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https://github.com/google/brotli.git
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541 lines
21 KiB
C++
541 lines
21 KiB
C++
/* Copyright 2015 Google Inc. All Rights Reserved.
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Distributed under MIT license.
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See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
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*/
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/* Algorithms for distributing the literals and commands of a metablock between
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block types and contexts. */
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#include "./metablock.h"
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#include "../common/types.h"
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#include "./block_splitter.h"
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#include "./cluster.h"
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#include "./context.h"
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#include "./histogram.h"
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namespace brotli {
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void BuildMetaBlock(const uint8_t* ringbuffer,
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const size_t pos,
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const size_t mask,
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uint8_t prev_byte,
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uint8_t prev_byte2,
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const Command* cmds,
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size_t num_commands,
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ContextType literal_context_mode,
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MetaBlockSplit* mb) {
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SplitBlock(cmds, num_commands,
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ringbuffer, pos, mask,
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&mb->literal_split,
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&mb->command_split,
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&mb->distance_split);
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std::vector<ContextType> literal_context_modes(mb->literal_split.num_types,
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literal_context_mode);
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size_t num_literal_contexts =
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mb->literal_split.num_types << kLiteralContextBits;
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size_t num_distance_contexts =
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mb->distance_split.num_types << kDistanceContextBits;
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std::vector<HistogramLiteral> literal_histograms(num_literal_contexts);
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mb->command_histograms.resize(mb->command_split.num_types);
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std::vector<HistogramDistance> distance_histograms(num_distance_contexts);
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BuildHistograms(cmds, num_commands,
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mb->literal_split,
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mb->command_split,
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mb->distance_split,
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ringbuffer,
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pos,
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mask,
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prev_byte,
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prev_byte2,
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literal_context_modes,
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&literal_histograms,
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&mb->command_histograms,
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&distance_histograms);
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/* Histogram ids need to fit in one byte. */
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static const size_t kMaxNumberOfHistograms = 256;
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ClusterHistograms(literal_histograms,
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1u << kLiteralContextBits,
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mb->literal_split.num_types,
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kMaxNumberOfHistograms,
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&mb->literal_histograms,
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&mb->literal_context_map);
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ClusterHistograms(distance_histograms,
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1u << kDistanceContextBits,
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mb->distance_split.num_types,
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kMaxNumberOfHistograms,
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&mb->distance_histograms,
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&mb->distance_context_map);
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}
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// Greedy block splitter for one block category (literal, command or distance).
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template<typename HistogramType>
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class BlockSplitter {
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public:
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BlockSplitter(size_t alphabet_size,
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size_t min_block_size,
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double split_threshold,
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size_t num_symbols,
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BlockSplit* split,
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std::vector<HistogramType>* histograms)
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: alphabet_size_(alphabet_size),
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min_block_size_(min_block_size),
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split_threshold_(split_threshold),
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num_blocks_(0),
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split_(split),
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histograms_(histograms),
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target_block_size_(min_block_size),
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block_size_(0),
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curr_histogram_ix_(0),
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merge_last_count_(0) {
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size_t max_num_blocks = num_symbols / min_block_size + 1;
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// We have to allocate one more histogram than the maximum number of block
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// types for the current histogram when the meta-block is too big.
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size_t max_num_types = std::min<size_t>(max_num_blocks, kMaxBlockTypes + 1);
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split_->lengths.resize(max_num_blocks);
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split_->types.resize(max_num_blocks);
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histograms_->resize(max_num_types);
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last_histogram_ix_[0] = last_histogram_ix_[1] = 0;
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}
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// Adds the next symbol to the current histogram. When the current histogram
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// reaches the target size, decides on merging the block.
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void AddSymbol(size_t symbol) {
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(*histograms_)[curr_histogram_ix_].Add(symbol);
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++block_size_;
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if (block_size_ == target_block_size_) {
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FinishBlock(/* is_final = */ false);
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}
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}
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// Does either of three things:
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// (1) emits the current block with a new block type;
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// (2) emits the current block with the type of the second last block;
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// (3) merges the current block with the last block.
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void FinishBlock(bool is_final) {
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if (block_size_ < min_block_size_) {
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block_size_ = min_block_size_;
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}
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if (num_blocks_ == 0) {
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// Create first block.
