text: // Copyright (c) 2012 The LevelDB Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file. See the AUTHORS file for names of contributors.
  
  #include "leveldb/filter_policy.h"
  
  #include "leveldb/slice.h"
  #include "util/hash.h"
  
  namespace leveldb {
  
  namespace {
  static uint32_t BloomHash(const Slice& key) {
    return Hash(key.data(), key.size(), 0xbc9f1d34);
  }
  
  class BloomFilterPolicy : public FilterPolicy {
   private:
    size_t bits_per_key_;
    size_t k_;
  
   public:
    explicit BloomFilterPolicy(int bits_per_key)
        : bits_per_key_(bits_per_key) {
      // We intentionally round down to reduce probing cost a little bit
      k_ = static_cast<size_t>(bits_per_key * 0.69);  // 0.69 =~ ln(2)
      if (k_ < 1) k_ = 1;
      if (k_ > 30) k_ = 30;
    }
  
    virtual const char* Name() const {
      return "leveldb.BuiltinBloomFilter2";
    }
  
    virtual void CreateFilter(const Slice* keys, int n, std::string* dst) const {
      // Compute bloom filter size (in both bits and bytes)
      size_t bits = n * bits_per_key_;
  
      // For small n, we can see a very high false positive rate.  Fix it
      // by enforcing a minimum bloom filter length.
      if (bits < 64) bits = 64;
  
      size_t bytes = (bits + 7) / 8;
      bits = bytes * 8;
  
      const size_t init_size = dst->size();
      dst->resize(init_size + bytes, 0);
      dst->push_back(static_cast<char>(k_));  // Remember # of probes in filter
      char* array = &(*dst)[init_size];
      for (int i = 0; i < n; i++) {
        // Use double-hashing to generate a sequence of hash values.
        // See analysis in [Kirsch,Mitzenmacher 2006].
        uint32_t h = BloomHash(keys[i]);
        const uint32_t delta = (h >> 17) | (h << 15);  // Rotate right 17 bits
        for (size_t j = 0; j < k_; j++) {
          const uint32_t bitpos = h % bits;
          array[bitpos/8] |= (1 << (bitpos % 8));
          h += delta;
        }
      }
    }
  
    virtual bool KeyMayMatch(const Slice& key, const Slice& bloom_filter) const {
      const size_t len = bloom_filter.size();
      if (len < 2) return false;
  
      const char* array = bloom_filter.data();
      const size_t bits = (len - 1) * 8;
  
      // Use the encoded k so that we can read filters generated by
      // bloom filters created using different parameters.
      const size_t k = array[len-1];
      if (k > 30) {
        // Reserved for potentially new encodings for short bloom filters.
        // Consider it a match.
        return true;
      }
  
      uint32_t h = BloomHash(key);
      const uint32_t delta = (h >> 17) | (h << 15);  // Rotate right 17 bits
      for (size_t j = 0; j < k; j++) {
        const uint32_t bitpos = h % bits;
        if ((array[bitpos/8] & (1 << (bitpos % 8))) == 0) return false;
        h += delta;
      }
      return true;
    }
  };
  }
  
  const FilterPolicy* NewBloomFilterPolicy(int bits_per_key) {
    return new BloomFilterPolicy(bits_per_key);
  }
  
  }  // namespace leveldb
  