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    //! The string Pattern API.
    //!
    //! For more details, see the traits [`Pattern`], [`Searcher`],
    //! [`ReverseSearcher`], and [`DoubleEndedSearcher`].

    #![unstable(feature = "pattern",
                reason = "API not fully fleshed out and ready to be stabilized",
                issue = "27721")]

    use crate::cmp;
    use crate::fmt;
    use crate::slice::memchr;
    use crate::usize;

    // Pattern

    /// A string pattern.
    ///
    /// A `Pattern<'a>` expresses that the implementing type
    /// can be used as a string pattern for searching in a `&'a str`.
    ///
    /// For example, both `'a'` and `"aa"` are patterns that
    /// would match at index `1` in the string `"baaaab"`.
    ///
    /// The trait itself acts as a builder for an associated
    /// `Searcher` type, which does the actual work of finding
    /// occurrences of the pattern in a string.
    pub trait Pattern<'a>: Sized {
        /// Associated searcher for this pattern
        type Searcher: Searcher<'a>;

        /// Constructs the associated searcher from
        /// `self` and the `haystack` to search in.
        fn into_searcher(self, haystack: &'a str) -> Self::Searcher;

        /// Checks whether the pattern matches anywhere in the haystack
        #[inline]
        fn is_contained_in(self, haystack: &'a str) -> bool {
            self.into_searcher(haystack).next_match().is_some()
        }

        /// Checks whether the pattern matches at the front of the haystack
        #[inline]
        fn is_prefix_of(self, haystack: &'a str) -> bool {
            match self.into_searcher(haystack).next() {
                SearchStep::Match(0, _) => true,
                _ => false,
            }
        }

        /// Checks whether the pattern matches at the back of the haystack
        #[inline]
        fn is_suffix_of(self, haystack: &'a str) -> bool
            where Self::Searcher: ReverseSearcher<'a>
        {
            match self.into_searcher(haystack).next_back() {
                SearchStep::Match(_, j) if haystack.len() == j => true,
                _ => false,
            }
        }
    }

    // Searcher

    /// Result of calling `Searcher::next()` or `ReverseSearcher::next_back()`.
    #[derive(Copy, Clone, Eq, PartialEq, Debug)]
    pub enum SearchStep {
        /// Expresses that a match of the pattern has been found at
        /// `haystack[a..b]`.
        Match(usize, usize),
        /// Expresses that `haystack[a..b]` has been rejected as a possible match
        /// of the pattern.
        ///
        /// Note that there might be more than one `Reject` between two `Match`es,
        /// there is no requirement for them to be combined into one.
        Reject(usize, usize),
        /// Expresses that every byte of the haystack has been visited, ending
        /// the iteration.
        Done
    }

    /// A searcher for a string pattern.
    ///
    /// This trait provides methods for searching for non-overlapping
    /// matches of a pattern starting from the front (left) of a string.
    ///
    /// It will be implemented by associated `Searcher`
    /// types of the `Pattern` trait.
    ///
    /// The trait is marked unsafe because the indices returned by the
    /// `next()` methods are required to lie on valid utf8 boundaries in
    /// the haystack. This enables consumers of this trait to
    /// slice the haystack without additional runtime checks.
    pub unsafe trait Searcher<'a> {
        /// Getter for the underlying string to be searched in
        ///
        /// Will always return the same `&str`
        fn haystack(&self) -> &'a str;

        /// Performs the next search step starting from the front.
        ///
        /// - Returns `Match(a, b)` if `haystack[a..b]` matches the pattern.
        /// - Returns `Reject(a, b)` if `haystack[a..b]` can not match the
        ///   pattern, even partially.
        /// - Returns `Done` if every byte of the haystack has been visited
        ///
        /// The stream of `Match` and `Reject` values up to a `Done`
        /// will contain index ranges that are adjacent, non-overlapping,
        /// covering the whole haystack, and laying on utf8 boundaries.
        ///
        /// A `Match` result needs to contain the whole matched pattern,
        /// however `Reject` results may be split up into arbitrary
        /// many adjacent fragments. Both ranges may have zero length.
        ///
        /// As an example, the pattern `"aaa"` and the haystack `"cbaaaaab"`
        /// might produce the stream
        /// `[Reject(0, 1), Reject(1, 2), Match(2, 5), Reject(5, 8)]`
        fn next(&mut self) -> SearchStep;

        /// Finds the next `Match` result. See `next()`
        ///
        /// Unlike next(), there is no guarantee that the returned ranges
        /// of this and next_reject will overlap. This will return (start_match, end_match),
        /// where start_match is the index of where the match begins, and end_match is
        /// the index after the end of the match.
        #[inline]
        fn next_match(&mut self) -> Option<(usize, usize)> {
            loop {
                match self.next() {
                    SearchStep::Match(a, b) => return Some((a, b)),
                    SearchStep::Done => return None,
                    _ => continue,
                }
            }
        }

        /// Finds the next `Reject` result. See `next()` and `next_match()`
        ///
        /// Unlike next(), there is no guarantee that the returned ranges
        /// of this and next_match will overlap.
        #[inline]
        fn next_reject(&mut self) -> Option<(usize, usize)> {
            loop {
                match self.next() {
                    SearchStep::Reject(a, b) => return Some((a, b)),
                    SearchStep::Done => return None,
                    _ => continue,
                }
            }
        }
    }