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split_->lengths[0] = static_cast<uint32_t>(block_size_);
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split_->types[0] = 0;
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last_entropy_[0] =
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BitsEntropy(&(*histograms_)[0].data_[0], alphabet_size_);
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last_entropy_[1] = last_entropy_[0];
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++num_blocks_;
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++split_->num_types;
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++curr_histogram_ix_;
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block_size_ = 0;
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} else if (block_size_ > 0) {
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double entropy = BitsEntropy(&(*histograms_)[curr_histogram_ix_].data_[0],
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alphabet_size_);
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HistogramType combined_histo[2];
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double combined_entropy[2];
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double diff[2];
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for (size_t j = 0; j < 2; ++j) {
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size_t last_histogram_ix = last_histogram_ix_[j];
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combined_histo[j] = (*histograms_)[curr_histogram_ix_];
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combined_histo[j].AddHistogram((*histograms_)[last_histogram_ix]);
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combined_entropy[j] = BitsEntropy(
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&combined_histo[j].data_[0], alphabet_size_);
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diff[j] = combined_entropy[j] - entropy - last_entropy_[j];
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}
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if (split_->num_types < kMaxBlockTypes &&
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diff[0] > split_threshold_ &&
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diff[1] > split_threshold_) {
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// Create new block.
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split_->lengths[num_blocks_] = static_cast<uint32_t>(block_size_);
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split_->types[num_blocks_] = static_cast<uint8_t>(split_->num_types);
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last_histogram_ix_[1] = last_histogram_ix_[0];
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last_histogram_ix_[0] = static_cast<uint8_t>(split_->num_types);
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last_entropy_[1] = last_entropy_[0];
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last_entropy_[0] = entropy;
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++num_blocks_;
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++split_->num_types;
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++curr_histogram_ix_;
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block_size_ = 0;
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merge_last_count_ = 0;
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target_block_size_ = min_block_size_;
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} else if (diff[1] < diff[0] - 20.0) {
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// Combine this block with second last block.
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split_->lengths[num_blocks_] = static_cast<uint32_t>(block_size_);
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split_->types[num_blocks_] = split_->types[num_blocks_ - 2];
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std::swap(last_histogram_ix_[0], last_histogram_ix_[1]);
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(*histograms_)[last_histogram_ix_[0]] = combined_histo[1];
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last_entropy_[1] = last_entropy_[0];
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last_entropy_[0] = combined_entropy[1];
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++num_blocks_;
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block_size_ = 0;
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(*histograms_)[curr_histogram_ix_].Clear();
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merge_last_count_ = 0;
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target_block_size_ = min_block_size_;
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} else {
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// Combine this block with last block.
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split_->lengths[num_blocks_ - 1] += static_cast<uint32_t>(block_size_);
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(*histograms_)[last_histogram_ix_[0]] = combined_histo[0];
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last_entropy_[0] = combined_entropy[0];
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if (split_->num_types == 1) {
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last_entropy_[1] = last_entropy_[0];
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}
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block_size_ = 0;
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(*histograms_)[curr_histogram_ix_].Clear();
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if (++merge_last_count_ > 1) {
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target_block_size_ += min_block_size_;
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}
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}
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}
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if (is_final) {
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(*histograms_).resize(split_->num_types);
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split_->types.resize(num_blocks_);
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split_->lengths.resize(num_blocks_);
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}
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}
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private:
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static const uint16_t kMaxBlockTypes = 256;
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/* Alphabet size of particular block category. */
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const size_t alphabet_size_;
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/* We collect at least this many symbols for each block. */
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const size_t min_block_size_;
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/* We merge histograms A and B if
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entropy(A+B) < entropy(A) + entropy(B) + split_threshold_,
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where A is the current histogram and B is the histogram of the last or the
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second last block type. */
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const double split_threshold_;
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size_t num_blocks_;
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BlockSplit* split_; /* not owned */
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std::vector<HistogramType>* histograms_; /* not owned */
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/* The number of symbols that we want to collect before deciding on whether
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or not to merge the block with a previous one or emit a new block. */
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size_t target_block_size_;
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/* The number of symbols in the current histogram. */
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size_t block_size_;
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/* Offset of the current histogram. */
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size_t curr_histogram_ix_;
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/* Offset of the histograms of the previous two block types. */
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size_t last_histogram_ix_[2];
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/* Entropy of the previous two block types. */
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double last_entropy_[2];
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/* The number of times we merged the current block with the last one. */
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size_t merge_last_count_;
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};
