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89 lines
3.0 KiB
89 lines
3.0 KiB
2 years ago
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// Copyright 2010 Google Inc. All Rights Reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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// Entropy encoding (Huffman) utilities.
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#ifndef BROTLI_ENC_ENTROPY_ENCODE_H_
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#define BROTLI_ENC_ENTROPY_ENCODE_H_
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#include <stdint.h>
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#include <string.h>
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#include <vector>
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#include "./histogram.h"
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#include "./prefix.h"
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namespace brotli {
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// This function will create a Huffman tree.
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//
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// The (data,length) contains the population counts.
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// The tree_limit is the maximum bit depth of the Huffman codes.
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//
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// The depth contains the tree, i.e., how many bits are used for
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// the symbol.
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//
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// See http://en.wikipedia.org/wiki/Huffman_coding
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void CreateHuffmanTree(const int *data,
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const int length,
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const int tree_limit,
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uint8_t *depth);
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// Change the population counts in a way that the consequent
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// Hufmann tree compression, especially its rle-part will be more
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// likely to compress this data more efficiently.
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//
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// length contains the size of the histogram.
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// counts contains the population counts.
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int OptimizeHuffmanCountsForRle(int length, int* counts);
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// Write a huffman tree from bit depths into the bitstream representation
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// of a Huffman tree. The generated Huffman tree is to be compressed once
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// more using a Huffman tree
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void WriteHuffmanTree(const uint8_t* depth,
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uint32_t num,
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std::vector<uint8_t> *tree,
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std::vector<uint8_t> *extra_bits_data);
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// Get the actual bit values for a tree of bit depths.
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void ConvertBitDepthsToSymbols(const uint8_t *depth, int len, uint16_t *bits);
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template<int kSize>
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struct EntropyCode {
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// How many bits for symbol.
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uint8_t depth_[kSize];
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// Actual bits used to represent the symbol.
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uint16_t bits_[kSize];
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// How many non-zero depth.
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int count_;
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// First four symbols with non-zero depth.
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int symbols_[4];
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};
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static const int kCodeLengthCodes = 18;
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// Literal entropy code.
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typedef EntropyCode<256> EntropyCodeLiteral;
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// Prefix entropy codes.
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typedef EntropyCode<kNumCommandPrefixes> EntropyCodeCommand;
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typedef EntropyCode<kNumDistancePrefixes> EntropyCodeDistance;
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typedef EntropyCode<kNumBlockLenPrefixes> EntropyCodeBlockLength;
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// Context map entropy code, 256 Huffman tree indexes + 16 run length codes.
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typedef EntropyCode<272> EntropyCodeContextMap;
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// Block type entropy code, 256 block types + 2 special symbols.
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typedef EntropyCode<258> EntropyCodeBlockType;
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} // namespace brotli
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#endif // BROTLI_ENC_ENTROPY_ENCODE_H_
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