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374 lines
15 KiB
374 lines
15 KiB
// 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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// Function to find backward reference copies. |
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#include "./backward_references.h" |
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#include <algorithm> |
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#include <vector> |
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#include "./command.h" |
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namespace brotli { |
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template<typename Hasher, bool kUseCostModel, bool kUseDictionary> |
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void CreateBackwardReferences(size_t num_bytes, |
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size_t position, |
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const uint8_t* ringbuffer, |
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size_t ringbuffer_mask, |
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const float* literal_cost, |
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size_t literal_cost_mask, |
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const size_t max_backward_limit, |
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const double base_min_score, |
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const int quality, |
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Hasher* hasher, |
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int* dist_cache, |
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int* last_insert_len, |
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Command* commands, |
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int* num_commands) { |
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if (num_bytes >= 3 && position >= 3) { |
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// Prepare the hashes for three last bytes of the last write. |
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// These could not be calculated before, since they require knowledge |
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// of both the previous and the current block. |
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hasher->Store(&ringbuffer[(position - 3) & ringbuffer_mask], |
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position - 3); |
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hasher->Store(&ringbuffer[(position - 2) & ringbuffer_mask], |
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position - 2); |
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hasher->Store(&ringbuffer[(position - 1) & ringbuffer_mask], |
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position - 1); |
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} |
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const Command * const orig_commands = commands; |
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int insert_length = *last_insert_len; |
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size_t i = position & ringbuffer_mask; |
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const int i_diff = position - i; |
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const size_t i_end = i + num_bytes; |
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// For speed up heuristics for random data. |
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const int random_heuristics_window_size = quality < 9 ? 64 : 512; |
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int apply_random_heuristics = i + random_heuristics_window_size; |
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double average_cost = 5.4; |
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if (kUseCostModel) { |
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average_cost = 0.0; |
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for (int k = position; k < position + num_bytes; ++k) { |
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average_cost += literal_cost[k & literal_cost_mask]; |
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} |
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if (num_bytes > 0) { |
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average_cost /= num_bytes; |
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} |
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} |
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// M1 match is for considering for two repeated copies, if moving |
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// one literal form the previous copy to the current one allows the |
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// current copy to be more efficient (because the way static dictionary |
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// codes words). M1 matching improves text compression density by ~0.15 %. |
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bool match_found_M1 = false; |
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int best_len_M1 = 0; |
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int best_len_code_M1 = 0; |
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int best_dist_M1 = 0; |
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double best_score_M1 = 0; |
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while (i + 3 < i_end) { |
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int max_length = i_end - i; |
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size_t max_distance = std::min(i + i_diff, max_backward_limit); |
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double min_score = base_min_score; |
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if (kUseCostModel && insert_length < 8) { |
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double cost_diff[8] = |
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{ 0.1, 0.038, 0.019, 0.013, 0.001, 0.001, 0.001, 0.001 }; |
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min_score += cost_diff[insert_length]; |
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} |
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int best_len = 0; |
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int best_len_code = 0; |
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int best_dist = 0; |
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double best_score = min_score; |
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bool match_found = hasher->FindLongestMatch( |
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ringbuffer, ringbuffer_mask, |
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literal_cost, literal_cost_mask, average_cost, |
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dist_cache, i + i_diff, max_length, max_distance, |
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&best_len, &best_len_code, &best_dist, &best_score); |
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if (match_found) { |
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if (kUseDictionary && match_found_M1 && best_score_M1 > best_score) { |
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// Two copies after each other. Take the last literal from the |
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// last copy, and use it as the first of this one. |
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Command prev_cmd = commands[-1]; |
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commands[-1] = Command(prev_cmd.insert_len_, |
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prev_cmd.copy_len_ - 1, |
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prev_cmd.copy_len_ - 1, |
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prev_cmd.DistanceCode()); |
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hasher->Store(ringbuffer + i, i + i_diff); |
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--i; |
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best_len = best_len_M1; |
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best_len_code = best_len_code_M1; |
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best_dist = best_dist_M1; |
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best_score = best_score_M1; |
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} else { |
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// Found a match. Let's look for something even better ahead. |
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int delayed_backward_references_in_row = 0; |
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for (;;) { |
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--max_length; |
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int best_len_2 = quality < 4 ? std::min(best_len - 1, max_length) : 0; |
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int best_len_code_2 = 0; |
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int best_dist_2 = 0; |
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double best_score_2 = min_score; |
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max_distance = std::min(i + i_diff + 1, max_backward_limit); |
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hasher->Store(ringbuffer + i, i + i_diff); |
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match_found = hasher->FindLongestMatch( |
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ringbuffer, ringbuffer_mask, |
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literal_cost, literal_cost_mask, average_cost, |
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dist_cache, i + i_diff + 1, max_length, max_distance, |
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&best_len_2, &best_len_code_2, &best_dist_2, &best_score_2); |
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double cost_diff_lazy = 7.0; |
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if (kUseCostModel) { |
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cost_diff_lazy = 0.0; |
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if (best_len >= 4) { |
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cost_diff_lazy += |
