jpeg_huffman_decode.h

/*
 * Copyright 2016 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// Utility function for building a Huffman lookup table for the jpeg decoder.

#ifndef KNUSPERLI_JPEG_HUFFMAN_DECODE_H_
#define KNUSPERLI_JPEG_HUFFMAN_DECODE_H_

#include <inttypes.h>

namespace knusperli {

static const int kJpegHuffmanRootTableBits = 8;
// Maximum huffman lookup table size.
// According to zlib/examples/enough.c, 758 entries are always enough for
// an alphabet of 257 symbols (256 + 1 special symbol for the all 1s code) and
// max bit length 16 if the root table has 8 bits.
static const int kJpegHuffmanLutSize = 758;

struct HuffmanTableEntry {
  // Initialize the value to an invalid symbol so that we can recognize it
  // when reading the bit stream using a Huffman code with space > 0.
  HuffmanTableEntry() : bits(0), value(0xffff) {}

  uint8_t bits;     // number of bits used for this symbol
  uint16_t value;   // symbol value or table offset
};

// Builds jpeg-style Huffman lookup table from the given symbols.
// The symbols are in order of increasing bit lengths. The number of symbols
// with bit length n is given in counts[n] for each n >= 1.
// Returns the size of the lookup table.
int BuildJpegHuffmanTable(const int* counts, const int* symbols,
                          HuffmanTableEntry* lut);

}  // namespace knusperli

#endif  // KNUSPERLI_JPEG_HUFFMAN_DECODE_H_