1 // Copyright (c) 2012 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 // 5 // A read-only set implementation for |SBPrefix| items. Prefixes are 6 // sorted and stored as 16-bit deltas from the previous prefix. An 7 // index structure provides quick random access, and also handles 8 // cases where 16 bits cannot encode a delta. 9 // 10 // For example, the sequence {20, 25, 41, 65432, 150000, 160000} would 11 // be stored as: 12 // A pair {20, 0} in |index_|. 13 // 5, 16, 65391 in |deltas_|. 14 // A pair {150000, 3} in |index_|. 15 // 10000 in |deltas_|. 16 // |index_.size()| will be 2, |deltas_.size()| will be 4. 17 // 18 // This structure is intended for storage of sparse uniform sets of 19 // prefixes of a certain size. As of this writing, my safe-browsing 20 // database contains: 21 // 653132 add prefixes 22 // 6446 are duplicates (from different chunks) 23 // 24301 w/in 2^8 of the prior prefix 24 // 622337 w/in 2^16 of the prior prefix 25 // 47 further than 2^16 from the prior prefix 26 // For this input, the memory usage is approximately 2 bytes per 27 // prefix, a bit over 1.2M. The bloom filter used 25 bits per prefix, 28 // a bit over 1.9M on this data. 29 // 30 // Experimenting with random selections of the above data, storage 31 // size drops almost linearly as prefix count drops, until the index 32 // overhead starts to become a problem a bit under 200k prefixes. The 33 // memory footprint gets worse than storing the raw prefix data around 34 // 75k prefixes. Fortunately, the actual memory footprint also falls. 35 // If the prefix count increases the memory footprint should increase 36 // approximately linearly. The worst-case would be 2^16 items all 37 // 2^16 apart, which would need 512k (versus 256k to store the raw 38 // data). 39 // 40 // The on-disk format looks like: 41 // 4 byte magic number 42 // 4 byte version number 43 // 4 byte |index_.size()| 44 // 4 byte |deltas_.size()| 45 // n * 8 byte |&index_[0]..&index_[n]| 46 // m * 2 byte |&deltas_[0]..&deltas_[m]| 47 // 16 byte digest 48 49 #ifndef CHROME_BROWSER_SAFE_BROWSING_PREFIX_SET_H_ 50 #define CHROME_BROWSER_SAFE_BROWSING_PREFIX_SET_H_ 51 52 #include <vector> 53 54 #include "chrome/browser/safe_browsing/safe_browsing_util.h" 55 56 namespace base { 57 class FilePath; 58 } 59 60 namespace safe_browsing { 61 62 class PrefixSet { 63 public: 64 explicit PrefixSet(const std::vector<SBPrefix>& sorted_prefixes); 65 ~PrefixSet(); 66 67 // |true| if |prefix| was in |prefixes| passed to the constructor. 68 bool Exists(SBPrefix prefix) const; 69 70 // Persist the set on disk. 71 static PrefixSet* LoadFile(const base::FilePath& filter_name); 72 bool WriteFile(const base::FilePath& filter_name) const; 73 74 // Regenerate the vector of prefixes passed to the constructor into 75 // |prefixes|. Prefixes will be added in sorted order. 76 void GetPrefixes(std::vector<SBPrefix>* prefixes) const; 77 78 private: 79 // Maximum number of consecutive deltas to encode before generating 80 // a new index entry. This helps keep the worst-case performance 81 // for |Exists()| under control. 82 static const size_t kMaxRun = 100; 83 84 // Helper for |LoadFile()|. Steals the contents of |index| and 85 // |deltas| using |swap()|. 86 PrefixSet(std::vector<std::pair<SBPrefix,size_t> > *index, 87 std::vector<uint16> *deltas); 88 89 // Top-level index of prefix to offset in |deltas_|. Each pair 90 // indicates a base prefix and where the deltas from that prefix 91 // begin in |deltas_|. The deltas for a pair end at the next pair's 92 // index into |deltas_|. 93 std::vector<std::pair<SBPrefix,size_t> > index_; 94 95 // Deltas which are added to the prefix in |index_| to generate 96 // prefixes. Deltas are only valid between consecutive items from 97 // |index_|, or the end of |deltas_| for the last |index_| pair. 98 std::vector<uint16> deltas_; 99 100 DISALLOW_COPY_AND_ASSIGN(PrefixSet); 101 }; 102 103 } // namespace safe_browsing 104 105 #endif // CHROME_BROWSER_SAFE_BROWSING_PREFIX_SET_H_ 106