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void BuildMetaBlockGreedy(const uint8_t* ringbuffer,
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size_t pos,
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size_t mask,
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const Command *commands,
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size_t n_commands,
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MetaBlockSplit* mb) {
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size_t num_literals = 0;
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for (size_t i = 0; i < n_commands; ++i) {
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num_literals += commands[i].insert_len_;
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}
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BlockSplitter<HistogramLiteral> lit_blocks(
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256, 512, 400.0, num_literals,
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&mb->literal_split, &mb->literal_histograms);
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BlockSplitter<HistogramCommand> cmd_blocks(
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kNumCommandPrefixes, 1024, 500.0, n_commands,
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&mb->command_split, &mb->command_histograms);
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BlockSplitter<HistogramDistance> dist_blocks(
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64, 512, 100.0, n_commands,
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&mb->distance_split, &mb->distance_histograms);
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for (size_t i = 0; i < n_commands; ++i) {
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const Command cmd = commands[i];
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cmd_blocks.AddSymbol(cmd.cmd_prefix_);
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for (size_t j = cmd.insert_len_; j != 0; --j) {
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lit_blocks.AddSymbol(ringbuffer[pos & mask]);
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++pos;
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}
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pos += cmd.copy_len();
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if (cmd.copy_len() && cmd.cmd_prefix_ >= 128) {
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dist_blocks.AddSymbol(cmd.dist_prefix_);
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}
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}
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lit_blocks.FinishBlock(/* is_final = */ true);
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cmd_blocks.FinishBlock(/* is_final = */ true);
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dist_blocks.FinishBlock(/* is_final = */ true);
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}
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// Greedy block splitter for one block category (literal, command or distance).
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// Gathers histograms for all context buckets.
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template<typename HistogramType>
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class ContextBlockSplitter {
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public:
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ContextBlockSplitter(size_t alphabet_size,
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size_t num_contexts,
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size_t min_block_size,
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double split_threshold,
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size_t num_symbols,
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BlockSplit* split,
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std::vector<HistogramType>* histograms)
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: alphabet_size_(alphabet_size),
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num_contexts_(num_contexts),
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max_block_types_(kMaxBlockTypes / num_contexts),
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min_block_size_(min_block_size),
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split_threshold_(split_threshold),
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num_blocks_(0),
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split_(split),
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histograms_(histograms),
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target_block_size_(min_block_size),
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block_size_(0),
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curr_histogram_ix_(0),
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last_entropy_(2 * num_contexts),
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merge_last_count_(0) {
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size_t max_num_blocks = num_symbols / min_block_size + 1;
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// We have to allocate one more histogram than the maximum number of block
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// types for the current histogram when the meta-block is too big.
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size_t max_num_types = std::min(max_num_blocks, max_block_types_ + 1);
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split_->lengths.resize(max_num_blocks);
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split_->types.resize(max_num_blocks);
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histograms_->resize(max_num_types * num_contexts);
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last_histogram_ix_[0] = last_histogram_ix_[1] = 0;
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}
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// Adds the next symbol to the current block type and context. When the
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// current block reaches the target size, decides on merging the block.
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void AddSymbol(size_t symbol, size_t context) {
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(*histograms_)[curr_histogram_ix_ + context].Add(symbol);
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++block_size_;
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if (block_size_ == target_block_size_) {
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FinishBlock(/* is_final = */ false);
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}
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}
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/* Does either of three things:
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(1) emits the current block with a new block type;
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(2) emits the current block with the type of the second last block;
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(3) merges the current block with the last block. */
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void FinishBlock(bool is_final) {
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if (block_size_ < min_block_size_) {
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block_size_ = min_block_size_;
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}
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if (num_blocks_ == 0) {
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// Create first block.
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split_->lengths[0] = static_cast<uint32_t>(block_size_);
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split_->types[0] = 0;
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for (size_t i = 0; i < num_contexts_; ++i) {
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last_entropy_[i] =
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BitsEntropy(&(*histograms_)[i].data_[0], alphabet_size_);
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last_entropy_[num_contexts_ + i] = last_entropy_[i];
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}
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++num_blocks_;
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++split_->num_types;
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curr_histogram_ix_ += num_contexts_;
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block_size_ = 0;
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} else if (block_size_ > 0) {
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/* Try merging the set of histograms for the current block type with the
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respective set of histograms for the last and second last block types.