    /// A reverse searcher for a string pattern.
    ///
    /// This trait provides methods for searching for non-overlapping
    /// matches of a pattern starting from the back (right) of a string.
    ///
    /// It will be implemented by associated `Searcher`
    /// types of the `Pattern` trait if the pattern supports searching
    /// for it from the back.
    ///
    /// The index ranges returned by this trait are not required
    /// to exactly match those of the forward search in reverse.
    ///
    /// For the reason why this trait is marked unsafe, see them
    /// parent trait `Searcher`.
    pub unsafe trait ReverseSearcher<'a>: Searcher<'a> {
        /// Performs the next search step starting from the back.
        ///
        /// - Returns `Match(a, b)` if `haystack[a..b]` matches the pattern.
        /// - Returns `Reject(a, b)` if `haystack[a..b]` can not match the
        ///   pattern, even partially.
        /// - Returns `Done` if every byte of the haystack has been visited
        ///
        /// The stream of `Match` and `Reject` values up to a `Done`
        /// will contain index ranges that are adjacent, non-overlapping,
        /// covering the whole haystack, and laying on utf8 boundaries.
        ///
        /// A `Match` result needs to contain the whole matched pattern,
        /// however `Reject` results may be split up into arbitrary
        /// many adjacent fragments. Both ranges may have zero length.
        ///
        /// As an example, the pattern `"aaa"` and the haystack `"cbaaaaab"`
        /// might produce the stream
        /// `[Reject(7, 8), Match(4, 7), Reject(1, 4), Reject(0, 1)]`
        fn next_back(&mut self) -> SearchStep;

        /// Finds the next `Match` result. See `next_back()`
        #[inline]
        fn next_match_back(&mut self) -> Option<(usize, usize)>{
            loop {
                match self.next_back() {
                    SearchStep::Match(a, b) => return Some((a, b)),
                    SearchStep::Done => return None,
                    _ => continue,
                }
            }
        }

        /// Finds the next `Reject` result. See `next_back()`
        #[inline]
        fn next_reject_back(&mut self) -> Option<(usize, usize)>{
            loop {
                match self.next_back() {
                    SearchStep::Reject(a, b) => return Some((a, b)),
                    SearchStep::Done => return None,
                    _ => continue,
                }
            }
        }
    }

    /// A marker trait to express that a `ReverseSearcher`
    /// can be used for a `DoubleEndedIterator` implementation.
    ///
    /// For this, the impl of `Searcher` and `ReverseSearcher` need
    /// to follow these conditions:
    ///
    /// - All results of `next()` need to be identical
    ///   to the results of `next_back()` in reverse order.
    /// - `next()` and `next_back()` need to behave as
    ///   the two ends of a range of values, that is they
    ///   can not "walk past each other".
    ///
    /// # Examples
    ///
    /// `char::Searcher` is a `DoubleEndedSearcher` because searching for a
    /// `char` only requires looking at one at a time, which behaves the same
    /// from both ends.
    ///
    /// `(&str)::Searcher` is not a `DoubleEndedSearcher` because
    /// the pattern `"aa"` in the haystack `"aaa"` matches as either
    /// `"[aa]a"` or `"a[aa]"`, depending from which side it is searched.
    pub trait DoubleEndedSearcher<'a>: ReverseSearcher<'a> {}

    /////////////////////////////////////////////////////////////////////////////
    // Impl for char
    /////////////////////////////////////////////////////////////////////////////

    /// Associated type for `<char as Pattern<'a>>::Searcher`.
    #[derive(Clone, Debug)]
    pub struct CharSearcher<'a> {
        haystack: &'a str,
        // safety invariant: `finger`/`finger_back` must be a valid utf8 byte index of `haystack`
        // This invariant can be broken *within* next_match and next_match_back, however
        // they must exit with fingers on valid code point boundaries.

        /// `finger` is the current byte index of the forward search.
        /// Imagine that it exists before the byte at its index, i.e.
        /// `haystack[finger]` is the first byte of the slice we must inspect during
        /// forward searching
        finger: usize,
        /// `finger_back` is the current byte index of the reverse search.
        /// Imagine that it exists after the byte at its index, i.e.
        /// haystack[finger_back - 1] is the last byte of the slice we must inspect during
        /// forward searching (and thus the first byte to be inspected when calling next_back())
        finger_back: usize,
        /// The character being searched for
        needle: char,

        // safety invariant: `utf8_size` must be less than 5
        /// The number of bytes `needle` takes up when encoded in utf8
        utf8_size: usize,
        /// A utf8 encoded copy of the `needle`
        utf8_encoded: [u8; 4],
    }

    unsafe impl<'a> Searcher<'a> for CharSearcher<'a> {
        #[inline]
        fn haystack(&self) -> &'a str {
            self.haystack
        }
        #[inline]
        fn next(&mut self) -> SearchStep {
            let old_finger = self.finger;
            let slice = unsafe { self.haystack.get_unchecked(old_finger..self.finger_back) };
            let mut iter = slice.chars();
            let old_len = iter.iter.len();
            if let Some(ch) = iter.next() {
                // add byte offset of current character
                // without re-encoding as utf-8
                self.finger += old_len - iter.iter.len();
                if ch == self.needle {
                    SearchStep::Match(old_finger, self.finger)
                } else {
                    SearchStep::Reject(old_finger, self.finger)
                }
            } else {
                SearchStep::Done
            }
        }
        #[inline]
        fn next_match(&mut self) -> Option<(usize, usize)> {
            loop {
                // get the haystack after the last character found
                let bytes = if let Some(slice) = self.haystack.as_bytes()
                                                     .get(self.finger..self.finger_back) {
                    slice
                } else {
                    return None;
                };
                // the last byte of the utf8 encoded needle
                let last_byte = unsafe { *self.utf8_encoded.get_unchecked(self.utf8_size - 1) };
                if let Some(index) = memchr::memchr(last_byte, bytes) {
                    // The new finger is the index of the byte we found,
                    // plus one, since we memchr'd for the last byte of the character.
                    //
                    // Note that this doesn't always give us a finger on a UTF8 boundary.
                    // If we *didn't* find our character
                    // we may have indexed to the non-last byte of a 3-byte or 4-byte character.
                    // We can't just skip to the next valid starting byte because a character like
                    // ꁁ (U+A041 YI SYLLABLE PA), utf-8 `EA 81 81` will have us always find
                    // the second byte when searching for the third.
                    //
                    // However, this is totally okay. While we have the invariant that
                    // self.finger is on a UTF8 boundary, this invariant is not relied upon
                    // within this method (it is relied upon in CharSearcher::next()).
                    //
                    // We only exit this method when we reach the end of the string, or if we
                    // find something. When we find something the `finger` will be set
                    // to a UTF8 boundary.
                    self.finger += index + 1;
                    if self.finger >= self.utf8_size {
                        let found_char = self.finger - self.utf8_size;
                        if let Some(slice) = self.haystack.as_bytes().get(found_char..self.finger) {
                            if slice == &self.utf8_encoded[0..self.utf8_size] {
                                return Some((found_char, self.finger));
                            }
                        }
                    }
                } else {
                    // found nothing, exit
                    self.finger = self.finger_back;
                    return None;
                }
            }
        }

