| // 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. |
| // |
| // Function to find backward reference copies. |
| |
| #include "./backward_references.h" |
| |
| #include <algorithm> |
| #include <vector> |
| |
| #include "./command.h" |
| |
| namespace brotli { |
| |
| template<typename Hasher> |
| void CreateBackwardReferences(size_t num_bytes, |
| size_t position, |
| const uint8_t* ringbuffer, |
| const float* literal_cost, |
| size_t ringbuffer_mask, |
| const size_t max_backward_limit, |
| Hasher* hasher, |
| std::vector<Command>* commands) { |
| // Length heuristic that seems to help probably by better selection |
| // of lazy matches of similar lengths. |
| int insert_length = 0; |
| size_t i = position & ringbuffer_mask; |
| const int i_diff = position - i; |
| const size_t i_end = i + num_bytes; |
| |
| const int random_heuristics_window_size = 512; |
| int apply_random_heuristics = i + random_heuristics_window_size; |
| |
| double average_cost = 0.0; |
| for (int k = position; k < position + num_bytes; ++k) { |
| average_cost += literal_cost[k & ringbuffer_mask]; |
| } |
| average_cost /= num_bytes; |
| hasher->set_average_cost(average_cost); |
| |
| // M1 match is for considering for two repeated copies, if moving |
| // one literal form the previous copy to the current one allows the |
| // current copy to be more efficient (because the way static dictionary |
| // codes words). M1 matching improves text compression density by ~0.15 %. |
| bool match_found_M1 = false; |
| size_t best_len_M1 = 0; |
| size_t best_len_code_M1 = 0; |
| size_t best_dist_M1 = 0; |
| double best_score_M1 = 0; |
| while (i + 2 < i_end) { |
| size_t best_len = 0; |
| size_t best_len_code = 0; |
| size_t best_dist = 0; |
| double best_score = 0; |
| size_t max_distance = std::min(i + i_diff, max_backward_limit); |
| bool in_dictionary; |
| hasher->set_insert_length(insert_length); |
| bool match_found = hasher->FindLongestMatch( |
| ringbuffer, literal_cost, ringbuffer_mask, |
| i + i_diff, i_end - i, max_distance, |
| &best_len, &best_len_code, &best_dist, &best_score, |
| &in_dictionary); |
| bool best_in_dictionary = in_dictionary; |
| if (match_found) { |
| if (match_found_M1 && best_score_M1 > best_score) { |
| // Two copies after each other. Take the last literal from the |
| // last copy, and use it as the first of this one. |
| (commands->rbegin())->copy_length_ -= 1; |
| (commands->rbegin())->copy_length_code_ -= 1; |
| hasher->Store(ringbuffer + i, i + i_diff); |
| --i; |
| best_len = best_len_M1; |
| best_len_code = best_len_code_M1; |
| best_dist = best_dist_M1; |
| best_score = best_score_M1; |
| // in_dictionary doesn't need to be correct, but it is the only |
| // reason why M1 matching should be beneficial here. Setting it here |
| // will only disable further M1 matching against this copy. |
| best_in_dictionary = true; |
| in_dictionary = true; |
| } else { |
| // Found a match. Let's look for something even better ahead. |
| int delayed_backward_references_in_row = 0; |
| while (i + 4 < i_end && |
| delayed_backward_references_in_row < 4) { |
| size_t best_len_2 = 0; |
| size_t best_len_code_2 = 0; |
| size_t best_dist_2 = 0; |
| double best_score_2 = 0; |
| max_distance = std::min(i + i_diff + 1, max_backward_limit); |
| hasher->Store(ringbuffer + i, i + i_diff); |
| match_found = hasher->FindLongestMatch( |
| ringbuffer, literal_cost, ringbuffer_mask, |
| i + i_diff + 1, i_end - i - 1, max_distance, |
| &best_len_2, &best_len_code_2, &best_dist_2, &best_score_2, |
| &in_dictionary); |
| double cost_diff_lazy = 0; |
| if (best_len >= 4) { |
| cost_diff_lazy += |
| literal_cost[(i + 4) & ringbuffer_mask] - average_cost; |
| } |
| { |
| const int tail_length = best_len_2 - best_len + 1; |
| for (int k = 0; k < tail_length; ++k) { |
| cost_diff_lazy -= |
| literal_cost[(i + best_len + k) & ringbuffer_mask] - |
| average_cost; |
| } |
| } |
| // If we are not inserting any symbols, inserting one is more |
| // expensive than if we were inserting symbols anyways. |
| if (insert_length < 1) { |
| cost_diff_lazy += 0.97; |
| } |
| // Add bias to slightly avoid lazy matching. |
| cost_diff_lazy += 2.0 + delayed_backward_references_in_row * 0.2; |
| cost_diff_lazy += 0.04 * literal_cost[i & ringbuffer_mask]; |