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Decide over the split based on the total reduction of entropy across
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all contexts. */
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std::vector<double> entropy(num_contexts_);
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std::vector<HistogramType> combined_histo(2 * num_contexts_);
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std::vector<double> combined_entropy(2 * num_contexts_);
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double diff[2] = { 0.0 };
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for (size_t i = 0; i < num_contexts_; ++i) {
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size_t curr_histo_ix = curr_histogram_ix_ + i;
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entropy[i] = BitsEntropy(&(*histograms_)[curr_histo_ix].data_[0],
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alphabet_size_);
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for (size_t j = 0; j < 2; ++j) {
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size_t jx = j * num_contexts_ + i;
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size_t last_histogram_ix = last_histogram_ix_[j] + i;
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combined_histo[jx] = (*histograms_)[curr_histo_ix];
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combined_histo[jx].AddHistogram((*histograms_)[last_histogram_ix]);
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combined_entropy[jx] = BitsEntropy(
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&combined_histo[jx].data_[0], alphabet_size_);
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diff[j] += combined_entropy[jx] - entropy[i] - last_entropy_[jx];
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}
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}
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if (split_->num_types < max_block_types_ &&
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diff[0] > split_threshold_ &&
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diff[1] > split_threshold_) {
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// Create new block.
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split_->lengths[num_blocks_] = static_cast<uint32_t>(block_size_);
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split_->types[num_blocks_] = static_cast<uint8_t>(split_->num_types);
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last_histogram_ix_[1] = last_histogram_ix_[0];
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last_histogram_ix_[0] = split_->num_types * num_contexts_;
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for (size_t i = 0; i < num_contexts_; ++i) {
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last_entropy_[num_contexts_ + i] = last_entropy_[i];
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last_entropy_[i] = entropy[i];
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}
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++num_blocks_;
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++split_->num_types;
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curr_histogram_ix_ += num_contexts_;
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block_size_ = 0;
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merge_last_count_ = 0;
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target_block_size_ = min_block_size_;
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} else if (diff[1] < diff[0] - 20.0) {
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// Combine this block with second last block.
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split_->lengths[num_blocks_] = static_cast<uint32_t>(block_size_);
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split_->types[num_blocks_] = split_->types[num_blocks_ - 2];
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std::swap(last_histogram_ix_[0], last_histogram_ix_[1]);
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for (size_t i = 0; i < num_contexts_; ++i) {
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(*histograms_)[last_histogram_ix_[0] + i] =
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combined_histo[num_contexts_ + i];
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last_entropy_[num_contexts_ + i] = last_entropy_[i];
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last_entropy_[i] = combined_entropy[num_contexts_ + i];
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(*histograms_)[curr_histogram_ix_ + i].Clear();
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}
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++num_blocks_;
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block_size_ = 0;
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merge_last_count_ = 0;
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target_block_size_ = min_block_size_;
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} else {
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// Combine this block with last block.
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split_->lengths[num_blocks_ - 1] += static_cast<uint32_t>(block_size_);
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for (size_t i = 0; i < num_contexts_; ++i) {
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(*histograms_)[last_histogram_ix_[0] + i] = combined_histo[i];
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last_entropy_[i] = combined_entropy[i];
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if (split_->num_types == 1) {
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last_entropy_[num_contexts_ + i] = last_entropy_[i];
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}
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(*histograms_)[curr_histogram_ix_ + i].Clear();
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}
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block_size_ = 0;
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if (++merge_last_count_ > 1) {
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target_block_size_ += min_block_size_;
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}
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}
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}
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if (is_final) {
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(*histograms_).resize(split_->num_types * num_contexts_);
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split_->types.resize(num_blocks_);
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split_->lengths.resize(num_blocks_);
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}
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}
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private:
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static const int kMaxBlockTypes = 256;
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// Alphabet size of particular block category.
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const size_t alphabet_size_;
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const size_t num_contexts_;
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const size_t max_block_types_;
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// We collect at least this many symbols for each block.
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const size_t min_block_size_;
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// We merge histograms A and B if
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// entropy(A+B) < entropy(A) + entropy(B) + split_threshold_,
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// where A is the current histogram and B is the histogram of the last or the
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// second last block type.
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const double split_threshold_;
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size_t num_blocks_;
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BlockSplit* split_; // not owned
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std::vector<HistogramType>* histograms_; // not owned
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|
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// The number of symbols that we want to collect before deciding on whether
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// or not to merge the block with a previous one or emit a new block.
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|
size_t target_block_size_;
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// The number of symbols in the current histogram.
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size_t block_size_;
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// Offset of the current histogram.
