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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::cmp;
    use std::mem;

    use aho_corasick::{self, AhoCorasick, AhoCorasickBuilder};
    use memchr::{memchr, memchr2, memchr3};
    use syntax::hir::literal::{Literal, Literals};

    use freqs::BYTE_FREQUENCIES;
    use self::teddy_avx2::{Teddy as TeddyAVX2};
    use self::teddy_ssse3::{Teddy as TeddySSSE3};

    mod teddy_avx2;
    mod teddy_ssse3;

    /// 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. Similarly
    /// for a regex suffix.
    #[derive(Clone, Debug)]
    pub struct LiteralSearcher {
        complete: bool,
        lcp: FreqyPacked,
        lcs: FreqyPacked,
        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, find using memchr and frequency analysis.
        FreqyPacked(FreqyPacked),
        /// A single substring, find using Boyer-Moore.
        BoyerMoore(BoyerMooreSearch),
        /// An Aho-Corasick automaton.
        AC { ac: AhoCorasick<u32>, lits: Vec<Literal> },
        /// A simd accelerated multiple string matcher. Used only for a small
        /// number of small literals.
        TeddySSSE3(TeddySSSE3),
        /// A simd accelerated multiple string matcher. Used only for a small
        /// number of small literals. This uses 256-bit vectors.
        TeddyAVX2(TeddyAVX2),
    }

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

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

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

        /// 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)),
                FreqyPacked(ref s) => s.find(haystack).map(|i| (i, i + s.len())),
                BoyerMoore(ref s) => s.find(haystack).map(|i| (i, i + s.len())),
                AC { ref ac, .. } => {
                    ac.find(haystack).map(|m| (m.start(), m.end()))
                }
                TeddySSSE3(ref t) => t.find(haystack).map(|m| (m.start, m.end)),
                TeddyAVX2(ref t) => t.find(haystack).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::FreqyPacked(ref s) => LiteralIter::Single(&s.pat),
                Matcher::BoyerMoore(ref s) => LiteralIter::Single(&s.pattern),
                Matcher::AC { ref lits, .. } => LiteralIter::AC(lits),
                Matcher::TeddySSSE3(ref ted) => {
                    LiteralIter::TeddySSSE3(ted.patterns())
                }
                Matcher::TeddyAVX2(ref ted) => {
                    LiteralIter::TeddyAVX2(ted.patterns())
                }
            }
        }

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

        /// Returns a matcher for the longest common suffix of this matcher.
        pub fn lcs(&self) -> &FreqyPacked {
            &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(),
                FreqyPacked(_) => 1,
                BoyerMoore(_) => 1,
                AC { ref ac, .. } => ac.pattern_count(),
                TeddySSSE3(ref ted) => ted.len(),
                TeddyAVX2(ref ted) => ted.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(),
                FreqyPacked(ref single) => single.approximate_size(),
                BoyerMoore(ref single) => single.approximate_size(),
                AC { ref ac, .. } => ac.heap_bytes(),
                TeddySSSE3(ref ted) => ted.approximate_size(),
                TeddyAVX2(ref ted) => ted.approximate_size(),
            }
        }
    }

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

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

        fn new(lits: &Literals, sset: SingleByteSet) -> Self {
            if lits.literals().is_empty() {
                return Matcher::Empty;
            }
            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
                return Matcher::Empty;
            }
            if sset.complete {
                return Matcher::Bytes(sset);
            }
            if lits.literals().len() == 1 {
                let lit = lits.literals()[0].to_vec();
                if BoyerMooreSearch::should_use(lit.as_slice()) {
                    return Matcher::BoyerMoore(BoyerMooreSearch::new(lit));
                } else {
                    return Matcher::FreqyPacked(FreqyPacked::new(lit));
                }
            }
            let is_aho_corasick_fast = sset.dense.len() == 1 && sset.all_ascii;
            if TeddyAVX2::available() && !is_aho_corasick_fast {
                const MAX_TEDDY_LITERALS: usize = 32;
                if lits.literals().len() <= MAX_TEDDY_LITERALS {
                    if let Some(ted) = TeddyAVX2::new(lits) {
                        return Matcher::TeddyAVX2(ted);
                    }
                }
            }
            if TeddySSSE3::available() && !is_aho_corasick_fast {
                // Only try Teddy if Aho-Corasick can't use memchr on an ASCII
                // byte. Also, in its current form, Teddy doesn't scale well to
                // lots of literals.
                //
                // We impose the ASCII restriction since an alternation of
                // non-ASCII string literals in the same language is likely to all
                // start with the same byte. Even worse, the corpus being searched
                // probably has a similar composition, which ends up completely
                // negating the benefit of memchr.
                const MAX_TEDDY_LITERALS: usize = 32;
                if lits.literals().len() <= MAX_TEDDY_LITERALS {
                    if let Some(ted) = TeddySSSE3::new(lits) {
                        return Matcher::TeddySSSE3(ted);
                    }
                }
                // Fallthrough to ol' reliable Aho-Corasick...
            }
            let pats = lits.literals().to_owned();
            let ac = AhoCorasickBuilder::new()
                .match_kind(aho_corasick::MatchKind::LeftmostFirst)
                .dfa(true)
                .build_with_size::<u32, _, _>(&pats)
                .unwrap();
            Matcher::AC { ac, lits: pats }
        }
    }

