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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.

    //! Functions for computing canonical and compatible decompositions for Unicode characters.
    use crate::lookups::{
        canonical_fully_decomposed, cjk_compat_variants_fully_decomposed,
        compatibility_fully_decomposed, composition_table,
    };

    use core::{char, ops::FnMut};

    /// Compute canonical Unicode decomposition for character.
    /// See [Unicode Standard Annex #15](http://www.unicode.org/reports/tr15/)
    /// for more information.
    #[inline]
    pub fn decompose_canonical<F>(c: char, emit_char: F)
    where
        F: FnMut(char),
    {
        decompose(c, canonical_fully_decomposed, emit_char)
    }

    /// Compute canonical or compatible Unicode decomposition for character.
    /// See [Unicode Standard Annex #15](http://www.unicode.org/reports/tr15/)
    /// for more information.
    #[inline]
    pub fn decompose_compatible<F: FnMut(char)>(c: char, emit_char: F) {
        let decompose_char =
            |c| compatibility_fully_decomposed(c).or_else(|| canonical_fully_decomposed(c));
        decompose(c, decompose_char, emit_char)
    }

    /// Compute standard-variation decomposition for character.
    ///
    /// [Standardized Variation Sequences] are used instead of the standard canonical
    /// decompositions, notably for CJK codepoints with singleton canonical decompositions,
    /// to avoid losing information. See the
    /// [Unicode Variation Sequence FAQ](http://unicode.org/faq/vs.html) and the
    /// "Other Enhancements" section of the
    /// [Unicode 6.3 Release Summary](https://www.unicode.org/versions/Unicode6.3.0/#Summary)
    /// for more information.
    #[inline]
    pub fn decompose_cjk_compat_variants<F>(c: char, mut emit_char: F)
    where
        F: FnMut(char),
    {
        // 7-bit ASCII never decomposes
        if c <= '\x7f' {
            emit_char(c);
            return;
        }

        // Don't perform decomposition for Hangul

        if let Some(decomposed) = cjk_compat_variants_fully_decomposed(c) {
            for &d in decomposed {
                emit_char(d);
            }
            return;
        }

        // Finally bottom out.
        emit_char(c);
    }

    #[inline]
    fn decompose<D, F>(c: char, decompose_char: D, mut emit_char: F)
    where
        D: Fn(char) -> Option<&'static [char]>,
        F: FnMut(char),
    {
        // 7-bit ASCII never decomposes
        if c <= '\x7f' {
            emit_char(c);
            return;
        }

        // Perform decomposition for Hangul
        if is_hangul_syllable(c) {
            decompose_hangul(c, emit_char);
            return;
        }

        if let Some(decomposed) = decompose_char(c) {
            for &d in decomposed {
                emit_char(d);
            }
            return;
        }

        // Finally bottom out.
        emit_char(c);
    }

    /// Compose two characters into a single character, if possible.
    /// See [Unicode Standard Annex #15](http://www.unicode.org/reports/tr15/)
    /// for more information.
    pub fn compose(a: char, b: char) -> Option<char> {
        compose_hangul(a, b).or_else(|| composition_table(a, b))
    }

    // Constants from Unicode 9.0.0 Section 3.12 Conjoining Jamo Behavior
    // http://www.unicode.org/versions/Unicode9.0.0/ch03.pdf#M9.32468.Heading.310.Combining.Jamo.Behavior
    const S_BASE: u32 = 0xAC00;
    const L_BASE: u32 = 0x1100;
    const V_BASE: u32 = 0x1161;
    const T_BASE: u32 = 0x11A7;
    const L_COUNT: u32 = 19;
    const V_COUNT: u32 = 21;
    const T_COUNT: u32 = 28;
    const N_COUNT: u32 = V_COUNT * T_COUNT;
    const S_COUNT: u32 = L_COUNT * N_COUNT;

    const S_LAST: u32 = S_BASE + S_COUNT - 1;
    const L_LAST: u32 = L_BASE + L_COUNT - 1;
    const V_LAST: u32 = V_BASE + V_COUNT - 1;
    const T_LAST: u32 = T_BASE + T_COUNT - 1;

    // Composition only occurs for `TPart`s in `U+11A8 ... U+11C2`,
    // i.e. `T_BASE + 1 ... T_LAST`.
    const T_FIRST: u32 = T_BASE + 1;

    pub(crate) fn is_hangul_syllable(c: char) -> bool {
        (c as u32) >= S_BASE && (c as u32) < (S_BASE + S_COUNT)
    }

    // Decompose a precomposed Hangul syllable
    #[allow(unsafe_code)]
    #[inline(always)]
    fn decompose_hangul<F>(s: char, mut emit_char: F)
    where
        F: FnMut(char),
    {
        let s_index = s as u32 - S_BASE;
        let l_index = s_index / N_COUNT;
        unsafe {
            emit_char(char::from_u32_unchecked(L_BASE + l_index));

            let v_index = (s_index % N_COUNT) / T_COUNT;
            emit_char(char::from_u32_unchecked(V_BASE + v_index));

            let t_index = s_index % T_COUNT;
            if t_index > 0 {
                emit_char(char::from_u32_unchecked(T_BASE + t_index));
            }
        }
    }

    #[inline]
    pub(crate) fn hangul_decomposition_length(s: char) -> usize {
        let si = s as u32 - S_BASE;
        let ti = si % T_COUNT;
        if ti > 0 {
            3
        } else {
            2
        }
    }

    // Compose a pair of Hangul Jamo
    #[allow(unsafe_code)]
    #[inline(always)]
    #[allow(ellipsis_inclusive_range_patterns)]
    fn compose_hangul(a: char, b: char) -> Option<char> {
        let (a, b) = (a as u32, b as u32);
        match (a, b) {
            // Compose a leading consonant and a vowel together into an LV_Syllable
            (L_BASE...L_LAST, V_BASE...V_LAST) => {
                let l_index = a - L_BASE;
                let v_index = b - V_BASE;
                let lv_index = l_index * N_COUNT + v_index * T_COUNT;
                let s = S_BASE + lv_index;
                Some(unsafe { char::from_u32_unchecked(s) })
            }
            // Compose an LV_Syllable and a trailing consonant into an LVT_Syllable
            (S_BASE...S_LAST, T_FIRST...T_LAST) if (a - S_BASE) % T_COUNT == 0 => {
                Some(unsafe { char::from_u32_unchecked(a + (b - T_BASE)) })
            }
            _ => None,
        }
    }

    #[cfg(test)]
    mod tests {
        use super::compose_hangul;

        // Regression test from a bugfix where we were composing an LV_Syllable with
        // T_BASE directly. (We should only compose an LV_Syllable with a character
        // in the range `T_BASE + 1 ... T_LAST`.)
        #[test]
        fn test_hangul_composition() {
            assert_eq!(compose_hangul('\u{c8e0}', '\u{11a7}'), None);
        }
    }