brotli/enc/metablock.c

/* Copyright 2015 Google Inc. All Rights Reserved.

   Distributed under MIT license.
   See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
*/

/* Algorithms for distributing the literals and commands of a metablock between
   block types and contexts. */

#include "./metablock.h"

#include "../common/types.h"
#include "./block_splitter.h"
#include "./cluster.h"
#include "./context.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,
                    ContextType literal_context_mode,
                    MetaBlockSplit* mb) {
  SplitBlock(cmds, num_commands,
             ringbuffer, pos, mask,
             &mb->literal_split,
             &mb->command_split,
             &mb->distance_split);

  std::vector<ContextType> literal_context_modes(mb->literal_split.num_types,
                                                 literal_context_mode);

  size_t num_literal_contexts =
      mb->literal_split.num_types << kLiteralContextBits;
  size_t num_distance_contexts =
      mb->distance_split.num_types << kDistanceContextBits;
  std::vector<HistogramLiteral> literal_histograms(num_literal_contexts);
  mb->command_histograms.resize(mb->command_split.num_types);
  std::vector<HistogramDistance> 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 size_t kMaxNumberOfHistograms = 256;

  ClusterHistograms(literal_histograms,
                    1u << kLiteralContextBits,
                    mb->literal_split.num_types,
                    kMaxNumberOfHistograms,
                    &mb->literal_histograms,
                    &mb->literal_context_map);

  ClusterHistograms(distance_histograms,
                    1u << 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<typename HistogramType>
class BlockSplitter {
 public:
  BlockSplitter(size_t alphabet_size,
                size_t min_block_size,
                double split_threshold,
                size_t num_symbols,
                BlockSplit* split,
                std::vector<HistogramType>* 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) {
    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 = std::min<size_t>(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(size_t 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] = static_cast<uint32_t>(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 (size_t j = 0; j < 2; ++j) {
        size_t 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_] = static_cast<uint32_t>(block_size_);
        split_->types[num_blocks_] = static_cast<uint8_t>(split_->num_types);
        last_histogram_ix_[1] = last_histogram_ix_[0];
        last_histogram_ix_[0] = static_cast<uint8_t>(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_] = static_cast<uint32_t>(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] += static_cast<uint32_t>(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 uint16_t kMaxBlockTypes = 256;

  /* Alphabet size of particular block category. */
  const size_t alphabet_size_;
  /* We collect at least this many symbols for each block. */
  const 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. */
  const double split_threshold_;

  size_t num_blocks_;
  BlockSplit* split_;  /* not owned */
  std::vector<HistogramType>* 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. */
  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_;
};

void BuildMetaBlockGreedy(const uint8_t* ringbuffer,
                          size_t pos,
                          size_t mask,
                          const Command *commands,
                          size_t n_commands,
                          MetaBlockSplit* mb) {
  size_t num_literals = 0;
  for (size_t i = 0; i < n_commands; ++i) {
    num_literals += commands[i].insert_len_;
  }

  BlockSplitter<HistogramLiteral> lit_blocks(
      256, 512, 400.0, num_literals,
      &mb->literal_split, &mb->literal_histograms);
  BlockSplitter<HistogramCommand> cmd_blocks(
      kNumCommandPrefixes, 1024, 500.0, n_commands,
      &mb->command_split, &mb->command_histograms);
  BlockSplitter<HistogramDistance> dist_blocks(
      64, 512, 100.0, n_commands,
      &mb->distance_split, &mb->distance_histograms);

  for (size_t i = 0; i < n_commands; ++i) {
    const Command cmd = commands[i];
    cmd_blocks.AddSymbol(cmd.cmd_prefix_);
    for (size_t j = cmd.insert_len_; j != 0; --j) {
      lit_blocks.AddSymbol(ringbuffer[pos & mask]);
      ++pos;
    }
    pos += cmd.copy_len();
    if (cmd.copy_len() && 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);
}

// Greedy block splitter for one block category (literal, command or distance).
// Gathers histograms for all context buckets.
template<typename HistogramType>
class ContextBlockSplitter {
 public:
  ContextBlockSplitter(size_t alphabet_size,
                       size_t num_contexts,
                       size_t min_block_size,
                       double split_threshold,
                       size_t num_symbols,
                       BlockSplit* split,
                       std::vector<HistogramType>* histograms)
      : alphabet_size_(alphabet_size),
        num_contexts_(num_contexts),
        max_block_types_(kMaxBlockTypes / num_contexts),
        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),
        last_entropy_(2 * num_contexts),
        merge_last_count_(0) {
    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 = std::min(max_num_blocks, max_block_types_ + 1);
    split_->lengths.resize(max_num_blocks);
    split_->types.resize(max_num_blocks);
    histograms_->resize(max_num_types * num_contexts);
    last_histogram_ix_[0] = last_histogram_ix_[1] = 0;
  }

