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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.

    use std::mem;

    use aho_corasick::{Automaton, AcAutomaton, FullAcAutomaton};
    use memchr::{memchr, memchr2, memchr3};
    use syntax;

    use freqs::BYTE_FREQUENCIES;

    /// A prefix extracted from a compiled regular expression.
    ///
    /// A regex prefix is a set of literal strings that *must* be matched at the
    /// beginning of a regex in order for the entire regex to match.
    ///
    /// There are a variety of ways to efficiently scan the search text for a
    /// prefix. Currently, there are three implemented:
    ///
    /// 1. The prefix is a single byte. Just use memchr.
    /// 2. If the prefix is a set of two or more single byte prefixes, then
    ///    a single sparse map is created. Checking if there is a match is a lookup
    ///    in this map for each byte in the search text.
    /// 3. In all other cases, build an Aho-Corasick automaton.
    ///
    /// It's possible that there's room here for other substring algorithms,
    /// such as Boyer-Moore for single-set prefixes greater than 1, or Rabin-Karp
    /// for small sets of same-length prefixes.
    #[derive(Clone, Debug)]
    pub struct LiteralSearcher {
        complete: bool,
        lcp: SingleSearch,
        lcs: SingleSearch,
        matcher: Matcher,
    }

    #[derive(Clone, Debug)]
    enum Matcher {
        /// No literals. (Never advances through the input.)
        Empty,
        /// A set of four or more single byte literals.
        Bytes(SingleByteSet),
        /// A single substring. (Likely using Boyer-Moore with memchr.)
        Single(SingleSearch),
        /// An Aho-Corasick automaton.
        AC(FullAcAutomaton<syntax::Lit>),
    }

    impl LiteralSearcher {
        /// Returns a matcher that never matches and never advances the input.
        pub fn empty() -> Self {
            Self::new(syntax::Literals::empty(), Matcher::Empty)
        }

        /// Returns a matcher for literal prefixes from the given set.
        pub fn prefixes(lits: syntax::Literals) -> Self {
            let matcher = Matcher::prefixes(&lits);
            Self::new(lits, matcher)
        }

        /// Returns a matcher for literal suffixes from the given set.
        pub fn suffixes(lits: syntax::Literals) -> Self {
            let matcher = Matcher::suffixes(&lits);
            Self::new(lits, matcher)
        }

        fn new(lits: syntax::Literals, matcher: Matcher) -> Self {
            let complete = lits.all_complete();
            LiteralSearcher {
                complete: complete,
                lcp: SingleSearch::new(lits.longest_common_prefix().to_vec()),
                lcs: SingleSearch::new(lits.longest_common_suffix().to_vec()),
                matcher: matcher,
            }
        }

        /// Returns true if all matches comprise the entire regular expression.
        ///
        /// This does not necessarily mean that a literal match implies a match
        /// of the regular expression. For example, the regular expresison `^a`
        /// is comprised of a single complete literal `a`, but the regular
        /// expression demands that it only match at the beginning of a string.
        pub fn complete(&self) -> bool {
            self.complete && self.len() > 0
        }

        /// Find the position of a literal in `haystack` if it exists.
        #[inline(always)] // reduces constant overhead
        pub fn find(&self, haystack: &[u8]) -> Option<(usize, usize)> {
            use self::Matcher::*;
            match self.matcher {
                Empty => Some((0, 0)),
                Bytes(ref sset) => sset.find(haystack).map(|i| (i, i + 1)),
                Single(ref s) => s.find(haystack).map(|i| (i, i + s.len())),
                AC(ref aut) => aut.find(haystack).next().map(|m| (m.start, m.end)),
            }
        }

        /// Like find, except matches must start at index `0`.
        pub fn find_start(&self, haystack: &[u8]) -> Option<(usize, usize)> {
            for lit in self.iter() {
                if lit.len() > haystack.len() {
                    continue;
                }
                if lit == &haystack[0..lit.len()] {
                    return Some((0, lit.len()));
                }
            }
            None
        }

        /// Like find, except matches must end at index `haystack.len()`.
        pub fn find_end(&self, haystack: &[u8]) -> Option<(usize, usize)> {
            for lit in self.iter() {
                if lit.len() > haystack.len() {
                    continue;
                }
                if lit == &haystack[haystack.len() - lit.len()..] {
                    return Some((haystack.len() - lit.len(), haystack.len()));
                }
            }
            None
        }

        /// Returns an iterator over all literals to be matched.
        pub fn iter(&self) -> LiteralIter {
            match self.matcher {
                Matcher::Empty => LiteralIter::Empty,
                Matcher::Bytes(ref sset) => LiteralIter::Bytes(&sset.dense),
                Matcher::Single(ref s) => LiteralIter::Single(&s.pat),
                Matcher::AC(ref ac) => LiteralIter::AC(ac.patterns()),
            }
        }

        /// Returns a matcher for the longest common prefix of this matcher.
        pub fn lcp(&self) -> &SingleSearch {
            &self.lcp
        }

