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apbf.go
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apbf.go
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// Copyright 2020 CrowdStrike Holdings, Inc.
//
// Use of this source code is governed by the MIT License.
package apbf
import (
"math"
"sync"
"sync/atomic"
"time"
"github.com/spaolacci/murmur3"
)
//go:generate protoc --gogo_out=. snapshot.proto
// Filter represents the Age-Partitioned Bloom Filter (APBF).
// The implementation is safe for concurrent use.
type Filter struct {
k uint
l uint
h uint // slice count
g uint // generation size
r int64 // refresh interval in nano
m uint // slice bit count
lock sync.RWMutex // guards all below
base uint // current slice 1-based index
buffer []byte // circular buffer
count uint // current generation count
lastRefresh int64
hasherPool *sync.Pool
}
type hash [2]uint
// New returns the APBF with k + l slices and g generation size.
func New(k, l, g uint) *Filter {
return new(k, l, g, 0)
}
// NewWithRefresh returns the APBF with k + l slices, g generation size, and r refresh interval.
func NewWithRefresh(k, l, g uint, r time.Duration) *Filter {
return new(k, l, g, r)
}
func new(k, l, g uint, r time.Duration) *Filter {
if k == 0 || l == 0 || g == 0 || r < 0 {
panic("invalid parameters")
}
h, m, bufferLen := deriveParams(k, l, g)
return &Filter{
k: k,
l: l,
h: h,
g: g,
r: int64(r),
m: m,
base: 1,
buffer: make([]byte, bufferLen),
count: 0,
hasherPool: getHasherPool(),
lastRefresh: time.Now().UnixNano(),
}
}
// NewFromSnapshot recreates the matching filter from the provided snapshot.
func NewFromSnapshot(s Snapshot) *Filter {
if s.K == 0 || s.L == 0 || s.G == 0 || s.R < 0 {
panic("invalid snapshot")
}
h, m, bufferLen := deriveParams(uint(s.K), uint(s.L), uint(s.G))
validBuffer := len(s.Buffer) == bufferLen
validBase := uint(s.Base) >= 1 && uint(s.Base) <= h
validCount := s.Count <= s.G
if !validBuffer || !validBase || !validCount {
panic("invalid snapshot")
}
return &Filter{
k: uint(s.K),
l: uint(s.L),
h: h,
g: uint(s.G),
r: int64(s.R),
m: m,
base: uint(s.Base),
buffer: cloneSlice(s.Buffer),
count: uint(s.Count),
hasherPool: getHasherPool(),
lastRefresh: time.Now().UnixNano(),
}
}
func deriveParams(k, l, g uint) (uint, uint, int) {
h := k + l // slice count
n := k * g // slice capacity
m := uint(math.Ceil(1.442695 * float64(n))) // slice bit count
mt := h * m // total bit count
bufferLen := int((mt + 7) / 8)
return h, m, bufferLen
}
// Add item to the set.
func (f *Filter) Add(item []byte) {
f.refresh()
hash := f.getHash(item)
f.lock.Lock()
if f.count == f.g {
f.shift()
}
slice := f.base
for i := uint(0); i < f.k; i++ {
bit := f.location(slice, hash)
f.setBit(bit)
slice = f.nextSlice(slice)
}
f.count++
f.lock.Unlock()
}
// Query returns true if the item is in the set and false otherwise. A true value might be a false positive whereas false is always correct.
func (f *Filter) Query(item []byte) bool {
f.refresh()
hash := f.getHash(item)
f.lock.RLock()
slice := f.base
matched := uint(0)
for i := f.h; i >= f.k-matched; i-- {
bit := f.location(slice, hash)
if f.hasBit(bit) {
matched++
if matched == f.k {
break
}
} else {
matched = 0
}
slice = f.nextSlice(slice)
}
f.lock.RUnlock()
return matched == f.k
}
// NextGeneration transitions to next generation.
func (f *Filter) NextGeneration() {
f.lock.Lock()
f.shift()
f.lock.Unlock()
}
// Snapshot returns a consistent snapshot of filter state.
func (f *Filter) Snapshot() Snapshot {
f.lock.RLock()
result := Snapshot{
K: uint64(f.k),
L: uint64(f.l),
G: uint64(f.g),
R: uint64(f.r),
Base: uint64(f.base),
Count: uint64(f.count),
Buffer: cloneSlice(f.buffer),
}
f.lock.RUnlock()
return result
}
// MaxCapacity returns filter max capacity.
func (f *Filter) MaxCapacity() int {
return int(f.g) * f.MaxGenerations()
}
// MaxGenerations returns filter max generations count.
func (f *Filter) MaxGenerations() int {
return int(f.l + 1)
}
func (f *Filter) refresh() {
if f.r == 0 {
return
}
now := time.Now().UnixNano()
// fast path
lastRefresh := atomic.LoadInt64(&f.lastRefresh)
if lastRefresh+f.r > now {
return
}
// slow path
f.lock.Lock()
for {
next := f.lastRefresh + f.r
if next > now {
break
}
f.shift()
f.lastRefresh = next
}
f.lock.Unlock()
}
func (f *Filter) shift() {
f.count = 0
f.base = f.prevSlice(f.base)
bit := (f.base - 1) * f.m
endBit := bit + f.m
for bit < endBit && bit%8 != 0 {
f.clearBit(bit)
bit++
}
for bit < endBit && bit+8 < endBit {
f.buffer[bit/8] = 0
bit += 8
}
for bit < endBit {
f.clearBit(bit)
bit++
}
}
func (f *Filter) getHash(item []byte) hash {
hasher := f.hasherPool.Get().(murmur3.Hash128)
hasher.Reset()
hasher.Write(item)
h1, h2 := hasher.Sum128()
f.hasherPool.Put(hasher)
return hash{uint(h1), uint(h2)}
}
func (f *Filter) location(i uint, h hash) uint {
t := (i*i*i - i) / 6
return (i-1)*f.m + (h[0]+i*h[1]+t)%f.m // enhanced double hashing
}
func (f *Filter) nextSlice(i uint) uint {
if i == f.h {
return 1
}
return i + 1
}
func (f *Filter) prevSlice(i uint) uint {
if i == 1 {
return f.h
}
return i - 1
}
func (f *Filter) setBit(index uint) {
f.buffer[index/8] |= 1 << (index % 8)
}
func (f *Filter) clearBit(index uint) {
f.buffer[index/8] &^= 1 << (index % 8)
}
func (f *Filter) hasBit(index uint) bool {
return f.buffer[index/8]&(1<<(index%8)) != 0
}
// CalculateFalsePositiveRate computes the false positive rate for given k and l parameters.
func CalculateFalsePositiveRate(k, l uint) float64 {
if k == 0 || l == 0 {
panic("invalid parameters")
}
type key struct {
a, i uint
}
cache := map[key]float64{}
var calculate func(a, i uint) float64
calculate = func(a, i uint) float64 {
if a == k {
return 1
} else if i > l+a {
return 0
}
ck := key{a, i}
if val, ok := cache[ck]; ok {
return val
}
ri := 0.5
if i < k {
ri = float64(i+1) / float64(2*k)
}
val := ri*calculate(a+1, i+1) + (1-ri)*calculate(0, i+1)
cache[ck] = val
return val
}
return calculate(0, 0)
}
func cloneSlice(src []byte) []byte {
dest := make([]byte, len(src))
copy(dest, src)
return dest
}
func getHasherPool() *sync.Pool {
return &sync.Pool{
New: func() interface{} { return murmur3.New128() },
}
}