Add a faster but less dense compression mode.

The new mode can be used by setting the greedy_block_split
field of BrotliParams to true.

This commit moves all the meta-block processing code
into its own library and moves the meta-block encoding
code to brotli_bit_stream.cc from encode.cc

enc/Makefile

@@ -2,7 +2,7 @@
 
 include ../shared.mk
 
-OBJS = backward_references.o block_splitter.o brotli_bit_stream.o encode.o entropy_encode.o histogram.o literal_cost.o
+OBJS = backward_references.o block_splitter.o brotli_bit_stream.o encode.o entropy_encode.o histogram.o literal_cost.o metablock.o
 
 all : $(OBJS)
 

enc/bit_cost.h

@@ -24,6 +24,31 @@
 
 namespace brotli {
 
+static inline double BitsEntropy(const int *population, int size) {
+  int sum = 0;
+  double retval = 0;
+  const int *population_end = population + size;
+  int p;
+  if (size & 1) {
+    goto odd_number_of_elements_left;
+  }
+  while (population < population_end) {
+    p = *population++;
+    sum += p;
+    retval -= p * FastLog2(p);
+ odd_number_of_elements_left:
+    p = *population++;
+    sum += p;
+    retval -= p * FastLog2(p);
+  }
+  if (sum) retval -= sum * log(sum);
+  if (retval < sum) {
+    // At least one bit per literal is needed.
+    retval = sum;
+  }
+  return retval;
+}
+
 static const int kHuffmanExtraBits[kCodeLengthCodes] = {
   0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 3,
 };

enc/block_splitter.cc

@@ -284,21 +284,21 @@ void ClusterBlocks(const DataType* data, const size_t length,
 void BuildBlockSplit(const std::vector<uint8_t>& block_ids, BlockSplit* split) {
   int cur_id = block_ids[0];
   int cur_length = 1;
-  split->num_types_ = -1;
+  split->num_types = -1;
   for (int i = 1; i < block_ids.size(); ++i) {
     if (block_ids[i] != cur_id) {
-      split->types_.push_back(cur_id);
-      split->lengths_.push_back(cur_length);
-      split->num_types_ = std::max(split->num_types_, cur_id);
+      split->types.push_back(cur_id);
+      split->lengths.push_back(cur_length);
+      split->num_types = std::max(split->num_types, cur_id);
       cur_id = block_ids[i];
       cur_length = 0;
     }
     ++cur_length;
   }
-  split->types_.push_back(cur_id);
-  split->lengths_.push_back(cur_length);
-  split->num_types_ = std::max(split->num_types_, cur_id);
-  ++split->num_types_;
+  split->types.push_back(cur_id);
+  split->lengths.push_back(cur_length);
+  split->num_types = std::max(split->num_types, cur_id);
+  ++split->num_types;
 }
 
 template<typename HistogramType, typename DataType>
@@ -309,12 +309,12 @@ void SplitByteVector(const std::vector<DataType>& data,
                      const double block_switch_cost,
                      BlockSplit* split) {
   if (data.empty()) {
-    split->num_types_ = 1;
+    split->num_types = 1;
     return;
   } else if (data.size() < kMinLengthForBlockSplitting) {
-    split->num_types_ = 1;
-    split->types_.push_back(0);
-    split->lengths_.push_back(data.size());
+    split->num_types = 1;
+    split->types.push_back(0);
+    split->lengths.push_back(data.size());
     return;
   }
   std::vector<HistogramType> histograms;
@@ -356,7 +356,6 @@ void SplitBlock(const std::vector<Command>& cmds,
                           &insert_and_copy_codes,
                           &distance_prefixes);
 
-
   SplitByteVector<HistogramLiteral>(
       literals,
       kSymbolsPerLiteralHistogram, kMaxLiteralHistograms,

enc/block_splitter.h

@@ -24,28 +24,23 @@
 #include <utility>
 
 #include "./command.h"
+#include "./metablock.h"
 
 namespace brotli {
 
-struct BlockSplit {
-  int num_types_;
-  std::vector<int> types_;
-  std::vector<int> lengths_;
-};
-
 struct BlockSplitIterator {
   explicit BlockSplitIterator(const BlockSplit& split)
       : split_(split), idx_(0), type_(0), length_(0) {
-    if (!split.lengths_.empty()) {
-      length_ = split.lengths_[0];
+    if (!split.lengths.empty()) {
+      length_ = split.lengths[0];
     }
   }
 
   void Next() {
     if (length_ == 0) {
       ++idx_;
-      type_ = split_.types_[idx_];
-      length_ = split_.lengths_[idx_];
+      type_ = split_.types[idx_];
+      length_ = split_.lengths[idx_];
     }
     --length_;
   }

enc/brotli_bit_stream.cc

@@ -23,6 +23,7 @@
 #include <vector>
 
 #include "./bit_cost.h"
+#include "./context.h"
 #include "./entropy_encode.h"
 #include "./fast_log.h"
 #include "./prefix.h"
@@ -572,4 +573,253 @@ void StoreTrivialContextMap(int num_types,
   }
 }
 
