1 // Copyright (c) the JPEG XL Project Authors. All rights reserved. 2 // 3 // Use of this source code is governed by a BSD-style 4 // license that can be found in the LICENSE file. 5 6 // Utility function for building a Huffman lookup table for the jpeg decoder. 7 8 #ifndef LIB_JXL_JPEG_ENC_JPEG_HUFFMAN_DECODE_H_ 9 #define LIB_JXL_JPEG_ENC_JPEG_HUFFMAN_DECODE_H_ 10 11 #include <stdint.h> 12 13 namespace jxl { 14 namespace jpeg { 15 16 constexpr int kJpegHuffmanRootTableBits = 8; 17 // Maximum huffman lookup table size. 18 // According to zlib/examples/enough.c, 758 entries are always enough for 19 // an alphabet of 257 symbols (256 + 1 special symbol for the all 1s code) and 20 // max bit length 16 if the root table has 8 bits. 21 constexpr int kJpegHuffmanLutSize = 758; 22 23 struct HuffmanTableEntry { 24 // Initialize the value to an invalid symbol so that we can recognize it 25 // when reading the bit stream using a Huffman code with space > 0. HuffmanTableEntryHuffmanTableEntry26 HuffmanTableEntry() : bits(0), value(0xffff) {} 27 28 uint8_t bits; // number of bits used for this symbol 29 uint16_t value; // symbol value or table offset 30 }; 31 32 // Builds jpeg-style Huffman lookup table from the given symbols. 33 // The symbols are in order of increasing bit lengths. The number of symbols 34 // with bit length n is given in counts[n] for each n >= 1. 35 void BuildJpegHuffmanTable(const uint32_t* counts, const uint32_t* symbols, 36 HuffmanTableEntry* lut); 37 38 } // namespace jpeg 39 } // namespace jxl 40 41 #endif // LIB_JXL_JPEG_ENC_JPEG_HUFFMAN_DECODE_H_ 42