regex_automata/nfa/

compiler.rs

// This module provides an NFA compiler using Thompson's construction
// algorithm. The compiler takes a regex-syntax::Hir as input and emits an NFA
// graph as output. The NFA graph is structured in a way that permits it to be
// executed by a virtual machine and also used to efficiently build a DFA.
//
// The compiler deals with a slightly expanded set of NFA states that notably
// includes an empty node that has exactly one epsilon transition to the next
// state. In other words, it's a "goto" instruction if one views Thompson's NFA
// as a set of bytecode instructions. These goto instructions are removed in
// a subsequent phase before returning the NFA to the caller. The purpose of
// these empty nodes is that they make the construction algorithm substantially
// simpler to implement. We remove them before returning to the caller because
// they can represent substantial overhead when traversing the NFA graph
// (either while searching using the NFA directly or while building a DFA).
//
// In the future, it would be nice to provide a Glushkov compiler as well,
// as it would work well as a bit-parallel NFA for smaller regexes. But
// the Thompson construction is one I'm more familiar with and seems more
// straight-forward to deal with when it comes to large Unicode character
// classes.
//
// Internally, the compiler uses interior mutability to improve composition
// in the face of the borrow checker. In particular, we'd really like to be
// able to write things like this:
//
//     self.c_concat(exprs.iter().map(|e| self.c(e)))
//
// Which elegantly uses iterators to build up a sequence of compiled regex
// sub-expressions and then hands it off to the concatenating compiler
// routine. Without interior mutability, the borrow checker won't let us
// borrow `self` mutably both inside and outside the closure at the same
// time.

use std::cell::RefCell;
use std::mem;

use regex_syntax::hir::{self, Hir, HirKind};
use regex_syntax::utf8::{Utf8Range, Utf8Sequences};

use classes::ByteClassSet;
use error::{Error, Result};
use nfa::map::{Utf8BoundedMap, Utf8SuffixKey, Utf8SuffixMap};
use nfa::range_trie::RangeTrie;
use nfa::{State, StateID, Transition, NFA};

/// Config knobs for the NFA compiler. See the builder's methods for more
/// docs on each one.
#[derive(Clone, Copy, Debug)]
struct Config {
    anchored: bool,
    allow_invalid_utf8: bool,
    reverse: bool,
    shrink: bool,
}

impl Default for Config {
    fn default() -> Config {
        Config {
            anchored: false,
            allow_invalid_utf8: false,
            reverse: false,
            shrink: true,
        }
    }
}

/// A builder for compiling an NFA.
#[derive(Clone, Debug)]
pub struct Builder {
    config: Config,
}

impl Builder {
    /// Create a new NFA builder with its default configuration.
    pub fn new() -> Builder {
        Builder { config: Config::default() }
    }

    /// Compile the given high level intermediate representation of a regular
    /// expression into an NFA.
    ///
    /// If there was a problem building the NFA, then an error is returned.
    /// For example, if the regex uses unsupported features (such as zero-width
    /// assertions), then an error is returned.
    pub fn build(&self, expr: &Hir) -> Result<NFA> {
        let mut nfa = NFA::always_match();
        self.build_with(&mut Compiler::new(), &mut nfa, expr)?;
        Ok(nfa)
    }

    /// Compile the given high level intermediate representation of a regular
    /// expression into the NFA given using the given compiler. Callers may
    /// prefer this over `build` if they would like to reuse allocations while
    /// compiling many regular expressions.
    ///
    /// On success, the given NFA is completely overwritten with the NFA
    /// produced by the compiler.
    ///
    /// If there was a problem building the NFA, then an error is returned. For
    /// example, if the regex uses unsupported features (such as zero-width
    /// assertions), then an error is returned. When an error is returned,
    /// the contents of `nfa` are unspecified and should not be relied upon.
    /// However, it can still be reused in subsequent calls to this method.
    pub fn build_with(
        &self,
        compiler: &mut Compiler,
        nfa: &mut NFA,
        expr: &Hir,
    ) -> Result<()> {
        compiler.clear();
        compiler.configure(self.config);
        compiler.compile(nfa, expr)
    }

    /// Set whether matching must be anchored at the beginning of the input.
    ///
    /// When enabled, a match must begin at the start of the input. When
    /// disabled, the NFA will act as if the pattern started with a `.*?`,
    /// which enables a match to appear anywhere.
    ///
    /// By default this is disabled.
    pub fn anchored(&mut self, yes: bool) -> &mut Builder {
        self.config.anchored = yes;
        self
    }

    /// When enabled, the builder will permit the construction of an NFA that
    /// may match invalid UTF-8.
    ///
    /// When disabled (the default), the builder is guaranteed to produce a
    /// regex that will only ever match valid UTF-8 (otherwise, the builder
    /// will return an error).
    pub fn allow_invalid_utf8(&mut self, yes: bool) -> &mut Builder {
        self.config.allow_invalid_utf8 = yes;
        self
    }

