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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.

    /// Slot is a single saved capture location. Note that there are two slots for
    /// every capture in a regular expression (one slot each for the start and end
    /// of the capture).
    pub type Slot = Option<usize>;

    /// RegularExpression describes types that can implement regex searching.
    ///
    /// This trait is my attempt at reducing code duplication and to standardize
    /// the internal API. Specific duplication that is avoided are the `find`
    /// and `capture` iterators, which are slightly tricky.
    ///
    /// It's not clear whether this trait is worth it, and it also isn't
    /// clear whether it's useful as a public trait or not. Methods like
    /// `next_after_empty` reak of bad design, but the rest of the methods seem
    /// somewhat reasonable. One particular thing this trait would expose would be
    /// the ability to start the search of a regex anywhere in a haystack, which
    /// isn't possible in the current public API.
    pub trait RegularExpression: Sized {
        /// The type of the haystack.
        type Text: ?Sized;

        /// The number of capture slots in the compiled regular expression. This is
        /// always two times the number of capture groups (two slots per group).
        fn slots_len(&self) -> usize;

        /// Returns the position of the next character after `i`.
        ///
        /// For example, a haystack with type `&[u8]` probably returns `i+1`,
        /// whereas a haystack with type `&str` probably returns `i` plus the
        /// length of the next UTF-8 sequence.
        fn next_after_empty(&self, text: &Self::Text, i: usize) -> usize;

        /// Returns the location of the shortest match.
        fn shortest_match_at(
            &self,
            text: &Self::Text,
            start: usize,
        ) -> Option<usize>;

        /// Returns whether the regex matches the text given.
        fn is_match_at(
            &self,
            text: &Self::Text,
            start: usize,
        ) -> bool;

        /// Returns the leftmost-first match location if one exists.
        fn find_at(
            &self,
            text: &Self::Text,
            start: usize,
        ) -> Option<(usize, usize)>;

        /// Returns the leftmost-first match location if one exists, and also
        /// fills in any matching capture slot locations.
        fn read_captures_at(
            &self,
            slots: &mut [Slot],
            text: &Self::Text,
            start: usize,
        ) -> Option<(usize, usize)>;

        /// Returns an iterator over all non-overlapping successive leftmost-first
        /// matches.
        fn find_iter<'t>(
            self,
            text: &'t Self::Text,
        ) -> FindMatches<'t, Self> {
            FindMatches {
                re: self,
                text: text,
                last_end: 0,
                last_match: None,
            }
        }

        /// Returns an iterator over all non-overlapping successive leftmost-first
        /// matches with captures.
        fn captures_iter<'t>(
            self,
            text: &'t Self::Text,
        ) -> FindCaptures<'t, Self> {
            FindCaptures(self.find_iter(text))
        }
    }

    /// An iterator over all non-overlapping successive leftmost-first matches.
    pub struct FindMatches<'t, R> where R: RegularExpression, R::Text: 't {
        re: R,
        text: &'t R::Text,
        last_end: usize,
        last_match: Option<usize>,
    }

    impl<'t, R> FindMatches<'t, R> where R: RegularExpression, R::Text: 't {
        /// Return the text being searched.
        pub fn text(&self) -> &'t R::Text {
            self.text
        }

        /// Return the underlying regex.
        pub fn regex(&self) -> &R {
            &self.re
        }
    }

    impl<'t, R> Iterator for FindMatches<'t, R>
            where R: RegularExpression, R::Text: 't + AsRef<[u8]> {
        type Item = (usize, usize);

        fn next(&mut self) -> Option<(usize, usize)> {
            if self.last_end > self.text.as_ref().len() {
                return None;
            }
            let (s, e) = match self.re.find_at(self.text, self.last_end) {
                None => return None,
                Some((s, e)) => (s, e),
            };
            if s == e {
                // This is an empty match. To ensure we make progress, start
                // the next search at the smallest possible starting position
                // of the next match following this one.
                self.last_end = self.re.next_after_empty(&self.text, e);
                // Don't accept empty matches immediately following a match.
                // Just move on to the next match.
                if Some(e) == self.last_match {
                    return self.next();
                }
            } else {
                self.last_end = e;
            }
            self.last_match = Some(e);
            Some((s, e))
        }
    }

    /// An iterator over all non-overlapping successive leftmost-first matches with
    /// captures.
    pub struct FindCaptures<'t, R>(FindMatches<'t, R>)
        where R: RegularExpression, R::Text: 't;

    impl<'t, R> FindCaptures<'t, R> where R: RegularExpression, R::Text: 't {
        /// Return the text being searched.
        pub fn text(&self) -> &'t R::Text {
            self.0.text()
        }

        /// Return the underlying regex.
        pub fn regex(&self) -> &R {
            self.0.regex()
        }
    }

    impl<'t, R> Iterator for FindCaptures<'t, R>
            where R: RegularExpression, R::Text: 't + AsRef<[u8]> {
        type Item = Vec<Slot>;

        fn next(&mut self) -> Option<Vec<Slot>> {
            if self.0.last_end > self.0.text.as_ref().len() {
                return None
            }
            let mut slots = vec![None; self.0.re.slots_len()];
            let (s, e) = match self.0.re.read_captures_at(
                &mut slots,
                self.0.text,
                self.0.last_end,
            ) {
                None => return None,
                Some((s, e)) => (s, e),
            };
            if s == e {
                self.0.last_end = self.0.re.next_after_empty(&self.0.text, e);
                if Some(e) == self.0.last_match {
                    return self.next();
                }
            } else {
                self.0.last_end = e;
            }
            self.0.last_match = Some(e);
            Some(slots)
        }
    }

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