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LibGfx/WebPLossless: Update spec comments for reading max_symbol

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This used to be misleading in the spec, but that was fixed in
https://chromium-review.googlesource.com/c/webm/libwebp/+/4674031

Update the comments to match.

Also, a check we already had got added to the spec in
https://chromium-review.googlesource.com/c/webm/libwebp/+/4674032
so add a citation for that.

No behavior change.
This commit is contained in:
Nico Weber 2024-05-07 07:30:05 -04:00 • committed by Tim Flynn
commit 3662eea0ac
Notes: sideshowbarker 2024-07-17 10:10:18 +09:00
1 changed files with 16 additions and 11 deletions
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27
Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossless.cpp
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@ -165,18 +165,22 @@ static ErrorOr<CanonicalCode> decode_webp_chunk_VP8L_prefix_code(LittleEndianInp
for (int i = 0; i < num_code_lengths; ++i)
code_length_code_lengths[kCodeLengthCodeOrder[i]] = TRY(bit_stream.read_bits(3));
// The spec is at best misleading here, suggesting that max_symbol should be set to "num_code_lengths" if it's not explicitly stored.
// But num_code_lengths doesn't mean the num_code_lengths mentioned a few lines further up in the spec (and in scope here),
// but alphabet_size!
//
// Since the spec doesn't mention it, see libwebp vp8l_dec.c, ReadHuffmanCode()
// which passes alphabet_size to ReadHuffmanCodeLengths() as num_symbols,
// and ReadHuffmanCodeLengths() then sets max_symbol to that.)
unsigned max_symbol = alphabet_size;
if (TRY(bit_stream.read_bits(1))) {
// "Next, if `ReadBits(1) == 0`, the maximum number of different read symbols
// (`max_symbol`) for each symbol type (A, R, G, B, and distance) is set to its
// alphabet size:"
unsigned max_symbol;
if (TRY(bit_stream.read_bits(1)) == 0) {
max_symbol = alphabet_size;
}
// "Otherwise, it is defined as:"
else {
// "int length_nbits = 2 + 2 * ReadBits(3);"
int length_nbits = 2 + 2 * TRY(bit_stream.read_bits(3));
// "int max_symbol = 2 + ReadBits(length_nbits);"
max_symbol = 2 + TRY(bit_stream.read_bits(length_nbits));
dbgln_if(WEBP_DEBUG, " extended, length_nbits {} max_symbol {}", length_nbits, max_symbol);
// "If `max_symbol` is larger than the size of the alphabet for the symbol type, the bitstream is invalid."
if (max_symbol > alphabet_size)
return Error::from_string_literal("WebPImageDecoderPlugin: invalid max_symbol");
}
@ -238,8 +242,9 @@ static ErrorOr<PrefixCodeGroup> decode_webp_chunk_VP8L_prefix_code_group(u16 col
// ; understand what each of these five prefix
// ; codes are for.
// "Once code lengths are read, a prefix code for each symbol type (A, R, G, B, distance) is formed using their respective alphabet sizes:
// * G channel: 256 + 24 + color_cache_size
// "Once code lengths are read, a prefix code for each symbol type (A, R, G, B, distance) is formed using their respective alphabet sizes."
// ...
// "* G channel: 256 + 24 + color_cache_size
// * other literals (A,R,B): 256
// * distance code: 40"
Array<size_t, 5> const alphabet_sizes { 256 + 24 + static_cast<size_t>(color_cache_size), 256, 256, 256, 40 };