    /// Reverse the NFA.
    ///
    /// A NFA reversal is performed by reversing all of the concatenated
    /// sub-expressions in the original pattern, recursively. The resulting
    /// NFA can be used to match the pattern starting from the end of a string
    /// instead of the beginning of a string.
    ///
    /// Reversing the NFA is useful for building a reverse DFA, which is most
    /// useful for finding the start of a match.
    pub fn reverse(&mut self, yes: bool) -> &mut Builder {
        self.config.reverse = yes;
        self
    }

    /// Apply best effort heuristics to shrink the NFA at the expense of more
    /// time/memory.
    ///
    /// This is enabled by default. Generally speaking, if one is using an NFA
    /// to compile DFA, then the extra time used to shrink the NFA will be
    /// more than made up for during DFA construction (potentially by a lot).
    /// In other words, enabling this can substantially decrease the overall
    /// amount of time it takes to build a DFA.
    ///
    /// The only reason to disable this if you want to compile an NFA and start
    /// using it as quickly as possible without needing to build a DFA.
    pub fn shrink(&mut self, yes: bool) -> &mut Builder {
        self.config.shrink = yes;
        self
    }
}

/// A compiler that converts a regex abstract syntax to an NFA via Thompson's
/// construction. Namely, this compiler permits epsilon transitions between
/// states.
///
/// Users of this crate cannot use a compiler directly. Instead, all one can
/// do is create one and use it via the
/// [`Builder::build_with`](struct.Builder.html#method.build_with)
/// method. This permits callers to reuse compilers in order to amortize
/// allocations.
#[derive(Clone, Debug)]
pub struct Compiler {
    /// The set of compiled NFA states. Once a state is compiled, it is
    /// assigned a state ID equivalent to its index in this list. Subsequent
    /// compilation can modify previous states by adding new transitions.
    states: RefCell<Vec<CState>>,
    /// The configuration from the builder.
    config: Config,
    /// State used for compiling character classes to UTF-8 byte automata.
    /// State is not retained between character class compilations. This just
    /// serves to amortize allocation to the extent possible.
    utf8_state: RefCell<Utf8State>,
    /// State used for arranging character classes in reverse into a trie.
    trie_state: RefCell<RangeTrie>,
    /// State used for caching common suffixes when compiling reverse UTF-8
    /// automata (for Unicode character classes).
    utf8_suffix: RefCell<Utf8SuffixMap>,
    /// A map used to re-map state IDs when translating the compiler's internal
    /// NFA state representation to the external NFA representation.
    remap: RefCell<Vec<StateID>>,
    /// A set of compiler internal state IDs that correspond to states that are
    /// exclusively epsilon transitions, i.e., goto instructions, combined with
    /// the state that they point to. This is used to record said states while
    /// transforming the compiler's internal NFA representation to the external
    /// form.
    empties: RefCell<Vec<(StateID, StateID)>>,
}

/// A compiler intermediate state representation for an NFA that is only used
/// during compilation. Once compilation is done, `CState`s are converted to
/// `State`s, which have a much simpler representation.
#[derive(Clone, Debug, Eq, PartialEq)]
enum CState {
    /// An empty state whose only purpose is to forward the automaton to
    /// another state via en epsilon transition. These are useful during
    /// compilation but are otherwise removed at the end.
    Empty { next: StateID },
    /// A state that only transitions to `next` if the current input byte is
    /// in the range `[start, end]` (inclusive on both ends).
    Range { range: Transition },
    /// A state with possibly many transitions, represented in a sparse
    /// fashion. Transitions are ordered lexicographically by input range.
    /// As such, this may only be used when every transition has equal
    /// priority. (In practice, this is only used for encoding large UTF-8
    /// automata.)
    Sparse { ranges: Vec<Transition> },
    /// An alternation such that there exists an epsilon transition to all
    /// states in `alternates`, where matches found via earlier transitions
    /// are preferred over later transitions.
    Union { alternates: Vec<StateID> },
    /// An alternation such that there exists an epsilon transition to all
    /// states in `alternates`, where matches found via later transitions
    /// are preferred over earlier transitions.
    ///
    /// This "reverse" state exists for convenience during compilation that
    /// permits easy construction of non-greedy combinations of NFA states.
    /// At the end of compilation, Union and UnionReverse states are merged
    /// into one Union type of state, where the latter has its epsilon
    /// transitions reversed to reflect the priority inversion.
    UnionReverse { alternates: Vec<StateID> },
    /// A match state. There is exactly one such occurrence of this state in
    /// an NFA.
    Match,
}

/// A value that represents the result of compiling a sub-expression of a
/// regex's HIR. Specifically, this represents a sub-graph of the NFA that
/// has an initial state at `start` and a final state at `end`.
#[derive(Clone, Copy, Debug)]
pub struct ThompsonRef {
    start: StateID,
    end: StateID,
}

impl Compiler {
    /// Create a new compiler.
    pub fn new() -> Compiler {
        Compiler {
            states: RefCell::new(vec![]),
            config: Config::default(),
            utf8_state: RefCell::new(Utf8State::new()),
            trie_state: RefCell::new(RangeTrie::new()),
            utf8_suffix: RefCell::new(Utf8SuffixMap::new(1000)),
            remap: RefCell::new(vec![]),
            empties: RefCell::new(vec![]),
        }
    }

