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}
135