        /// Returns a matcher for the longest common suffix of this matcher.
        pub fn lcs(&self) -> &SingleSearch {
            &self.lcs
        }

        /// Returns true iff this prefix is empty.
        pub fn is_empty(&self) -> bool {
            self.len() == 0
        }

        /// Returns the number of prefixes in this machine.
        pub fn len(&self) -> usize {
            use self::Matcher::*;
            match self.matcher {
                Empty => 0,
                Bytes(ref sset) => sset.dense.len(),
                Single(_) => 1,
                AC(ref aut) => aut.len(),
            }
        }

        /// Return the approximate heap usage of literals in bytes.
        pub fn approximate_size(&self) -> usize {
            use self::Matcher::*;
            match self.matcher {
                Empty => 0,
                Bytes(ref sset) => sset.approximate_size(),
                Single(ref single) => single.approximate_size(),
                AC(ref aut) => aut.heap_bytes(),
            }
        }
    }

    impl Matcher {
        fn prefixes(lits: &syntax::Literals) -> Self {
            let sset = SingleByteSet::prefixes(&lits);
            Matcher::new(lits, sset)
        }

        fn suffixes(lits: &syntax::Literals) -> Self {
            let sset = SingleByteSet::suffixes(&lits);
            Matcher::new(lits, sset)
        }

        fn new(lits: &syntax::Literals, sset: SingleByteSet) -> Self {
            if lits.literals().is_empty() {
                Matcher::Empty
            } else if sset.dense.len() >= 26 {
                // Avoid trying to match a large number of single bytes.
                // This is *very* sensitive to a frequency analysis comparison
                // between the bytes in sset and the composition of the haystack.
                // No matter the size of sset, if its members all are rare in the
                // haystack, then it'd be worth using it. How to tune this... IDK.
                // ---AG
                Matcher::Empty
            } else if sset.complete {
                Matcher::Bytes(sset)
            } else if lits.literals().len() == 1 {
                Matcher::Single(SingleSearch::new(lits.literals()[0].to_vec()))
            } else {
                let pats = lits.literals().to_owned();
                Matcher::AC(AcAutomaton::new(pats).into_full())
            }
        }
    }

    pub enum LiteralIter<'a> {
        Empty,
        Bytes(&'a [u8]),
        Single(&'a [u8]),
        AC(&'a [syntax::Lit]),
    }

    impl<'a> Iterator for LiteralIter<'a> {
        type Item = &'a [u8];

        fn next(&mut self) -> Option<Self::Item> {
            match *self {
                LiteralIter::Empty => None,
                LiteralIter::Bytes(ref mut many) => {
                    if many.is_empty() {
                        None
                    } else {
                        let next = &many[0..1];
                        *many = &many[1..];
                        Some(next)
                    }
                }
                LiteralIter::Single(ref mut one) => {
                    if one.is_empty() {
                        None
                    } else {
                        let next = &one[..];
                        *one = &[];
                        Some(next)
                    }
                }
                LiteralIter::AC(ref mut lits) => {
                    if lits.is_empty() {
                        None
                    } else {
                        let next = &lits[0];
                        *lits = &lits[1..];
                        Some(&**next)
                    }
                }
            }
        }
    }

    #[derive(Clone, Debug)]
    struct SingleByteSet {
        sparse: Vec<bool>,
        dense: Vec<u8>,
        complete: bool,
    }

    impl SingleByteSet {
        fn new() -> SingleByteSet {
            SingleByteSet {
                sparse: vec![false; 256],
                dense: vec![],
                complete: true,
            }
        }

        fn prefixes(lits: &syntax::Literals) -> SingleByteSet {
            let mut sset = SingleByteSet::new();
            for lit in lits.literals() {
                sset.complete = sset.complete && lit.len() == 1;
                if let Some(&b) = lit.get(0) {
                    if !sset.sparse[b as usize] {
                        sset.dense.push(b);
                        sset.sparse[b as usize] = true;
                    }
                }
            }
            sset
        }

        fn suffixes(lits: &syntax::Literals) -> SingleByteSet {
            let mut sset = SingleByteSet::new();
            for lit in lits.literals() {
                sset.complete = sset.complete && lit.len() == 1;
                if let Some(&b) = lit.get(lit.len().checked_sub(1).unwrap()) {
                    if !sset.sparse[b as usize] {
                        sset.dense.push(b);
                        sset.sparse[b as usize] = true;
                    }
                }
            }
            sset
        }

        /// Faster find that special cases certain sizes to use memchr.
        #[inline(always)] // reduces constant overhead
        fn find(&self, text: &[u8]) -> Option<usize> {
            match self.dense.len() {
                0 => None,
                1 => memchr(self.dense[0], text),
                2 => memchr2(self.dense[0], self.dense[1], text),
                3 => memchr3(self.dense[0], self.dense[1], self.dense[2], text),
                _ => self._find(text),
            }
        }