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literal_cost[(i + 4) & literal_cost_mask] - average_cost; |
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} |
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{ |
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const int tail_length = best_len_2 - best_len + 1; |
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for (int k = 0; k < tail_length; ++k) { |
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cost_diff_lazy -= |
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literal_cost[(i + best_len + k) & literal_cost_mask] - |
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average_cost; |
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} |
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} |
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// If we are not inserting any symbols, inserting one is more |
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// expensive than if we were inserting symbols anyways. |
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if (insert_length < 1) { |
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cost_diff_lazy += 0.97; |
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} |
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// Add bias to slightly avoid lazy matching. |
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cost_diff_lazy += 2.0 + delayed_backward_references_in_row * 0.2; |
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cost_diff_lazy += 0.04 * literal_cost[i & literal_cost_mask]; |
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} |
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if (match_found && best_score_2 >= best_score + cost_diff_lazy) { |
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// Ok, let's just write one byte for now and start a match from the |
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// next byte. |
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++i; |
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++insert_length; |
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best_len = best_len_2; |
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best_len_code = best_len_code_2; |
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best_dist = best_dist_2; |
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best_score = best_score_2; |
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if (++delayed_backward_references_in_row < 4) { |
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continue; |
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} |
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} |
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break; |
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} |
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} |
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apply_random_heuristics = |
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i + 2 * best_len + random_heuristics_window_size; |
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max_distance = std::min(i + i_diff, max_backward_limit); |
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int distance_code = best_dist + 16; |
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if (best_dist <= max_distance) { |
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if (best_dist == dist_cache[0]) { |
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distance_code = 1; |
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} else if (best_dist == dist_cache[1]) { |
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distance_code = 2; |
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} else if (best_dist == dist_cache[2]) { |
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distance_code = 3; |
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} else if (best_dist == dist_cache[3]) { |
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distance_code = 4; |
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} else if (quality > 1 && best_dist >= 6) { |
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for (int k = 4; k < kNumDistanceShortCodes; ++k) { |
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int idx = kDistanceCacheIndex[k]; |
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int candidate = dist_cache[idx] + kDistanceCacheOffset[k]; |
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static const int kLimits[16] = { 0, 0, 0, 0, |
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6, 6, 11, 11, |
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11, 11, 11, 11, |
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12, 12, 12, 12 }; |
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if (best_dist == candidate && best_dist >= kLimits[k]) { |
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distance_code = k + 1; |
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break; |
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} |
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} |
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} |
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if (distance_code > 1) { |
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dist_cache[3] = dist_cache[2]; |
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dist_cache[2] = dist_cache[1]; |
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dist_cache[1] = dist_cache[0]; |
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dist_cache[0] = best_dist; |
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} |
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} |
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Command cmd(insert_length, best_len, best_len_code, distance_code); |
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*commands++ = cmd; |
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insert_length = 0; |
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if (kUseDictionary) { |
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++i; |
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// Copy all copied literals to the hasher, except the last one. |
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// We cannot store the last one yet, otherwise we couldn't find |
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// the possible M1 match. |
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for (int j = 1; j < best_len - 1; ++j) { |
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if (i + 3 < i_end) { |
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hasher->Store(ringbuffer + i, i + i_diff); |
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} |
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++i; |
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} |
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// Prepare M1 match. |
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if (hasher->HasStaticDictionary() && |
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best_len >= 4 && i + 20 < i_end && best_dist <= max_distance) { |
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max_distance = std::min(i + i_diff, max_backward_limit); |
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best_score_M1 = min_score; |
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match_found_M1 = hasher->FindLongestMatch( |
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ringbuffer, ringbuffer_mask, |
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literal_cost, literal_cost_mask, average_cost, |
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dist_cache, i + i_diff, i_end - i, max_distance, |
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&best_len_M1, &best_len_code_M1, &best_dist_M1, &best_score_M1); |
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} else { |
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match_found_M1 = false; |
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} |
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if (kUseCostModel) { |
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// This byte is just moved from the previous copy to the current, |
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// that is no gain. |
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best_score_M1 -= literal_cost[i & literal_cost_mask]; |
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// Adjust for losing the opportunity for lazy matching. |
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best_score_M1 -= 3.75; |
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} |
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// Store the last one of the match. |
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if (i + 3 < i_end) { |
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hasher->Store(ringbuffer + i, i + i_diff); |
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} |
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++i; |
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} else { |
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// Put the hash keys into the table, if there are enough |
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// bytes left. |
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for (int j = 1; j < best_len; ++j) { |
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hasher->Store(&ringbuffer[i + j], i + i_diff + j); |
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} |
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i += best_len; |
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} |
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} else { |
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match_found_M1 = false; |
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++insert_length; |
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hasher->Store(ringbuffer + i, i + i_diff); |
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++i; |
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// If we have not seen matches for a long time, we can skip some |