    /// Clear any memory used by this compiler such that it is ready to compile
    /// a new regex.
    ///
    /// It is preferrable to reuse a compiler if possible in order to reuse
    /// allocations.
    fn clear(&self) {
        self.states.borrow_mut().clear();
        // We don't need to clear anything else since they are cleared on
        // their own and only when they are used.
    }

    /// Configure this compiler from the builder's knobs.
    ///
    /// The compiler is always reconfigured by the builder before using it to
    /// build an NFA.
    fn configure(&mut self, config: Config) {
        self.config = config;
    }

    /// Convert the current intermediate NFA to its final compiled form.
    fn compile(&self, nfa: &mut NFA, expr: &Hir) -> Result<()> {
        nfa.anchored = self.config.anchored;

        let mut start = self.add_empty();
        if !nfa.anchored {
            let compiled = if self.config.allow_invalid_utf8 {
                self.c_unanchored_prefix_invalid_utf8()?
            } else {
                self.c_unanchored_prefix_valid_utf8()?
            };
            self.patch(start, compiled.start);
            start = compiled.end;
        }
        let compiled = self.c(&expr)?;
        let match_id = self.add_match();
        self.patch(start, compiled.start);
        self.patch(compiled.end, match_id);
        self.finish(nfa);
        Ok(())
    }

    /// Finishes the compilation process and populates the provide NFA with
    /// the final graph.
    fn finish(&self, nfa: &mut NFA) {
        let mut bstates = self.states.borrow_mut();
        let mut remap = self.remap.borrow_mut();
        remap.resize(bstates.len(), 0);
        let mut empties = self.empties.borrow_mut();
        empties.clear();

        // We don't reuse allocations here becuase this is what we're
        // returning.
        nfa.states.clear();
        let mut byteset = ByteClassSet::new();

        // The idea here is to convert our intermediate states to their final
        // form. The only real complexity here is the process of converting
        // transitions, which are expressed in terms of state IDs. The new
        // set of states will be smaller because of partial epsilon removal,
        // so the state IDs will not be the same.
        for (id, bstate) in bstates.iter_mut().enumerate() {
            match *bstate {
                CState::Empty { next } => {
                    // Since we're removing empty states, we need to handle
                    // them later since we don't yet know which new state this
                    // empty state will be mapped to.
                    empties.push((id, next));
                }
                CState::Range { ref range } => {
                    remap[id] = nfa.states.len();
                    byteset.set_range(range.start, range.end);
                    nfa.states.push(State::Range { range: range.clone() });
                }
                CState::Sparse { ref mut ranges } => {
                    remap[id] = nfa.states.len();

                    let ranges = mem::replace(ranges, vec![]);
                    for r in &ranges {
                        byteset.set_range(r.start, r.end);
                    }
                    nfa.states.push(State::Sparse {
                        ranges: ranges.into_boxed_slice(),
                    });
                }
                CState::Union { ref mut alternates } => {
                    remap[id] = nfa.states.len();

                    let alternates = mem::replace(alternates, vec![]);
                    nfa.states.push(State::Union {
                        alternates: alternates.into_boxed_slice(),
                    });
                }
                CState::UnionReverse { ref mut alternates } => {
                    remap[id] = nfa.states.len();

                    let mut alternates = mem::replace(alternates, vec![]);
                    alternates.reverse();
                    nfa.states.push(State::Union {
                        alternates: alternates.into_boxed_slice(),
                    });
                }
                CState::Match => {
                    remap[id] = nfa.states.len();
                    nfa.states.push(State::Match);
                }
            }
        }
        for &(empty_id, mut empty_next) in empties.iter() {
            // empty states can point to other empty states, forming a chain.
            // So we must follow the chain until the end, which must end at
            // a non-empty state, and therefore, a state that is correctly
            // remapped. We are guaranteed to terminate because our compiler
            // never builds a loop among empty states.
            while let CState::Empty { next } = bstates[empty_next] {
                empty_next = next;
            }
            remap[empty_id] = remap[empty_next];
        }
        for state in &mut nfa.states {
            state.remap(&remap);
        }
        // The compiler always begins the NFA at the first state.
        nfa.start = remap[0];
        nfa.byte_classes = byteset.byte_classes();
    }

    fn c(&self, expr: &Hir) -> Result<ThompsonRef> {
        match *expr.kind() {
            HirKind::Empty => {
                let id = self.add_empty();
                Ok(ThompsonRef { start: id, end: id })
            }
            HirKind::Literal(hir::Literal::Unicode(ch)) => {
                let mut buf = [0; 4];
                let it = ch
                    .encode_utf8(&mut buf)
                    .as_bytes()
                    .iter()
                    .map(|&b| Ok(self.c_range(b, b)));
                self.c_concat(it)
            }
            HirKind::Literal(hir::Literal::Byte(b)) => Ok(self.c_range(b, b)),
            HirKind::Class(hir::Class::Bytes(ref cls)) => {
                self.c_byte_class(cls)
            }
            HirKind::Class(hir::Class::Unicode(ref cls)) => {
                self.c_unicode_class(cls)
            }
            HirKind::Repetition(ref rep) => self.c_repetition(rep),
            HirKind::Group(ref group) => self.c(&*group.hir),
            HirKind::Concat(ref exprs) => {
                self.c_concat(exprs.iter().map(|e| self.c(e)))
            }
            HirKind::Alternation(ref exprs) => {
                self.c_alternation(exprs.iter().map(|e| self.c(e)))
            }
            HirKind::Anchor(_) => Err(Error::unsupported_anchor()),
            HirKind::WordBoundary(_) => Err(Error::unsupported_word()),
        }
    }

