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
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
// This file is part of ICU4X. For terms of use, please see the file
// called LICENSE at the top level of the ICU4X source tree
// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).

pub mod errors;
mod langid;
mod locale;

pub use errors::ParserError;
pub use langid::{
    parse_language_identifier, parse_language_identifier_from_iter,
    parse_language_identifier_with_single_variant,
    parse_locale_with_single_variant_single_keyword_unicode_extension_from_iter, ParserMode,
};

pub use locale::{
    parse_locale, parse_locale_with_single_variant_single_keyword_unicode_keyword_extension,
};

#[inline]
const fn is_separator(slice: &[u8], idx: usize) -> bool {
    #[allow(clippy::indexing_slicing)]
    let b = slice[idx];
    b == b'-' || b == b'_'
}

const fn get_current_subtag(slice: &[u8], idx: usize) -> (usize, usize) {
    debug_assert!(idx < slice.len());

    // This function is called only on the idx == 0 or on a separator.
    let (start, mut end) = if is_separator(slice, idx) {
        // If it's a separator, set the start to idx+1 and advance the idx to the next char.
        (idx + 1, idx + 1)
    } else {
        // If it's idx=0, start is 0 and end is set to 1
        debug_assert!(idx == 0);
        (0, 1)
    };

    while end < slice.len() && !is_separator(slice, end) {
        // Advance until we reach end of slice or a separator.
        end += 1;
    }
    // Notice: this slice may be empty (start == end) for cases like `"en-"` or `"en--US"`
    (start, end)
}

// `SubtagIterator` is a helper iterator for [`LanguageIdentifier`] and [`Locale`] parsing.
//
// It is quite extraordinary due to focus on performance and Rust limitations for `const`
// functions.
//
// The iterator is eager and fallible allowing it to reject invalid slices such as `"-"`, `"-en"`,
// `"en-"` etc.
//
// The iterator provides methods available for static users - `next_manual` and `peek_manual`,
// as well as typical `Peekable` iterator APIs - `next` and `peek`.
//
// All methods return an `Option` of a `Result`.
#[derive(Copy, Clone, Debug)]
pub struct SubtagIterator<'a> {
    pub slice: &'a [u8],
    done: bool,
    // done + subtag is faster than Option<(usize, usize)>
    // at the time of writing.
    subtag: (usize, usize),
}

impl<'a> SubtagIterator<'a> {
    pub const fn new(slice: &'a [u8]) -> Self {
        let subtag = if slice.is_empty() || is_separator(slice, 0) {
            // This returns (0, 0) which returns Some(b"") for slices like `"-en"` or `"-"`
            (0, 0)
        } else {
            get_current_subtag(slice, 0)
        };
        Self {
            slice,
            done: false,
            subtag,
        }
    }

    pub const fn next_manual(mut self) -> (Self, Option<(usize, usize)>) {
        if self.done {
            return (self, None);
        }
        let result = self.subtag;
        if result.1 < self.slice.len() {
            self.subtag = get_current_subtag(self.slice, result.1);
        } else {
            self.done = true;
        }
        (self, Some(result))
    }

    pub const fn peek_manual(&self) -> Option<(usize, usize)> {
        if self.done {
            return None;
        }
        Some(self.subtag)
    }

    pub fn peek(&self) -> Option<&'a [u8]> {
        #[allow(clippy::indexing_slicing)] // peek_manual returns valid indices
        self.peek_manual().map(|(s, e)| &self.slice[s..e])
    }
}

impl<'a> Iterator for SubtagIterator<'a> {
    type Item = &'a [u8];

    fn next(&mut self) -> Option<Self::Item> {
        let (s, res) = self.next_manual();
        *self = s;
        #[allow(clippy::indexing_slicing)] // next_manual returns valid indices
        res.map(|(s, e)| &self.slice[s..e])
    }
}

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

    fn slice_to_str(input: &[u8]) -> &str {
        std::str::from_utf8(input).unwrap()
    }

