| /* |
| * Copyright (C) 2012 The Android Open Source Project |
| * |
| * 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. |
| */ |
| |
| #ifndef LATINIME_BLOOM_FILTER_H |
| #define LATINIME_BLOOM_FILTER_H |
| |
| #include <bitset> |
| |
| #include "defines.h" |
| |
| namespace latinime { |
| |
| // This bloom filter is used for optimizing bigram retrieval. |
| // Execution times with previous word "this" are as follows: |
| // without bloom filter (use only hash_map): |
| // Total 147792.34 (sum of others 147771.57) |
| // with bloom filter: |
| // Total 145900.64 (sum of others 145874.30) |
| // always read binary dictionary: |
| // Total 148603.14 (sum of others 148579.90) |
| class BloomFilter { |
| public: |
| BloomFilter() : mFilter() {} |
| |
| AK_FORCE_INLINE void setInFilter(const int position) { |
| mFilter.set(getIndex(position)); |
| } |
| |
| AK_FORCE_INLINE bool isInFilter(const int position) const { |
| return mFilter.test(getIndex(position)); |
| } |
| |
| private: |
| DISALLOW_ASSIGNMENT_OPERATOR(BloomFilter); |
| |
| AK_FORCE_INLINE size_t getIndex(const int position) const { |
| return static_cast<size_t>(position) % BIGRAM_FILTER_MODULO; |
| } |
| |
| // Size, in bits, of the bloom filter index for bigrams |
| // The probability of false positive is (1 - e ** (-kn/m))**k, |
| // where k is the number of hash functions, n the number of bigrams, and m the number of |
| // bits we can test. |
| // At the moment 100 is the maximum number of bigrams for a word with the current main |
| // dictionaries, so n = 100. 1024 buckets give us m = 1024. |
| // With 1 hash function, our false positive rate is about 9.3%, which should be enough for |
| // our uses since we are only using this to increase average performance. For the record, |
| // k = 2 gives 3.1% and k = 3 gives 1.6%. With k = 1, making m = 2048 gives 4.8%, |
| // and m = 4096 gives 2.4%. |
| // This is assigned here because it is used for bitset size. |
| // 1021 is the largest prime under 1024. |
| static const size_t BIGRAM_FILTER_MODULO = 1021; |
| std::bitset<BIGRAM_FILTER_MODULO> mFilter; |
| }; |
| } // namespace latinime |
| #endif // LATINIME_BLOOM_FILTER_H |