1 /* 2 * Copyright (C) 2012 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 #ifndef LATINIME_BLOOM_FILTER_H 18 #define LATINIME_BLOOM_FILTER_H 19 20 #include <bitset> 21 22 #include "defines.h" 23 24 namespace latinime { 25 26 // This bloom filter is used for optimizing bigram retrieval. 27 // Execution times with previous word "this" are as follows: 28 // without bloom filter (use only hash_map): 29 // Total 147792.34 (sum of others 147771.57) 30 // with bloom filter: 31 // Total 145900.64 (sum of others 145874.30) 32 // always read binary dictionary: 33 // Total 148603.14 (sum of others 148579.90) 34 class BloomFilter { 35 public: 36 BloomFilter() : mFilter() {} 37 38 AK_FORCE_INLINE void setInFilter(const int position) { 39 mFilter.set(getIndex(position)); 40 } 41 42 AK_FORCE_INLINE bool isInFilter(const int position) const { 43 return mFilter.test(getIndex(position)); 44 } 45 46 private: 47 DISALLOW_ASSIGNMENT_OPERATOR(BloomFilter); 48 49 AK_FORCE_INLINE size_t getIndex(const int position) const { 50 return static_cast<size_t>(position) % BIGRAM_FILTER_MODULO; 51 } 52 53 // Size, in bits, of the bloom filter index for bigrams 54 // The probability of false positive is (1 - e ** (-kn/m))**k, 55 // where k is the number of hash functions, n the number of bigrams, and m the number of 56 // bits we can test. 57 // At the moment 100 is the maximum number of bigrams for a word with the current main 58 // dictionaries, so n = 100. 1024 buckets give us m = 1024. 59 // With 1 hash function, our false positive rate is about 9.3%, which should be enough for 60 // our uses since we are only using this to increase average performance. For the record, 61 // k = 2 gives 3.1% and k = 3 gives 1.6%. With k = 1, making m = 2048 gives 4.8%, 62 // and m = 4096 gives 2.4%. 63 // This is assigned here because it is used for bitset size. 64 // 1021 is the largest prime under 1024. 65 static const size_t BIGRAM_FILTER_MODULO = 1021; 66 std::bitset<BIGRAM_FILTER_MODULO> mFilter; 67 }; 68 } // namespace latinime 69 #endif // LATINIME_BLOOM_FILTER_H 70
