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Diffstat (limited to 'modules/brotli/enc/metablock_inc.h')
| -rw-r--r-- | modules/brotli/enc/metablock_inc.h | 183 |
1 files changed, 183 insertions, 0 deletions
diff --git a/modules/brotli/enc/metablock_inc.h b/modules/brotli/enc/metablock_inc.h new file mode 100644 index 000000000..dcc9d3c4a --- /dev/null +++ b/modules/brotli/enc/metablock_inc.h @@ -0,0 +1,183 @@ +/* NOLINT(build/header_guard) */ +/* Copyright 2015 Google Inc. All Rights Reserved. + + Distributed under MIT license. + See file LICENSE for detail or copy at https://opensource.org/licenses/MIT +*/ + +/* template parameters: FN */ + +#define HistogramType FN(Histogram) + +/* Greedy block splitter for one block category (literal, command or distance). +*/ +typedef struct FN(BlockSplitter) { + /* Alphabet size of particular block category. */ + size_t alphabet_size_; + /* We collect at least this many symbols for each block. */ + size_t 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. */ + double split_threshold_; + + size_t num_blocks_; + BlockSplit* split_; /* not owned */ + HistogramType* histograms_; /* not owned */ + size_t* histograms_size_; /* 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. */ + size_t target_block_size_; + /* The number of symbols in the current histogram. */ + size_t block_size_; + /* Offset of the current histogram. */ + size_t curr_histogram_ix_; + /* Offset of the histograms of the previous two block types. */ + size_t 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. */ + size_t merge_last_count_; +} FN(BlockSplitter); + +static void FN(InitBlockSplitter)( + MemoryManager* m, FN(BlockSplitter)* self, size_t alphabet_size, + size_t min_block_size, double split_threshold, size_t num_symbols, + BlockSplit* split, HistogramType** histograms, size_t* histograms_size) { + size_t 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. */ + size_t max_num_types = + BROTLI_MIN(size_t, max_num_blocks, BROTLI_MAX_NUMBER_OF_BLOCK_TYPES + 1); + self->alphabet_size_ = alphabet_size; + self->min_block_size_ = min_block_size; + self->split_threshold_ = split_threshold; + self->num_blocks_ = 0; + self->split_ = split; + self->histograms_size_ = histograms_size; + self->target_block_size_ = min_block_size; + self->block_size_ = 0; + self->curr_histogram_ix_ = 0; + self->merge_last_count_ = 0; + BROTLI_ENSURE_CAPACITY(m, uint8_t, + split->types, split->types_alloc_size, max_num_blocks); + BROTLI_ENSURE_CAPACITY(m, uint32_t, + split->lengths, split->lengths_alloc_size, max_num_blocks); + if (BROTLI_IS_OOM(m)) return; + self->split_->num_blocks = max_num_blocks; + BROTLI_DCHECK(*histograms == 0); + *histograms_size = max_num_types; + *histograms = BROTLI_ALLOC(m, HistogramType, *histograms_size); + self->histograms_ = *histograms; + if (BROTLI_IS_OOM(m)) return; + /* Clear only current histogram. */ + FN(HistogramClear)(&self->histograms_[0]); + self->last_histogram_ix_[0] = self->last_histogram_ix_[1] = 0; +} + +/* 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. */ +static void FN(BlockSplitterFinishBlock)( + FN(BlockSplitter)* self, BROTLI_BOOL is_final) { + BlockSplit* split = self->split_; + double* last_entropy = self->last_entropy_; + HistogramType* histograms = self->histograms_; + self->block_size_ = + BROTLI_MAX(size_t, self->block_size_, self->min_block_size_); + if (self->num_blocks_ == 0) { + /* Create first block. */ + split->lengths[0] = (uint32_t)self->block_size_; + split->types[0] = 0; + last_entropy[0] = + BitsEntropy(histograms[0].data_, self->alphabet_size_); + last_entropy[1] = last_entropy[0]; + ++self->num_blocks_; + ++split->num_types; + ++self->curr_histogram_ix_; + if (self->curr_histogram_ix_ < *self->histograms_size_) + FN(HistogramClear)(&histograms[self->curr_histogram_ix_]); + self->block_size_ = 0; + } else if (self->block_size_ > 0) { + double entropy = BitsEntropy(histograms[self->curr_histogram_ix_].data_, + self->alphabet_size_); + HistogramType combined_histo[2]; + double combined_entropy[2]; + double diff[2]; + size_t j; + for (j = 0; j < 2; ++j) { + size_t last_histogram_ix = self->last_histogram_ix_[j]; + combined_histo[j] = histograms[self->curr_histogram_ix_]; + FN(HistogramAddHistogram)(&combined_histo[j], + &histograms[last_histogram_ix]); + combined_entropy[j] = BitsEntropy( + &combined_histo[j].data_[0], self->alphabet_size_); + diff[j] = combined_entropy[j] - entropy - last_entropy[j]; + } + + if (split->num_types < BROTLI_MAX_NUMBER_OF_BLOCK_TYPES && + diff[0] > self->split_threshold_ && + diff[1] > self->split_threshold_) { + /* Create new block. */ + split->lengths[self->num_blocks_] = (uint32_t)self->block_size_; + split->types[self->num_blocks_] = (uint8_t)split->num_types; + self->last_histogram_ix_[1] = self->last_histogram_ix_[0]; + self->last_histogram_ix_[0] = (uint8_t)split->num_types; + last_entropy[1] = last_entropy[0]; + last_entropy[0] = entropy; + ++self->num_blocks_; + ++split->num_types; + ++self->curr_histogram_ix_; + if (self->curr_histogram_ix_ < *self->histograms_size_) + FN(HistogramClear)(&histograms[self->curr_histogram_ix_]); + self->block_size_ = 0; + self->merge_last_count_ = 0; + self->target_block_size_ = self->min_block_size_; + } else if (diff[1] < diff[0] - 20.0) { + /* Combine this block with second last block. */ + split->lengths[self->num_blocks_] = (uint32_t)self->block_size_; + split->types[self->num_blocks_] = split->types[self->num_blocks_ - 2]; + BROTLI_SWAP(size_t, self->last_histogram_ix_, 0, 1); + histograms[self->last_histogram_ix_[0]] = combined_histo[1]; + last_entropy[1] = last_entropy[0]; + last_entropy[0] = combined_entropy[1]; + ++self->num_blocks_; + self->block_size_ = 0; + FN(HistogramClear)(&histograms[self->curr_histogram_ix_]); + self->merge_last_count_ = 0; + self->target_block_size_ = self->min_block_size_; + } else { + /* Combine this block with last block. */ + split->lengths[self->num_blocks_ - 1] += (uint32_t)self->block_size_; + histograms[self->last_histogram_ix_[0]] = combined_histo[0]; + last_entropy[0] = combined_entropy[0]; + if (split->num_types == 1) { + last_entropy[1] = last_entropy[0]; + } + self->block_size_ = 0; + FN(HistogramClear)(&histograms[self->curr_histogram_ix_]); + if (++self->merge_last_count_ > 1) { + self->target_block_size_ += self->min_block_size_; + } + } + } + if (is_final) { + *self->histograms_size_ = split->num_types; + split->num_blocks = self->num_blocks_; + } +} + +/* Adds the next symbol to the current histogram. When the current histogram + reaches the target size, decides on merging the block. */ +static void FN(BlockSplitterAddSymbol)(FN(BlockSplitter)* self, size_t symbol) { + FN(HistogramAdd)(&self->histograms_[self->curr_histogram_ix_], symbol); + ++self->block_size_; + if (self->block_size_ == self->target_block_size_) { + FN(BlockSplitterFinishBlock)(self, /* is_final = */ BROTLI_FALSE); + } +} + +#undef HistogramType |