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// match lookups. Unsuccessful match lookups are very very expensive |
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// and this kind of a heuristic speeds up compression quite |
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// a lot. |
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if (i > apply_random_heuristics) { |
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// Going through uncompressible data, jump. |
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if (i > apply_random_heuristics + 4 * random_heuristics_window_size) { |
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// It is quite a long time since we saw a copy, so we assume |
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// that this data is not compressible, and store hashes less |
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// often. Hashes of non compressible data are less likely to |
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// turn out to be useful in the future, too, so we store less of |
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// them to not to flood out the hash table of good compressible |
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// data. |
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int i_jump = std::min(i + 16, i_end - 4); |
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for (; i < i_jump; i += 4) { |
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hasher->Store(ringbuffer + i, i + i_diff); |
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insert_length += 4; |
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} |
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} else { |
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int i_jump = std::min(i + 8, i_end - 3); |
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for (; i < i_jump; i += 2) { |
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hasher->Store(ringbuffer + i, i + i_diff); |
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insert_length += 2; |
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} |
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} |
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} |
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} |
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} |
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insert_length += (i_end - i); |
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*last_insert_len = insert_length; |
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*num_commands += (commands - orig_commands); |
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} |
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void CreateBackwardReferences(size_t num_bytes, |
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size_t position, |
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const uint8_t* ringbuffer, |
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size_t ringbuffer_mask, |
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const float* literal_cost, |
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size_t literal_cost_mask, |
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const size_t max_backward_limit, |
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const double base_min_score, |
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const int quality, |
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Hashers* hashers, |
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int hash_type, |
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int* dist_cache, |
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int* last_insert_len, |
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Command* commands, |
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int* num_commands) { |
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switch (hash_type) { |
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case 1: |
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CreateBackwardReferences<Hashers::H1, false, false>( |
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num_bytes, position, ringbuffer, ringbuffer_mask, |
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literal_cost, literal_cost_mask, max_backward_limit, base_min_score, |
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quality, hashers->hash_h1.get(), dist_cache, last_insert_len, |
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commands, num_commands); |
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break; |
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case 2: |
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CreateBackwardReferences<Hashers::H2, false, false>( |
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num_bytes, position, ringbuffer, ringbuffer_mask, |
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literal_cost, literal_cost_mask, max_backward_limit, base_min_score, |
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quality, hashers->hash_h2.get(), dist_cache, last_insert_len, |
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commands, num_commands); |
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break; |
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case 3: |
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CreateBackwardReferences<Hashers::H3, false, false>( |
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num_bytes, position, ringbuffer, ringbuffer_mask, |
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literal_cost, literal_cost_mask, max_backward_limit, base_min_score, |
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quality, hashers->hash_h3.get(), dist_cache, last_insert_len, |
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commands, num_commands); |
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break; |
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case 4: |
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CreateBackwardReferences<Hashers::H4, false, false>( |
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num_bytes, position, ringbuffer, ringbuffer_mask, |
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literal_cost, literal_cost_mask, max_backward_limit, base_min_score, |
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quality, hashers->hash_h4.get(), dist_cache, last_insert_len, |
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commands, num_commands); |
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break; |
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case 5: |
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CreateBackwardReferences<Hashers::H5, false, false>( |
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num_bytes, position, ringbuffer, ringbuffer_mask, |
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literal_cost, literal_cost_mask, max_backward_limit, base_min_score, |
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quality, hashers->hash_h5.get(), dist_cache, last_insert_len, |
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commands, num_commands); |
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break; |
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case 6: |
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CreateBackwardReferences<Hashers::H6, false, false>( |
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num_bytes, position, ringbuffer, ringbuffer_mask, |
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literal_cost, literal_cost_mask, max_backward_limit, base_min_score, |
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quality, hashers->hash_h6.get(), dist_cache, last_insert_len, |
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commands, num_commands); |
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break; |
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case 7: |
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CreateBackwardReferences<Hashers::H7, false, false>( |
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num_bytes, position, ringbuffer, ringbuffer_mask, |
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literal_cost, literal_cost_mask, max_backward_limit, base_min_score, |
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quality, hashers->hash_h7.get(), dist_cache, last_insert_len, |
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commands, num_commands); |
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break; |
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case 8: |
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CreateBackwardReferences<Hashers::H8, true, true>( |
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num_bytes, position, ringbuffer, ringbuffer_mask, |
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literal_cost, literal_cost_mask, max_backward_limit, base_min_score, |
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quality, hashers->hash_h8.get(), dist_cache, last_insert_len, |
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commands, num_commands); |
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break; |
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case 9: |
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CreateBackwardReferences<Hashers::H9, true, false>( |
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num_bytes, position, ringbuffer, ringbuffer_mask, |
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literal_cost, literal_cost_mask, max_backward_limit, base_min_score, |
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quality, hashers->hash_h9.get(), dist_cache, last_insert_len, |
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commands, num_commands); |
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break; |
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default: |
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break; |
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} |
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} |
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} // namespace brotli
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