    fn c_concat<I>(&self, mut it: I) -> Result<ThompsonRef>
    where
        I: DoubleEndedIterator<Item = Result<ThompsonRef>>,
    {
        let first =
            if self.config.reverse { it.next_back() } else { it.next() };
        let ThompsonRef { start, mut end } = match first {
            Some(result) => result?,
            None => return Ok(self.c_empty()),
        };
        loop {
            let next =
                if self.config.reverse { it.next_back() } else { it.next() };
            let compiled = match next {
                Some(result) => result?,
                None => break,
            };
            self.patch(end, compiled.start);
            end = compiled.end;
        }
        Ok(ThompsonRef { start, end })
    }

    fn c_alternation<I>(&self, mut it: I) -> Result<ThompsonRef>
    where
        I: Iterator<Item = Result<ThompsonRef>>,
    {
        let first = it.next().expect("alternations must be non-empty")?;
        let second = match it.next() {
            None => return Ok(first),
            Some(result) => result?,
        };

        let union = self.add_union();
        let end = self.add_empty();
        self.patch(union, first.start);
        self.patch(first.end, end);
        self.patch(union, second.start);
        self.patch(second.end, end);
        for result in it {
            let compiled = result?;
            self.patch(union, compiled.start);
            self.patch(compiled.end, end);
        }
        Ok(ThompsonRef { start: union, end })
    }

    fn c_repetition(&self, rep: &hir::Repetition) -> Result<ThompsonRef> {
        match rep.kind {
            hir::RepetitionKind::ZeroOrOne => {
                self.c_zero_or_one(&rep.hir, rep.greedy)
            }
            hir::RepetitionKind::ZeroOrMore => {
                self.c_at_least(&rep.hir, rep.greedy, 0)
            }
            hir::RepetitionKind::OneOrMore => {
                self.c_at_least(&rep.hir, rep.greedy, 1)
            }
            hir::RepetitionKind::Range(ref rng) => match *rng {
                hir::RepetitionRange::Exactly(count) => {
                    self.c_exactly(&rep.hir, count)
                }
                hir::RepetitionRange::AtLeast(m) => {
                    self.c_at_least(&rep.hir, rep.greedy, m)
                }
                hir::RepetitionRange::Bounded(min, max) => {
                    self.c_bounded(&rep.hir, rep.greedy, min, max)
                }
            },
        }
    }

    fn c_bounded(
        &self,
        expr: &Hir,
        greedy: bool,
        min: u32,
        max: u32,
    ) -> Result<ThompsonRef> {
        let prefix = self.c_exactly(expr, min)?;
        if min == max {
            return Ok(prefix);
        }

        // It is tempting here to compile the rest here as a concatenation
        // of zero-or-one matches. i.e., for `a{2,5}`, compile it as if it
        // were `aaa?a?a?`. The problem here is that it leads to this program:
        //
        //     >000000: 61 => 01
        //     000001: 61 => 02
        //     000002: alt(03, 04)
        //     000003: 61 => 04
        //     000004: alt(05, 06)
        //     000005: 61 => 06
        //     000006: alt(07, 08)
        //     000007: 61 => 08
        //     000008: MATCH
        //
        // And effectively, once you hit state 2, the epsilon closure will
        // include states 3, 5, 5, 6, 7 and 8, which is quite a bit. It is
        // better to instead compile it like so:
        //
        //     >000000: 61 => 01
        //      000001: 61 => 02
        //      000002: alt(03, 08)
        //      000003: 61 => 04
        //      000004: alt(05, 08)
        //      000005: 61 => 06
        //      000006: alt(07, 08)
        //      000007: 61 => 08
        //      000008: MATCH
        //
        // So that the epsilon closure of state 2 is now just 3 and 8.
        let empty = self.add_empty();
        let mut prev_end = prefix.end;
        for _ in min..max {
            let union = if greedy {
                self.add_union()
            } else {
                self.add_reverse_union()
            };
            let compiled = self.c(expr)?;
            self.patch(prev_end, union);
            self.patch(union, compiled.start);
            self.patch(union, empty);
            prev_end = compiled.end;
        }
        self.patch(prev_end, empty);
        Ok(ThompsonRef { start: prefix.start, end: empty })
    }