    pub enum LiteralIter<'a> {
        Empty,
        Bytes(&'a [u8]),
        Single(&'a [u8]),
        AC(&'a [Literal]),
        TeddySSSE3(&'a [Vec<u8>]),
        TeddyAVX2(&'a [Vec<u8>]),
    }

    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)
                    }
                }
                LiteralIter::TeddySSSE3(ref mut lits) => {
                    if lits.is_empty() {
                        None
                    } else {
                        let next = &lits[0];
                        *lits = &lits[1..];
                        Some(&**next)
                    }
                }
                LiteralIter::TeddyAVX2(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,
        all_ascii: bool,
    }

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

        fn prefixes(lits: &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] {
                        if b > 0x7F {
                            sset.all_ascii = false;
                        }
                        sset.dense.push(b);
                        sset.sparse[b as usize] = true;
                    }
                }
            }
            sset
        }

        fn suffixes(lits: &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] {
                        if b > 0x7F {
                            sset.all_ascii = false;
                        }
                        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 using frequency
    /// analysis.
    ///
    /// 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.
    #[derive(Clone, Debug)]
    pub struct FreqyPacked {
        /// 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 *all*
        /// pathological cases.)
        rare2: u8,
        /// The offset of the second rarest byte in `pat`.
        rare2i: usize,
    }

    impl FreqyPacked {
        fn new(pat: Vec<u8>) -> FreqyPacked {
            if pat.is_empty() {
                return FreqyPacked::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);
            FreqyPacked {
                pat: pat,
                char_len: char_len,
                rare1: rare1,
                rare1i: rare1i,
                rare2: rare2,
                rare2i: rare2i,
            }
        }

        fn empty() -> FreqyPacked {
            FreqyPacked {
                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()
        }

        pub fn char_len(&self) -> usize {
            self.char_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()
    }

    /// An implementation of Tuned Boyer-Moore as laid out by
    /// Andrew Hume and Daniel Sunday in "Fast String Searching".
    /// O(n) in the size of the input.
    ///
    /// Fast string searching algorithms come in many variations,
    /// but they can generally be described in terms of three main
    /// components.
    ///
    /// The skip loop is where the string searcher wants to spend
    /// as much time as possible. Exactly which character in the
    /// pattern the skip loop examines varies from algorithm to
    /// algorithm, but in the simplest case this loop repeated
    /// looks at the last character in the pattern and jumps
    /// forward in the input if it is not in the pattern.
    /// Robert Boyer and J Moore called this the "fast" loop in
    /// their original paper.
    ///
    /// The match loop is responsible for actually examining the
    /// whole potentially matching substring. In order to fail
    /// faster, the match loop sometimes has a guard test attached.
    /// The guard test uses frequency analysis of the different
    /// characters in the pattern to choose the least frequency
    /// occurring character and use it to find match failures
    /// as quickly as possible.
    ///
    /// The shift rule governs how the algorithm will shuffle its
    /// test window in the event of a failure during the match loop.
    /// Certain shift rules allow the worst-case run time of the
    /// algorithm to be shown to be O(n) in the size of the input
    /// rather than O(nm) in the size of the input and the size
    /// of the pattern (as naive Boyer-Moore is).
    ///
    /// "Fast String Searching", in addition to presenting a tuned
    /// algorithm, provides a comprehensive taxonomy of the many
    /// different flavors of string searchers. Under that taxonomy
    /// TBM, the algorithm implemented here, uses an unrolled fast
    /// skip loop with memchr fallback, a forward match loop with guard,
    /// and the mini Sunday's delta shift rule. To unpack that you'll have to
    /// read the paper.
    #[derive(Clone, Debug)]
    pub struct BoyerMooreSearch {
        /// The pattern we are going to look for in the haystack.
        pattern: Vec<u8>,

