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LibGfx/JPEG: Use a lookup table to retrieve huffman symbols

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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.
This commit is contained in:
Lucas CHOLLET 2023-04-30 00:16:14 -04:00 • committed by Andreas Kling
parent 9389177e5f
commit 677386bfaa
1 changed files with 65 additions and 14 deletions
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79
Userland/Libraries/LibGfx/ImageFormats/JPEGLoader.cpp
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@ -187,6 +187,13 @@ struct HuffmanTable {
Vector<u8> symbols;
Vector<u16> codes;
// Note: The value 8 is chosen quite arbitrarily, the only current constraint
// is that both the symbol and the size fit in an u16. I've tested more
// values but none stand out, and 8 is the value used by libjpeg-turbo.
static constexpr u8 bits_per_cached_code = 8;
static constexpr u8 maximum_bits_per_code = 16;
u8 first_non_cached_code_index {};
void generate_codes()
{
unsigned code = 0;
@ -195,7 +202,59 @@ struct HuffmanTable {
codes.append(code++);
code <<= 1;
}
generate_lookup_table();
}
struct SymbolAndSize {
u8 symbol {};
u8 size {};
};
ErrorOr<SymbolAndSize> symbol_from_code(u16 code) const
{
static constexpr u8 shift_for_cache = maximum_bits_per_code - bits_per_cached_code;
if (lookup_table[code >> shift_for_cache] != invalid_entry) {
u8 const code_length = lookup_table[code >> shift_for_cache] >> bits_per_cached_code;
return SymbolAndSize { static_cast<u8>(lookup_table[code >> shift_for_cache]), code_length };
}
u64 code_cursor = first_non_cached_code_index;
for (u8 i = HuffmanTable::bits_per_cached_code; i < 16; i++) {
auto const result = code >> (maximum_bits_per_code - 1 - i);
for (u32 j = 0; j < code_counts[i]; j++) {
if (result == codes[code_cursor])
return SymbolAndSize { symbols[code_cursor], static_cast<u8>(i + 1) };
code_cursor++;
}
}
return Error::from_string_literal("This kind of JPEG is not yet supported by the decoder");
}
private:
static constexpr u16 invalid_entry = 0xFF;
void generate_lookup_table()
{
lookup_table.fill(invalid_entry);
u32 code_offset = 0;
for (u8 code_length = 1; code_length <= bits_per_cached_code; code_length++) {
for (u32 i = 0; i < code_counts[code_length - 1]; i++, code_offset++) {
u32 code_key = codes[code_offset] << (bits_per_cached_code - code_length);
for (u8 duplicate_count = 1 << (bits_per_cached_code - code_length); duplicate_count > 0; duplicate_count--) {
lookup_table[code_key] = (code_length << bits_per_cached_code) | symbols[code_offset];
code_key++;
}
}
}
}
Array<u16, 1 << bits_per_cached_code> lookup_table {};
};
class HuffmanStream {
@ -239,22 +298,12 @@ public:
ErrorOr<u8> next_symbol(HuffmanTable const& table)
{
u16 const code = peek_bits(16);
u64 code_cursor = 0;
u16 const code = peek_bits(HuffmanTable::maximum_bits_per_code);
for (int i = 0; i < 16; i++) { // Codes can't be longer than 16 bits.
auto const result = code >> (15 - i);
for (int j = 0; j < table.code_counts[i]; j++) {
if (result == table.codes[code_cursor]) {
discard_bits(i + 1);
return table.symbols[code_cursor];
}
code_cursor++;
}
}
auto const symbol_and_size = TRY(table.symbol_from_code(code));
dbgln_if(JPEG_DEBUG, "If you're seeing this...the jpeg decoder needs to support more kinds of JPEGs!");
return Error::from_string_literal("This kind of JPEG is not yet supported by the decoder");
discard_bits(symbol_and_size.size);
return symbol_and_size.symbol;
}
ErrorOr<u16> read_bits(u8 count = 1)
@ -879,6 +928,8 @@ static ErrorOr<void> read_huffman_table(Stream& stream, JPEGLoadingContext& cont
// Read code counts. At each index K, the value represents the number of K+1 bit codes in this header.
for (int i = 0; i < 16; i++) {
if (i == HuffmanTable::bits_per_cached_code)
table.first_non_cached_code_index = total_codes;
u8 count = TRY(stream.read_value<u8>());
total_codes += count;
table.code_counts[i] = count;