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/* 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
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