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// Copyright (c) 2012, Suryandaru Triandana <syndtr@gmail.com>
// All rights reserved.
//
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
package filter
import (
"github.com/syndtr/goleveldb/leveldb/util"
)
func bloomHash(key []byte) uint32 {
return util.Hash(key, 0xbc9f1d34)
}
type bloomFilter int
// The bloom filter serializes its parameters and is backward compatible
// with respect to them. Therefor, its parameters are not added to its
// name.
func (bloomFilter) Name() string {
return "leveldb.BuiltinBloomFilter"
}
func (f bloomFilter) Contains(filter, key []byte) bool {
nBytes := len(filter) - 1
if nBytes < 1 {
return false
}
nBits := uint32(nBytes * 8)
// Use the encoded k so that we can read filters generated by
// bloom filters created using different parameters.
k := filter[nBytes]
if k > 30 {
// Reserved for potentially new encodings for short bloom filters.
// Consider it a match.
return true
}
kh := bloomHash(key)
delta := (kh >> 17) | (kh << 15) // Rotate right 17 bits
for j := uint8(0); j < k; j++ {
bitpos := kh % nBits
if (uint32(filter[bitpos/8]) & (1 << (bitpos % 8))) == 0 {
return false
}
kh += delta
}
return true
}
func (f bloomFilter) NewGenerator() FilterGenerator {
// Round down to reduce probing cost a little bit.
k := uint8(f * 69 / 100) // 0.69 =~ ln(2)
if k < 1 {
k = 1
} else if k > 30 {
k = 30
}
return &bloomFilterGenerator{
n: int(f),
k: k,
}
}
type bloomFilterGenerator struct {
n int
k uint8
keyHashes []uint32
}
func (g *bloomFilterGenerator) Add(key []byte) {
// Use double-hashing to generate a sequence of hash values.
// See analysis in [Kirsch,Mitzenmacher 2006].
g.keyHashes = append(g.keyHashes, bloomHash(key))
}
func (g *bloomFilterGenerator) Generate(b Buffer) {
// Compute bloom filter size (in both bits and bytes)
nBits := uint32(len(g.keyHashes) * g.n)
// For small n, we can see a very high false positive rate. Fix it
// by enforcing a minimum bloom filter length.
if nBits < 64 {
nBits = 64
}
nBytes := (nBits + 7) / 8
nBits = nBytes * 8
dest := b.Alloc(int(nBytes) + 1)
dest[nBytes] = g.k
for _, kh := range g.keyHashes {
delta := (kh >> 17) | (kh << 15) // Rotate right 17 bits
for j := uint8(0); j < g.k; j++ {
bitpos := kh % nBits
dest[bitpos/8] |= (1 << (bitpos % 8))
kh += delta
}
}
g.keyHashes = g.keyHashes[:0]
}
// NewBloomFilter creates a new initialized bloom filter for given
// bitsPerKey.
//
// Since bitsPerKey is persisted individually for each bloom filter
// serialization, bloom filters are backwards compatible with respect to
// changing bitsPerKey. This means that no big performance penalty will
// be experienced when changing the parameter. See documentation for
// opt.Options.Filter for more information.
func NewBloomFilter(bitsPerKey int) Filter {
return bloomFilter(bitsPerKey)
}
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