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    // Copyright 2012-2015 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 core::fmt::{self, Write};
    use core::iter::Fuse;
    use core::ops::Range;
    use tinyvec::TinyVec;

    #[derive(Clone)]
    enum DecompositionType {
        Canonical,
        Compatible,
    }

    /// External iterator for a string decomposition's characters.
    #[derive(Clone)]
    pub struct Decompositions<I> {
        kind: DecompositionType,
        iter: Fuse<I>,

        // This buffer stores pairs of (canonical combining class, character),
        // pushed onto the end in text order.
        //
        // It's divided into up to three sections:
        // 1) A prefix that is free space;
        // 2) "Ready" characters which are sorted and ready to emit on demand;
        // 3) A "pending" block which stills needs more characters for us to be able
        //    to sort in canonical order and is not safe to emit.
        buffer: TinyVec<[(u8, char); 4]>,
        ready: Range<usize>,
    }

    #[inline]
    pub fn new_canonical<I: Iterator<Item = char>>(iter: I) -> Decompositions<I> {
        Decompositions {
            kind: self::DecompositionType::Canonical,
            iter: iter.fuse(),
            buffer: TinyVec::new(),
            ready: 0..0,
        }
    }

    #[inline]
    pub fn new_compatible<I: Iterator<Item = char>>(iter: I) -> Decompositions<I> {
        Decompositions {
            kind: self::DecompositionType::Compatible,
            iter: iter.fuse(),
            buffer: TinyVec::new(),
            ready: 0..0,
        }
    }

    impl<I> Decompositions<I> {
        #[inline]
        fn push_back(&mut self, ch: char) {
            let class = super::char::canonical_combining_class(ch);

            if class == 0 {
                self.sort_pending();
                self.buffer.push((class, ch));
                self.ready.end = self.buffer.len();
            } else {
                self.buffer.push((class, ch));
            }
        }

        #[inline]
        fn sort_pending(&mut self) {
            // NB: `sort_by_key` is stable, so it will preserve the original text's
            // order within a combining class.
            self.buffer[self.ready.end..].sort_by_key(|k| k.0);
        }

        #[inline]
        fn reset_buffer(&mut self) {
            // Equivalent to `self.buffer.drain(0..self.ready.end)`
            // but faster than drain() if the buffer is a SmallVec or TinyVec
            let pending = self.buffer.len() - self.ready.end;
            for i in 0..pending {
                self.buffer[i] = self.buffer[i + self.ready.end];
            }
            self.buffer.truncate(pending);
            self.ready = 0..0;
        }

        #[inline]
        fn increment_next_ready(&mut self) {
            let next = self.ready.start + 1;
            if next == self.ready.end {
                self.reset_buffer();
            } else {
                self.ready.start = next;
            }
        }
    }

    impl<I: Iterator<Item = char>> Iterator for Decompositions<I> {
        type Item = char;

        #[inline]
        fn next(&mut self) -> Option<char> {
            while self.ready.end == 0 {
                match (self.iter.next(), &self.kind) {
                    (Some(ch), &DecompositionType::Canonical) => {
                        super::char::decompose_canonical(ch, |d| self.push_back(d));
                    }
                    (Some(ch), &DecompositionType::Compatible) => {
                        super::char::decompose_compatible(ch, |d| self.push_back(d));
                    }
                    (None, _) => {
                        if self.buffer.is_empty() {
                            return None;
                        } else {
                            self.sort_pending();
                            self.ready.end = self.buffer.len();

                            // This implementation means that we can call `next`
                            // on an exhausted iterator; the last outer `next` call
                            // will result in an inner `next` call. To make this
                            // safe, we use `fuse`.
                            break;
                        }
                    }
                }
            }

            // We can assume here that, if `self.ready.end` is greater than zero,
            // it's also greater than `self.ready.start`. That's because we only
            // increment `self.ready.start` inside `increment_next_ready`, and
            // whenever it reaches equality with `self.ready.end`, we reset both
            // to zero, maintaining the invariant that:
            //      self.ready.start < self.ready.end || self.ready.end == self.ready.start == 0
            //
            // This less-than-obviously-safe implementation is chosen for performance,
            // minimizing the number & complexity of branches in `next` in the common
            // case of buffering then unbuffering a single character with each call.
            let (_, ch) = self.buffer[self.ready.start];
            self.increment_next_ready();
            Some(ch)
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            let (lower, _) = self.iter.size_hint();
            (lower, None)
        }
    }

    impl<I: Iterator<Item = char> + Clone> fmt::Display for Decompositions<I> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            for c in self.clone() {
                f.write_char(c)?;
            }
            Ok(())
        }
    }