        // let next_reject use the default implementation from the Searcher trait
    }

    unsafe impl<'a> ReverseSearcher<'a> for CharSearcher<'a> {
        #[inline]
        fn next_back(&mut self) -> SearchStep {
            let old_finger = self.finger_back;
            let slice = unsafe { self.haystack.get_unchecked(self.finger..old_finger) };
            let mut iter = slice.chars();
            let old_len = iter.iter.len();
            if let Some(ch) = iter.next_back() {
                // subtract byte offset of current character
                // without re-encoding as utf-8
                self.finger_back -= old_len - iter.iter.len();
                if ch == self.needle {
                    SearchStep::Match(self.finger_back, old_finger)
                } else {
                    SearchStep::Reject(self.finger_back, old_finger)
                }
            } else {
                SearchStep::Done
            }
        }
        #[inline]
        fn next_match_back(&mut self) -> Option<(usize, usize)> {
            let haystack = self.haystack.as_bytes();
            loop {
                // get the haystack up to but not including the last character searched
                let bytes = if let Some(slice) = haystack.get(self.finger..self.finger_back) {
                    slice
                } else {
                    return None;
                };
                // the last byte of the utf8 encoded needle
                let last_byte = unsafe { *self.utf8_encoded.get_unchecked(self.utf8_size - 1) };
                if let Some(index) = memchr::memrchr(last_byte, bytes) {
                    // we searched a slice that was offset by self.finger,
                    // add self.finger to recoup the original index
                    let index = self.finger + index;
                    // memrchr will return the index of the byte we wish to
                    // find. In case of an ASCII character, this is indeed
                    // were we wish our new finger to be ("after" the found
                    // char in the paradigm of reverse iteration). For
                    // multibyte chars we need to skip down by the number of more
                    // bytes they have than ASCII
                    let shift = self.utf8_size - 1;
                    if index >= shift {
                        let found_char = index - shift;
                        if let Some(slice) = haystack.get(found_char..(found_char + self.utf8_size)) {
                            if slice == &self.utf8_encoded[0..self.utf8_size] {
                                // move finger to before the character found (i.e., at its start index)
                                self.finger_back = found_char;
                                return Some((self.finger_back, self.finger_back + self.utf8_size));
                            }
                        }
                    }
                    // We can't use finger_back = index - size + 1 here. If we found the last char
                    // of a different-sized character (or the middle byte of a different character)
                    // we need to bump the finger_back down to `index`. This similarly makes
                    // `finger_back` have the potential to no longer be on a boundary,
                    // but this is OK since we only exit this function on a boundary
                    // or when the haystack has been searched completely.
                    //
                    // Unlike next_match this does not
                    // have the problem of repeated bytes in utf-8 because
                    // we're searching for the last byte, and we can only have
                    // found the last byte when searching in reverse.
                    self.finger_back = index;
                } else {
                    self.finger_back = self.finger;
                    // found nothing, exit
                    return None;
                }
            }
        }

        // let next_reject_back use the default implementation from the Searcher trait
    }

    impl<'a> DoubleEndedSearcher<'a> for CharSearcher<'a> {}

    /// Searches for chars that are equal to a given char
    impl<'a> Pattern<'a> for char {
        type Searcher = CharSearcher<'a>;

        #[inline]
        fn into_searcher(self, haystack: &'a str) -> Self::Searcher {
            let mut utf8_encoded = [0; 4];
            let utf8_size = self.encode_utf8(&mut utf8_encoded).len();
            CharSearcher {
                haystack,
                finger: 0,
                finger_back: haystack.len(),
                needle: self,
                utf8_size,
                utf8_encoded
            }
        }

        #[inline]
        fn is_contained_in(self, haystack: &'a str) -> bool {
            if (self as u32) < 128 {
                haystack.as_bytes().contains(&(self as u8))
            } else {
                let mut buffer = [0u8; 4];
                self.encode_utf8(&mut buffer).is_contained_in(haystack)
            }
        }

        #[inline]
        fn is_prefix_of(self, haystack: &'a str) -> bool {
            if let Some(ch) = haystack.chars().next() {
                self == ch
            } else {
                false
            }
        }

        #[inline]
        fn is_suffix_of(self, haystack: &'a str) -> bool where Self::Searcher: ReverseSearcher<'a>
        {
            if let Some(ch) = haystack.chars().next_back() {
                self == ch
            } else {
                false
            }
        }
    }

    /////////////////////////////////////////////////////////////////////////////
    // Impl for a MultiCharEq wrapper
    /////////////////////////////////////////////////////////////////////////////

    #[doc(hidden)]
    trait MultiCharEq {
        fn matches(&mut self, c: char) -> bool;
    }

    impl<F> MultiCharEq for F where F: FnMut(char) -> bool {
        #[inline]
        fn matches(&mut self, c: char) -> bool { (*self)(c) }
    }

    impl MultiCharEq for &[char] {
        #[inline]
        fn matches(&mut self, c: char) -> bool {
            self.iter().any(|&m| { m == c })
        }
    }

    struct MultiCharEqPattern<C: MultiCharEq>(C);

    #[derive(Clone, Debug)]
    struct MultiCharEqSearcher<'a, C: MultiCharEq> {
        char_eq: C,
        haystack: &'a str,
        char_indices: super::CharIndices<'a>,
    }

    impl<'a, C: MultiCharEq> Pattern<'a> for MultiCharEqPattern<C> {
        type Searcher = MultiCharEqSearcher<'a, C>;

