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61 lines
1.7 KiB
C++
61 lines
1.7 KiB
C++
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// Copyright 2013 Google Inc. All Rights Reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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// Literal cost model to allow backward reference replacement to be efficient.
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#include "./literal_cost.h"
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#include <math.h>
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#include <stdint.h>
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#include <algorithm>
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namespace brotli {
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void EstimateBitCostsForLiterals(size_t len, const uint8_t *data, float *cost) {
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int histogram[256] = { 0 };
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int window_half = 2000;
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int in_window = std::min(static_cast<size_t>(window_half), len);
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// Bootstrap histogram.
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for (int i = 0; i < in_window; ++i) {
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++histogram[data[i]];
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}
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// Compute bit costs with sliding window.
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for (int i = 0; i < len; ++i) {
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if (i - window_half >= 0) {
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// Remove a byte in the past.
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--histogram[data[i - window_half]];
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--in_window;
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}
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if (i + window_half < len) {
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// Add a byte in the future.
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++histogram[data[i + window_half]];
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++in_window;
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}
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int histo = histogram[data[i]];
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if (histo == 0) {
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histo = 1;
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}
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cost[i] = log2(static_cast<double>(in_window) / histo);
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cost[i] += 0.03;
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if (cost[i] < 1.0) {
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cost[i] *= 0.5;
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cost[i] += 0.5;
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}
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}
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}
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} // namespace brotli
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