        /// Generic find that works on any sized set.
        fn _find(&self, haystack: &[u8]) -> Option<usize> {
            for (i, &b) in haystack.iter().enumerate() {
                if self.sparse[b as usize] {
                    return Some(i);
                }
            }
            None
        }

        fn approximate_size(&self) -> usize {
            (self.dense.len() * mem::size_of::<u8>())
            + (self.sparse.len() * mem::size_of::<bool>())
        }
    }

    /// Provides an implementation of fast subtring search.
    ///
    /// This particular implementation is a Boyer-Moore variant, based on the
    /// "tuned boyer moore" search from (Hume & Sunday, 1991). It has been tweaked
    /// slightly to better use memchr.
    ///
    /// memchr is so fast that we do everything we can to keep the loop in memchr
    /// for as long as possible. The easiest way to do this is to intelligently
    /// pick the byte to send to memchr. The best byte is the byte that occurs
    /// least frequently in the haystack. Since doing frequency analysis on the
    /// haystack is far too expensive, we compute a set of fixed frequencies up
    /// front and hard code them in src/freqs.rs. Frequency analysis is done via
    /// scripts/frequencies.py.
    ///
    /// TODO(burntsushi): Add some amount of shifting to this.
    #[derive(Clone, Debug)]
    pub struct SingleSearch {
        /// The pattern.
        pat: Vec<u8>,
        /// The number of Unicode characters in the pattern. This is useful for
        /// determining the effective length of a pattern when deciding which
        /// optimizations to perform. A trailing incomplete UTF-8 sequence counts
        /// as one character.
        char_len: usize,
        /// The rarest byte in the pattern, according to pre-computed frequency
        /// analysis.
        rare1: u8,
        /// The offset of the rarest byte in `pat`.
        rare1i: usize,
        /// The second rarest byte in the pattern, according to pre-computed
        /// frequency analysis. (This may be equivalent to the rarest byte.)
        ///
        /// The second rarest byte is used as a type of guard for quickly detecting
        /// a mismatch after memchr locates an instance of the rarest byte. This
        /// is a hedge against pathological cases where the pre-computed frequency
        /// analysis may be off. (But of course, does not prevent pathological
        /// cases.)
        rare2: u8,
        /// The offset of the second rarest byte in `pat`.
        rare2i: usize,
    }

    impl SingleSearch {
        fn new(pat: Vec<u8>) -> SingleSearch {
            fn freq_rank(b: u8) -> usize { BYTE_FREQUENCIES[b as usize] as usize }

            if pat.is_empty() {
                return SingleSearch::empty();
            }

            // Find the rarest two bytes. Try to make them distinct (but it's not
            // required).
            let mut rare1 = pat[0];
            let mut rare2 = pat[0];
            for b in pat[1..].iter().cloned() {
                if freq_rank(b) < freq_rank(rare1) {
                    rare1 = b;
                }
            }
            for &b in &pat {
                if rare1 == rare2 {
                    rare2 = b
                } else if b != rare1 && freq_rank(b) < freq_rank(rare2) {
                    rare2 = b;
                }
            }

            // And find the offsets of their last occurrences.
            let rare1i = pat.iter().rposition(|&b| b == rare1).unwrap();
            let rare2i = pat.iter().rposition(|&b| b == rare2).unwrap();

            let char_len = char_len_lossy(&pat);
            SingleSearch {
                pat: pat,
                char_len: char_len,
                rare1: rare1,
                rare1i: rare1i,
                rare2: rare2,
                rare2i: rare2i,
            }
        }

        fn empty() -> SingleSearch {
            SingleSearch {
                pat: vec![],
                char_len: 0,
                rare1: 0,
                rare1i: 0,
                rare2: 0,
                rare2i: 0,
            }
        }

        #[inline(always)] // reduces constant overhead
        pub fn find(&self, haystack: &[u8]) -> Option<usize> {
            let pat = &*self.pat;
            if haystack.len() < pat.len() || pat.is_empty() {
                return None;
            }
            let mut i = self.rare1i;
            while i < haystack.len() {
                i += match memchr(self.rare1, &haystack[i..]) {
                    None => return None,
                    Some(i) => i,
                };
                let start = i - self.rare1i;
                let end = start + pat.len();
                if end > haystack.len() {
                    return None;
                }
                let aligned = &haystack[start..end];
                if aligned[self.rare2i] == self.rare2 && aligned == &*self.pat {
                    return Some(start);
                }
                i += 1;
            }
            None
        }

        #[inline(always)] // reduces constant overhead
        pub fn is_suffix(&self, text: &[u8]) -> bool {
            if text.len() < self.len() {
                return false;
            }
            &text[text.len() - self.len()..] == &*self.pat
        }

        pub fn len(&self) -> usize {
            self.pat.len()
        }

        fn approximate_size(&self) -> usize {
            self.pat.len() * mem::size_of::<u8>()
        }
    }

    fn char_len_lossy(bytes: &[u8]) -> usize {
        String::from_utf8_lossy(bytes).chars().count()
    }

# Help

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## Search Tricks

Prefix searches with a type followed by a colon (e.g. `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g. `vec -> usize` or `* -> vec`)