        /// The skip table for the skip loop.
        ///
        /// Maps the character at the end of the input
        /// to a shift.
        skip_table: Vec<usize>,

        /// The guard character (least frequently occurring char).
        guard: u8,
        /// The reverse-index of the guard character in the pattern.
        guard_reverse_idx: usize,

        /// Daniel Sunday's mini generalized delta2 shift table.
        ///
        /// We use a skip loop, so we only have to provide a shift
        /// for the skip char (last char). This is why it is a mini
        /// shift rule.
        md2_shift: usize,
    }

    impl BoyerMooreSearch {
        /// Create a new string searcher, performing whatever
        /// compilation steps are required.
        fn new(pattern: Vec<u8>) -> Self {
            debug_assert!(pattern.len() > 0);

            let (g, gi) = Self::select_guard(pattern.as_slice());
            let skip_table = Self::compile_skip_table(pattern.as_slice());
            let md2_shift = Self::compile_md2_shift(pattern.as_slice());
            BoyerMooreSearch {
                pattern: pattern,
                skip_table: skip_table,
                guard: g,
                guard_reverse_idx: gi,
                md2_shift: md2_shift,
            }
        }

        /// Find the pattern in `haystack`, returning the offset
        /// of the start of the first occurrence of the pattern
        /// in `haystack`.
        #[inline]
        fn find(&self, haystack: &[u8]) -> Option<usize> {
            if haystack.len() < self.pattern.len() {
                return None;
            }

            let mut window_end = self.pattern.len() - 1;

            // Inspired by the grep source. It is a way
            // to do correct loop unrolling without having to place
            // a crashpad of terminating charicters at the end in
            // the way described in the Fast String Searching paper.
            const NUM_UNROLL: usize = 10;
            // 1 for the initial position, and 1 for the md2 shift
            let short_circut = (NUM_UNROLL + 2) * self.pattern.len();

            if haystack.len() > short_circut {
                // just 1 for the md2 shift
                let backstop = haystack.len() - ((NUM_UNROLL + 1) * self.pattern.len());
                loop {
                    window_end = match self.skip_loop(haystack, window_end, backstop) {
                        Some(i) => i,
                        None => return None,
                    };
                    if window_end >= backstop {
                        break;
                    }

                    if self.check_match(haystack, window_end) {
                        return Some(window_end - (self.pattern.len() - 1));
                    } else {
                        let skip = self.skip_table[haystack[window_end] as usize];
                        window_end +=
                            if skip == 0 { self.md2_shift } else { skip };
                        continue;
                    }
                }
            }

            // now process the input after the backstop
            while window_end < haystack.len() {
                let mut skip = self.skip_table[haystack[window_end] as usize];
                if skip == 0 {
                    if self.check_match(haystack, window_end) {
                        return Some(window_end - (self.pattern.len() - 1));
                    } else {
                        skip = self.md2_shift;
                    }
                }
                window_end += skip;
            }

            None
        }

        fn len(&self) -> usize {
            return self.pattern.len()
        }

        /// The key heuristic behind which the BoyerMooreSearch lives.
        ///
        /// See `rust-lang/regex/issues/408`.
        ///
        /// Tuned Boyer-Moore is actually pretty slow! It turns out a handrolled
        /// platform-specific memchr routine with a bit of frequency
        /// analysis sprinkled on top actually wins most of the time.
        /// However, there are a few cases where Tuned Boyer-Moore still
        /// wins.
        ///
        /// If the haystack is random, frequency analysis doesn't help us,
        /// so Boyer-Moore will win for sufficiently large needles.
        /// Unfortunately, there is no obvious way to determine this
        /// ahead of time.
        ///
        /// If the pattern itself consists of very common characters,
        /// frequency analysis won't get us anywhere. The most extreme
        /// example of this is a pattern like `eeeeeeeeeeeeeeee`. Fortunately,
        /// this case is wholly determined by the pattern, so we can actually
        /// implement the heuristic.
        ///
        /// A third case is if the pattern is sufficiently long. The idea
        /// here is that once the pattern gets long enough the Tuned
        /// Boyer-Moore skip loop will start making strides long enough
        /// to beat the asm deep magic that is memchr.
        fn should_use(pattern: &[u8]) -> bool {
            // The minimum pattern length required to use TBM.
            const MIN_LEN: usize = 9;
            // The minimum frequency rank (lower is rarer) that every byte in the
            // pattern must have in order to use TBM. That is, if the pattern
            // contains _any_ byte with a lower rank, then TBM won't be used.
            const MIN_CUTOFF: usize = 150;
            // The maximum frequency rank for any byte.
            const MAX_CUTOFF: usize = 255;
            // The scaling factor used to determine the actual cutoff frequency
            // to use (keeping in mind that the minimum frequency rank is bounded
            // by MIN_CUTOFF). This scaling factor is an attempt to make TBM more
            // likely to be used as the pattern grows longer. That is, longer
            // patterns permit somewhat less frequent bytes than shorter patterns,
            // under the assumption that TBM gets better as the pattern gets
            // longer.
            const LEN_CUTOFF_PROPORTION: usize = 4;

