1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174

use core::iter::FusedIterator;

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)
    }
}

impl<I: Iterator<Item = char> + FusedIterator> FusedIterator for StreamSafe<I> {}

#[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 alloc::{string::String, vec::Vec};

    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);
        }
    }
}