+// Manages the encoding of one block category (literal, command or distance).
+class BlockEncoder {
+ public:
+  BlockEncoder(int alphabet_size,
+               int num_block_types,
+               const std::vector<int>& block_types,
+               const std::vector<int>& block_lengths)
+      : alphabet_size_(alphabet_size),
+        num_block_types_(num_block_types),
+        block_types_(block_types),
+        block_lengths_(block_lengths),
+        block_ix_(0),
+        block_len_(block_lengths.empty() ? 0 : block_lengths[0]),
+        entropy_ix_(0) {}
+
+  // Creates entropy codes of block lengths and block types and stores them
+  // to the bit stream.
+  void BuildAndStoreBlockSwitchEntropyCodes(int quality,
+                                            int* storage_ix, uint8_t* storage) {
+    BuildAndStoreBlockSplitCode(
+        block_types_, block_lengths_, num_block_types_,
+        quality, &block_split_code_, storage_ix, storage);
+  }
+
+  // Creates entropy codes for all block types and stores them to the bit
+  // stream.
+  template<int kSize>
+  void BuildAndStoreEntropyCodes(
+      const std::vector<Histogram<kSize> >& histograms,
+      int quality,
+      int* storage_ix, uint8_t* storage) {
+    depths_.resize(histograms.size() * alphabet_size_);
+    bits_.resize(histograms.size() * alphabet_size_);
+    for (int i = 0; i < histograms.size(); ++i) {
+      int ix = i * alphabet_size_;
+      BuildAndStoreHuffmanTree(&histograms[i].data_[0], alphabet_size_,
+                               quality,
+                               &depths_[ix], &bits_[ix],
+                               storage_ix, storage);
+    }
+  }
+
+  // Stores the next symbol with the entropy code of the current block type.
+  // Updates the block type and block length at block boundaries.
+  void StoreSymbol(int symbol, int* storage_ix, uint8_t* storage) {
+    if (block_len_ == 0) {
+      ++block_ix_;
+      block_len_ = block_lengths_[block_ix_];
+      entropy_ix_ = block_types_[block_ix_] * alphabet_size_;
+      StoreBlockSwitch(block_split_code_, block_ix_, storage_ix, storage);
+    }
+    --block_len_;
+    int ix = entropy_ix_ + symbol;
+    WriteBits(depths_[ix], bits_[ix], storage_ix, storage);
+  }
+
+  // Stores the next symbol with the entropy code of the current block type and
+  // context value.
+  // Updates the block type and block length at block boundaries.
+  template<int kContextBits>
+  void StoreSymbolWithContext(int symbol, int context,
+                              const std::vector<int>& context_map,
+                              int* storage_ix, uint8_t* storage) {
+    if (block_len_ == 0) {
+      ++block_ix_;
+      block_len_ = block_lengths_[block_ix_];
+      entropy_ix_ = block_types_[block_ix_] << kContextBits;
+      StoreBlockSwitch(block_split_code_, block_ix_, storage_ix, storage);
+    }
+    --block_len_;
+    int histo_ix = context_map[entropy_ix_ + context];
+    int ix = histo_ix * alphabet_size_ + symbol;
+    WriteBits(depths_[ix], bits_[ix], storage_ix, storage);
+  }
+
+ private:
+  const int alphabet_size_;
+  const int num_block_types_;
+  const std::vector<int>& block_types_;
+  const std::vector<int>& block_lengths_;
+  BlockSplitCode block_split_code_;
+  int block_ix_;
+  int block_len_;
+  int entropy_ix_;
+  std::vector<uint8_t> depths_;
+  std::vector<uint16_t> bits_;
+};
+
+bool StoreMetaBlock(const uint8_t* input,
+                    size_t start_pos,
+                    size_t length,
+                    size_t mask,
+                    bool is_last,
+                    int quality,
+                    int num_direct_distance_codes,
+                    int distance_postfix_bits,
+                    int literal_context_mode,
+                    const brotli::Command *commands,
+                    size_t n_commands,
+                    const MetaBlockSplit& mb,
+                    int *storage_ix,
+                    uint8_t *storage) {
+  if (!StoreCompressedMetaBlockHeader(is_last, length, storage_ix, storage)) {
+    return false;
+  }
+
+  if (length == 0) {
+    return true;
+  }
+
+  int num_distance_codes =
+      kNumDistanceShortCodes + num_direct_distance_codes +
+      (48 << distance_postfix_bits);
+
+  BlockEncoder literal_enc(256,
+                           mb.literal_split.num_types,
+                           mb.literal_split.types,
+                           mb.literal_split.lengths);
+  BlockEncoder command_enc(kNumCommandPrefixes,
+                           mb.command_split.num_types,
+                           mb.command_split.types,
+                           mb.command_split.lengths);
+  BlockEncoder distance_enc(num_distance_codes,
+                            mb.distance_split.num_types,
+                            mb.distance_split.types,
+                            mb.distance_split.lengths);
+
+  literal_enc.BuildAndStoreBlockSwitchEntropyCodes(