            let scaled_rank = pattern.len().wrapping_mul(LEN_CUTOFF_PROPORTION);
            let cutoff = cmp::max(
                MIN_CUTOFF,
                MAX_CUTOFF - cmp::min(MAX_CUTOFF, scaled_rank),
            );
            // The pattern must be long enough to be worthwhile. e.g., memchr will
            // be faster on `e` because it is short even though e is quite common.
            pattern.len() > MIN_LEN
                // all the bytes must be more common than the cutoff.
                && pattern.iter().all(|c| freq_rank(*c) >= cutoff)
        }

        /// Check to see if there is a match at the given position
        #[inline]
        fn check_match(&self, haystack: &[u8], window_end: usize) -> bool {
            // guard test
            if haystack[window_end - self.guard_reverse_idx] != self.guard {
                return false;
            }

            // match loop
            let window_start = window_end - (self.pattern.len() - 1);
            for i in 0..self.pattern.len() {
                if self.pattern[i] != haystack[window_start + i] {
                    return false;
                }
            }

            true
        }

        /// Skip forward according to the shift table.
        ///
        /// Returns the offset of the next occurrence
        /// of the last char in the pattern, or the none
        /// if it never reappears. If `skip_loop` hits the backstop
        /// it will leave early.
        #[inline]
        fn skip_loop(&self,
            haystack: &[u8],
            mut window_end: usize,
            backstop: usize,
        ) -> Option<usize> {
            let window_end_snapshot = window_end;
            let skip_of = |we: usize| -> usize {
                // Unsafe might make this faster, but the benchmarks
                // were hard to interpret.
                self.skip_table[haystack[we] as usize]
            };

            loop {
                let mut skip = skip_of(window_end); window_end += skip;
                skip = skip_of(window_end); window_end += skip;
                if skip != 0 {
                    skip = skip_of(window_end); window_end += skip;
                    skip = skip_of(window_end); window_end += skip;
                    skip = skip_of(window_end); window_end += skip;
                    if skip != 0 {
                        skip = skip_of(window_end); window_end += skip;
                        skip = skip_of(window_end); window_end += skip;
                        skip = skip_of(window_end); window_end += skip;
                        if skip != 0 {
                            skip = skip_of(window_end); window_end += skip;
                            skip = skip_of(window_end); window_end += skip;

                            // If ten iterations did not make at least 16 words
                            // worth of progress, we just fall back on memchr.
                            if window_end - window_end_snapshot >
                                 16 * mem::size_of::<usize>() {

                                // Returning a window_end >= backstop will immediatly
                                // break us out of the inner loop in `find`.
                                if window_end >= backstop {
                                    return Some(window_end);
                                }

                                continue; // we made enough progress
                            } else {
                                // In case we are already there, and so that
                                // we will catch the guard char.
                                window_end = window_end
                                    .checked_sub(1 + self.guard_reverse_idx)
                                    .unwrap_or(0);

                                match memchr(self.guard, &haystack[window_end..]) {
                                    None => return None,
                                    Some(g_idx) => {
                                        return Some(window_end + g_idx + self.guard_reverse_idx);
                                    }
                                }
                            }
                        }
                    }
                }

                return Some(window_end);
            }
        }

        /// Compute the ufast skip table.
        fn compile_skip_table(pattern: &[u8]) -> Vec<usize> {
            let mut tab = vec![pattern.len(); 256];

