src/leveldb/util/bloom.cc

   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
  10 namespace leveldb {
  11
  12 namespace {
  13 static uint32_t BloomHash(const Slice& key) {
  14   return Hash(key.data(), key.size(), 0xbc9f1d34);
  15 }
  16
  17 class BloomFilterPolicy : public FilterPolicy {
  18  private:
  19   size_t bits_per_key_;
  20   size_t k_;
  21
  22  public:
  23   explicit BloomFilterPolicy(int bits_per_key)
  24       : bits_per_key_(bits_per_key) {
  25     // We intentionally round down to reduce probing cost a little bit
  26     k_ = static_cast<size_t>(bits_per_key * 0.69);  // 0.69 =~ ln(2)
  27     if (k_ < 1) k_ = 1;
  28     if (k_ > 30) k_ = 30;
  29   }
  30
  31   virtual const char* Name() const {
  32     return "leveldb.BuiltinBloomFilter2";
  33   }
  34
  35   virtual void CreateFilter(const Slice* keys, int n, std::string* dst) const {
  36     // Compute bloom filter size (in both bits and bytes)
  37     size_t bits = n * bits_per_key_;
  38
  39     // For small n, we can see a very high false positive rate.  Fix it
  40     // by enforcing a minimum bloom filter length.
  41     if (bits < 64) bits = 64;
  42
  43     size_t bytes = (bits + 7) / 8;
  44     bits = bytes * 8;
  45
  46     const size_t init_size = dst->size();
  47     dst->resize(init_size + bytes, 0);
  48     dst->push_back(static_cast<char>(k_));  // Remember # of probes in filter
  49     char* array = &(*dst)[init_size];
  50     for (int i = 0; i < n; i++) {
  51       // Use double-hashing to generate a sequence of hash values.
  52       // See analysis in [Kirsch,Mitzenmacher 2006].
  53       uint32_t h = BloomHash(keys[i]);
  54       const uint32_t delta = (h >> 17) | (h << 15);  // Rotate right 17 bits
  55       for (size_t j = 0; j < k_; j++) {
  56         const uint32_t bitpos = h % bits;
  57         array[bitpos/8] |= (1 << (bitpos % 8));
  58         h += delta;
  59       }
  60     }
  61   }
  62
  63   virtual bool KeyMayMatch(const Slice& key, const Slice& bloom_filter) const {
  64     const size_t len = bloom_filter.size();
  65     if (len < 2) return false;
  66
  67     const char* array = bloom_filter.data();
  68     const size_t bits = (len - 1) * 8;
  69
  70     // Use the encoded k so that we can read filters generated by
  71     // bloom filters created using different parameters.
  72     const size_t k = array[len-1];
  73     if (k > 30) {
  74       // Reserved for potentially new encodings for short bloom filters.
  75       // Consider it a match.
  76       return true;
  77     }
  78
  79     uint32_t h = BloomHash(key);
  80     const uint32_t delta = (h >> 17) | (h << 15);  // Rotate right 17 bits
  81     for (size_t j = 0; j < k; j++) {
  82       const uint32_t bitpos = h % bits;
  83       if ((array[bitpos/8] & (1 << (bitpos % 8))) == 0) return false;
  84       h += delta;
  85     }
  86     return true;
  87   }
  88 };
  89 }
  90
  91 const FilterPolicy* NewBloomFilterPolicy(int bits_per_key) {
  92   return new BloomFilterPolicy(bits_per_key);
  93 }
  94
  95 }  // namespace leveldb