mirror of
https://github.com/google/brotli.git
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60c24c0c2d
This commit contains a batch of changes that were made to the Brotli
compression algorithm in the last month. Most important changes:
* Updated spec
* Changed Huffman code length alphabet to use run length codes more
efficiently, based on a suggestion by Robert Obryk
* Changed encoding of the number of Huffman code lengths (HLEN)
* Changed encoding of the number of Huffman trees (NTREES)
* Added support for uncompressed meta-blocks
115 lines
3.1 KiB
C++
115 lines
3.1 KiB
C++
// Copyright 2013 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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// Models the histograms of literals, commands and distance codes.
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#ifndef BROTLI_ENC_HISTOGRAM_H_
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#define BROTLI_ENC_HISTOGRAM_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 <utility>
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#include "./command.h"
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#include "./fast_log.h"
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#include "./prefix.h"
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namespace brotli {
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class BlockSplit;
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// A simple container for histograms of data in blocks.
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template<int kDataSize>
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struct Histogram {
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Histogram() {
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Clear();
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}
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void Clear() {
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memset(data_, 0, sizeof(data_));
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total_count_ = 0;
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}
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void Add(int val) {
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++data_[val];
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++total_count_;
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}
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void Remove(int val) {
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--data_[val];
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--total_count_;
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}
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template<typename DataType>
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void Add(const DataType *p, size_t n) {
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total_count_ += n;
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n += 1;
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while(--n) ++data_[*p++];
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}
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void AddHistogram(const Histogram& v) {
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total_count_ += v.total_count_;
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for (int i = 0; i < kDataSize; ++i) {
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data_[i] += v.data_[i];
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}
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}
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double EntropyBitCost() const {
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double retval = total_count_ * FastLog2(total_count_);
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for (int i = 0; i < kDataSize; ++i) {
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retval -= data_[i] * FastLog2(data_[i]);
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}
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return retval;
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}
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int data_[kDataSize];
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int total_count_;
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double bit_cost_;
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};
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// Literal histogram.
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typedef Histogram<256> HistogramLiteral;
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// Prefix histograms.
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typedef Histogram<kNumCommandPrefixes> HistogramCommand;
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typedef Histogram<kNumDistancePrefixes> HistogramDistance;
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typedef Histogram<kNumBlockLenPrefixes> HistogramBlockLength;
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// Context map histogram, 256 Huffman tree indexes + 16 run length codes.
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typedef Histogram<272> HistogramContextMap;
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// Block type histogram, 256 block types + 2 special symbols.
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typedef Histogram<258> HistogramBlockType;
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static const int kLiteralContextBits = 6;
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static const int kDistanceContextBits = 2;
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void BuildHistograms(
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const std::vector<Command>& cmds,
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const BlockSplit& literal_split,
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const BlockSplit& insert_and_copy_split,
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const BlockSplit& dist_split,
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const uint8_t* ringbuffer,
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size_t pos,
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size_t mask,
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const std::vector<int>& context_modes,
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std::vector<HistogramLiteral>* literal_histograms,
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std::vector<HistogramCommand>* insert_and_copy_histograms,
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std::vector<HistogramDistance>* copy_dist_histograms);
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void BuildLiteralHistogramsForBlockType(
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const std::vector<Command>& cmds,
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const BlockSplit& literal_split,
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const uint8_t* ringbuffer,
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size_t pos,
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size_t mask,
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int block_type,
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int context_mode,
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std::vector<HistogramLiteral>* histograms);
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} // namespace brotli
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#endif // BROTLI_ENC_HISTOGRAM_H_
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