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LibGfx/JPEG: Use a lookup table to retrieve huffman symbols
Instead of testing all possible code to find the good symbol, we use a lookup table to directly find the expected symbol. This method is used by most Huffman decoder (gzip or libjpeg-turbo). In order to use the correct key when peeking a constant number of bits from the stream, we generate duplicates in the table. As an example, for the code 110, all entries with that pattern 110***** will be set to 110's symbol. So, when you read this code plus garbage from following codes, you still find the correct symbol.
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1 changed files with 65 additions and 14 deletions
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@ -187,6 +187,13 @@ struct HuffmanTable {
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Vector<u8> symbols;
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Vector<u16> codes;
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// Note: The value 8 is chosen quite arbitrarily, the only current constraint
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// is that both the symbol and the size fit in an u16. I've tested more
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// values but none stand out, and 8 is the value used by libjpeg-turbo.
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static constexpr u8 bits_per_cached_code = 8;
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static constexpr u8 maximum_bits_per_code = 16;
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u8 first_non_cached_code_index {};
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void generate_codes()
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{
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unsigned code = 0;
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@ -195,7 +202,59 @@ struct HuffmanTable {
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codes.append(code++);
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code <<= 1;
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}
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generate_lookup_table();
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}
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struct SymbolAndSize {
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u8 symbol {};
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u8 size {};
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};
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ErrorOr<SymbolAndSize> symbol_from_code(u16 code) const
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{
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static constexpr u8 shift_for_cache = maximum_bits_per_code - bits_per_cached_code;
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if (lookup_table[code >> shift_for_cache] != invalid_entry) {
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u8 const code_length = lookup_table[code >> shift_for_cache] >> bits_per_cached_code;
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return SymbolAndSize { static_cast<u8>(lookup_table[code >> shift_for_cache]), code_length };
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}
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u64 code_cursor = first_non_cached_code_index;
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for (u8 i = HuffmanTable::bits_per_cached_code; i < 16; i++) {
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auto const result = code >> (maximum_bits_per_code - 1 - i);
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for (u32 j = 0; j < code_counts[i]; j++) {
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if (result == codes[code_cursor])
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return SymbolAndSize { symbols[code_cursor], static_cast<u8>(i + 1) };
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code_cursor++;
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}
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}
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return Error::from_string_literal("This kind of JPEG is not yet supported by the decoder");
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}
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private:
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static constexpr u16 invalid_entry = 0xFF;
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void generate_lookup_table()
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{
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lookup_table.fill(invalid_entry);
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u32 code_offset = 0;
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for (u8 code_length = 1; code_length <= bits_per_cached_code; code_length++) {
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for (u32 i = 0; i < code_counts[code_length - 1]; i++, code_offset++) {
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u32 code_key = codes[code_offset] << (bits_per_cached_code - code_length);
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for (u8 duplicate_count = 1 << (bits_per_cached_code - code_length); duplicate_count > 0; duplicate_count--) {
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lookup_table[code_key] = (code_length << bits_per_cached_code) | symbols[code_offset];
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code_key++;
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}
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}
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}
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}
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Array<u16, 1 << bits_per_cached_code> lookup_table {};
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};
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class HuffmanStream {
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@ -239,22 +298,12 @@ public:
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ErrorOr<u8> next_symbol(HuffmanTable const& table)
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{
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u16 const code = peek_bits(16);
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u64 code_cursor = 0;
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u16 const code = peek_bits(HuffmanTable::maximum_bits_per_code);
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for (int i = 0; i < 16; i++) { // Codes can't be longer than 16 bits.
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auto const result = code >> (15 - i);
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for (int j = 0; j < table.code_counts[i]; j++) {
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if (result == table.codes[code_cursor]) {
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discard_bits(i + 1);
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return table.symbols[code_cursor];
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}
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code_cursor++;
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}
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}
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auto const symbol_and_size = TRY(table.symbol_from_code(code));
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dbgln_if(JPEG_DEBUG, "If you're seeing this...the jpeg decoder needs to support more kinds of JPEGs!");
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return Error::from_string_literal("This kind of JPEG is not yet supported by the decoder");
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discard_bits(symbol_and_size.size);
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return symbol_and_size.symbol;
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}
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ErrorOr<u16> read_bits(u8 count = 1)
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@ -879,6 +928,8 @@ static ErrorOr<void> read_huffman_table(Stream& stream, JPEGLoadingContext& cont
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// Read code counts. At each index K, the value represents the number of K+1 bit codes in this header.
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for (int i = 0; i < 16; i++) {
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if (i == HuffmanTable::bits_per_cached_code)
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table.first_non_cached_code_index = total_codes;
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u8 count = TRY(stream.read_value<u8>());
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total_codes += count;
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table.code_counts[i] = count;
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