Home | History | Annotate | Line # | Download | only in enc 
      1 /* Copyright 2013 Google Inc. All Rights Reserved.
      2 
      3    Distributed under MIT license.
      4    See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
      5 */
      6 
      7 /* Build per-context histograms of literals, commands and distance codes. */
      8 
      9 #include "./histogram.h"
     10 
     11 #include "../common/context.h"
     12 #include "./block_splitter.h"
     13 #include "./command.h"
     14 
     15 #if defined(__cplusplus) || defined(c_plusplus)
     16 extern "C" {
     17 #endif
     18 
     19 typedef struct BlockSplitIterator {
     20   const BlockSplit* split_;  /* Not owned. */
     21   size_t idx_;
     22   size_t type_;
     23   size_t length_;
     24 } BlockSplitIterator;
     25 
     26 static void InitBlockSplitIterator(BlockSplitIterator* self,
     27     const BlockSplit* split) {
     28   self->split_ = split;
     29   self->idx_ = 0;
     30   self->type_ = 0;
     31   self->length_ = split->lengths ? split->lengths[0] : 0;
     32 }
     33 
     34 static void BlockSplitIteratorNext(BlockSplitIterator* self) {
     35   if (self->length_ == 0) {
     36     ++self->idx_;
     37     self->type_ = self->split_->types[self->idx_];
     38     self->length_ = self->split_->lengths[self->idx_];
     39   }
     40   --self->length_;
     41 }
     42 
     43 void BrotliBuildHistogramsWithContext(
     44     const Command* cmds, const size_t num_commands,
     45     const BlockSplit* literal_split, const BlockSplit* insert_and_copy_split,
     46     const BlockSplit* dist_split, const uint8_t* ringbuffer, size_t start_pos,
     47     size_t mask, uint8_t prev_byte, uint8_t prev_byte2,
     48     const ContextType* context_modes, HistogramLiteral* literal_histograms,
     49     HistogramCommand* insert_and_copy_histograms,
     50     HistogramDistance* copy_dist_histograms) {
     51   size_t pos = start_pos;
     52   BlockSplitIterator literal_it;
     53   BlockSplitIterator insert_and_copy_it;
     54   BlockSplitIterator dist_it;
     55   size_t i;
     56 
     57   InitBlockSplitIterator(&literal_it, literal_split);
     58   InitBlockSplitIterator(&insert_and_copy_it, insert_and_copy_split);
     59   InitBlockSplitIterator(&dist_it, dist_split);
     60   for (i = 0; i < num_commands; ++i) {
     61     const Command* cmd = &cmds[i];
     62     size_t j;
     63     BlockSplitIteratorNext(&insert_and_copy_it);
     64     HistogramAddCommand(&insert_and_copy_histograms[insert_and_copy_it.type_],
     65         cmd->cmd_prefix_);
     66     /* TODO: unwrap iterator blocks. */
     67     for (j = cmd->insert_len_; j != 0; --j) {
     68       size_t context;
     69       BlockSplitIteratorNext(&literal_it);
     70       context = literal_it.type_;
     71       if (context_modes) {
     72         ContextLut lut = BROTLI_CONTEXT_LUT(context_modes[context]);
     73         context = (context << BROTLI_LITERAL_CONTEXT_BITS) +
     74             BROTLI_CONTEXT(prev_byte, prev_byte2, lut);
     75       }
     76       HistogramAddLiteral(&literal_histograms[context],
     77           ringbuffer[pos & mask]);
     78       prev_byte2 = prev_byte;
     79       prev_byte = ringbuffer[pos & mask];
     80       ++pos;
     81     }
     82     pos += CommandCopyLen(cmd);
     83     if (CommandCopyLen(cmd)) {
     84       prev_byte2 = ringbuffer[(pos - 2) & mask];
     85       prev_byte = ringbuffer[(pos - 1) & mask];
     86       if (cmd->cmd_prefix_ >= 128) {
     87         size_t context;
     88         BlockSplitIteratorNext(&dist_it);
     89         context = (dist_it.type_ << BROTLI_DISTANCE_CONTEXT_BITS) +
     90             CommandDistanceContext(cmd);
     91         HistogramAddDistance(&copy_dist_histograms[context],
     92             cmd->dist_prefix_ & 0x3FF);
     93       }
     94     }
     95   }
     96 }
     97 
     98 #if defined(__cplusplus) || defined(c_plusplus)
     99 }  /* extern "C" */
    100 #endif
    101