Copy, Debug, Default, Eq, Hash, PartialEq)]
    struct SuffixCacheEntry {
        key: SuffixCacheKey,
        pc: InstPtr,
    }

    #[derive(Clone, Copy, Debug, Default, Eq, Hash, PartialEq)]
    struct SuffixCacheKey {
        from_inst: InstPtr,
        start: u8,
        end: u8,
    }

    impl SuffixCache {
        fn new(size: usize) -> Self {
            SuffixCache {
                sparse: vec![0usize; size].into(),
                dense: Vec::with_capacity(size),
            }
        }

        fn get(&mut self, key: SuffixCacheKey, pc: InstPtr) -> Option<InstPtr> {
            let hash = self.hash(&key);
            let pos = &mut self.sparse[hash];
            if let Some(entry) = self.dense.get(*pos) {
                if entry.key == key {
                    return Some(entry.pc);
                }
            }
            *pos = self.dense.len();
            self.dense.push(SuffixCacheEntry {
                key: key,
                pc: pc,
            });
            None
        }

        fn clear(&mut self) {
            self.dense.clear();
        }

        fn hash(&self, suffix: &SuffixCacheKey) -> usize {
            // Basic FNV-1a hash as described:
            // https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function
            const FNV_PRIME: u64 = 1099511628211;
            let mut h = 14695981039346656037;
            h = (h ^ (suffix.from_inst as u64)).wrapping_mul(FNV_PRIME);
            h = (h ^ (suffix.start as u64)).wrapping_mul(FNV_PRIME);
            h = (h ^ (suffix.end as u64)).wrapping_mul(FNV_PRIME);
            (h as usize) % self.sparse.len()
        }
    }

    struct ByteClassSet([bool; 256]);

    impl ByteClassSet {
        fn new() -> Self {
            ByteClassSet([false; 256])
        }

        fn set_range(&mut self, start: u8, end: u8) {
            debug_assert!(start <= end);
            if start > 0 {
                self.0[start as usize - 1] = true;
            }
            self.0[end as usize] = true;
        }

        fn set_word_boundary(&mut self) {
            // We need to mark all ranges of bytes whose pairs result in
            // evaluating \b differently.
            let iswb = is_word_byte;
            let mut b1: u16 = 0;
            let mut b2: u16;
            while b1 <= 255 {
                b2 = b1 + 1;
                while b2 <= 255 && iswb(b1 as u8) == iswb(b2 as u8) {
                    b2 += 1;
                }
                self.set_range(b1 as u8, (b2 - 1) as u8);
                b1 = b2;
            }
        }

        fn byte_classes(&self) -> Vec<u8> {
            // N.B. If you're debugging the DFA, it's useful to simply return
            // `(0..256).collect()`, which effectively removes the byte classes
            // and makes the transitions easier to read.
            // (0usize..256).map(|x| x as u8).collect()
            let mut byte_classes = vec![0; 256];
            let mut class = 0u8;
            let mut i = 0;
            loop {
                byte_classes[i] = class as u8;
                if i >= 255 {
                    break;
                }
                if self.0[i] {
                    class = class.checked_add(1).unwrap();
                }
                i += 1;
            }
            byte_classes
        }
    }

    fn u32_to_usize(n: u32) -> usize {
        // In case usize is less than 32 bits, we need to guard against overflow.
        // On most platforms this compiles to nothing.
        // TODO Use `std::convert::TryFrom` once it's stable.
        if (n as u64) > (::std::usize::MAX as u64) {
            panic!("BUG: {} is too big to be pointer sized", n)
        }
        n as usize
    }

    #[cfg(test)]
    mod tests {
        use super::ByteClassSet;

        #[test]
        fn byte_classes() {
            let mut set = ByteClassSet::new();
            set.set_range(b'a', b'z');
            let classes = set.byte_classes();
            assert_eq!(classes[0], 0);
            assert_eq!(classes[1], 0);
            assert_eq!(classes[2], 0);
            assert_eq!(classes[b'a' as usize - 1], 0);
            assert_eq!(classes[b'a' as usize], 1);
            assert_eq!(classes[b'm' as usize], 1);
            assert_eq!(classes[b'z' as usize], 1);
            assert_eq!(classes[b'z' as usize + 1], 2);
            assert_eq!(classes[254], 2);
            assert_eq!(classes[255], 2);

            let mut set = ByteClassSet::new();
            set.set_range(0, 2);
            set.set_range(4, 6);
            let classes = set.byte_classes();
            assert_eq!(classes[0], 0);
            assert_eq!(classes[1], 0);
            assert_eq!(classes[2], 0);
            assert_eq!(classes[3], 1);
            assert_eq!(classes[4], 2);
            assert_eq!(classes[5], 2);
            assert_eq!(classes[6], 2);
            assert_eq!(classes[7], 3);
            assert_eq!(classes[255], 3);
        }

        #[test]
        fn full_byte_classes() {
            let mut set = ByteClassSet::new();
            for i in 0..256u16 {
                set.set_range(i as u8, i as u8);
            }
            assert_eq!(set.byte_classes().len(), 256);
        }
    }

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