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// Copyright 2021 the Blobloom authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package blobloom
import "sync/atomic"
// A SyncFilter is a Bloom filter that can be accessed and updated
// by multiple goroutines concurrently.
//
// A SyncFilter mostly behaves as a regular filter protected by a lock,
//
// type SyncFilter struct {
// Filter
// lock sync.Mutex
// }
//
// with each method taking and releasing the lock,
// but is implemented much more efficiently.
// See the method descriptions for exceptions to the previous rule.
type SyncFilter struct {
b []block // Shards.
k int // Number of hash functions required.
}
// NewSync constructs a Bloom filter with given numbers of bits and hash functions.
//
// The number of bits should be at least BlockBits; smaller values are silently
// increased.
//
// The number of hashes reflects the number of hashes synthesized from the
// single hash passed in by the client. It is silently increased to two if
// a lower value is given.
func NewSync(nbits uint64, nhashes int) *SyncFilter {
return (*SyncFilter)(New(nbits, nhashes))
}
// Add insert a key with hash value h into f.
func (f *SyncFilter) Add(h uint64) {
h1, h2 := uint32(h>>32), uint32(h)
b := (*Filter)(f).getblock(h2)
for i := 1; i < f.k; i++ {
h1, h2 = doublehash(h1, h2, i)
b.setbitAtomic(h1)
}
}
// Cardinality estimates the number of distinct keys added to f.
//
// The estimate is most reliable when f is filled to roughly its capacity.
// It gets worse as f gets more densely filled. When one of the blocks is
// entirely filled, the estimate becomes +Inf.
//
// The return value is the maximum likelihood estimate of Papapetrou, Siberski
// and Nejdl, summed over the blocks
// (https://www.win.tue.nl/~opapapetrou/papers/Bloomfilters-DAPD.pdf).
//
// If other goroutines are concurrently adding keys,
// the estimate may lie in between what would have been returned
// before the concurrent updates started and what is returned
// after the updates complete.
func (f *SyncFilter) Cardinality() float64 {
return (*Filter)(f).cardinality(onescountAtomic)
}
// Empty reports whether f contains no keys.
//
// If other goroutines are concurrently adding keys,
// Empty may return a false positive.
func (f *SyncFilter) Empty() bool {
for i := 0; i < len(f.b); i++ {
for j := 0; j < blockWords; j++ {
if atomic.LoadUint32(&f.b[i][j]) != 0 {
return false
}
}
}
return true
}
// Fill sets f to a completely full filter.
// After Fill, Has returns true for any key.
func (f *SyncFilter) Fill() {
for i := 0; i < len(f.b); i++ {
for j := 0; j < blockWords; j++ {
atomic.StoreUint32(&f.b[i][j], ^uint32(0))
}
}
}
// Has reports whether a key with hash value h has been added.
// It may return a false positive.
func (f *SyncFilter) Has(h uint64) bool {
h1, h2 := uint32(h>>32), uint32(h)
b := (*Filter)(f).getblock(h2)
for i := 1; i < f.k; i++ {
h1, h2 = doublehash(h1, h2, i)
if !b.getbitAtomic(h1) {
return false
}
}
return true
}
// getbitAtomic reports whether bit (i modulo BlockBits) is set.
func (b *block) getbitAtomic(i uint32) bool {
bit := uint32(1) << (i % wordSize)
x := atomic.LoadUint32(&(*b)[(i/wordSize)%blockWords])
return x&bit != 0
}
// setbit sets bit (i modulo BlockBits) of b, atomically.
func (b *block) setbitAtomic(i uint32) {
bit := uint32(1) << (i % wordSize)
p := &(*b)[(i/wordSize)%blockWords]
for {
old := atomic.LoadUint32(p)
if old&bit != 0 {
// Checking here instead of checking the return value from
// the CAS is between 50% and 80% faster on the benchmark.
return
}
atomic.CompareAndSwapUint32(p, old, old|bit)
}
}
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