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LibGfx/JPEGXL: Store distributions in a Variant

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JPEG XL supports two types of entropy encoding: the first one is
Huffman-based (Brotli) and the second one is based on ANS. To introduce
the latter, we start by storing the `Vector` of distributions in a
`Variant`. This will allow us to choose which entropy decoder we use
during execution.
This commit is contained in:
Lucas CHOLLET 2023-06-28 12:25:55 -04:00 committed by Jelle Raaijmakers
parent fb4db096cb
commit 4da8c45e43
Notes: sideshowbarker 2024-07-17 06:20:50 +09:00 
Author: https://github.com/LucasChollet
Commit: https://github.com/SerenityOS/serenity/commit/4da8c45e43
Pull-request: https://github.com/SerenityOS/serenity/pull/20305
Reviewed-by: https://github.com/gmta ✅
1 changed files with 19 additions and 12 deletions
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31
Userland/Libraries/LibGfx/ImageFormats/JPEGXLLoader.cpp
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@ -834,11 +834,14 @@ public:
for (auto& config : entropy_decoder.m_configs)
config = TRY(entropy_decoder.read_config(stream));
Vector<u16> counts;
TRY(counts.try_resize(entropy_decoder.m_configs.size()));
TRY(entropy_decoder.m_distributions.try_resize(entropy_decoder.m_configs.size()));
if (use_prefix_code) {
entropy_decoder.m_distributions = Vector<BrotliCanonicalCode> {};
auto& distributions = entropy_decoder.m_distributions.get<Vector<BrotliCanonicalCode>>();
TRY(distributions.try_resize(entropy_decoder.m_configs.size()));
Vector<u16> counts;
TRY(counts.try_resize(entropy_decoder.m_configs.size()));
for (auto& count : counts) {
if (TRY(stream.read_bit())) {
auto const n = TRY(stream.read_bits(4));
@ -850,15 +853,14 @@ public:
// After reading the counts, the decoder reads each D[i] (implicitly
// described by a prefix code) as specified in C.2.4, with alphabet_size = count[i].
for (u32 i {}; i < entropy_decoder.m_distributions.size(); ++i) {
for (u32 i {}; i < distributions.size(); ++i) {
// The alphabet size mentioned in the [Brotli] RFC is explicitly specified as parameter alphabet_size
// when the histogram is being decoded, except in the special case of alphabet_size == 1, where no
// histogram is read, and all decoded symbols are zero without reading any bits at all.
if (counts[i] != 1) {
entropy_decoder.m_distributions[i] = TRY(BrotliCanonicalCode::read_prefix_code(stream, counts[i]));
} else {
entropy_decoder.m_distributions[i] = BrotliCanonicalCode { { 1 }, { 0 } };
}
if (counts[i] != 1)
distributions[i] = TRY(BrotliCanonicalCode::read_prefix_code(stream, counts[i]));
else
distributions[i] = BrotliCanonicalCode { { 1 }, { 0 } };
}
} else {
TODO();
@ -875,7 +877,12 @@ public:
TODO();
// Read symbol from entropy coded stream using D[clusters[ctx]]
auto const token = TRY(m_distributions[m_clusters[context]].read_symbol(stream));
u32 token {};
TRY(m_distributions.visit(
[&](Vector<BrotliCanonicalCode> const& distributions) -> ErrorOr<void> {
token = TRY(distributions[m_clusters[context]].read_symbol(stream));
return {};
}));
auto r = TRY(read_uint(stream, m_configs[m_clusters[context]], token));
return r;
@ -965,7 +972,7 @@ private:
u8 m_log_alphabet_size { 15 };
Vector<BrotliCanonicalCode> m_distributions; // D in the spec
Variant<Vector<BrotliCanonicalCode>> m_distributions { Vector<BrotliCanonicalCode> {} }; // D in the spec
};
///