public static unsafe void InitBlockSplitter( ref MemoryManager m, BlockSplitterDistance *self, size_t alphabet_size, size_t min_block_size, double split_threshold, size_t num_symbols, BlockSplit *split, HistogramDistance **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 = Math.Min(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; BrotliEnsureCapacity(ref m, sizeof(byte), (void **)&split->types, &split->types_alloc_size, max_num_blocks); BrotliEnsureCapacity(ref m, sizeof(uint), (void **)&split->lengths, &split->lengths_alloc_size, max_num_blocks); self->split_->num_blocks = max_num_blocks; *histograms_size = max_num_types; *histograms = (HistogramDistance *)BrotliAllocate(ref m, *histograms_size * sizeof(HistogramDistance)); self->histograms_ = *histograms; /* Clear only current histogram. */ HistogramDistance.HistogramClear(&self->histograms_[0]); self->last_histogram_ix_0 = self->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. */ public static unsafe void BlockSplitterAddSymbol(BlockSplitterDistance *self, size_t symbol) { HistogramDistance.HistogramAdd(&self->histograms_[self->curr_histogram_ix_], symbol); ++self->block_size_; if (self->block_size_ == self->target_block_size_) { BlockSplitterFinishBlock(self, /* 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. */ public static unsafe void BlockSplitterFinishBlock( BlockSplitterDistance *self, bool is_final) { BlockSplit * split = self->split_; double * last_entropy = self->last_entropy_; HistogramDistance *histograms = self->histograms_; self->block_size_ = Math.Max(self->block_size_, self->min_block_size_); if (self->num_blocks_ == 0) { /* Create first block. */ split->lengths[0] = (uint)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_) { HistogramDistance.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_); HistogramDistance *combined_histo = stackalloc HistogramDistance[2]; double * combined_entropy = stackalloc double[2]; double * diff = stackalloc double[2]; size_t j; for (j = 0; j < 2; ++j) { size_t last_histogram_ix = j == 0 ? self->last_histogram_ix_0 : self->last_histogram_ix_1; combined_histo[j] = histograms[self->curr_histogram_ix_]; HistogramDistance.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_] = (uint)self->block_size_; split->types[self->num_blocks_] = (byte)split->num_types; self->last_histogram_ix_1 = self->last_histogram_ix_0; self->last_histogram_ix_0 = (byte)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_) { HistogramDistance.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_] = (uint)self->block_size_; split->types[self->num_blocks_] = split->types[self->num_blocks_ - 2]; size_t tmp = self->last_histogram_ix_0; self->last_histogram_ix_0 = self->last_histogram_ix_1; self->last_histogram_ix_1 = tmp; 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; HistogramDistance.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] += (uint)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; HistogramDistance.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_; } }