        #[inline]
        fn into_searcher(self, haystack: &'a str) -> MultiCharEqSearcher<'a, C> {
            MultiCharEqSearcher {
                haystack,
                char_eq: self.0,
                char_indices: haystack.char_indices(),
            }
        }
    }

    unsafe impl<'a, C: MultiCharEq> Searcher<'a> for MultiCharEqSearcher<'a, C> {
        #[inline]
        fn haystack(&self) -> &'a str {
            self.haystack
        }

        #[inline]
        fn next(&mut self) -> SearchStep {
            let s = &mut self.char_indices;
            // Compare lengths of the internal byte slice iterator
            // to find length of current char
            let pre_len = s.iter.iter.len();
            if let Some((i, c)) = s.next() {
                let len = s.iter.iter.len();
                let char_len = pre_len - len;
                if self.char_eq.matches(c) {
                    return SearchStep::Match(i, i + char_len);
                } else {
                    return SearchStep::Reject(i, i + char_len);
                }
            }
            SearchStep::Done
        }
    }

    unsafe impl<'a, C: MultiCharEq> ReverseSearcher<'a> for MultiCharEqSearcher<'a, C> {
        #[inline]
        fn next_back(&mut self) -> SearchStep {
            let s = &mut self.char_indices;
            // Compare lengths of the internal byte slice iterator
            // to find length of current char
            let pre_len = s.iter.iter.len();
            if let Some((i, c)) = s.next_back() {
                let len = s.iter.iter.len();
                let char_len = pre_len - len;
                if self.char_eq.matches(c) {
                    return SearchStep::Match(i, i + char_len);
                } else {
                    return SearchStep::Reject(i, i + char_len);
                }
            }
            SearchStep::Done
        }
    }

    impl<'a, C: MultiCharEq> DoubleEndedSearcher<'a> for MultiCharEqSearcher<'a, C> {}

    /////////////////////////////////////////////////////////////////////////////

    macro_rules! pattern_methods {
        ($t:ty, $pmap:expr, $smap:expr) => {
            type Searcher = $t;

            #[inline]
            fn into_searcher(self, haystack: &'a str) -> $t {
                ($smap)(($pmap)(self).into_searcher(haystack))
            }

            #[inline]
            fn is_contained_in(self, haystack: &'a str) -> bool {
                ($pmap)(self).is_contained_in(haystack)
            }

            #[inline]
            fn is_prefix_of(self, haystack: &'a str) -> bool {
                ($pmap)(self).is_prefix_of(haystack)
            }

            #[inline]
            fn is_suffix_of(self, haystack: &'a str) -> bool
                where $t: ReverseSearcher<'a>
            {
                ($pmap)(self).is_suffix_of(haystack)
            }
        }
    }

    macro_rules! searcher_methods {
        (forward) => {
            #[inline]
            fn haystack(&self) -> &'a str {
                self.0.haystack()
            }
            #[inline]
            fn next(&mut self) -> SearchStep {
                self.0.next()
            }
            #[inline]
            fn next_match(&mut self) -> Option<(usize, usize)> {
                self.0.next_match()
            }
            #[inline]
            fn next_reject(&mut self) -> Option<(usize, usize)> {
                self.0.next_reject()
            }
        };
        (reverse) => {
            #[inline]
            fn next_back(&mut self) -> SearchStep {
                self.0.next_back()
            }
            #[inline]
            fn next_match_back(&mut self) -> Option<(usize, usize)> {
                self.0.next_match_back()
            }
            #[inline]
            fn next_reject_back(&mut self) -> Option<(usize, usize)> {
                self.0.next_reject_back()
            }
        }
    }

    /////////////////////////////////////////////////////////////////////////////
    // Impl for &[char]
    /////////////////////////////////////////////////////////////////////////////

    // Todo: Change / Remove due to ambiguity in meaning.

    /// Associated type for `<&[char] as Pattern<'a>>::Searcher`.
    #[derive(Clone, Debug)]
    pub struct CharSliceSearcher<'a, 'b>(<MultiCharEqPattern<&'b [char]> as Pattern<'a>>::Searcher);

    unsafe impl<'a, 'b> Searcher<'a> for CharSliceSearcher<'a, 'b> {
        searcher_methods!(forward);
    }

    unsafe impl<'a, 'b> ReverseSearcher<'a> for CharSliceSearcher<'a, 'b> {
        searcher_methods!(reverse);
    }

    impl<'a, 'b> DoubleEndedSearcher<'a> for CharSliceSearcher<'a, 'b> {}

    /// Searches for chars that are equal to any of the chars in the array
    impl<'a, 'b> Pattern<'a> for &'b [char] {
        pattern_methods!(CharSliceSearcher<'a, 'b>, MultiCharEqPattern, CharSliceSearcher);
    }

    /////////////////////////////////////////////////////////////////////////////
    // Impl for F: FnMut(char) -> bool
    /////////////////////////////////////////////////////////////////////////////

    /// Associated type for `<F as Pattern<'a>>::Searcher`.
    #[derive(Clone)]
    pub struct CharPredicateSearcher<'a, F>(<MultiCharEqPattern<F> as Pattern<'a>>::Searcher)
        where F: FnMut(char) -> bool;

    impl<F> fmt::Debug for CharPredicateSearcher<'_, F>
        where F: FnMut(char) -> bool
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("CharPredicateSearcher")
                .field("haystack", &self.0.haystack)
                .field("char_indices", &self.0.char_indices)
                .finish()
        }
    }
    unsafe impl<'a, F> Searcher<'a> for CharPredicateSearcher<'a, F>
        where F: FnMut(char) -> bool
    {
        searcher_methods!(forward);
    }

    unsafe impl<'a, F> ReverseSearcher<'a> for CharPredicateSearcher<'a, F>
        where F: FnMut(char) -> bool
    {
        searcher_methods!(reverse);
    }

    impl<'a, F> DoubleEndedSearcher<'a> for CharPredicateSearcher<'a, F>
        where F: FnMut(char) -> bool {}

    /// Searches for chars that match the given predicate
    impl<'a, F> Pattern<'a> for F where F: FnMut(char) -> bool {
        pattern_methods!(CharPredicateSearcher<'a, F>, MultiCharEqPattern, CharPredicateSearcher);
    }

