enc_jpeg_huffman_decode.h (1915B)
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 // Requirements: alphabet of 257 symbols (256 + 1 special symbol for the all 1s 19 // code) and max bit length 16, the root table has 8 bits. 20 // zlib/examples/enough.c works with an assumption that Huffman code is 21 // "complete". Input JPEGs might have this assumption broken, hence the 22 // following sum is used as estimate: 23 // + number of 1-st level cells 24 // + number of symbols 25 // + asymptotic amount of repeated 2nd level cells 26 // The third number is 1 + 3 + ... + 255 i.e. it is assumed that sub-table of 27 // each "size" might be almost completely be filled with repetitions. 28 // Total sum is slightly less than 1024,... 29 constexpr int kJpegHuffmanLutSize = 1024; 30 31 struct HuffmanTableEntry { 32 // Initialize the value to an invalid symbol so that we can recognize it 33 // when reading the bit stream using a Huffman code with space > 0. 34 HuffmanTableEntry() : bits(0), value(0xffff) {} 35 36 uint8_t bits; // number of bits used for this symbol 37 uint16_t value; // symbol value or table offset 38 }; 39 40 // Builds jpeg-style Huffman lookup table from the given symbols. 41 // The symbols are in order of increasing bit lengths. The number of symbols 42 // with bit length n is given in counts[n] for each n >= 1. 43 void BuildJpegHuffmanTable(const uint32_t* counts, const uint32_t* symbols, 44 HuffmanTableEntry* lut); 45 46 } // namespace jpeg 47 } // namespace jxl 48 49 #endif // LIB_JXL_JPEG_ENC_JPEG_HUFFMAN_DECODE_H_