    #[test]
    fn subtag_iterator_peek_test() {
        let slice = "de_at-u-ca-foobar";
        let mut si = SubtagIterator::new(slice.as_bytes());

        assert_eq!(si.peek().map(slice_to_str), Some("de"));
        assert_eq!(si.peek().map(slice_to_str), Some("de"));
        assert_eq!(si.next().map(slice_to_str), Some("de"));

        assert_eq!(si.peek().map(slice_to_str), Some("at"));
        assert_eq!(si.peek().map(slice_to_str), Some("at"));
        assert_eq!(si.next().map(slice_to_str), Some("at"));
    }

    #[test]
    fn subtag_iterator_test() {
        let slice = "";
        let mut si = SubtagIterator::new(slice.as_bytes());
        assert_eq!(si.next().map(slice_to_str), Some(""));

        let slice = "-";
        let mut si = SubtagIterator::new(slice.as_bytes());
        assert_eq!(si.next().map(slice_to_str), Some(""));

        let slice = "-en";
        let mut si = SubtagIterator::new(slice.as_bytes());
        assert_eq!(si.next().map(slice_to_str), Some(""));
        assert_eq!(si.next().map(slice_to_str), Some("en"));
        assert_eq!(si.next(), None);

        let slice = "en";
        let si = SubtagIterator::new(slice.as_bytes());
        assert_eq!(si.map(slice_to_str).collect::<Vec<_>>(), vec!["en",]);

        let slice = "en-";
        let si = SubtagIterator::new(slice.as_bytes());
        assert_eq!(si.map(slice_to_str).collect::<Vec<_>>(), vec!["en", "",]);

        let slice = "--";
        let mut si = SubtagIterator::new(slice.as_bytes());
        assert_eq!(si.next().map(slice_to_str), Some(""));
        assert_eq!(si.next().map(slice_to_str), Some(""));
        assert_eq!(si.next().map(slice_to_str), Some(""));
        assert_eq!(si.next(), None);

        let slice = "-en-";
        let mut si = SubtagIterator::new(slice.as_bytes());
        assert_eq!(si.next().map(slice_to_str), Some(""));
        assert_eq!(si.next().map(slice_to_str), Some("en"));
        assert_eq!(si.next().map(slice_to_str), Some(""));
        assert_eq!(si.next(), None);

        let slice = "de_at-u-ca-foobar";
        let si = SubtagIterator::new(slice.as_bytes());
        assert_eq!(
            si.map(slice_to_str).collect::<Vec<_>>(),
            vec!["de", "at", "u", "ca", "foobar",]
        );
    }

    #[test]
    fn get_current_subtag_test() {
        let slice = "-";
        let current = get_current_subtag(slice.as_bytes(), 0);
        assert_eq!(current, (1, 1));

        let slice = "-en";
        let current = get_current_subtag(slice.as_bytes(), 0);
        assert_eq!(current, (1, 3));

        let slice = "-en-";
        let current = get_current_subtag(slice.as_bytes(), 3);
        assert_eq!(current, (4, 4));

        let slice = "en-";
        let current = get_current_subtag(slice.as_bytes(), 0);
        assert_eq!(current, (0, 2));

        let current = get_current_subtag(slice.as_bytes(), 2);
        assert_eq!(current, (3, 3));

        let slice = "en--US";
        let current = get_current_subtag(slice.as_bytes(), 0);
        assert_eq!(current, (0, 2));

        let current = get_current_subtag(slice.as_bytes(), 2);
        assert_eq!(current, (3, 3));

        let current = get_current_subtag(slice.as_bytes(), 3);
        assert_eq!(current, (4, 6));

        let slice = "--";
        let current = get_current_subtag(slice.as_bytes(), 0);
        assert_eq!(current, (1, 1));

        let current = get_current_subtag(slice.as_bytes(), 1);
        assert_eq!(current, (2, 2));

        let slice = "-";
        let current = get_current_subtag(slice.as_bytes(), 0);
        assert_eq!(current, (1, 1));
    }
}