mirror of
https://github.com/google/brotli.git
synced 2024-11-23 12:10:07 +00:00
63 lines
1.9 KiB
C++
63 lines
1.9 KiB
C++
// 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] += 0.029;
|
|
if (cost[masked_pos] < 1.0) {
|
|
cost[masked_pos] *= 0.5;
|
|
cost[masked_pos] += 0.5;
|
|
}
|
|
}
|
|
}
|
|
|
|
} // namespace brotli
|