    /////////////////////////////////////////////////////////////////////////////
    // Impl for &&str
    /////////////////////////////////////////////////////////////////////////////

    /// Delegates to the `&str` impl.
    impl<'a, 'b, 'c> Pattern<'a> for &'c &'b str {
        pattern_methods!(StrSearcher<'a, 'b>, |&s| s, |s| s);
    }

    /////////////////////////////////////////////////////////////////////////////
    // Impl for &str
    /////////////////////////////////////////////////////////////////////////////

    /// Non-allocating substring search.
    ///
    /// Will handle the pattern `""` as returning empty matches at each character
    /// boundary.
    impl<'a, 'b> Pattern<'a> for &'b str {
        type Searcher = StrSearcher<'a, 'b>;

        #[inline]
        fn into_searcher(self, haystack: &'a str) -> StrSearcher<'a, 'b> {
            StrSearcher::new(haystack, self)
        }

        /// Checks whether the pattern matches at the front of the haystack
        #[inline]
        fn is_prefix_of(self, haystack: &'a str) -> bool {
            haystack.is_char_boundary(self.len()) &&
                self == &haystack[..self.len()]
        }

        /// Checks whether the pattern matches at the back of the haystack
        #[inline]
        fn is_suffix_of(self, haystack: &'a str) -> bool {
            self.len() <= haystack.len() &&
                haystack.is_char_boundary(haystack.len() - self.len()) &&
                self == &haystack[haystack.len() - self.len()..]
        }
    }

    /////////////////////////////////////////////////////////////////////////////
    // Two Way substring searcher
    /////////////////////////////////////////////////////////////////////////////

    #[derive(Clone, Debug)]
    /// Associated type for `<&str as Pattern<'a>>::Searcher`.
    pub struct StrSearcher<'a, 'b> {
        haystack: &'a str,
        needle: &'b str,

        searcher: StrSearcherImpl,
    }

    #[derive(Clone, Debug)]
    enum StrSearcherImpl {
        Empty(EmptyNeedle),
        TwoWay(TwoWaySearcher),
    }

    #[derive(Clone, Debug)]
    struct EmptyNeedle {
        position: usize,
        end: usize,
        is_match_fw: bool,
        is_match_bw: bool,
    }

    impl<'a, 'b> StrSearcher<'a, 'b> {
        fn new(haystack: &'a str, needle: &'b str) -> StrSearcher<'a, 'b> {
            if needle.is_empty() {
                StrSearcher {
                    haystack,
                    needle,
                    searcher: StrSearcherImpl::Empty(EmptyNeedle {
                        position: 0,
                        end: haystack.len(),
                        is_match_fw: true,
                        is_match_bw: true,
                    }),
                }
            } else {
                StrSearcher {
                    haystack,
                    needle,
                    searcher: StrSearcherImpl::TwoWay(
                        TwoWaySearcher::new(needle.as_bytes(), haystack.len())
                    ),
                }
            }
        }
    }

    unsafe impl<'a, 'b> Searcher<'a> for StrSearcher<'a, 'b> {
        #[inline]
        fn haystack(&self) -> &'a str {
            self.haystack
        }

        #[inline]
        fn next(&mut self) -> SearchStep {
            match self.searcher {
                StrSearcherImpl::Empty(ref mut searcher) => {
                    // empty needle rejects every char and matches every empty string between them
                    let is_match = searcher.is_match_fw;
                    searcher.is_match_fw = !searcher.is_match_fw;
                    let pos = searcher.position;
                    match self.haystack[pos..].chars().next() {
                        _ if is_match => SearchStep::Match(pos, pos),
                        None => SearchStep::Done,
                        Some(ch) => {
                            searcher.position += ch.len_utf8();
                            SearchStep::Reject(pos, searcher.position)
                        }
                    }
                }
                StrSearcherImpl::TwoWay(ref mut searcher) => {
                    // TwoWaySearcher produces valid *Match* indices that split at char boundaries
                    // as long as it does correct matching and that haystack and needle are
                    // valid UTF-8
                    // *Rejects* from the algorithm can fall on any indices, but we will walk them
                    // manually to the next character boundary, so that they are utf-8 safe.
                    if searcher.position == self.haystack.len() {
                        return SearchStep::Done;
                    }
                    let is_long = searcher.memory == usize::MAX;
                    match searcher.next::<RejectAndMatch>(self.haystack.as_bytes(),
                                                          self.needle.as_bytes(),
                                                          is_long)
                    {
                        SearchStep::Reject(a, mut b) => {
                            // skip to next char boundary
                            while !self.haystack.is_char_boundary(b) {
                                b += 1;
                            }
                            searcher.position = cmp::max(b, searcher.position);
                            SearchStep::Reject(a, b)
                        }
                        otherwise => otherwise,
                    }
                }
            }
        }

        #[inline]
        fn next_match(&mut self) -> Option<(usize, usize)> {
            match self.searcher {
                StrSearcherImpl::Empty(..) => {
                    loop {
                        match self.next() {
                            SearchStep::Match(a, b) => return Some((a, b)),
                            SearchStep::Done => return None,
                            SearchStep::Reject(..) => { }
                        }
                    }
                }
                StrSearcherImpl::TwoWay(ref mut searcher) => {
                    let is_long = searcher.memory == usize::MAX;
                    // write out `true` and `false` cases to encourage the compiler
                    // to specialize the two cases separately.
                    if is_long {
                        searcher.next::<MatchOnly>(self.haystack.as_bytes(),
                                                   self.needle.as_bytes(),
                                                   true)
                    } else {
                        searcher.next::<MatchOnly>(self.haystack.as_bytes(),
                                                   self.needle.as_bytes(),
                                                   false)
                    }
                }
            }
        }
    }