| |
| if (match_found && best_score_2 >= best_score + cost_diff_lazy) { |
| // Ok, let's just write one byte for now and start a match from the |
| // next byte. |
| ++insert_length; |
| ++delayed_backward_references_in_row; |
| best_len = best_len_2; |
| best_len_code = best_len_code_2; |
| best_dist = best_dist_2; |
| best_score = best_score_2; |
| best_in_dictionary = in_dictionary; |
| i++; |
| } else { |
| break; |
| } |
| } |
| } |
| apply_random_heuristics = |
| i + 2 * best_len + random_heuristics_window_size; |
| Command cmd; |
| cmd.insert_length_ = insert_length; |
| cmd.copy_length_ = best_len; |
| cmd.copy_length_code_ = best_len_code; |
| cmd.copy_distance_ = best_dist; |
| commands->push_back(cmd); |
| insert_length = 0; |
| ++i; |
| if (best_dist <= std::min(i + i_diff, max_backward_limit)) { |
| hasher->set_last_distance(best_dist); |
| } |
| |
| // Copy all copied literals to the hasher, except the last one. |
| // We cannot store the last one yet, otherwise we couldn't find |
| // the possible M1 match. |
| for (int j = 1; j < best_len - 1; ++j) { |
| if (i + 2 < i_end) { |
| hasher->Store(ringbuffer + i, i + i_diff); |
| } |
| ++i; |
| } |
| // Prepare M1 match. |
| if (hasher->HasStaticDictionary() && |
| best_len >= 4 && i + 20 < i_end && !best_in_dictionary) { |
| max_distance = std::min(i + i_diff, max_backward_limit); |
| match_found_M1 = hasher->FindLongestMatch( |
| ringbuffer, literal_cost, ringbuffer_mask, |
| i + i_diff, i_end - i, max_distance, |
| &best_len_M1, &best_len_code_M1, &best_dist_M1, &best_score_M1, |
| &in_dictionary); |
| } else { |
| match_found_M1 = false; |
| in_dictionary = false; |
| } |
| // This byte is just moved from the previous copy to the current, |
| // that is no gain. |
| best_score_M1 -= literal_cost[i & ringbuffer_mask]; |
| // Adjust for losing the opportunity for lazy matching. |
| best_score_M1 -= 3.75; |
| |
| // Store the last one of the match. |
| if (i + 2 < i_end) { |
| hasher->Store(ringbuffer + i, i + i_diff); |
| } |
| ++i; |
| } else { |
| match_found_M1 = false; |
| ++insert_length; |
| hasher->Store(ringbuffer + i, i + i_diff); |
| ++i; |
| // If we have not seen matches for a long time, we can skip some |
| // match lookups. Unsuccessful match lookups are very very expensive |
| // and this kind of a heuristic speeds up compression quite |
| // a lot. |
| if (i > apply_random_heuristics) { |
| // Going through uncompressible data, jump. |
| if (i > apply_random_heuristics + 4 * random_heuristics_window_size) { |
| // It is quite a long time since we saw a copy, so we assume |
| // that this data is not compressible, and store hashes less |
| // often. Hashes of non compressible data are less likely to |
| // turn out to be useful in the future, too, so we store less of |
| // them to not to flood out the hash table of good compressible |
| // data. |
| int i_jump = std::min(i + 16, i_end - 4); |
| for (; i < i_jump; i += 4) { |
| hasher->Store(ringbuffer + i, i + i_diff); |
| insert_length += 4; |
| } |
| } else { |
| int i_jump = std::min(i + 8, i_end - 2); |
| for (; i < i_jump; i += 2) { |
| hasher->Store(ringbuffer + i, i + i_diff); |
| insert_length += 2; |
| } |
| } |
| } |
| } |
| } |
| insert_length += (i_end - i); |
| |
| if (insert_length > 0) { |
| Command cmd; |
| cmd.insert_length_ = insert_length; |
| cmd.copy_length_ = 0; |
| cmd.copy_distance_ = 0; |
| commands->push_back(cmd); |
| } |
| } |
| |
| void CreateBackwardReferences(size_t num_bytes, |
| size_t position, |
| const uint8_t* ringbuffer, |
| const float* literal_cost, |
| size_t ringbuffer_mask, |
| const size_t max_backward_limit, |
| Hashers* hashers, |
| Hashers::Type hash_type, |
| std::vector<Command>* commands) { |
| switch (hash_type) { |
| case Hashers::HASH_15_8_4: |
| CreateBackwardReferences( |
| num_bytes, position, ringbuffer, literal_cost, |
| ringbuffer_mask, max_backward_limit, |
| hashers->hash_15_8_4.get(), |
| commands); |
| break; |
| case Hashers::HASH_15_8_2: |
| CreateBackwardReferences( |
| num_bytes, position, ringbuffer, literal_cost, |
| ringbuffer_mask, max_backward_limit, |
| hashers->hash_15_8_2.get(), |
| commands); |
| break; |
| default: |
| break; |
| } |
| } |
| |
| |
| } // namespace brotli |