+      quality, storage_ix, storage);
+  command_enc.BuildAndStoreBlockSwitchEntropyCodes(
+      quality, storage_ix, storage);
+  distance_enc.BuildAndStoreBlockSwitchEntropyCodes(
+      quality, storage_ix, storage);
+
+  WriteBits(2, distance_postfix_bits, storage_ix, storage);
+  WriteBits(4, num_direct_distance_codes >> distance_postfix_bits,
+            storage_ix, storage);
+  for (int i = 0; i < mb.literal_split.num_types; ++i) {
+    WriteBits(2, literal_context_mode, storage_ix, storage);
+  }
+
+  if (mb.literal_context_map.empty()) {
+    StoreTrivialContextMap(mb.literal_histograms.size(), kLiteralContextBits,
+                           storage_ix, storage);
+  } else {
+    EncodeContextMap(mb.literal_context_map, mb.literal_histograms.size(),
+                     storage_ix, storage);
+  }
+
+  if (mb.distance_context_map.empty()) {
+    StoreTrivialContextMap(mb.distance_histograms.size(), kDistanceContextBits,
+                           storage_ix, storage);
+  } else {
+    EncodeContextMap(mb.distance_context_map, mb.distance_histograms.size(),
+                     storage_ix, storage);
+  }
+
+  literal_enc.BuildAndStoreEntropyCodes(mb.literal_histograms, quality,
+                                        storage_ix, storage);
+  command_enc.BuildAndStoreEntropyCodes(mb.command_histograms, quality,
+                                        storage_ix, storage);
+  distance_enc.BuildAndStoreEntropyCodes(mb.distance_histograms, quality,
+                                         storage_ix, storage);
+
+  size_t pos = start_pos;
+  for (int i = 0; i < n_commands; ++i) {
+    const Command cmd = commands[i];
+    int cmd_code = cmd.cmd_prefix_;
+    int lennumextra = cmd.cmd_extra_ >> 48;
+    uint64_t lenextra = cmd.cmd_extra_ & 0xffffffffffffULL;
+    command_enc.StoreSymbol(cmd_code, storage_ix, storage);
+    WriteBits(lennumextra, lenextra, storage_ix, storage);
+    if (mb.literal_context_map.empty()) {
+      for (int j = 0; j < cmd.insert_len_; j++) {
+        literal_enc.StoreSymbol(input[pos & mask], storage_ix, storage);
+        ++pos;
+      }
+    } else {
+      for (int j = 0; j < cmd.insert_len_; ++j) {
+        uint8_t prev_byte = pos > 0 ? input[(pos - 1) & mask] : 0;
+        uint8_t prev_byte2 = pos > 1 ? input[(pos - 2) & mask] : 0;
+        int context = Context(prev_byte, prev_byte2,
+                              literal_context_mode);
+        int literal = input[pos & mask];
+        literal_enc.StoreSymbolWithContext<kLiteralContextBits>(
+            literal, context, mb.literal_context_map, storage_ix, storage);
+        ++pos;
+      }
+    }
+    if (cmd.copy_len_ > 0 && cmd.cmd_prefix_ >= 128) {
+      int dist_code = cmd.dist_prefix_;
+      int distnumextra = cmd.dist_extra_ >> 24;
+      int distextra = cmd.dist_extra_ & 0xffffff;
+      if (mb.distance_context_map.empty()) {
+        distance_enc.StoreSymbol(dist_code, storage_ix, storage);
+      } else {
+        int context = cmd.DistanceContext();
+        distance_enc.StoreSymbolWithContext<kDistanceContextBits>(
+            dist_code, context, mb.distance_context_map, storage_ix, storage);
+      }
+      brotli::WriteBits(distnumextra, distextra, storage_ix, storage);
+    }
+    pos += cmd.copy_len_;
+  }
+  return true;
+}
+
+// This is for storing uncompressed blocks (simple raw storage of
+// bytes-as-bytes).
+bool StoreUncompressedMetaBlock(bool final_block,
+                                const uint8_t * __restrict input,
+                                size_t position, size_t mask,
+                                size_t len,
+                                int * __restrict storage_ix,
+                                uint8_t * __restrict storage) {
+  if (!brotli::StoreUncompressedMetaBlockHeader(len, storage_ix, storage)) {
+    return false;
+  }
+  *storage_ix = ((*storage_ix + 7) / 8) * 8;  // Go to next byte
+
+  size_t masked_pos = position & mask;
+  if (masked_pos + len > mask + 1) {
+    size_t len1 = mask + 1 - masked_pos;
+    memcpy(&storage[*storage_ix >> 3], &input[masked_pos], len1);
+    *storage_ix += len1 << 3;
+    len -= len1;
+    masked_pos = 0;
+  }
+  memcpy(&storage[*storage_ix >> 3], &input[masked_pos], len);
+  *storage_ix += len << 3;
+
+  // We need to clear the next 4 bytes to continue to be
+  // compatible with WriteBits.
+  brotli::WriteBitsPrepareStorage(*storage_ix, storage);
+
+  // Since the uncomressed block itself may not be the final block, add an empty
+  // one after this.
+  if (final_block) {
+    brotli::WriteBits(1, 1, storage_ix, storage);  // islast
+    brotli::WriteBits(1, 1, storage_ix, storage);  // isempty
+    *storage_ix = ((*storage_ix + 7) / 8) * 8;  // Go to next byte
+
+    // We need to clear the next 4 bytes to continue to be
+    // compatible with WriteBits.
+    brotli::WriteBitsPrepareStorage(*storage_ix, storage);
+  }
+  return true;
+}
+
 }  // namespace brotli

