core/char/
decode.rs

1//! UTF-8 and UTF-16 decoding iterators
2
3use crate::error::Error;
4use crate::fmt;
5use crate::iter::FusedIterator;
6
7/// An iterator that decodes UTF-16 encoded code points from an iterator of `u16`s.
8///
9/// This `struct` is created by the [`decode_utf16`] method on [`char`]. See its
10/// documentation for more.
11///
12/// [`decode_utf16`]: char::decode_utf16
13#[stable(feature = "decode_utf16", since = "1.9.0")]
14#[derive(Clone, Debug)]
15pub struct DecodeUtf16<I>
16where
17    I: Iterator<Item = u16>,
18{
19    iter: I,
20    buf: Option<u16>,
21}
22
23/// An error that can be returned when decoding UTF-16 code points.
24///
25/// This `struct` is created when using the [`DecodeUtf16`] type.
26#[stable(feature = "decode_utf16", since = "1.9.0")]
27#[derive(Debug, Clone, Eq, PartialEq)]
28pub struct DecodeUtf16Error {
29    code: u16,
30}
31
32/// Creates an iterator over the UTF-16 encoded code points in `iter`,
33/// returning unpaired surrogates as `Err`s. See [`char::decode_utf16`].
34#[inline]
35pub(super) fn decode_utf16<I: IntoIterator<Item = u16>>(iter: I) -> DecodeUtf16<I::IntoIter> {
36    DecodeUtf16 { iter: iter.into_iter(), buf: None }
37}
38
39#[stable(feature = "decode_utf16", since = "1.9.0")]
40impl<I: Iterator<Item = u16>> Iterator for DecodeUtf16<I> {
41    type Item = Result<char, DecodeUtf16Error>;
42
43    fn next(&mut self) -> Option<Result<char, DecodeUtf16Error>> {
44        let u = match self.buf.take() {
45            Some(buf) => buf,
46            None => self.iter.next()?,
47        };
48
49        if !u.is_utf16_surrogate() {
50            // SAFETY: not a surrogate
51            Some(Ok(unsafe { char::from_u32_unchecked(u as u32) }))
52        } else if u >= 0xDC00 {
53            // a trailing surrogate
54            Some(Err(DecodeUtf16Error { code: u }))
55        } else {
56            let u2 = match self.iter.next() {
57                Some(u2) => u2,
58                // eof
59                None => return Some(Err(DecodeUtf16Error { code: u })),
60            };
61            if u2 < 0xDC00 || u2 > 0xDFFF {
62                // not a trailing surrogate so we're not a valid
63                // surrogate pair, so rewind to redecode u2 next time.
64                self.buf = Some(u2);
65                return Some(Err(DecodeUtf16Error { code: u }));
66            }
67
68            // all ok, so lets decode it.
69            let c = (((u & 0x3ff) as u32) << 10 | (u2 & 0x3ff) as u32) + 0x1_0000;
70            // SAFETY: we checked that it's a legal unicode value
71            Some(Ok(unsafe { char::from_u32_unchecked(c) }))
72        }
73    }
74
75    #[inline]
76    fn size_hint(&self) -> (usize, Option<usize>) {
77        let (low, high) = self.iter.size_hint();
78
79        let (low_buf, high_buf) = match self.buf {
80            // buf is empty, no additional elements from it.
81            None => (0, 0),
82            // `u` is a non surrogate, so it's always an additional character.
83            Some(u) if !u.is_utf16_surrogate() => (1, 1),
84            // `u` is a leading surrogate (it can never be a trailing surrogate and
85            // it's a surrogate due to the previous branch) and `self.iter` is empty.
86            //
87            // `u` can't be paired, since the `self.iter` is empty,
88            // so it will always become an additional element (error).
89            Some(_u) if high == Some(0) => (1, 1),
90            // `u` is a leading surrogate and `iter` may be non-empty.
91            //
92            // `u` can either pair with a trailing surrogate, in which case no additional elements
93            // are produced, or it can become an error, in which case it's an additional character (error).
94            Some(_u) => (0, 1),
95        };
96
97        // `self.iter` could contain entirely valid surrogates (2 elements per
98        // char), or entirely non-surrogates (1 element per char).
99        //
100        // On odd lower bound, at least one element must stay unpaired
101        // (with other elements from `self.iter`), so we round up.
102        let low = low.div_ceil(2) + low_buf;
103        let high = high.and_then(|h| h.checked_add(high_buf));
104
105        (low, high)
106    }
107}
108
109#[stable(feature = "decode_utf16_fused_iterator", since = "1.75.0")]
110impl<I: Iterator<Item = u16> + FusedIterator> FusedIterator for DecodeUtf16<I> {}
111
112impl DecodeUtf16Error {
113    /// Returns the unpaired surrogate which caused this error.
114    #[must_use]
115    #[stable(feature = "decode_utf16", since = "1.9.0")]
116    pub fn unpaired_surrogate(&self) -> u16 {
117        self.code
118    }
119}
120
121#[stable(feature = "decode_utf16", since = "1.9.0")]
122impl fmt::Display for DecodeUtf16Error {
123    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
124        write!(f, "unpaired surrogate found: {:x}", self.code)
125    }
126}
127
128#[stable(feature = "decode_utf16", since = "1.9.0")]
129impl Error for DecodeUtf16Error {
130    #[allow(deprecated)]
131    fn description(&self) -> &str {
132        "unpaired surrogate found"
133    }
134}
core/char/decode.rs

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core/char/
decode.rs
