184 lines
7.3 KiB
C
184 lines
7.3 KiB
C
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/* NOLINT(build/header_guard) */
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/* 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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/* template parameters: FN */
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#define HistogramType FN(Histogram)
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/* Greedy block splitter for one block category (literal, command or distance).
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*/
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typedef struct FN(BlockSplitter) {
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/* Alphabet size of particular block category. */
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size_t alphabet_size_;
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/* We collect at least this many symbols for each block. */
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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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double split_threshold_;
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size_t num_blocks_;
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BlockSplit* split_; /* not owned */
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HistogramType* histograms_; /* not owned */
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size_t* histograms_size_; /* 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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} FN(BlockSplitter);
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static void FN(InitBlockSplitter)(
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MemoryManager* m, FN(BlockSplitter)* self, size_t alphabet_size,
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size_t min_block_size, double split_threshold, size_t num_symbols,
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BlockSplit* split, HistogramType** histograms, size_t* histograms_size) {
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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 =
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BROTLI_MIN(size_t, max_num_blocks, BROTLI_MAX_NUMBER_OF_BLOCK_TYPES + 1);
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self->alphabet_size_ = alphabet_size;
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self->min_block_size_ = min_block_size;
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self->split_threshold_ = split_threshold;
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self->num_blocks_ = 0;
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self->split_ = split;
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self->histograms_size_ = histograms_size;
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self->target_block_size_ = min_block_size;
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self->block_size_ = 0;
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self->curr_histogram_ix_ = 0;
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self->merge_last_count_ = 0;
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BROTLI_ENSURE_CAPACITY(m, uint8_t,
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split->types, split->types_alloc_size, max_num_blocks);
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BROTLI_ENSURE_CAPACITY(m, uint32_t,
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split->lengths, split->lengths_alloc_size, max_num_blocks);
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if (BROTLI_IS_OOM(m)) return;
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self->split_->num_blocks = max_num_blocks;
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assert(*histograms == 0);
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*histograms_size = max_num_types;
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*histograms = BROTLI_ALLOC(m, HistogramType, *histograms_size);
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self->histograms_ = *histograms;
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if (BROTLI_IS_OOM(m)) return;
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/* Clear only current histogram. */
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FN(HistogramClear)(&self->histograms_[0]);
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self->last_histogram_ix_[0] = self->last_histogram_ix_[1] = 0;
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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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static void FN(BlockSplitterFinishBlock)(
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FN(BlockSplitter)* self, BROTLI_BOOL is_final) {
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BlockSplit* split = self->split_;
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double* last_entropy = self->last_entropy_;
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HistogramType* histograms = self->histograms_;
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self->block_size_ =
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BROTLI_MAX(size_t, self->block_size_, self->min_block_size_);
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if (self->num_blocks_ == 0) {
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/* Create first block. */
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split->lengths[0] = (uint32_t)self->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_, self->alphabet_size_);
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last_entropy[1] = last_entropy[0];
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++self->num_blocks_;
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++split->num_types;
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++self->curr_histogram_ix_;
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if (self->curr_histogram_ix_ < *self->histograms_size_)
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FN(HistogramClear)(&histograms[self->curr_histogram_ix_]);
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self->block_size_ = 0;
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} else if (self->block_size_ > 0) {
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double entropy = BitsEntropy(histograms[self->curr_histogram_ix_].data_,
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self->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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size_t j;
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for (j = 0; j < 2; ++j) {
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size_t last_histogram_ix = self->last_histogram_ix_[j];
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combined_histo[j] = histograms[self->curr_histogram_ix_];
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FN(HistogramAddHistogram)(&combined_histo[j],
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&histograms[last_histogram_ix]);
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combined_entropy[j] = BitsEntropy(
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&combined_histo[j].data_[0], self->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 < BROTLI_MAX_NUMBER_OF_BLOCK_TYPES &&
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diff[0] > self->split_threshold_ &&
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diff[1] > self->split_threshold_) {
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/* Create new block. */
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split->lengths[self->num_blocks_] = (uint32_t)self->block_size_;
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split->types[self->num_blocks_] = (uint8_t)split->num_types;
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self->last_histogram_ix_[1] = self->last_histogram_ix_[0];
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self->last_histogram_ix_[0] = (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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++self->num_blocks_;
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++split->num_types;
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++self->curr_histogram_ix_;
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if (self->curr_histogram_ix_ < *self->histograms_size_)
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FN(HistogramClear)(&histograms[self->curr_histogram_ix_]);
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self->block_size_ = 0;
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self->merge_last_count_ = 0;
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self->target_block_size_ = self->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[self->num_blocks_] = (uint32_t)self->block_size_;
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split->types[self->num_blocks_] = split->types[self->num_blocks_ - 2];
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BROTLI_SWAP(size_t, self->last_histogram_ix_, 0, 1);
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histograms[self->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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++self->num_blocks_;
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self->block_size_ = 0;
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FN(HistogramClear)(&histograms[self->curr_histogram_ix_]);
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self->merge_last_count_ = 0;
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self->target_block_size_ = self->min_block_size_;
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} else {
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/* Combine this block with last block. */
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split->lengths[self->num_blocks_ - 1] += (uint32_t)self->block_size_;
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histograms[self->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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self->block_size_ = 0;
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FN(HistogramClear)(&histograms[self->curr_histogram_ix_]);
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if (++self->merge_last_count_ > 1) {
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self->target_block_size_ += self->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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*self->histograms_size_ = split->num_types;
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split->num_blocks = self->num_blocks_;
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}
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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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static void FN(BlockSplitterAddSymbol)(FN(BlockSplitter)* self, size_t symbol) {
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FN(HistogramAdd)(&self->histograms_[self->curr_histogram_ix_], symbol);
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++self->block_size_;
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if (self->block_size_ == self->target_block_size_) {
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FN(BlockSplitterFinishBlock)(self, /* is_final = */ BROTLI_FALSE);
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}
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}
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#undef HistogramType
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