Inside Bitcoin

1// Copyright (c) 2012 The LevelDB 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. See the AUTHORS file for names of contributors.
4
5#include "leveldb/filter_policy.h"
6
7#include "leveldb/slice.h"
8#include "util/hash.h"
9
10namespace leveldb {
11
12namespace {
13static uint32_t BloomHash(const Slice& key) {
14  return Hash(key.data(), key.size(), 0xbc9f1d34);
15}
16
17class BloomFilterPolicy : public FilterPolicy {
18 public:
19  explicit BloomFilterPolicy(int bits_per_key) : bits_per_key_(bits_per_key) {
20    // We intentionally round down to reduce probing cost a little bit
21    k_ = static_cast<size_t>(bits_per_key * 0.69);  // 0.69 =~ ln(2)
22    if (k_ < 1) k_ = 1;
23    if (k_ > 30) k_ = 30;
24  }
25
26  const char* Name() const override { return "leveldb.BuiltinBloomFilter2"; }
27
28  void CreateFilter(const Slice* keys, int n, std::string* dst) const override {
29    // Compute bloom filter size (in both bits and bytes)
30    size_t bits = n * bits_per_key_;
31
32    // For small n, we can see a very high false positive rate.  Fix it
33    // by enforcing a minimum bloom filter length.
34    if (bits < 64) bits = 64;
35
36    size_t bytes = (bits + 7) / 8;
37    bits = bytes * 8;
38
39    const size_t init_size = dst->size();
40    dst->resize(init_size + bytes, 0);
41    dst->push_back(static_cast<char>(k_));  // Remember # of probes in filter
42    char* array = &(*dst)[init_size];
43    for (int i = 0; i < n; i++) {
44      // Use double-hashing to generate a sequence of hash values.
45      // See analysis in [Kirsch,Mitzenmacher 2006].
46      uint32_t h = BloomHash(keys[i]);
47      const uint32_t delta = (h >> 17) | (h << 15);  // Rotate right 17 bits
48      for (size_t j = 0; j < k_; j++) {
49        const uint32_t bitpos = h % bits;
50        array[bitpos / 8] |= (1 << (bitpos % 8));
51        h += delta;
52      }
53    }
54  }
55
56  bool KeyMayMatch(const Slice& key, const Slice& bloom_filter) const override {
57    const size_t len = bloom_filter.size();
58    if (len < 2) return false;
59
60    const char* array = bloom_filter.data();
61    const size_t bits = (len - 1) * 8;
62
63    // Use the encoded k so that we can read filters generated by
64    // bloom filters created using different parameters.
65    const size_t k = array[len - 1];
66    if (k > 30) {
67      // Reserved for potentially new encodings for short bloom filters.
68      // Consider it a match.
69      return true;
70    }
71
72    uint32_t h = BloomHash(key);
73    const uint32_t delta = (h >> 17) | (h << 15);  // Rotate right 17 bits
74    for (size_t j = 0; j < k; j++) {
75      const uint32_t bitpos = h % bits;
76      if ((array[bitpos / 8] & (1 << (bitpos % 8))) == 0) return false;
77      h += delta;
78    }
79    return true;
80  }
81
82 private:
83  size_t bits_per_key_;
84  size_t k_;
85};
86}  // namespace
87
88const FilterPolicy* NewBloomFilterPolicy(int bits_per_key) {
89  return new BloomFilterPolicy(bits_per_key);
90}
91
92}  // namespace leveldb
93