enc/brotli_bit_stream.h

@@ -28,6 +28,8 @@
 #include <stdint.h>
 #include <vector>
 
+#include "./metablock.h"
+
 namespace brotli {
 
 // All Store functions here will use a storage_ix, which is always the bit
@@ -104,6 +106,30 @@ void StoreBlockSwitch(const BlockSplitCode& code,
                       int* storage_ix,
                       uint8_t* storage);
 
+bool StoreMetaBlock(const uint8_t* input,
+                    size_t start_pos,
+                    size_t length,
+                    size_t mask,
+                    bool final_block,
+                    int quality,
+                    int num_direct_distance_codes,
+                    int distance_postfix_bits,
+                    int literal_context_mode,
+                    const brotli::Command *commands,
+                    size_t n_commands,
+                    const MetaBlockSplit& mb,
+                    int *storage_ix,
+                    uint8_t *storage);
+
+// This is for storing uncompressed blocks (simple raw storage of
+// bytes-as-bytes).
+bool StoreUncompressedMetaBlock(bool final_block,
+                                const uint8_t* input,
+                                size_t position, size_t mask,
+                                size_t len,
+                                int* storage_ix,
+                                uint8_t* storage);
+
 }  // namespace brotli
 
 #endif  // BROTLI_ENC_BROTLI_BIT_STREAM_H_

enc/encode.cc

@@ -25,6 +25,7 @@
 #include "./brotli_bit_stream.h"
 #include "./cluster.h"
 #include "./context.h"
+#include "./metablock.h"
 #include "./transform.h"
 #include "./entropy_encode.h"
 #include "./fast_log.h"
@@ -48,24 +49,6 @@ static const int kMetaBlockSizeBits = 21;
 static const int kRingBufferBits = 23;
 static const int kRingBufferMask = (1 << kRingBufferBits) - 1;
 
-template<int kSize>
-double Entropy(const std::vector<Histogram<kSize> >& histograms) {
-  double retval = 0;
-  for (int i = 0; i < histograms.size(); ++i) {
-    retval += histograms[i].EntropyBitCost();
-  }
-  return retval;
-}
-
-template<int kSize>
-double TotalBitCost(const std::vector<Histogram<kSize> >& histograms) {
-  double retval = 0;
-  for (int i = 0; i < histograms.size(); ++i) {
-    retval += PopulationCost(histograms[i]);
-  }
-  return retval;
-}
-
 int ParseAsUTF8(int* symbol, const uint8_t* input, int size) {
   // ASCII
   if ((input[0] & 0x80) == 0) {
@@ -128,55 +111,6 @@ bool IsMostlyUTF8(const uint8_t* data, size_t length, double min_fraction) {
   return size_utf8 > min_fraction * length;
 }
 