    unsafe impl<'a, 'b> ReverseSearcher<'a> for StrSearcher<'a, 'b> {
        #[inline]
        fn next_back(&mut self) -> SearchStep {
            match self.searcher {
                StrSearcherImpl::Empty(ref mut searcher) => {
                    let is_match = searcher.is_match_bw;
                    searcher.is_match_bw = !searcher.is_match_bw;
                    let end = searcher.end;
                    match self.haystack[..end].chars().next_back() {
                        _ if is_match => SearchStep::Match(end, end),
                        None => SearchStep::Done,
                        Some(ch) => {
                            searcher.end -= ch.len_utf8();
                            SearchStep::Reject(searcher.end, end)
                        }
                    }
                }
                StrSearcherImpl::TwoWay(ref mut searcher) => {
                    if searcher.end == 0 {
                        return SearchStep::Done;
                    }
                    let is_long = searcher.memory == usize::MAX;
                    match searcher.next_back::<RejectAndMatch>(self.haystack.as_bytes(),
                                                               self.needle.as_bytes(),
                                                               is_long)
                    {
                        SearchStep::Reject(mut a, b) => {
                            // skip to next char boundary
                            while !self.haystack.is_char_boundary(a) {
                                a -= 1;
                            }
                            searcher.end = cmp::min(a, searcher.end);
                            SearchStep::Reject(a, b)
                        }
                        otherwise => otherwise,
                    }
                }
            }
        }

        #[inline]
        fn next_match_back(&mut self) -> Option<(usize, usize)> {
            match self.searcher {
                StrSearcherImpl::Empty(..) => {
                    loop {
                        match self.next_back() {
                            SearchStep::Match(a, b) => return Some((a, b)),
                            SearchStep::Done => return None,
                            SearchStep::Reject(..) => { }
                        }
                    }
                }
                StrSearcherImpl::TwoWay(ref mut searcher) => {
                    let is_long = searcher.memory == usize::MAX;
                    // write out `true` and `false`, like `next_match`
                    if is_long {
                        searcher.next_back::<MatchOnly>(self.haystack.as_bytes(),
                                                        self.needle.as_bytes(),
                                                        true)
                    } else {
                        searcher.next_back::<MatchOnly>(self.haystack.as_bytes(),
                                                        self.needle.as_bytes(),
                                                        false)
                    }
                }
            }
        }
    }

    /// The internal state of the two-way substring search algorithm.
    #[derive(Clone, Debug)]
    struct TwoWaySearcher {
        // constants
        /// critical factorization index
        crit_pos: usize,
        /// critical factorization index for reversed needle
        crit_pos_back: usize,
        period: usize,
        /// `byteset` is an extension (not part of the two way algorithm);
        /// it's a 64-bit "fingerprint" where each set bit `j` corresponds
        /// to a (byte & 63) == j present in the needle.
        byteset: u64,

        // variables
        position: usize,
        end: usize,
        /// index into needle before which we have already matched
        memory: usize,
        /// index into needle after which we have already matched
        memory_back: usize,
    }

    /*
        This is the Two-Way search algorithm, which was introduced in the paper:
        Crochemore, M., Perrin, D., 1991, Two-way string-matching, Journal of the ACM 38(3):651-675.

        Here's some background information.

        A *word* is a string of symbols. The *length* of a word should be a familiar
        notion, and here we denote it for any word x by |x|.
        (We also allow for the possibility of the *empty word*, a word of length zero).

        If x is any non-empty word, then an integer p with 0 < p <= |x| is said to be a
        *period* for x iff for all i with 0 <= i <= |x| - p - 1, we have x[i] == x[i+p].
        For example, both 1 and 2 are periods for the string "aa". As another example,
        the only period of the string "abcd" is 4.

        We denote by period(x) the *smallest* period of x (provided that x is non-empty).
        This is always well-defined since every non-empty word x has at least one period,
        |x|. We sometimes call this *the period* of x.

        If u, v and x are words such that x = uv, where uv is the concatenation of u and
        v, then we say that (u, v) is a *factorization* of x.

        Let (u, v) be a factorization for a word x. Then if w is a non-empty word such
        that both of the following hold

          - either w is a suffix of u or u is a suffix of w
          - either w is a prefix of v or v is a prefix of w

        then w is said to be a *repetition* for the factorization (u, v).

        Just to unpack this, there are four possibilities here. Let w = "abc". Then we
        might have:

          - w is a suffix of u and w is a prefix of v. ex: ("lolabc", "abcde")
          - w is a suffix of u and v is a prefix of w. ex: ("lolabc", "ab")
          - u is a suffix of w and w is a prefix of v. ex: ("bc", "abchi")
          - u is a suffix of w and v is a prefix of w. ex: ("bc", "a")

        Note that the word vu is a repetition for any factorization (u,v) of x = uv,
        so every factorization has at least one repetition.

        If x is a string and (u, v) is a factorization for x, then a *local period* for
        (u, v) is an integer r such that there is some word w such that |w| = r and w is
        a repetition for (u, v).

        We denote by local_period(u, v) the smallest local period of (u, v). We sometimes
        call this *the local period* of (u, v). Provided that x = uv is non-empty, this
        is well-defined (because each non-empty word has at least one factorization, as
        noted above).

        It can be proven that the following is an equivalent definition of a local period
        for a factorization (u, v): any positive integer r such that x[i] == x[i+r] for
        all i such that |u| - r <= i <= |u| - 1 and such that both x[i] and x[i+r] are
        defined. (i.e., i > 0 and i + r < |x|).

        Using the above reformulation, it is easy to prove that

            1 <= local_period(u, v) <= period(uv)

        A factorization (u, v) of x such that local_period(u,v) = period(x) is called a
        *critical factorization*.

        The algorithm hinges on the following theorem, which is stated without proof:

        **Critical Factorization Theorem** Any word x has at least one critical
        factorization (u, v) such that |u| < period(x).

        The purpose of maximal_suffix is to find such a critical factorization.

        If the period is short, compute another factorization x = u' v' to use
        for reverse search, chosen instead so that |v'| < period(x).

