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    use crate::lookups::{
        canonical_combining_class, canonical_fully_decomposed, compatibility_fully_decomposed,
        stream_safe_trailing_nonstarters,
    };
    use crate::normalize::{hangul_decomposition_length, is_hangul_syllable};
    use crate::tables::stream_safe_leading_nonstarters;

    pub(crate) const MAX_NONSTARTERS: usize = 30;
    const COMBINING_GRAPHEME_JOINER: char = '\u{034F}';

    /// UAX15-D4: This iterator keeps track of how many non-starters there have been
    /// since the last starter in *NFKD* and will emit a Combining Grapheme Joiner
    /// (U+034F) if the count exceeds 30.
    pub struct StreamSafe<I> {
        iter: I,
        nonstarter_count: usize,
        buffer: Option<char>,
    }

    impl<I> StreamSafe<I> {
        pub(crate) fn new(iter: I) -> Self {
            Self {
                iter,
                nonstarter_count: 0,
                buffer: None,
            }
        }
    }

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

        #[inline]
        fn next(&mut self) -> Option<char> {
            let next_ch = match self.buffer.take().or_else(|| self.iter.next()) {
                None => return None,
                Some(c) => c,
            };
            let d = classify_nonstarters(next_ch);
            if self.nonstarter_count + d.leading_nonstarters > MAX_NONSTARTERS {
                // Since we're emitting a CGJ, the suffix of the emitted string in NFKD has no trailing
                // nonstarters, so we can reset the counter to zero. Put `next_ch` back into the
                // iterator (via `self.buffer`), and we'll reclassify it next iteration.
                self.nonstarter_count = 0;
                self.buffer = Some(next_ch);
                return Some(COMBINING_GRAPHEME_JOINER);
            }

            // Is the character all nonstarters in NFKD? If so, increment our counter of contiguous
            // nonstarters in NKFD.
            if d.leading_nonstarters == d.decomposition_len {
                self.nonstarter_count += d.decomposition_len;
            }
            // Otherwise, reset the counter to the decomposition's number of trailing nonstarters.
            else {
                self.nonstarter_count = d.trailing_nonstarters;
            }
            Some(next_ch)
        }
    }

    #[derive(Debug)]
    pub(crate) struct Decomposition {
        pub(crate) leading_nonstarters: usize,
        pub(crate) trailing_nonstarters: usize,
        pub(crate) decomposition_len: usize,
    }

    #[inline]
    pub(crate) fn classify_nonstarters(c: char) -> Decomposition {
        // As usual, fast path for ASCII (which is always a starter)
        if c <= '\x7f' {
            return Decomposition {
                leading_nonstarters: 0,
                trailing_nonstarters: 0,
                decomposition_len: 1,
            };
        }
        // Next, special case Hangul, since it's not handled by our tables.
        if is_hangul_syllable(c) {
            return Decomposition {
                leading_nonstarters: 0,
                trailing_nonstarters: 0,
                decomposition_len: hangul_decomposition_length(c),
            };
        }
        let decomp = compatibility_fully_decomposed(c).or_else(|| canonical_fully_decomposed(c));
        match decomp {
            Some(decomp) => Decomposition {
                leading_nonstarters: stream_safe_leading_nonstarters(c),
                trailing_nonstarters: stream_safe_trailing_nonstarters(c),
                decomposition_len: decomp.len(),
            },
            None => {
                let is_nonstarter = canonical_combining_class(c) != 0;
                let nonstarter = if is_nonstarter { 1 } else { 0 };
                Decomposition {
                    leading_nonstarters: nonstarter,
                    trailing_nonstarters: nonstarter,
                    decomposition_len: 1,
                }
            }
        }
    }

    #[cfg(test)]
    mod tests {
        use super::{classify_nonstarters, StreamSafe};
        use crate::lookups::canonical_combining_class;
        use crate::normalize::decompose_compatible;

        #[cfg(not(feature = "std"))]
        use crate::no_std_prelude::*;

        use core::char;

        fn stream_safe(s: &str) -> String {
            StreamSafe::new(s.chars()).collect()
        }