-template<int kSize>
-void BuildAndStoreEntropyCode(const Histogram<kSize>& histogram,
-                              const int tree_limit,
-                              const int alphabet_size,
-                              EntropyCode<kSize>* code,
-                              int* storage_ix, uint8_t* storage) {
-  memset(code->depth_, 0, sizeof(code->depth_));
-  memset(code->bits_, 0, sizeof(code->bits_));
-  BuildAndStoreHuffmanTree(histogram.data_, alphabet_size, 9,
-                           code->depth_, code->bits_, storage_ix, storage);
-}
-
-template<int kSize>
-void BuildAndStoreEntropyCodes(
-    const std::vector<Histogram<kSize> >& histograms,
-    int alphabet_size,
-    std::vector<EntropyCode<kSize> >* entropy_codes,
-    int* storage_ix, uint8_t* storage) {
-  entropy_codes->resize(histograms.size());
-  for (int i = 0; i < histograms.size(); ++i) {
-    BuildAndStoreEntropyCode(histograms[i], 15, alphabet_size,
-                             &(*entropy_codes)[i],
-                             storage_ix, storage);
-  }
-}
-
-void EncodeCommand(const Command& cmd,
-                   const EntropyCodeCommand& entropy,
-                   int* storage_ix, uint8_t* storage) {
-  int code = cmd.cmd_prefix_;
-  WriteBits(entropy.depth_[code], entropy.bits_[code], storage_ix, storage);
-  int nextra = cmd.cmd_extra_ >> 48;
-  uint64_t extra = cmd.cmd_extra_ & 0xffffffffffffULL;
-  if (nextra > 0) {
-    WriteBits(nextra, extra, storage_ix, storage);
-  }
-}
-
-void EncodeCopyDistance(const Command& cmd, const EntropyCodeDistance& entropy,
-                        int* storage_ix, uint8_t* storage) {
-  int code = cmd.dist_prefix_;
-  int extra_bits = cmd.dist_extra_ >> 24;
-  uint64_t extra_bits_val = cmd.dist_extra_ & 0xffffff;
-  WriteBits(entropy.depth_[code], entropy.bits_[code], storage_ix, storage);
-  if (extra_bits > 0) {
-    WriteBits(extra_bits, extra_bits_val, storage_ix, storage);
-  }
-}
-
 void RecomputeDistancePrefixes(std::vector<Command>* cmds,
                                int num_direct_distance_codes,
                                int distance_postfix_bits) {
@@ -196,101 +130,6 @@ void RecomputeDistancePrefixes(std::vector<Command>* cmds,
   }
 }
 
-void MoveAndEncode(const BlockSplitCode& code,
-                   BlockSplitIterator* it,
-                   int* storage_ix, uint8_t* storage) {
-  if (it->length_ == 0) {
-    ++it->idx_;
-    it->type_ = it->split_.types_[it->idx_];
-    it->length_ = it->split_.lengths_[it->idx_];
-    StoreBlockSwitch(code, it->idx_, storage_ix, storage);
-  }
-  --it->length_;
-}
-
-struct EncodingParams {
-  int num_direct_distance_codes;
-  int distance_postfix_bits;
-  int literal_context_mode;
-};
-
-struct MetaBlock {
-  std::vector<Command> cmds;
-  EncodingParams params;
-  BlockSplit literal_split;
-  BlockSplit command_split;
-  BlockSplit distance_split;
-  std::vector<int> literal_context_modes;
-  std::vector<int> literal_context_map;
-  std::vector<int> distance_context_map;
-  std::vector<HistogramLiteral> literal_histograms;
-  std::vector<HistogramCommand> command_histograms;
-  std::vector<HistogramDistance> distance_histograms;
-};
-
-void BuildMetaBlock(const EncodingParams& params,
-                    const std::vector<Command>& cmds,
-                    const uint8_t* ringbuffer,
-                    const size_t pos,
-                    const size_t mask,
-                    MetaBlock* mb) {
-  mb->cmds = cmds;
-  mb->params = params;
-  if (cmds.empty()) {
-    return;
-  }
-  RecomputeDistancePrefixes(&mb->cmds,
-                            mb->params.num_direct_distance_codes,
-                            mb->params.distance_postfix_bits);
-  SplitBlock(mb->cmds,
-             &ringbuffer[pos & mask],
-             &mb->literal_split,
-             &mb->command_split,
-             &mb->distance_split);
-
-  mb->literal_context_modes.resize(mb->literal_split.num_types_,
-                                   mb->params.literal_context_mode);
-
-
-  int num_literal_contexts =
-      mb->literal_split.num_types_ << kLiteralContextBits;
-  int 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(mb->cmds,
-                  mb->literal_split,
-                  mb->command_split,
-                  mb->distance_split,
-                  ringbuffer,
-                  pos,
-                  mask,
-                  mb->literal_context_modes,
-                  &literal_histograms,
-                  &mb->command_histograms,
-                  &distance_histograms);
-
-  // Histogram ids need to fit in one byte.
-  static const int kMaxNumberOfHistograms = 256;
-
-  mb->literal_histograms = literal_histograms;
-  ClusterHistograms(literal_histograms,
-                    1 << kLiteralContextBits,
-                    mb->literal_split.num_types_,
-                    kMaxNumberOfHistograms,
-                    &mb->literal_histograms,
-                    &mb->literal_context_map);
-
-  mb->distance_histograms = distance_histograms;
-  ClusterHistograms(distance_histograms,
-                    1 << kDistanceContextBits,
-                    mb->distance_split.num_types_,
-                    kMaxNumberOfHistograms,
-                    &mb->distance_histograms,
-                    &mb->distance_context_map);
-}
-
 size_t MetaBlockLength(const std::vector<Command>& cmds) {
   size_t length = 0;
   for (int i = 0; i < cmds.size(); ++i) {
@@ -300,100 +139,6 @@ size_t MetaBlockLength(const std::vector<Command>& cmds) {
   return length;
 }
 