    fn c_at_least(
        &self,
        expr: &Hir,
        greedy: bool,
        n: u32,
    ) -> Result<ThompsonRef> {
        if n == 0 {
            let union = if greedy {
                self.add_union()
            } else {
                self.add_reverse_union()
            };
            let compiled = self.c(expr)?;
            self.patch(union, compiled.start);
            self.patch(compiled.end, union);
            Ok(ThompsonRef { start: union, end: union })
        } else if n == 1 {
            let compiled = self.c(expr)?;
            let union = if greedy {
                self.add_union()
            } else {
                self.add_reverse_union()
            };
            self.patch(compiled.end, union);
            self.patch(union, compiled.start);
            Ok(ThompsonRef { start: compiled.start, end: union })
        } else {
            let prefix = self.c_exactly(expr, n - 1)?;
            let last = self.c(expr)?;
            let union = if greedy {
                self.add_union()
            } else {
                self.add_reverse_union()
            };
            self.patch(prefix.end, last.start);
            self.patch(last.end, union);
            self.patch(union, last.start);
            Ok(ThompsonRef { start: prefix.start, end: union })
        }
    }

    fn c_zero_or_one(&self, expr: &Hir, greedy: bool) -> Result<ThompsonRef> {
        let union =
            if greedy { self.add_union() } else { self.add_reverse_union() };
        let compiled = self.c(expr)?;
        let empty = self.add_empty();
        self.patch(union, compiled.start);
        self.patch(union, empty);
        self.patch(compiled.end, empty);
        Ok(ThompsonRef { start: union, end: empty })
    }

    fn c_exactly(&self, expr: &Hir, n: u32) -> Result<ThompsonRef> {
        let it = (0..n).map(|_| self.c(expr));
        self.c_concat(it)
    }

    fn c_byte_class(&self, cls: &hir::ClassBytes) -> Result<ThompsonRef> {
        let end = self.add_empty();
        let mut trans = Vec::with_capacity(cls.ranges().len());
        for r in cls.iter() {
            trans.push(Transition {
                start: r.start(),
                end: r.end(),
                next: end,
            });
        }
        Ok(ThompsonRef { start: self.add_sparse(trans), end })
    }

    fn c_unicode_class(&self, cls: &hir::ClassUnicode) -> Result<ThompsonRef> {
        // If all we have are ASCII ranges wrapped in a Unicode package, then
        // there is zero reason to bring out the big guns. We can fit all ASCII
        // ranges within a single sparse transition.
        if cls.is_all_ascii() {
            let end = self.add_empty();
            let mut trans = Vec::with_capacity(cls.ranges().len());
            for r in cls.iter() {
                assert!(r.start() <= '\x7F');
                assert!(r.end() <= '\x7F');
                trans.push(Transition {
                    start: r.start() as u8,
                    end: r.end() as u8,
                    next: end,
                });
            }
            Ok(ThompsonRef { start: self.add_sparse(trans), end })
        } else if self.config.reverse {
            if !self.config.shrink {
                // When we don't want to spend the extra time shrinking, we
                // compile the UTF-8 automaton in reverse using something like
                // the "naive" approach, but will attempt to re-use common
                // suffixes.
                self.c_unicode_class_reverse_with_suffix(cls)
            } else {
                // When we want to shrink our NFA for reverse UTF-8 automata,
                // we cannot feed UTF-8 sequences directly to the UTF-8
                // compiler, since the UTF-8 compiler requires all sequences
                // to be lexicographically sorted. Instead, we organize our
                // sequences into a range trie, which can then output our
                // sequences in the correct order. Unfortunately, building the
                // range trie is fairly expensive (but not nearly as expensive
                // as building a DFA). Hence the reason why the 'shrink' option
                // exists, so that this path can be toggled off.
                let mut trie = self.trie_state.borrow_mut();
                trie.clear();

                for rng in cls.iter() {
                    for mut seq in Utf8Sequences::new(rng.start(), rng.end()) {
                        seq.reverse();
                        trie.insert(seq.as_slice());
                    }
                }
                let mut utf8_state = self.utf8_state.borrow_mut();
                let mut utf8c = Utf8Compiler::new(self, &mut *utf8_state);
                trie.iter(|seq| {
                    utf8c.add(&seq);
                });
                Ok(utf8c.finish())
            }
        } else {
            // In the forward direction, we always shrink our UTF-8 automata
            // because we can stream it right into the UTF-8 compiler. There
            // is almost no downside (in either memory or time) to using this
            // approach.
            let mut utf8_state = self.utf8_state.borrow_mut();
            let mut utf8c = Utf8Compiler::new(self, &mut *utf8_state);
            for rng in cls.iter() {
                for seq in Utf8Sequences::new(rng.start(), rng.end()) {
                    utf8c.add(seq.as_slice());
                }
            }
            Ok(utf8c.finish())
        }