  // Adds the next symbol to the current block type and context. When the
  // current block reaches the target size, decides on merging the block.
  void AddSymbol(size_t symbol, size_t context) {
    (*histograms_)[curr_histogram_ix_ + context].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] = static_cast<uint32_t>(block_size_);
      split_->types[0] = 0;
      for (size_t i = 0; i < num_contexts_; ++i) {
        last_entropy_[i] =
            BitsEntropy(&(*histograms_)[i].data_[0], alphabet_size_);
        last_entropy_[num_contexts_ + i] = last_entropy_[i];
      }
      ++num_blocks_;
      ++split_->num_types;
      curr_histogram_ix_ += num_contexts_;
      block_size_ = 0;
    } else if (block_size_ > 0) {
    /* Try merging the set of histograms for the current block type with the
       respective set of histograms for the last and second last block types.
       Decide over the split based on the total reduction of entropy across
       all contexts. */
      std::vector<double> entropy(num_contexts_);
      std::vector<HistogramType> combined_histo(2 * num_contexts_);
      std::vector<double> combined_entropy(2 * num_contexts_);
      double diff[2] = { 0.0 };
      for (size_t i = 0; i < num_contexts_; ++i) {
        size_t curr_histo_ix = curr_histogram_ix_ + i;
        entropy[i] = BitsEntropy(&(*histograms_)[curr_histo_ix].data_[0],
                                 alphabet_size_);
        for (size_t j = 0; j < 2; ++j) {
          size_t jx = j * num_contexts_ + i;
          size_t last_histogram_ix = last_histogram_ix_[j] + i;
          combined_histo[jx] = (*histograms_)[curr_histo_ix];
          combined_histo[jx].AddHistogram((*histograms_)[last_histogram_ix]);
          combined_entropy[jx] = BitsEntropy(
              &combined_histo[jx].data_[0], alphabet_size_);
          diff[j] += combined_entropy[jx] - entropy[i] - last_entropy_[jx];
        }
      }

      if (split_->num_types < max_block_types_ &&
          diff[0] > split_threshold_ &&
          diff[1] > split_threshold_) {
        // Create new block.
        split_->lengths[num_blocks_] = static_cast<uint32_t>(block_size_);
        split_->types[num_blocks_] = static_cast<uint8_t>(split_->num_types);
        last_histogram_ix_[1] = last_histogram_ix_[0];
        last_histogram_ix_[0] = split_->num_types * num_contexts_;
        for (size_t i = 0; i < num_contexts_; ++i) {
          last_entropy_[num_contexts_ + i] = last_entropy_[i];
          last_entropy_[i] = entropy[i];
        }
        ++num_blocks_;
        ++split_->num_types;
        curr_histogram_ix_ += num_contexts_;
        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_] = static_cast<uint32_t>(block_size_);
        split_->types[num_blocks_] = split_->types[num_blocks_ - 2];
        std::swap(last_histogram_ix_[0], last_histogram_ix_[1]);
        for (size_t i = 0; i < num_contexts_; ++i) {
          (*histograms_)[last_histogram_ix_[0] + i] =
              combined_histo[num_contexts_ + i];
          last_entropy_[num_contexts_ + i] = last_entropy_[i];
          last_entropy_[i] = combined_entropy[num_contexts_ + i];
          (*histograms_)[curr_histogram_ix_ + i].Clear();
        }
        ++num_blocks_;
        block_size_ = 0;
        merge_last_count_ = 0;
        target_block_size_ = min_block_size_;
      } else {
        // Combine this block with last block.
        split_->lengths[num_blocks_ - 1] += static_cast<uint32_t>(block_size_);
        for (size_t i = 0; i < num_contexts_; ++i) {
          (*histograms_)[last_histogram_ix_[0] + i] = combined_histo[i];
          last_entropy_[i] = combined_entropy[i];
          if (split_->num_types == 1) {
            last_entropy_[num_contexts_ + i] = last_entropy_[i];
          }
          (*histograms_)[curr_histogram_ix_ + i].Clear();
        }
        block_size_ = 0;
        if (++merge_last_count_ > 1) {
          target_block_size_ += min_block_size_;
        }
      }
    }
    if (is_final) {
      (*histograms_).resize(split_->num_types * num_contexts_);
      split_->types.resize(num_blocks_);
      split_->lengths.resize(num_blocks_);
    }
  }

 private:
  static const int kMaxBlockTypes = 256;