-bool StoreMetaBlock(const MetaBlock& mb,
-                    const bool is_last,
-                    const uint8_t* ringbuffer,
-                    const size_t mask,
-                    size_t* pos,
-                    int* storage_ix, uint8_t* storage) {
-  size_t length = MetaBlockLength(mb.cmds);
-  const size_t end_pos = *pos + length;
-  if (!StoreCompressedMetaBlockHeader(is_last, length, storage_ix, storage)) {
-    return false;
-  }
-
-  if (length == 0) {
-    return true;
-  }
-  BlockSplitCode literal_split_code;
-  BlockSplitCode command_split_code;
-  BlockSplitCode distance_split_code;
-  BuildAndStoreBlockSplitCode(mb.literal_split.types_,
-                              mb.literal_split.lengths_,
-                              mb.literal_split.num_types_,
-                              9,  // quality
-                              &literal_split_code,
-                              storage_ix, storage);
-  BuildAndStoreBlockSplitCode(mb.command_split.types_,
-                              mb.command_split.lengths_,
-                              mb.command_split.num_types_,
-                              9,  // quality
-                              &command_split_code,
-                              storage_ix, storage);
-  BuildAndStoreBlockSplitCode(mb.distance_split.types_,
-                              mb.distance_split.lengths_,
-                              mb.distance_split.num_types_,
-                              9,  // quality
-                              &distance_split_code,
-                              storage_ix, storage);
-  WriteBits(2, mb.params.distance_postfix_bits, storage_ix, storage);
-  WriteBits(4,
-            mb.params.num_direct_distance_codes >>
-            mb.params.distance_postfix_bits,
-            storage_ix, storage);
-  int num_distance_codes =
-      kNumDistanceShortCodes + mb.params.num_direct_distance_codes +
-      (48 << mb.params.distance_postfix_bits);
-  for (int i = 0; i < mb.literal_split.num_types_; ++i) {
-    WriteBits(2, mb.literal_context_modes[i], storage_ix, storage);
-  }
-  EncodeContextMap(mb.literal_context_map, mb.literal_histograms.size(),
-                   storage_ix, storage);
-  EncodeContextMap(mb.distance_context_map, mb.distance_histograms.size(),
-                   storage_ix, storage);
-  std::vector<EntropyCodeLiteral> literal_codes;
-  std::vector<EntropyCodeCommand> command_codes;
-  std::vector<EntropyCodeDistance> distance_codes;
-  BuildAndStoreEntropyCodes(mb.literal_histograms, 256, &literal_codes,
-                            storage_ix, storage);
-  BuildAndStoreEntropyCodes(mb.command_histograms, kNumCommandPrefixes,
-                            &command_codes, storage_ix, storage);
-  BuildAndStoreEntropyCodes(mb.distance_histograms, num_distance_codes,
-                            &distance_codes, storage_ix, storage);
-  BlockSplitIterator literal_it(mb.literal_split);
-  BlockSplitIterator command_it(mb.command_split);
-  BlockSplitIterator distance_it(mb.distance_split);
-  for (int i = 0; i < mb.cmds.size(); ++i) {
-    const Command& cmd = mb.cmds[i];
-    MoveAndEncode(command_split_code, &command_it, storage_ix, storage);
-    EncodeCommand(cmd, command_codes[command_it.type_], storage_ix, storage);
-    for (int j = 0; j < cmd.insert_len_; ++j) {
-      MoveAndEncode(literal_split_code, &literal_it, storage_ix, storage);