        // For reference, the code below is the "naive" version of compiling a
        // UTF-8 automaton. It is deliciously simple (and works for both the
        // forward and reverse cases), but will unfortunately produce very
        // large NFAs. When compiling a forward automaton, the size difference
        // can sometimes be an order of magnitude. For example, the '\w' regex
        // will generate about ~3000 NFA states using the naive approach below,
        // but only 283 states when using the approach above. This is because
        // the approach above actually compiles a *minimal* (or near minimal,
        // because of the bounded hashmap) UTF-8 automaton.
        //
        // The code below is kept as a reference point in order to make it
        // easier to understand the higher level goal here.
        /*
        let it = cls
            .iter()
            .flat_map(|rng| Utf8Sequences::new(rng.start(), rng.end()))
            .map(|seq| {
                let it = seq
                    .as_slice()
                    .iter()
                    .map(|rng| Ok(self.c_range(rng.start, rng.end)));
                self.c_concat(it)
            });
        self.c_alternation(it);
        */
    }

    fn c_unicode_class_reverse_with_suffix(
        &self,
        cls: &hir::ClassUnicode,
    ) -> Result<ThompsonRef> {
        // N.B. It would likely be better to cache common *prefixes* in the
        // reverse direction, but it's not quite clear how to do that. The
        // advantage of caching suffixes is that it does give us a win, and
        // has a very small additional overhead.
        let mut cache = self.utf8_suffix.borrow_mut();
        cache.clear();

        let union = self.add_union();
        let alt_end = self.add_empty();
        for urng in cls.iter() {
            for seq in Utf8Sequences::new(urng.start(), urng.end()) {
                let mut end = alt_end;
                for brng in seq.as_slice() {
                    let key = Utf8SuffixKey {
                        from: end,
                        start: brng.start,
                        end: brng.end,
                    };
                    let hash = cache.hash(&key);
                    if let Some(id) = cache.get(&key, hash) {
                        end = id;
                        continue;
                    }

                    let compiled = self.c_range(brng.start, brng.end);
                    self.patch(compiled.end, end);
                    end = compiled.start;
                    cache.set(key, hash, end);
                }
                self.patch(union, end);
            }
        }
        Ok(ThompsonRef { start: union, end: alt_end })
    }

    fn c_range(&self, start: u8, end: u8) -> ThompsonRef {
        let id = self.add_range(start, end);
        ThompsonRef { start: id, end: id }
    }

    fn c_empty(&self) -> ThompsonRef {
        let id = self.add_empty();
        ThompsonRef { start: id, end: id }
    }

    fn c_unanchored_prefix_valid_utf8(&self) -> Result<ThompsonRef> {
        self.c(&Hir::repetition(hir::Repetition {
            kind: hir::RepetitionKind::ZeroOrMore,
            greedy: false,
            hir: Box::new(Hir::any(false)),
        }))
    }

    fn c_unanchored_prefix_invalid_utf8(&self) -> Result<ThompsonRef> {
        self.c(&Hir::repetition(hir::Repetition {
            kind: hir::RepetitionKind::ZeroOrMore,
            greedy: false,
            hir: Box::new(Hir::any(true)),
        }))
    }

    fn patch(&self, from: StateID, to: StateID) {
        match self.states.borrow_mut()[from] {
            CState::Empty { ref mut next } => {
                *next = to;
            }
            CState::Range { ref mut range } => {
                range.next = to;
            }
            CState::Sparse { .. } => {
                panic!("cannot patch from a sparse NFA state")
            }
            CState::Union { ref mut alternates } => {
                alternates.push(to);
            }
            CState::UnionReverse { ref mut alternates } => {
                alternates.push(to);
            }
            CState::Match => {}
        }
    }

    fn add_empty(&self) -> StateID {
        let id = self.states.borrow().len();
        self.states.borrow_mut().push(CState::Empty { next: 0 });
        id
    }

    fn add_range(&self, start: u8, end: u8) -> StateID {
        let id = self.states.borrow().len();
        let trans = Transition { start, end, next: 0 };
        let state = CState::Range { range: trans };
        self.states.borrow_mut().push(state);
        id
    }

    fn add_sparse(&self, ranges: Vec<Transition>) -> StateID {
        if ranges.len() == 1 {
            let id = self.states.borrow().len();
            let state = CState::Range { range: ranges[0] };
            self.states.borrow_mut().push(state);
            return id;
        }
        let id = self.states.borrow().len();
        let state = CState::Sparse { ranges };
        self.states.borrow_mut().push(state);
        id
    }

    fn add_union(&self) -> StateID {
        let id = self.states.borrow().len();
        let state = CState::Union { alternates: vec![] };
        self.states.borrow_mut().push(state);
        id
    }

    fn add_reverse_union(&self) -> StateID {
        let id = self.states.borrow().len();
        let state = CState::UnionReverse { alternates: vec![] };
        self.states.borrow_mut().push(state);
        id
    }

    fn add_match(&self) -> StateID {
        let id = self.states.borrow().len();
        self.states.borrow_mut().push(CState::Match);
        id
    }
}

#[derive(Debug)]
struct Utf8Compiler<'a> {
    nfac: &'a Compiler,
    state: &'a mut Utf8State,
    target: StateID,
}

#[derive(Clone, Debug)]
struct Utf8State {
    compiled: Utf8BoundedMap,
    uncompiled: Vec<Utf8Node>,
}

