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std: optimise utf8ByteSequenceLength

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Also tested (but not as fast):
```zig
pub fn utf8ByteSequenceLength(first_byte: u8) !u3 {
    const len = @clz(u8, ~first_byte);
    if (len == 0) return 1;
    if (len < 4) return @intCast(u3, len);
    return error.Utf8InvalidStartByte;
}
```
This commit is contained in:
daurnimator 2019-12-28 14:34:00 +11:00
parent 8b72eedc76
commit 5843a6e3bc
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1 changed files with 7 additions and 5 deletions
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12
lib/std/unicode.zig
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@ -18,11 +18,13 @@ pub fn utf8CodepointSequenceLength(c: u32) !u3 {
/// returns a number 1-4 indicating the total length of the codepoint in bytes.
/// If this byte does not match the form of a UTF-8 start byte, returns Utf8InvalidStartByte.
pub fn utf8ByteSequenceLength(first_byte: u8) !u3 {
if (first_byte < 0b10000000) return @as(u3, 1);
if (first_byte & 0b11100000 == 0b11000000) return @as(u3, 2);
if (first_byte & 0b11110000 == 0b11100000) return @as(u3, 3);
if (first_byte & 0b11111000 == 0b11110000) return @as(u3, 4);
return error.Utf8InvalidStartByte;
return switch (@clz(u8, ~first_byte)) {
0 => 1,
2 => 2,
3 => 3,
4 => 4,
else => error.Utf8InvalidStartByte,
};
}
/// Encodes the given codepoint into a UTF-8 byte sequence.