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size_t curr_histogram_ix_;
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// Offset of the histograms of the previous two block types.
|
|
size_t last_histogram_ix_[2];
|
|
// Entropy of the previous two block types.
|
|
std::vector<double> last_entropy_;
|
|
// The number of times we merged the current block with the last one.
|
|
size_t merge_last_count_;
|
|
};
|
|
|
|
void BuildMetaBlockGreedyWithContexts(const uint8_t* ringbuffer,
|
|
size_t pos,
|
|
size_t mask,
|
|
uint8_t prev_byte,
|
|
uint8_t prev_byte2,
|
|
ContextType literal_context_mode,
|
|
size_t num_contexts,
|
|
const uint32_t* static_context_map,
|
|
const Command *commands,
|
|
size_t n_commands,
|
|
MetaBlockSplit* mb) {
|
|
size_t num_literals = 0;
|
|
for (size_t i = 0; i < n_commands; ++i) {
|
|
num_literals += commands[i].insert_len_;
|
|
}
|
|
|
|
ContextBlockSplitter<HistogramLiteral> lit_blocks(
|
|
256, num_contexts, 512, 400.0, num_literals,
|
|
&mb->literal_split, &mb->literal_histograms);
|
|
BlockSplitter<HistogramCommand> cmd_blocks(
|
|
kNumCommandPrefixes, 1024, 500.0, n_commands,
|
|
&mb->command_split, &mb->command_histograms);
|
|
BlockSplitter<HistogramDistance> dist_blocks(
|
|
64, 512, 100.0, n_commands,
|
|
&mb->distance_split, &mb->distance_histograms);
|
|
|
|
for (size_t i = 0; i < n_commands; ++i) {
|
|
const Command cmd = commands[i];
|
|
cmd_blocks.AddSymbol(cmd.cmd_prefix_);
|
|
for (size_t j = cmd.insert_len_; j != 0; --j) {
|
|
size_t context = Context(prev_byte, prev_byte2, literal_context_mode);
|
|
uint8_t literal = ringbuffer[pos & mask];
|
|
lit_blocks.AddSymbol(literal, static_context_map[context]);
|
|
prev_byte2 = prev_byte;
|
|
prev_byte = literal;
|
|
++pos;
|
|
}
|
|
pos += cmd.copy_len();
|
|
if (cmd.copy_len()) {
|
|
prev_byte2 = ringbuffer[(pos - 2) & mask];
|
|
prev_byte = ringbuffer[(pos - 1) & mask];
|
|
if (cmd.cmd_prefix_ >= 128) {
|
|
dist_blocks.AddSymbol(cmd.dist_prefix_);
|
|
}
|
|
}
|
|
}
|
|
|
|
lit_blocks.FinishBlock(/* is_final = */ true);
|
|
cmd_blocks.FinishBlock(/* is_final = */ true);
|
|
dist_blocks.FinishBlock(/* is_final = */ true);
|
|
|
|
mb->literal_context_map.resize(
|
|
mb->literal_split.num_types << kLiteralContextBits);
|
|
for (size_t i = 0; i < mb->literal_split.num_types; ++i) {
|
|
for (size_t j = 0; j < (1u << kLiteralContextBits); ++j) {
|
|
mb->literal_context_map[(i << kLiteralContextBits) + j] =
|
|
static_cast<uint32_t>(i * num_contexts) + static_context_map[j];
|
|
}
|
|
}
|
|
}
|
|
|
|
void OptimizeHistograms(size_t num_direct_distance_codes,
|
|
size_t distance_postfix_bits,
|
|
MetaBlockSplit* mb) {
|
|
uint8_t* good_for_rle = new uint8_t[kNumCommandPrefixes];
|
|
for (size_t i = 0; i < mb->literal_histograms.size(); ++i) {
|
|
OptimizeHuffmanCountsForRle(256, &mb->literal_histograms[i].data_[0],
|
|
good_for_rle);
|
|
}
|
|
for (size_t i = 0; i < mb->command_histograms.size(); ++i) {
|
|
OptimizeHuffmanCountsForRle(kNumCommandPrefixes,
|
|
&mb->command_histograms[i].data_[0],
|
|
good_for_rle);
|
|
}
|
|
size_t num_distance_codes =
|
|
kNumDistanceShortCodes + num_direct_distance_codes +
|
|
(48u << distance_postfix_bits);
|
|
for (size_t i = 0; i < mb->distance_histograms.size(); ++i) {
|
|
OptimizeHuffmanCountsForRle(num_distance_codes,
|
|
&mb->distance_histograms[i].data_[0],
|
|
good_for_rle);
|
|
}
|
|
delete[] good_for_rle;
|
|
}
|
|
|
|
} // namespace brotli
|