            // For every char in the pattern, we write a skip
            // that will line us up with the rightmost occurrence.
            //
            // N.B. the sentinel (0) is written by the last
            // loop iteration.
            for (i, c) in pattern.iter().enumerate() {
                tab[*c as usize] = (pattern.len() - 1) - i;
            }

            tab
        }

        /// Select the guard character based off of the precomputed
        /// frequency table.
        fn select_guard(pattern: &[u8]) -> (u8, usize) {
            let mut rarest = pattern[0];
            let mut rarest_rev_idx = pattern.len() - 1;
            for (i, c) in pattern.iter().enumerate() {
                if freq_rank(*c) < freq_rank(rarest) {
                    rarest = *c;
                    rarest_rev_idx = (pattern.len() - 1) - i;
                }
            }

            (rarest, rarest_rev_idx)
        }

        /// If there is another occurrence of the skip
        /// char, shift to it, otherwise just shift to
        /// the next window.
        fn compile_md2_shift(pattern: &[u8]) -> usize {
            let shiftc = *pattern.last().unwrap();

            // For a pattern of length 1 we will never apply the
            // shift rule, so we use a poison value on the principle
            // that failing fast is a good thing.
            if pattern.len() == 1 {
                return 0xDEADBEAF;
            }

            let mut i = pattern.len() - 2;
            while i > 0 {
                if pattern[i] == shiftc {
                    return (pattern.len() - 1) - i;
                }
                i -= 1;
            }

            // The skip char never re-occurs in the pattern, so
            // we can just shift the whole window length.
            pattern.len() - 1
        }

        fn approximate_size(&self) -> usize {
            (self.pattern.len() * mem::size_of::<u8>())
                + (256 * mem::size_of::<usize>()) // skip table
        }
    }

    fn freq_rank(b: u8) -> usize {
        BYTE_FREQUENCIES[b as usize] as usize
    }

    #[cfg(test)]
    mod tests {
        use super::{BoyerMooreSearch, FreqyPacked};

        //
        // Unit Tests
        //

        // The "hello, world" of string searching
        #[test]
        fn bm_find_subs() {
            let searcher = BoyerMooreSearch::new(Vec::from(&b"pattern"[..]));
            let haystack = b"I keep seeing patterns in this text";
            assert_eq!(14, searcher.find(haystack).unwrap());
        }

        #[test]
        fn bm_find_no_subs() {
            let searcher = BoyerMooreSearch::new(Vec::from(&b"pattern"[..]));
            let haystack = b"I keep seeing needles in this text";
            assert_eq!(None, searcher.find(haystack));
        }

        //
        // Regression Tests
        //

        #[test]
        fn bm_skip_reset_bug() {
            let haystack = vec![0, 0, 0, 0, 0, 1, 1, 0];
            let needle = vec![0, 1, 1, 0];

            let searcher = BoyerMooreSearch::new(needle);
            let offset = searcher.find(haystack.as_slice()).unwrap();
            assert_eq!(4, offset);
        }

        #[test]
        fn bm_backstop_underflow_bug() {
            let haystack = vec![0, 0];
            let needle = vec![0, 0];

            let searcher = BoyerMooreSearch::new(needle);
            let offset = searcher.find(haystack.as_slice()).unwrap();
            assert_eq!(0, offset);
        }

        #[test]
        fn bm_naive_off_by_one_bug() {
            let haystack = vec![91];
            let needle = vec![91];

            let naive_offset = naive_find(&needle, &haystack).unwrap();
            assert_eq!(0, naive_offset);
        }

        #[test]
        fn bm_memchr_fallback_indexing_bug() {
            let mut haystack = vec![
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 87, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
            let needle = vec![1, 1, 1, 1, 32, 32, 87];
            let needle_start = haystack.len();
            haystack.extend(needle.clone());

            let searcher = BoyerMooreSearch::new(needle);
            assert_eq!(needle_start, searcher.find(haystack.as_slice()).unwrap());
        }

        #[test]
        fn bm_backstop_boundary() {
            let haystack = b"\
    // aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    e_data.clone_created(entity_id, entity_to_add.entity_id);
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    ".to_vec();
            let needle = b"clone_created".to_vec();

            let searcher = BoyerMooreSearch::new(needle);
            let result = searcher.find(&haystack);
            assert_eq!(Some(43), result);
        }

        #[test]
        fn bm_win_gnu_indexing_bug() {
            let haystack_raw = vec![
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
            let needle = vec![1, 1, 1, 1, 1, 1, 1];
            let haystack = haystack_raw.as_slice();

