kc3-lang/brotli/enc/literal_cost.cc

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• Hash : c6b9c7c5
  Author :
  Date : 2013-11-15T19:02:17
  

  Updates to Brotli compression format, decoder and encoder
  
  This commit contains a batch of changes that were made to the Brotli
  compression algorithm in the last three weeks. Most important changes:
  
    * Added UTF8 context model for good text compression.
    * Simplified context modeling by having only 4 context modes.
    * Per-block context mode selection.
    * Faster backward copying and bit reading functions.
    * More efficient histogram coding.
    * Streaming support for the decoder and encoder.
  

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• enc/literal_cost.cc

• // Copyright 2013 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.
  //
  // Literal cost model to allow backward reference replacement to be efficient.
  
  #include "./literal_cost.h"
  
  #include <math.h>
  #include <stdint.h>
  #include <algorithm>
  
  namespace brotli {
  
  void EstimateBitCostsForLiterals(size_t pos, size_t len, size_t mask,
                                   const uint8_t *data, float *cost) {
    int histogram[256] = { 0 };
    int window_half = 2000;
    int in_window = std::min(static_cast<size_t>(window_half), len);
  
    // Bootstrap histogram.
    for (int i = 0; i < in_window; ++i) {
      ++histogram[data[(pos + i) & mask]];
    }
  
    // Compute bit costs with sliding window.
    for (int i = 0; i < len; ++i) {
      if (i - window_half >= 0) {
        // Remove a byte in the past.
        --histogram[data[(pos + i - window_half) & mask]];
        --in_window;
      }
      if (i + window_half < len) {
        // Add a byte in the future.
        ++histogram[data[(pos + i + window_half) & mask]];
        ++in_window;
      }
      int masked_pos = (pos + i) & mask;
      int histo = histogram[data[masked_pos]];
      if (histo == 0) {
        histo = 1;
      }
      cost[masked_pos] = log2(static_cast<double>(in_window) / histo);
      cost[masked_pos] += 0.03;
      if (cost[masked_pos] < 1.0) {
        cost[masked_pos] *= 0.5;
        cost[masked_pos] += 0.5;
      }
    }
  }
  
  }  // namespace brotli
  

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