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LibAudio: Account for 0xFF start byte in FLAC UTF-8 decoder

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This specialized UTF-8 decoder is more powerful than a normal UTF-8
decoder anyways, but it couldn't account for the never spec-compliant
0xff start byte. This commit makes that byte behave as expected if
taking UTF-8 to its extreme, even if it is a little silly and likely not
relevant for real applications.
This commit is contained in:
kleines Filmröllchen 2023-06-27 20:28:37 +02:00 • committed by Jelle Raaijmakers
parent 8bc56c7fb0
commit e82bee86dd
1 changed files with 11 additions and 4 deletions
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15
Userland/Libraries/LibAudio/FlacLoader.cpp
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@ -958,13 +958,20 @@ ErrorOr<u64> read_utf8_char(BigEndianInputBitStream& input)
} else if ((start_byte & 0b11000000) == 0b10000000) {
return Error::from_string_literal("Illegal continuation byte");
}
// This algorithm is too good and supports the theoretical max 0xFF start byte
// This algorithm supports the theoretical max 0xFF start byte, which is not part of the regular UTF-8 spec.
u8 length = 1;
while (((start_byte << length) & 0b10000000) == 0b10000000)
++length;
u8 bits_from_start_byte = 8 - (length + 1);
u8 start_byte_bitmask = AK::exp2(bits_from_start_byte) - 1;
character = start_byte_bitmask & start_byte;
// This is technically not spec-compliant, but if we take UTF-8 to its logical extreme,
// we can say 0xFF means there's 7 following continuation bytes and no data at all in the leading character.
if (length == 8) [[unlikely]] {
character = 0;
} else {
u8 bits_from_start_byte = 8 - (length + 1);
u8 start_byte_bitmask = AK::exp2(bits_from_start_byte) - 1;
character = start_byte_bitmask & start_byte;
}
for (u8 i = length - 1; i > 0; --i) {
u8 current_byte = TRY(input.read_value<u8>());
character = (character << 6) | (current_byte & 0b00111111);