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;

}

private:

size_t bits_per_key_;

size_t k_;