1/// Decodes the next UTF-8 encoded codepoint from the given byte slice.
2///
3/// If no valid encoding of a codepoint exists at the beginning of the
4/// given byte slice, then the first byte is returned instead.
5///
6/// This returns `None` if and only if `bytes` is empty.
7///
8/// This never panics.
9///
10/// *WARNING*: This is not designed for performance. If you're looking for
11/// a fast UTF-8 decoder, this is not it. If you feel like you need one in
12/// this crate, then please file an issue and discuss your use case.
13pub(crate) fn decode(bytes: &[u8]) -> Option<Result<char, u8>> {
14 /// Given a UTF-8 leading byte, this returns the total number of code
15 /// units in the following encoded codepoint.
16 ///
17 /// If the given byte is not a valid UTF-8 leading byte, then this
18 /// returns `None`.
19 fn utf8_len(byte: u8) -> Option<usize> {
20 if byte <= 0x7F {
21 return Some(1);
22 } else if byte & 0b1100_0000 == 0b1000_0000 {
23 return None;
24 } else if byte <= 0b1101_1111 {
25 Some(2)
26 } else if byte <= 0b1110_1111 {
27 Some(3)
28 } else if byte <= 0b1111_0111 {
29 Some(4)
30 } else {
31 None
32 }
33 }
34
35 if bytes.is_empty() {
36 return None;
37 }
38 let len = match utf8_len(bytes[0]) {
39 None => return Some(Err(bytes[0])),
40 Some(len) if len > bytes.len() => return Some(Err(bytes[0])),
41 Some(1) => return Some(Ok(char::from(bytes[0]))),
42 Some(len) => len,
43 };
44 match core::str::from_utf8(&bytes[..len]) {
45 Ok(s) => Some(Ok(s.chars().next().unwrap())),
46 Err(_) => Some(Err(bytes[0])),
47 }
48}
49