-      int histogram_idx = literal_it.type_;
-      uint8_t prev_byte = *pos > 0 ? ringbuffer[(*pos - 1) & mask] : 0;
-      uint8_t prev_byte2 = *pos > 1 ? ringbuffer[(*pos - 2) & mask] : 0;
-      int context = ((literal_it.type_ << kLiteralContextBits) +
-                     Context(prev_byte, prev_byte2,
-                             mb.literal_context_modes[literal_it.type_]));
-      histogram_idx = mb.literal_context_map[context];
-      int literal = ringbuffer[*pos & mask];
-      WriteBits(literal_codes[histogram_idx].depth_[literal],
-                literal_codes[histogram_idx].bits_[literal],
-                storage_ix, storage);
-      ++(*pos);
-    }
-    if (*pos < end_pos && cmd.cmd_prefix_ >= 128) {
-      MoveAndEncode(distance_split_code, &distance_it, storage_ix, storage);
-      int context = (distance_it.type_ << 2) + cmd.DistanceContext();
-      int histogram_index = mb.distance_context_map[context];
-      EncodeCopyDistance(cmd, distance_codes[histogram_index],
-                         storage_ix, storage);
-    }
-    *pos += cmd.copy_len_;
-  }
-  return true;
-}
-
 BrotliCompressor::BrotliCompressor(BrotliParams params)
     : params_(params),
       window_bits_(kWindowBits),
@@ -499,20 +244,40 @@ bool BrotliCompressor::WriteMetaBlock(const size_t input_size,
       commands.push_back(Command(last_insert_len));
     }
   }
-  EncodingParams params;
-  params.num_direct_distance_codes =
+  int num_direct_distance_codes =
       params_.mode == BrotliParams::MODE_FONT ? 12 : 0;
-  params.distance_postfix_bits =
-      params_.mode == BrotliParams::MODE_FONT ? 1 : 0;
-  params.literal_context_mode = CONTEXT_SIGNED;
+  int distance_postfix_bits = params_.mode == BrotliParams::MODE_FONT ? 1 : 0;
+  int literal_context_mode = CONTEXT_SIGNED;
   const int storage_ix0 = storage_ix_;
-  MetaBlock mb;
-  BuildMetaBlock(params, commands, ringbuffer_.start(), input_pos_,
-                 kRingBufferMask, &mb);
-  if (!StoreMetaBlock(mb, is_last, ringbuffer_.start(), kRingBufferMask,
-                      &input_pos_, &storage_ix_, storage_)) {
+  MetaBlockSplit mb;
+  size_t len = MetaBlockLength(commands);
+  if (!commands.empty()) {
+    if (params_.greedy_block_split) {
+      BuildMetaBlockGreedy(ringbuffer_.start(), input_pos_, kRingBufferMask,
+                           commands.data(), commands.size(), 9, &mb);
+    } else {
+      RecomputeDistancePrefixes(&commands,
+                                num_direct_distance_codes,
+                                distance_postfix_bits);
+      BuildMetaBlock(ringbuffer_.start(), input_pos_, kRingBufferMask,
+                     commands,
+                     num_direct_distance_codes,
+                     distance_postfix_bits,
+                     literal_context_mode,
+                     &mb);
+    }
+  }
+  if (!StoreMetaBlock(ringbuffer_.start(), input_pos_, len, kRingBufferMask,
+                      is_last, 9,
+                      num_direct_distance_codes,
+                      distance_postfix_bits,
+                      literal_context_mode,
+                      commands.data(), commands.size(),
+                      mb,
+                      &storage_ix_, storage_)) {
     return false;
   }
+  input_pos_ += len;
   size_t output_size = is_last ? ((storage_ix_ + 7) >> 3) : (storage_ix_ >> 3);
   output_size -= (storage_ix0 >> 3);
   if (input_size + 4 < output_size) {