  // Alphabet size of particular block category.
  const size_t alphabet_size_;
  const size_t num_contexts_;
  const size_t max_block_types_;
  // We collect at least this many symbols for each block.
  const 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.
  const double split_threshold_;

  size_t num_blocks_;
  BlockSplit* split_;  // not owned
  std::vector<HistogramType>* 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.
  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.
  std::vector<double> last_entropy_;
  // The number of times we merged the current block with the last one.
  size_t merge_last_count_;
};

void BuildMetaBlockGreedyWithContexts(const uint8_t* ringbuffer,
                                      size_t pos,
                                      size_t mask,
                                      uint8_t prev_byte,
                                      uint8_t prev_byte2,
                                      ContextType literal_context_mode,
                                      size_t num_contexts,
                                      const uint32_t* static_context_map,
                                      const Command *commands,
                                      size_t n_commands,
                                      MetaBlockSplit* mb) {
  size_t num_literals = 0;
  for (size_t i = 0; i < n_commands; ++i) {
    num_literals += commands[i].insert_len_;
  }

  ContextBlockSplitter<HistogramLiteral> lit_blocks(
      256, num_contexts, 512, 400.0, num_literals,
      &mb->literal_split, &mb->literal_histograms);
  BlockSplitter<HistogramCommand> cmd_blocks(
      kNumCommandPrefixes, 1024, 500.0, n_commands,
      &mb->command_split, &mb->command_histograms);
  BlockSplitter<HistogramDistance> dist_blocks(
      64, 512, 100.0, n_commands,
      &mb->distance_split, &mb->distance_histograms);

  for (size_t i = 0; i < n_commands; ++i) {
    const Command cmd = commands[i];
    cmd_blocks.AddSymbol(cmd.cmd_prefix_);
    for (size_t j = cmd.insert_len_; j != 0; --j) {
      size_t context = Context(prev_byte, prev_byte2, literal_context_mode);
      uint8_t literal = ringbuffer[pos & mask];
      lit_blocks.AddSymbol(literal, static_context_map[context]);
      prev_byte2 = prev_byte;
      prev_byte = literal;
      ++pos;
    }
    pos += cmd.copy_len();
    if (cmd.copy_len()) {
      prev_byte2 = ringbuffer[(pos - 2) & mask];
      prev_byte = ringbuffer[(pos - 1) & mask];
      if (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);

  mb->literal_context_map.resize(
      mb->literal_split.num_types << kLiteralContextBits);
  for (size_t i = 0; i < mb->literal_split.num_types; ++i) {
    for (size_t j = 0; j < (1u << kLiteralContextBits); ++j) {
      mb->literal_context_map[(i << kLiteralContextBits) + j] =
          static_cast<uint32_t>(i * num_contexts) + static_context_map[j];
    }
  }
}

void OptimizeHistograms(size_t num_direct_distance_codes,
                        size_t distance_postfix_bits,
                        MetaBlockSplit* mb) {
  uint8_t* good_for_rle = new uint8_t[kNumCommandPrefixes];
  for (size_t i = 0; i < mb->literal_histograms.size(); ++i) {
    OptimizeHuffmanCountsForRle(256, &mb->literal_histograms[i].data_[0],
                                good_for_rle);
  }
  for (size_t i = 0; i < mb->command_histograms.size(); ++i) {
    OptimizeHuffmanCountsForRle(kNumCommandPrefixes,
                                &mb->command_histograms[i].data_[0],
                                good_for_rle);
  }
  size_t num_distance_codes =
      kNumDistanceShortCodes + num_direct_distance_codes +
      (48u << distance_postfix_bits);
  for (size_t i = 0; i < mb->distance_histograms.size(); ++i) {
    OptimizeHuffmanCountsForRle(num_distance_codes,
                                &mb->distance_histograms[i].data_[0],
                                good_for_rle);
  }
  delete[] good_for_rle;
}

}  // namespace brotli