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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 crate::decompose::Decompositions;
    use core::fmt::{self, Write};
    use tinyvec::TinyVec;

    #[derive(Clone)]
    enum RecompositionState {
        Composing,
        Purging(usize),
        Finished(usize),
    }

    /// External iterator for a string recomposition's characters.
    #[derive(Clone)]
    pub struct Recompositions<I> {
        iter: Decompositions<I>,
        state: RecompositionState,
        buffer: TinyVec<[char; 4]>,
        composee: Option<char>,
        last_ccc: Option<u8>,
    }

    #[inline]
    pub fn new_canonical<I: Iterator<Item = char>>(iter: I) -> Recompositions<I> {
        Recompositions {
            iter: super::decompose::new_canonical(iter),
            state: self::RecompositionState::Composing,
            buffer: TinyVec::new(),
            composee: None,
            last_ccc: None,
        }
    }

    #[inline]
    pub fn new_compatible<I: Iterator<Item = char>>(iter: I) -> Recompositions<I> {
        Recompositions {
            iter: super::decompose::new_compatible(iter),
            state: self::RecompositionState::Composing,
            buffer: TinyVec::new(),
            composee: None,
            last_ccc: None,
        }
    }

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

        #[inline]
        fn next(&mut self) -> Option<char> {
            use self::RecompositionState::*;

            loop {
                match self.state {
                    Composing => {
                        for ch in self.iter.by_ref() {
                            let ch_class = super::char::canonical_combining_class(ch);
                            let k = match self.composee {
                                None => {
                                    if ch_class != 0 {
                                        return Some(ch);
                                    }
                                    self.composee = Some(ch);
                                    continue;
                                }
                                Some(k) => k,
                            };
                            match self.last_ccc {
                                None => match super::char::compose(k, ch) {
                                    Some(r) => {
                                        self.composee = Some(r);
                                        continue;
                                    }
                                    None => {
                                        if ch_class == 0 {
                                            self.composee = Some(ch);
                                            return Some(k);
                                        }
                                        self.buffer.push(ch);
                                        self.last_ccc = Some(ch_class);
                                    }
                                },
                                Some(l_class) => {
                                    if l_class >= ch_class {
                                        // `ch` is blocked from `composee`
                                        if ch_class == 0 {
                                            self.composee = Some(ch);
                                            self.last_ccc = None;
                                            self.state = Purging(0);
                                            return Some(k);
                                        }
                                        self.buffer.push(ch);
                                        self.last_ccc = Some(ch_class);
                                        continue;
                                    }
                                    match super::char::compose(k, ch) {
                                        Some(r) => {
                                            self.composee = Some(r);
                                            continue;
                                        }
                                        None => {
                                            self.buffer.push(ch);
                                            self.last_ccc = Some(ch_class);
                                        }
                                    }
                                }
                            }
                        }
                        self.state = Finished(0);
                        if self.composee.is_some() {
                            return self.composee.take();
                        }
                    }
                    Purging(next) => match self.buffer.get(next).cloned() {
                        None => {
                            self.buffer.clear();
                            self.state = Composing;
                        }
                        s => {
                            self.state = Purging(next + 1);
                            return s;
                        }
                    },
                    Finished(next) => match self.buffer.get(next).cloned() {
                        None => {
                            self.buffer.clear();
                            return self.composee.take();
                        }
                        s => {
                            self.state = Finished(next + 1);
                            return s;
                        }
                    },
                }
            }
        }
    }

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