enc/encode.h

@@ -28,6 +28,11 @@
 namespace brotli {
 
 struct BrotliParams {
+  BrotliParams()
+      : mode(MODE_TEXT),
+        enable_transforms(false),
+        greedy_block_split(false) {}
+
   enum Mode {
     MODE_TEXT = 0,
     MODE_FONT = 1,
@@ -35,8 +40,7 @@ struct BrotliParams {
   Mode mode;
 
   bool enable_transforms;
-
-  BrotliParams() : mode(MODE_TEXT), enable_transforms(false) {}
+  bool greedy_block_split;
 };
 
 class BrotliCompressor {

enc/metablock.cc

@@ -0,0 +1,283 @@
+// Copyright 2015 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Algorithms for distributing the literals and commands of a metablock between
+// block types and contexts.
+
+#include "./metablock.h"
+
+#include "./block_splitter.h"
+#include "./cluster.h"
+#include "./histogram.h"
+
+namespace brotli {
+
+void BuildMetaBlock(const uint8_t* ringbuffer,
+                    const size_t pos,
+                    const size_t mask,
+                    const std::vector<Command>& cmds,
+                    int num_direct_distance_codes,
+                    int distance_postfix_bits,
+                    int literal_context_mode,
+                    MetaBlockSplit* mb) {
+  SplitBlock(cmds,
+             &ringbuffer[pos & mask],
+             &mb->literal_split,
+             &mb->command_split,
+             &mb->distance_split);
+
+  std::vector<int> literal_context_modes(mb->literal_split.num_types,
+                                         literal_context_mode);
+
+  int num_literal_contexts =
+      mb->literal_split.num_types << kLiteralContextBits;
+  int 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,
+                  mb->literal_split,
+                  mb->command_split,
+                  mb->distance_split,
+                  ringbuffer,
+                  pos,
+                  mask,
+                  literal_context_modes,
+                  &literal_histograms,
+                  &mb->command_histograms,
+                  &distance_histograms);
+
+  // Histogram ids need to fit in one byte.
+  static const int kMaxNumberOfHistograms = 256;
+
+  mb->literal_histograms = literal_histograms;
+  ClusterHistograms(literal_histograms,
+                    1 << kLiteralContextBits,
+                    mb->literal_split.num_types,
+                    kMaxNumberOfHistograms,
+                    &mb->literal_histograms,
+                    &mb->literal_context_map);
+
+  mb->distance_histograms = distance_histograms;
+  ClusterHistograms(distance_histograms,
+                    1 << 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(int alphabet_size,
+                int min_block_size,
+                double split_threshold,
+                int num_symbols,
+                int quality,
+                BlockSplit* split,
+                std::vector<HistogramType>* histograms)
+      : alphabet_size_(alphabet_size),
+        min_block_size_(min_block_size),
+        split_threshold_(split_threshold),
+        quality_(quality),
+        num_blocks_(0),
+        split_(split),
+        histograms_(histograms),
+        target_block_size_(min_block_size),
+        block_size_(0),
+        curr_histogram_ix_(0),
+        merge_last_count_(0) {
+    int 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.
+    int max_num_types = std::min(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(int 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] = 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 (int j = 0; j < 2; ++j) {
+        int 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_] = block_size_;
+        split_->types[num_blocks_] = split_->num_types;
+        last_histogram_ix_[1] = last_histogram_ix_[0];
+        last_histogram_ix_[0] = 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_] = 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] += 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 int kMaxBlockTypes = 256;
+
+  // Alphabet size of particular block category.
+  const int alphabet_size_;
+  // We collect at least this many symbols for each block.
+  const int 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_;
+  // Quality setting used for speed vs. compression ratio decisions.
+  const int quality_;
+
+  int 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.
+  int target_block_size_;
+  // The number of symbols in the current histogram.
+  int block_size_;
+  // Offset of the current histogram.
+  int curr_histogram_ix_;
+  // Offset of the histograms of the previous two block types.
+  int 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.
+  int merge_last_count_;
+};
+
+void BuildMetaBlockGreedy(const uint8_t* ringbuffer,
+                          size_t pos,
+                          size_t mask,
+                          const Command *commands,
+                          size_t n_commands,
+                          int quality,
+                          MetaBlockSplit* mb) {
+  int num_literals = 0;
+  for (int i = 0; i < n_commands; ++i) {
+    num_literals += commands[i].insert_len_;
+  }
+
+  BlockSplitter<HistogramLiteral> lit_blocks(
+      256, 512, 400.0, num_literals, quality,
+      &mb->literal_split, &mb->literal_histograms);
+  BlockSplitter<HistogramCommand> cmd_blocks(
+      kNumCommandPrefixes, 1024, 500.0, n_commands, quality,
+      &mb->command_split, &mb->command_histograms);
+  BlockSplitter<HistogramDistance> dist_blocks(
+      64, 512, 100.0, n_commands, quality,
+      &mb->distance_split, &mb->distance_histograms);
+
+  for (int i = 0; i < n_commands; ++i) {
+    const Command cmd = commands[i];
+    cmd_blocks.AddSymbol(cmd.cmd_prefix_);
+    for (int j = 0; j < cmd.insert_len_; ++j) {
+      lit_blocks.AddSymbol(ringbuffer[pos & mask]);
+      ++pos;
+    }
+    pos += cmd.copy_len_;
+    if (cmd.copy_len_ > 0 && 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);
+}
+
+}  // namespace brotli

enc/metablock.h

@@ -0,0 +1,66 @@
+// Copyright 2015 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Algorithms for distributing the literals and commands of a metablock between
+// block types and contexts.
+
+#ifndef BROTLI_ENC_METABLOCK_H_
+#define BROTLI_ENC_METABLOCK_H_
+
+#include <vector>
+
+#include "./command.h"
+#include "./histogram.h"
+
+namespace brotli {
+
+struct BlockSplit {
+  BlockSplit() : num_types(0) {}
+
+  int num_types;
+  std::vector<int> types;
+  std::vector<int> lengths;
+};
+
+struct MetaBlockSplit {
+  BlockSplit literal_split;
+  BlockSplit command_split;
+  BlockSplit distance_split;
+  std::vector<int> literal_context_map;
+  std::vector<int> distance_context_map;
+  std::vector<HistogramLiteral> literal_histograms;
+  std::vector<HistogramCommand> command_histograms;
+  std::vector<HistogramDistance> distance_histograms;
+};
+
+void BuildMetaBlock(const uint8_t* ringbuffer,
+                    const size_t pos,
+                    const size_t mask,
+                    const std::vector<Command>& cmds,
+                    int num_direct_distance_codes,
+                    int distance_postfix_bits,
+                    int literal_context_mode,
+                    MetaBlockSplit* mb);
+
+void BuildMetaBlockGreedy(const uint8_t* ringbuffer,
+                          size_t pos,
+                          size_t mask,
+                          const Command *commands,
+                          size_t n_commands,
+                          int quality,
+                          MetaBlockSplit* mb);
+
+}  // namespace brotli
+
+#endif  // BROTLI_ENC_METABLOCK_H_