#[derive(Clone, Debug)]
struct Utf8Node {
    trans: Vec<Transition>,
    last: Option<Utf8LastTransition>,
}

#[derive(Clone, Debug)]
struct Utf8LastTransition {
    start: u8,
    end: u8,
}

impl Utf8State {
    fn new() -> Utf8State {
        Utf8State { compiled: Utf8BoundedMap::new(5000), uncompiled: vec![] }
    }

    fn clear(&mut self) {
        self.compiled.clear();
        self.uncompiled.clear();
    }
}

impl<'a> Utf8Compiler<'a> {
    fn new(nfac: &'a Compiler, state: &'a mut Utf8State) -> Utf8Compiler<'a> {
        let target = nfac.add_empty();
        state.clear();
        let mut utf8c = Utf8Compiler { nfac, state, target };
        utf8c.add_empty();
        utf8c
    }

    fn finish(&mut self) -> ThompsonRef {
        self.compile_from(0);
        let node = self.pop_root();
        let start = self.compile(node);
        ThompsonRef { start, end: self.target }
    }

    fn add(&mut self, ranges: &[Utf8Range]) {
        let prefix_len = ranges
            .iter()
            .zip(&self.state.uncompiled)
            .take_while(|&(range, node)| {
                node.last.as_ref().map_or(false, |t| {
                    (t.start, t.end) == (range.start, range.end)
                })
            })
            .count();
        assert!(prefix_len < ranges.len());
        self.compile_from(prefix_len);
        self.add_suffix(&ranges[prefix_len..]);
    }

    fn compile_from(&mut self, from: usize) {
        let mut next = self.target;
        while from + 1 < self.state.uncompiled.len() {
            let node = self.pop_freeze(next);
            next = self.compile(node);
        }
        self.top_last_freeze(next);
    }

    fn compile(&mut self, node: Vec<Transition>) -> StateID {
        let hash = self.state.compiled.hash(&node);
        if let Some(id) = self.state.compiled.get(&node, hash) {
            return id;
        }
        let id = self.nfac.add_sparse(node.clone());
        self.state.compiled.set(node, hash, id);
        id
    }

    fn add_suffix(&mut self, ranges: &[Utf8Range]) {
        assert!(!ranges.is_empty());
        let last = self
            .state
            .uncompiled
            .len()
            .checked_sub(1)
            .expect("non-empty nodes");
        assert!(self.state.uncompiled[last].last.is_none());
        self.state.uncompiled[last].last = Some(Utf8LastTransition {
            start: ranges[0].start,
            end: ranges[0].end,
        });
        for r in &ranges[1..] {
            self.state.uncompiled.push(Utf8Node {
                trans: vec![],
                last: Some(Utf8LastTransition { start: r.start, end: r.end }),
            });
        }
    }

    fn add_empty(&mut self) {
        self.state.uncompiled.push(Utf8Node { trans: vec![], last: None });
    }

    fn pop_freeze(&mut self, next: StateID) -> Vec<Transition> {
        let mut uncompiled = self.state.uncompiled.pop().unwrap();
        uncompiled.set_last_transition(next);
        uncompiled.trans
    }

    fn pop_root(&mut self) -> Vec<Transition> {
        assert_eq!(self.state.uncompiled.len(), 1);
        assert!(self.state.uncompiled[0].last.is_none());
        self.state.uncompiled.pop().expect("non-empty nodes").trans
    }

    fn top_last_freeze(&mut self, next: StateID) {
        let last = self
            .state
            .uncompiled
            .len()
            .checked_sub(1)
            .expect("non-empty nodes");
        self.state.uncompiled[last].set_last_transition(next);
    }
}

impl Utf8Node {
    fn set_last_transition(&mut self, next: StateID) {
        if let Some(last) = self.last.take() {
            self.trans.push(Transition {
                start: last.start,
                end: last.end,
                next,
            });
        }
    }
}

#[cfg(test)]
mod tests {
    use regex_syntax::hir::Hir;
    use regex_syntax::ParserBuilder;

    use super::{Builder, State, StateID, Transition, NFA};

    fn parse(pattern: &str) -> Hir {
        ParserBuilder::new().build().parse(pattern).unwrap()
    }

    fn build(pattern: &str) -> NFA {
        Builder::new().anchored(true).build(&parse(pattern)).unwrap()
    }

    fn s_byte(byte: u8, next: StateID) -> State {
        let trans = Transition { start: byte, end: byte, next };
        State::Range { range: trans }
    }

    fn s_range(start: u8, end: u8, next: StateID) -> State {
        let trans = Transition { start, end, next };
        State::Range { range: trans }
    }

    fn s_sparse(ranges: &[(u8, u8, StateID)]) -> State {
        let ranges = ranges
            .iter()
            .map(|&(start, end, next)| Transition { start, end, next })
            .collect();
        State::Sparse { ranges }
    }

    fn s_union(alts: &[StateID]) -> State {
        State::Union { alternates: alts.to_vec().into_boxed_slice() }
    }

    fn s_match() -> State {
        State::Match
    }

    #[test]
    fn errors() {
        // unsupported anchors
        assert!(Builder::new().build(&parse(r"^")).is_err());
        assert!(Builder::new().build(&parse(r"$")).is_err());
        assert!(Builder::new().build(&parse(r"\A")).is_err());
        assert!(Builder::new().build(&parse(r"\z")).is_err());

