// Copyright 2015 Google Inc. All Rights Reserved. // // 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. // // Algorithms for distributing the literals and commands of a metablock between // block types and contexts. #include "./metablock.h" #include "./block_splitter.h" #include "./cluster.h" #include "./histogram.h" namespace brotli { void BuildMetaBlock(const uint8_t* ringbuffer, const size_t pos, const size_t mask, uint8_t prev_byte, uint8_t prev_byte2, const Command* cmds, size_t num_commands, int literal_context_mode, bool enable_context_modeling, MetaBlockSplit* mb) { SplitBlock(cmds, num_commands, &ringbuffer[pos & mask], &mb->literal_split, &mb->command_split, &mb->distance_split); std::vector literal_context_modes(mb->literal_split.num_types, literal_context_mode); int num_literal_contexts = mb->literal_split.num_types << kLiteralContextBits; int num_distance_contexts = mb->distance_split.num_types << kDistanceContextBits; std::vector literal_histograms(num_literal_contexts); mb->command_histograms.resize(mb->command_split.num_types); std::vector distance_histograms(num_distance_contexts); BuildHistograms(cmds, num_commands, mb->literal_split, mb->command_split, mb->distance_split, ringbuffer, pos, mask, prev_byte, prev_byte2, literal_context_modes, &literal_histograms, &mb->command_histograms, &distance_histograms); // Histogram ids need to fit in one byte. static const int kMaxNumberOfHistograms = 256; mb->literal_histograms = literal_histograms; if (enable_context_modeling) { ClusterHistograms(literal_histograms, 1 << kLiteralContextBits, mb->literal_split.num_types, kMaxNumberOfHistograms, &mb->literal_histograms, &mb->literal_context_map); } else { ClusterHistogramsTrivial(literal_histograms, 1 << kLiteralContextBits, mb->literal_split.num_types, kMaxNumberOfHistograms, &mb->literal_histograms, &mb->literal_context_map); } mb->distance_histograms = distance_histograms; if (enable_context_modeling) { ClusterHistograms(distance_histograms, 1 << kDistanceContextBits, mb->distance_split.num_types, kMaxNumberOfHistograms, &mb->distance_histograms, &mb->distance_context_map); } else { ClusterHistogramsTrivial(distance_histograms, 1 << kDistanceContextBits, mb->distance_split.num_types, kMaxNumberOfHistograms, &mb->distance_histograms, &mb->distance_context_map); } } // Greedy block splitter for one block category (literal, command or distance). template class BlockSplitter { public: BlockSplitter(int alphabet_size, int min_block_size, double split_threshold, int num_symbols, BlockSplit* split, std::vector* histograms) : alphabet_size_(alphabet_size), min_block_size_(min_block_size), split_threshold_(split_threshold), num_blocks_(0), split_(split), histograms_(histograms), target_block_size_(min_block_size), block_size_(0), curr_histogram_ix_(0), merge_last_count_(0) { int max_num_blocks = num_symbols / min_block_size + 1; // We have to allocate one more histogram than the maximum number of block // types for the current histogram when the meta-block is too big. int max_num_types = std::min(max_num_blocks, kMaxBlockTypes + 1); split_->lengths.resize(max_num_blocks); split_->types.resize(max_num_blocks); histograms_->resize(max_num_types); last_histogram_ix_[0] = last_histogram_ix_[1] = 0; } // Adds the next symbol to the current histogram. When the current histogram // reaches the target size, decides on merging the block. void AddSymbol(int symbol) { (*histograms_)[curr_histogram_ix_].Add(symbol); ++block_size_; if (block_size_ == target_block_size_) { FinishBlock(/* is_final = */ false); } } // Does either of three things: // (1) emits the current block with a new block type; // (2) emits the current block with the type of the second last block; // (3) merges the current block with the last block. void FinishBlock(bool is_final) { if (block_size_ < min_block_size_) { block_size_ = min_block_size_; } if (num_blocks_ == 0) { // Create first block. split_->lengths[0] = block_size_; split_->types[0] = 0; last_entropy_[0] = BitsEntropy(&(*histograms_)[0].data_[0], alphabet_size_); last_entropy_[1] = last_entropy_[0]; ++num_blocks_; ++split_->num_types; ++curr_histogram_ix_; block_size_ = 0; } else if (block_size_ > 0) { double entropy = BitsEntropy(&(*histograms_)[curr_histogram_ix_].data_[0], alphabet_size_); HistogramType combined_histo[2]; double combined_entropy[2]; double diff[2]; for (int j = 0; j < 2; ++j) { int last_histogram_ix = last_histogram_ix_[j]; combined_histo[j] = (*histograms_)[curr_histogram_ix_]; combined_histo[j].AddHistogram((*histograms_)[last_histogram_ix]); combined_entropy[j] = BitsEntropy( &combined_histo[j].data_[0], alphabet_size_); diff[j] = combined_entropy[j] - entropy - last_entropy_[j]; } if (split_->num_types < kMaxBlockTypes && diff[0] > split_threshold_ && diff[1] > split_threshold_) { // Create new block. split_->lengths[num_blocks_] = block_size_; split_->types[num_blocks_] = split_->num_types; last_histogram_ix_[1] = last_histogram_ix_[0]; last_histogram_ix_[0] = split_->num_types; last_entropy_[1] = last_entropy_[0]; last_entropy_[0] = entropy; ++num_blocks_; ++split_->num_types; ++curr_histogram_ix_; block_size_ = 0; merge_last_count_ = 0; target_block_size_ = min_block_size_; } else if (diff[1] < diff[0] - 20.0) { // Combine this block with second last block. split_->lengths[num_blocks_] = block_size_; split_->types[num_blocks_] = split_->types[num_blocks_ - 2]; std::swap(last_histogram_ix_[0], last_histogram_ix_[1]); (*histograms_)[last_histogram_ix_[0]] = combined_histo[1]; last_entropy_[1] = last_entropy_[0]; last_entropy_[0] = combined_entropy[1]; ++num_blocks_; block_size_ = 0; (*histograms_)[curr_histogram_ix_].Clear(); merge_last_count_ = 0; target_block_size_ = min_block_size_; } else { // Combine this block with last block. split_->lengths[num_blocks_ - 1] += block_size_; (*histograms_)[last_histogram_ix_[0]] = combined_histo[0]; last_entropy_[0] = combined_entropy[0]; if (split_->num_types == 1) { last_entropy_[1] = last_entropy_[0]; } block_size_ = 0; (*histograms_)[curr_histogram_ix_].Clear(); if (++merge_last_count_ > 1) { target_block_size_ += min_block_size_; } } } if (is_final) { (*histograms_).resize(split_->num_types); split_->types.resize(num_blocks_); split_->lengths.resize(num_blocks_); } } private: static const int kMaxBlockTypes = 256; // Alphabet size of particular block category. const int alphabet_size_; // We collect at least this many symbols for each block. const int min_block_size_; // We merge histograms A and B if // entropy(A+B) < entropy(A) + entropy(B) + split_threshold_, // where A is the current histogram and B is the histogram of the last or the // second last block type. const double split_threshold_; int num_blocks_; BlockSplit* split_; // not owned std::vector* histograms_; // not owned // The number of symbols that we want to collect before deciding on whether // or not to merge the block with a previous one or emit a new block. int target_block_size_; // The number of symbols in the current histogram. int block_size_; // Offset of the current histogram. int curr_histogram_ix_; // Offset of the histograms of the previous two block types. int last_histogram_ix_[2]; // Entropy of the previous two block types. double last_entropy_[2]; // The number of times we merged the current block with the last one. int merge_last_count_; }; void BuildMetaBlockGreedy(const uint8_t* ringbuffer, size_t pos, size_t mask, const Command *commands, size_t n_commands, MetaBlockSplit* mb) { int num_literals = 0; for (int i = 0; i < n_commands; ++i) { num_literals += commands[i].insert_len_; } BlockSplitter lit_blocks( 256, 512, 400.0, num_literals, &mb->literal_split, &mb->literal_histograms); BlockSplitter cmd_blocks( kNumCommandPrefixes, 1024, 500.0, n_commands, &mb->command_split, &mb->command_histograms); BlockSplitter dist_blocks( 64, 512, 100.0, n_commands, &mb->distance_split, &mb->distance_histograms); for (int i = 0; i < n_commands; ++i) { const Command cmd = commands[i]; cmd_blocks.AddSymbol(cmd.cmd_prefix_); for (int j = 0; j < cmd.insert_len_; ++j) { lit_blocks.AddSymbol(ringbuffer[pos & mask]); ++pos; } pos += cmd.copy_len_; if (cmd.copy_len_ > 0 && 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); } void OptimizeHistograms(int num_direct_distance_codes, int distance_postfix_bits, MetaBlockSplit* mb) { for (int i = 0; i < mb->literal_histograms.size(); ++i) { OptimizeHuffmanCountsForRle(256, &mb->literal_histograms[i].data_[0]); } for (int i = 0; i < mb->command_histograms.size(); ++i) { OptimizeHuffmanCountsForRle(kNumCommandPrefixes, &mb->command_histograms[i].data_[0]); } int num_distance_codes = kNumDistanceShortCodes + num_direct_distance_codes + (48 << distance_postfix_bits); for (int i = 0; i < mb->distance_histograms.size(); ++i) { OptimizeHuffmanCountsForRle(num_distance_codes, &mb->distance_histograms[i].data_[0]); } } } // namespace brotli