            BoyerMooreSearch::new(needle.clone()).find(haystack);
        }

        //
        // QuickCheck Properties
        //

        use quickcheck::TestResult;

        fn naive_find(needle: &[u8], haystack: &[u8]) -> Option<usize> {
            assert!(needle.len() <= haystack.len());

            for i in 0..(haystack.len() - (needle.len() - 1)) {
                if haystack[i] == needle[0]
                    && &haystack[i..(i+needle.len())] == needle {
                    return Some(i)
                }
            }

            None
        }

        quickcheck! {
            fn qc_bm_equals_nieve_find(pile1: Vec<u8>, pile2: Vec<u8>) -> TestResult {
                if pile1.len() == 0 || pile2.len() == 0 {
                    return TestResult::discard();
                }

                let (needle, haystack) = if pile1.len() < pile2.len() {
                    (pile1, pile2.as_slice())
                } else {
                    (pile2, pile1.as_slice())
                };

                let searcher = BoyerMooreSearch::new(needle.clone());
                TestResult::from_bool(
                    searcher.find(haystack) == naive_find(&needle, haystack))
            }

            fn qc_bm_equals_single(pile1: Vec<u8>, pile2: Vec<u8>) -> TestResult {
                if pile1.len() == 0 || pile2.len() == 0 {
                    return TestResult::discard();
                }

                let (needle, haystack) = if pile1.len() < pile2.len() {
                    (pile1, pile2.as_slice())
                } else {
                    (pile2, pile1.as_slice())
                };

                let bm_searcher = BoyerMooreSearch::new(needle.clone());
                let freqy_memchr = FreqyPacked::new(needle);
                TestResult::from_bool(
                    bm_searcher.find(haystack) == freqy_memchr.find(haystack))
            }

            fn qc_bm_finds_trailing_needle(
                haystack_pre: Vec<u8>,
                needle: Vec<u8>
            ) -> TestResult {
                if needle.len() == 0 {
                    return TestResult::discard();
                }

                let mut haystack = haystack_pre.clone();
                let searcher = BoyerMooreSearch::new(needle.clone());

                if haystack.len() >= needle.len() &&
                    searcher.find(haystack.as_slice()).is_some() {
                    return TestResult::discard();
                }

                haystack.extend(needle.clone());

                // What if the the tail of the haystack can start the
                // needle?
                let start = haystack_pre.len()
                    .checked_sub(needle.len())
                    .unwrap_or(0);
                for i in 0..(needle.len() - 1) {
                    if searcher.find(&haystack[(i + start)..]).is_some() {
                        return TestResult::discard();
                    }
                }

                TestResult::from_bool(
                    searcher.find(haystack.as_slice())
                            .map(|x| x == haystack_pre.len())
                            .unwrap_or(false))
            }

            // qc_equals_* is only testing the negative case as @burntsushi
            // pointed out in https://github.com/rust-lang/regex/issues/446.
            // This quickcheck prop represents an effort to force testing of
            // the positive case. qc_bm_finds_first and qc_bm_finds_trailing_needle
            // already check some of the positive cases, but they don't cover
            // cases where the needle is in the middle of haystack. This prop
            // fills that hole.
            fn qc_bm_finds_subslice(
                haystack: Vec<u8>,
                needle_start: usize,
                needle_length: usize
            ) -> TestResult {
                if haystack.len() == 0 {
                    return TestResult::discard();
                }

                let needle_start = needle_start % haystack.len();
                let needle_length = needle_length % (haystack.len() - needle_start);

                if needle_length == 0 {
                    return TestResult::discard();
                }

                let needle = &haystack[needle_start..(needle_start + needle_length)];

                let bm_searcher = BoyerMooreSearch::new(needle.to_vec());

                let start = naive_find(&needle, &haystack);
                match start {
                    None => TestResult::from_bool(false),
                    Some(nf_start) =>
                        TestResult::from_bool(
                            nf_start <= needle_start
                                && bm_searcher.find(&haystack) == start
                        )
                }
            }

            fn qc_bm_finds_first(needle: Vec<u8>) -> TestResult {
                if needle.len() == 0 {
                    return TestResult::discard();
                }

                let mut haystack = needle.clone();
                let searcher = BoyerMooreSearch::new(needle.clone());
                haystack.extend(needle);

                TestResult::from_bool(
                    searcher.find(haystack.as_slice())
                            .map(|x| x == 0)
                            .unwrap_or(false))
            }
        }
    }

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