    */
    impl TwoWaySearcher {
        fn new(needle: &[u8], end: usize) -> TwoWaySearcher {
            let (crit_pos_false, period_false) = TwoWaySearcher::maximal_suffix(needle, false);
            let (crit_pos_true, period_true) = TwoWaySearcher::maximal_suffix(needle, true);

            let (crit_pos, period) =
                if crit_pos_false > crit_pos_true {
                    (crit_pos_false, period_false)
                } else {
                    (crit_pos_true, period_true)
                };

            // A particularly readable explanation of what's going on here can be found
            // in Crochemore and Rytter's book "Text Algorithms", ch 13. Specifically
            // see the code for "Algorithm CP" on p. 323.
            //
            // What's going on is we have some critical factorization (u, v) of the
            // needle, and we want to determine whether u is a suffix of
            // &v[..period]. If it is, we use "Algorithm CP1". Otherwise we use
            // "Algorithm CP2", which is optimized for when the period of the needle
            // is large.
            if &needle[..crit_pos] == &needle[period.. period + crit_pos] {
                // short period case -- the period is exact
                // compute a separate critical factorization for the reversed needle
                // x = u' v' where |v'| < period(x).
                //
                // This is sped up by the period being known already.
                // Note that a case like x = "acba" may be factored exactly forwards
                // (crit_pos = 1, period = 3) while being factored with approximate
                // period in reverse (crit_pos = 2, period = 2). We use the given
                // reverse factorization but keep the exact period.
                let crit_pos_back = needle.len() - cmp::max(
                    TwoWaySearcher::reverse_maximal_suffix(needle, period, false),
                    TwoWaySearcher::reverse_maximal_suffix(needle, period, true));

                TwoWaySearcher {
                    crit_pos,
                    crit_pos_back,
                    period,
                    byteset: Self::byteset_create(&needle[..period]),

                    position: 0,
                    end,
                    memory: 0,
                    memory_back: needle.len(),
                }
            } else {
                // long period case -- we have an approximation to the actual period,
                // and don't use memorization.
                //
                // Approximate the period by lower bound max(|u|, |v|) + 1.
                // The critical factorization is efficient to use for both forward and
                // reverse search.

                TwoWaySearcher {
                    crit_pos,
                    crit_pos_back: crit_pos,
                    period: cmp::max(crit_pos, needle.len() - crit_pos) + 1,
                    byteset: Self::byteset_create(needle),

                    position: 0,
                    end,
                    memory: usize::MAX, // Dummy value to signify that the period is long
                    memory_back: usize::MAX,
                }
            }
        }

        #[inline]
        fn byteset_create(bytes: &[u8]) -> u64 {
            bytes.iter().fold(0, |a, &b| (1 << (b & 0x3f)) | a)
        }

        #[inline]
        fn byteset_contains(&self, byte: u8) -> bool {
            (self.byteset >> ((byte & 0x3f) as usize)) & 1 != 0
        }

        // One of the main ideas of Two-Way is that we factorize the needle into
        // two halves, (u, v), and begin trying to find v in the haystack by scanning
        // left to right. If v matches, we try to match u by scanning right to left.
        // How far we can jump when we encounter a mismatch is all based on the fact
        // that (u, v) is a critical factorization for the needle.
        #[inline]
        fn next<S>(&mut self, haystack: &[u8], needle: &[u8], long_period: bool)
            -> S::Output
            where S: TwoWayStrategy
        {
            // `next()` uses `self.position` as its cursor
            let old_pos = self.position;
            let needle_last = needle.len() - 1;
            'search: loop {
                // Check that we have room to search in
                // position + needle_last can not overflow if we assume slices
                // are bounded by isize's range.
                let tail_byte = match haystack.get(self.position + needle_last) {
                    Some(&b) => b,
                    None => {
                        self.position = haystack.len();
                        return S::rejecting(old_pos, self.position);
                    }
                };

                if S::use_early_reject() && old_pos != self.position {
                    return S::rejecting(old_pos, self.position);
                }

                // Quickly skip by large portions unrelated to our substring
                if !self.byteset_contains(tail_byte) {
                    self.position += needle.len();
                    if !long_period {
                        self.memory = 0;
                    }
                    continue 'search;
                }

                // See if the right part of the needle matches
                let start = if long_period { self.crit_pos }
                            else { cmp::max(self.crit_pos, self.memory) };
                for i in start..needle.len() {
                    if needle[i] != haystack[self.position + i] {
                        self.position += i - self.crit_pos + 1;
                        if !long_period {
                            self.memory = 0;
                        }
                        continue 'search;
                    }
                }

                // See if the left part of the needle matches
                let start = if long_period { 0 } else { self.memory };
                for i in (start..self.crit_pos).rev() {
                    if needle[i] != haystack[self.position + i] {
                        self.position += self.period;
                        if !long_period {
                            self.memory = needle.len() - self.period;
                        }
                        continue 'search;
                    }
                }

                // We have found a match!
                let match_pos = self.position;

                // Note: add self.period instead of needle.len() to have overlapping matches
                self.position += needle.len();
                if !long_period {
                    self.memory = 0; // set to needle.len() - self.period for overlapping matches
                }

                return S::matching(match_pos, match_pos + needle.len());
            }
        }

        // Follows the ideas in `next()`.
        //
        // The definitions are symmetrical, with period(x) = period(reverse(x))
        // and local_period(u, v) = local_period(reverse(v), reverse(u)), so if (u, v)
        // is a critical factorization, so is (reverse(v), reverse(u)).
        //
        // For the reverse case we have computed a critical factorization x = u' v'
        // (field `crit_pos_back`). We need |u| < period(x) for the forward case and
        // thus |v'| < period(x) for the reverse.
        //
        // To search in reverse through the haystack, we search forward through
        // a reversed haystack with a reversed needle, matching first u' and then v'.
        #[inline]
        fn next_back<S>(&mut self, haystack: &[u8], needle: &[u8], long_period: bool)
            -> S::Output
            where S: TwoWayStrategy
        {
            // `next_back()` uses `self.end` as its cursor -- so that `next()` and `next_back()`
            // are independent.
            let old_end = self.end;
            'search: loop {
                // Check that we have room to search in
                // end - needle.len() will wrap around when there is no more room,
                // but due to slice length limits it can never wrap all the way back
                // into the length of haystack.
                let front_byte = match haystack.get(self.end.wrapping_sub(needle.len())) {
                    Some(&b) => b,
                    None => {
                        self.end = 0;
                        return S::rejecting(0, old_end);
                    }
                };

                if S::use_early_reject() && old_end != self.end {
                    return S::rejecting(self.end, old_end);
                }