        // unsupported word boundaries
        assert!(Builder::new().build(&parse(r"\b")).is_err());
        assert!(Builder::new().build(&parse(r"\B")).is_err());
        assert!(Builder::new().build(&parse(r"(?-u)\b")).is_err());
    }

    // Test that building an unanchored NFA has an appropriate `.*?` prefix.
    #[test]
    fn compile_unanchored_prefix() {
        // When the machine can only match valid UTF-8.
        let nfa = Builder::new().anchored(false).build(&parse(r"a")).unwrap();
        // There should be many states since the `.` in `.*?` matches any
        // Unicode scalar value.
        assert_eq!(11, nfa.len());
        assert_eq!(nfa.states[10], s_match());
        assert_eq!(nfa.states[9], s_byte(b'a', 10));

        // When the machine can match invalid UTF-8.
        let nfa = Builder::new()
            .anchored(false)
            .allow_invalid_utf8(true)
            .build(&parse(r"a"))
            .unwrap();
        assert_eq!(
            nfa.states,
            &[
                s_union(&[2, 1]),
                s_range(0, 255, 0),
                s_byte(b'a', 3),
                s_match(),
            ]
        );
    }

    #[test]
    fn compile_empty() {
        assert_eq!(build("").states, &[s_match(),]);
    }

    #[test]
    fn compile_literal() {
        assert_eq!(build("a").states, &[s_byte(b'a', 1), s_match(),]);
        assert_eq!(
            build("ab").states,
            &[s_byte(b'a', 1), s_byte(b'b', 2), s_match(),]
        );
        assert_eq!(
            build("☃").states,
            &[s_byte(0xE2, 1), s_byte(0x98, 2), s_byte(0x83, 3), s_match(),]
        );

        // Check that non-UTF-8 literals work.
        let hir = ParserBuilder::new()
            .allow_invalid_utf8(true)
            .build()
            .parse(r"(?-u)\xFF")
            .unwrap();
        let nfa = Builder::new()
            .anchored(true)
            .allow_invalid_utf8(true)
            .build(&hir)
            .unwrap();
        assert_eq!(nfa.states, &[s_byte(b'\xFF', 1), s_match(),]);
    }

    #[test]
    fn compile_class() {
        assert_eq!(
            build(r"[a-z]").states,
            &[s_range(b'a', b'z', 1), s_match(),]
        );
        assert_eq!(
            build(r"[x-za-c]").states,
            &[s_sparse(&[(b'a', b'c', 1), (b'x', b'z', 1)]), s_match()]
        );
        assert_eq!(
            build(r"[\u03B1-\u03B4]").states,
            &[s_range(0xB1, 0xB4, 2), s_byte(0xCE, 0), s_match()]
        );
        assert_eq!(
            build(r"[\u03B1-\u03B4\u{1F919}-\u{1F91E}]").states,
            &[
                s_range(0xB1, 0xB4, 5),
                s_range(0x99, 0x9E, 5),
                s_byte(0xA4, 1),
                s_byte(0x9F, 2),
                s_sparse(&[(0xCE, 0xCE, 0), (0xF0, 0xF0, 3)]),
                s_match(),
            ]
        );
        assert_eq!(
            build(r"[a-z☃]").states,
            &[
                s_byte(0x83, 3),
                s_byte(0x98, 0),
                s_sparse(&[(b'a', b'z', 3), (0xE2, 0xE2, 1)]),
                s_match(),
            ]
        );
    }

    #[test]
    fn compile_repetition() {
        assert_eq!(
            build(r"a?").states,
            &[s_union(&[1, 2]), s_byte(b'a', 2), s_match(),]
        );
        assert_eq!(
            build(r"a??").states,
            &[s_union(&[2, 1]), s_byte(b'a', 2), s_match(),]
        );
    }

    #[test]
    fn compile_group() {
        assert_eq!(
            build(r"ab+").states,
            &[s_byte(b'a', 1), s_byte(b'b', 2), s_union(&[1, 3]), s_match(),]
        );
        assert_eq!(
            build(r"(ab)").states,
            &[s_byte(b'a', 1), s_byte(b'b', 2), s_match(),]
        );
        assert_eq!(
            build(r"(ab)+").states,
            &[s_byte(b'a', 1), s_byte(b'b', 2), s_union(&[0, 3]), s_match(),]
        );
    }

    #[test]
    fn compile_alternation() {
        assert_eq!(
            build(r"a|b").states,
            &[s_byte(b'a', 3), s_byte(b'b', 3), s_union(&[0, 1]), s_match(),]
        );
        assert_eq!(
            build(r"|b").states,
            &[s_byte(b'b', 2), s_union(&[2, 0]), s_match(),]
        );
        assert_eq!(
            build(r"a|").states,
            &[s_byte(b'a', 2), s_union(&[0, 2]), s_match(),]
        );
    }
}