        #[test]
        fn test_simple() {
            let technically_okay = "Da\u{0300}\u{0301}\u{0302}\u{0303}\u{0304}\u{0305}\u{0306}\u{0307}\u{0308}\u{0309}\u{030a}\u{030b}\u{030c}\u{030d}\u{030e}\u{030f}\u{0310}\u{0311}\u{0312}\u{0313}\u{0314}\u{0315}\u{0316}\u{0317}\u{0318}\u{0319}\u{031a}\u{031b}\u{031c}\u{031d}ngerzone";
            assert_eq!(stream_safe(technically_okay), technically_okay);

            let too_much = "Da\u{0300}\u{0301}\u{0302}\u{0303}\u{0304}\u{0305}\u{0306}\u{0307}\u{0308}\u{0309}\u{030a}\u{030b}\u{030c}\u{030d}\u{030e}\u{030f}\u{0310}\u{0311}\u{0312}\u{0313}\u{0314}\u{0315}\u{0316}\u{0317}\u{0318}\u{0319}\u{031a}\u{031b}\u{031c}\u{031d}\u{032e}ngerzone";
            let fixed_it = "Da\u{0300}\u{0301}\u{0302}\u{0303}\u{0304}\u{0305}\u{0306}\u{0307}\u{0308}\u{0309}\u{030a}\u{030b}\u{030c}\u{030d}\u{030e}\u{030f}\u{0310}\u{0311}\u{0312}\u{0313}\u{0314}\u{0315}\u{0316}\u{0317}\u{0318}\u{0319}\u{031a}\u{031b}\u{031c}\u{031d}\u{034f}\u{032e}ngerzone";
            assert_eq!(stream_safe(too_much), fixed_it);

            let woah_nelly = "Da\u{0300}\u{0301}\u{0302}\u{0303}\u{0304}\u{0305}\u{0306}\u{0307}\u{0308}\u{0309}\u{030a}\u{030b}\u{030c}\u{030d}\u{030e}\u{030f}\u{0310}\u{0311}\u{0312}\u{0313}\u{0314}\u{0315}\u{0316}\u{0317}\u{0318}\u{0319}\u{031a}\u{031b}\u{031c}\u{031d}\u{032e}\u{0300}\u{0301}\u{0302}\u{0303}\u{0304}\u{0305}\u{0306}\u{0307}\u{0308}\u{0309}\u{030a}\u{030b}\u{030c}\u{030d}\u{030e}\u{030f}\u{0310}\u{0311}\u{0312}\u{0313}\u{0314}\u{0315}\u{0316}\u{0317}\u{0318}\u{0319}\u{031a}\u{031b}\u{031c}\u{031d}\u{032e}ngerzone";
            let its_cool = "Da\u{0300}\u{0301}\u{0302}\u{0303}\u{0304}\u{0305}\u{0306}\u{0307}\u{0308}\u{0309}\u{030a}\u{030b}\u{030c}\u{030d}\u{030e}\u{030f}\u{0310}\u{0311}\u{0312}\u{0313}\u{0314}\u{0315}\u{0316}\u{0317}\u{0318}\u{0319}\u{031a}\u{031b}\u{031c}\u{031d}\u{034f}\u{032e}\u{0300}\u{0301}\u{0302}\u{0303}\u{0304}\u{0305}\u{0306}\u{0307}\u{0308}\u{0309}\u{030a}\u{030b}\u{030c}\u{030d}\u{030e}\u{030f}\u{0310}\u{0311}\u{0312}\u{0313}\u{0314}\u{0315}\u{0316}\u{0317}\u{0318}\u{0319}\u{031a}\u{031b}\u{031c}\u{034f}\u{031d}\u{032e}ngerzone";
            assert_eq!(stream_safe(woah_nelly), its_cool);
        }

        #[test]
        fn test_all_nonstarters() {
            let s = "\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}";
            let expected = "\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{034F}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}";
            assert_eq!(stream_safe(s), expected);
        }

        #[test]
        fn test_classify_nonstarters() {
            // Highest character in the `compat_fully_decomp` table is 2FA1D
            for ch in 0..0x2FA1E {
                let ch = match char::from_u32(ch) {
                    Some(c) => c,
                    None => continue,
                };
                let c = classify_nonstarters(ch);
                let mut s = Vec::new();
                decompose_compatible(ch, |c| s.push(c));

                assert_eq!(s.len(), c.decomposition_len);

                let num_leading = s
                    .iter()
                    .take_while(|&c| canonical_combining_class(*c) != 0)
                    .count();
                let num_trailing = s
                    .iter()
                    .rev()
                    .take_while(|&c| canonical_combining_class(*c) != 0)
                    .count();

                assert_eq!(num_leading, c.leading_nonstarters);
                assert_eq!(num_trailing, c.trailing_nonstarters);
            }
        }
    }