                // Quickly skip by large portions unrelated to our substring
                if !self.byteset_contains(front_byte) {
                    self.end -= needle.len();
                    if !long_period {
                        self.memory_back = needle.len();
                    }
                    continue 'search;
                }

                // See if the left part of the needle matches
                let crit = if long_period { self.crit_pos_back }
                           else { cmp::min(self.crit_pos_back, self.memory_back) };
                for i in (0..crit).rev() {
                    if needle[i] != haystack[self.end - needle.len() + i] {
                        self.end -= self.crit_pos_back - i;
                        if !long_period {
                            self.memory_back = needle.len();
                        }
                        continue 'search;
                    }
                }

                // See if the right part of the needle matches
                let needle_end = if long_period { needle.len() }
                                 else { self.memory_back };
                for i in self.crit_pos_back..needle_end {
                    if needle[i] != haystack[self.end - needle.len() + i] {
                        self.end -= self.period;
                        if !long_period {
                            self.memory_back = self.period;
                        }
                        continue 'search;
                    }
                }

                // We have found a match!
                let match_pos = self.end - needle.len();
                // Note: sub self.period instead of needle.len() to have overlapping matches
                self.end -= needle.len();
                if !long_period {
                    self.memory_back = needle.len();
                }

                return S::matching(match_pos, match_pos + needle.len());
            }
        }

        // Compute the maximal suffix of `arr`.
        //
        // The maximal suffix is a possible critical factorization (u, v) of `arr`.
        //
        // Returns (`i`, `p`) where `i` is the starting index of v and `p` is the
        // period of v.
        //
        // `order_greater` determines if lexical order is `<` or `>`. Both
        // orders must be computed -- the ordering with the largest `i` gives
        // a critical factorization.
        //
        // For long period cases, the resulting period is not exact (it is too short).
        #[inline]
        fn maximal_suffix(arr: &[u8], order_greater: bool) -> (usize, usize) {
            let mut left = 0; // Corresponds to i in the paper
            let mut right = 1; // Corresponds to j in the paper
            let mut offset = 0; // Corresponds to k in the paper, but starting at 0
                                // to match 0-based indexing.
            let mut period = 1; // Corresponds to p in the paper

            while let Some(&a) = arr.get(right + offset) {
                // `left` will be inbounds when `right` is.
                let b = arr[left + offset];
                if (a < b && !order_greater) || (a > b && order_greater) {
                    // Suffix is smaller, period is entire prefix so far.
                    right += offset + 1;
                    offset = 0;
                    period = right - left;
                } else if a == b {
                    // Advance through repetition of the current period.
                    if offset + 1 == period {
                        right += offset + 1;
                        offset = 0;
                    } else {
                        offset += 1;
                    }
                } else {
                    // Suffix is larger, start over from current location.
                    left = right;
                    right += 1;
                    offset = 0;
                    period = 1;
                }
            }
            (left, period)
        }

        // Compute the maximal suffix of the reverse of `arr`.
        //
        // The maximal suffix is a possible critical factorization (u', v') of `arr`.
        //
        // Returns `i` where `i` is the starting index of v', from the back;
        // returns immediately when a period of `known_period` is reached.
        //
        // `order_greater` determines if lexical order is `<` or `>`. Both
        // orders must be computed -- the ordering with the largest `i` gives
        // a critical factorization.
        //
        // For long period cases, the resulting period is not exact (it is too short).
        fn reverse_maximal_suffix(arr: &[u8], known_period: usize,
                                  order_greater: bool) -> usize
        {
            let mut left = 0; // Corresponds to i in the paper
            let mut right = 1; // Corresponds to j in the paper
            let mut offset = 0; // Corresponds to k in the paper, but starting at 0
                                // to match 0-based indexing.
            let mut period = 1; // Corresponds to p in the paper
            let n = arr.len();

            while right + offset < n {
                let a = arr[n - (1 + right + offset)];
                let b = arr[n - (1 + left + offset)];
                if (a < b && !order_greater) || (a > b && order_greater) {
                    // Suffix is smaller, period is entire prefix so far.
                    right += offset + 1;
                    offset = 0;
                    period = right - left;
                } else if a == b {
                    // Advance through repetition of the current period.
                    if offset + 1 == period {
                        right += offset + 1;
                        offset = 0;
                    } else {
                        offset += 1;
                    }
                } else {
                    // Suffix is larger, start over from current location.
                    left = right;
                    right += 1;
                    offset = 0;
                    period = 1;
                }
                if period == known_period {
                    break;
                }
            }
            debug_assert!(period <= known_period);
            left
        }
    }

    // TwoWayStrategy allows the algorithm to either skip non-matches as quickly
    // as possible, or to work in a mode where it emits Rejects relatively quickly.
    trait TwoWayStrategy {
        type Output;
        fn use_early_reject() -> bool;
        fn rejecting(a: usize, b: usize) -> Self::Output;
        fn matching(a: usize, b: usize) -> Self::Output;
    }

    /// Skip to match intervals as quickly as possible
    enum MatchOnly { }

    impl TwoWayStrategy for MatchOnly {
        type Output = Option<(usize, usize)>;

        #[inline]
        fn use_early_reject() -> bool { false }
        #[inline]
        fn rejecting(_a: usize, _b: usize) -> Self::Output { None }
        #[inline]
        fn matching(a: usize, b: usize) -> Self::Output { Some((a, b)) }
    }

    /// Emit Rejects regularly
    enum RejectAndMatch { }

    impl TwoWayStrategy for RejectAndMatch {
        type Output = SearchStep;

        #[inline]
        fn use_early_reject() -> bool { true }
        #[inline]
        fn rejecting(a: usize, b: usize) -> Self::Output { SearchStep::Reject(a, b) }
        #[inline]
        fn matching(a: usize, b: usize) -> Self::Output { SearchStep::Match(a, b) }
    }

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