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Merge commit '96a9518aee88a7301ab8894aaaa25c8964f90226' as 'vendor/'

Yawning Angel 4 years ago

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+// a2filter.go - A2 bloom filter
+// To the extent possible under law, the Yawning Angel waived all copyright
+// and related or neighboring rights to a2filter, using the creative
+// commons "cc0" public domain dedication. See LICENSE or
+// <> for full details.
+// Package a2filter implements a SipHash-2-4 based Active-Active Bloom Filter.
+// It is designed to be stable over time even when filled to max capacity by
+// implementing the active-active buffering (A2 buffering) scheme presented in
+// "Aging Bloom Filter with Two Active Buffers for Dynamic Sets" (MyungKeun
+// Yoon).
+// Note that none of the operations on the filter are constant time, and the
+// the max backing Bloom Filter size is limited to 2^31 bytes.  This package is
+// threadsafe.
+package a2filter
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+	"math"
+	"sync"
+	""
+const (
+	ln2         = 0.69314718055994529
+	ln2Sq       = 0.48045301391820139
+	maxMln2     = 31
+	maxNrHashes = 32
+// A2Filter is an Active-Active Bloom Filter.
+type A2Filter struct {
+	sync.Mutex
+	k1, k2 uint64
+	nrEntries    int
+	nrEntriesMax int
+	nrHashes int
+	hashMask uint32
+	active1  []byte
+	active2  []byte
+// New constructs a new A2Filter with a filter set size 2^mLn2, and false
+// postive rate p.  The actual in memory footprint of the datastructure will be
+// approximately 2^(mLn2+1) bits due to the double buffered nature of the
+// filter.
+func New(rand io.Reader, mLn2 int, p float64) (*A2Filter, error) {
+	var key [16]byte
+	if _, err := io.ReadFull(rand, key[:]); err != nil {
+		return nil, err
+	}
+	if mLn2 > maxMln2 {
+		return nil, fmt.Errorf("requested filter too large: %d", mLn2)
+	}
+	m := 1 << uint32(mLn2)
+	n := -1.0 * float64(m) * ln2Sq / math.Log(p)
+	k := int((float64(m) * ln2 / n) + 0.5)
+	f := new(A2Filter)
+	f.k1 = binary.BigEndian.Uint64(key[0:8])
+	f.k2 = binary.BigEndian.Uint64(key[8:16])
+	f.nrEntriesMax = int(n)
+	f.nrHashes = k
+	f.hashMask = uint32(m - 1)
+	if f.nrHashes < 2 {
+		f.nrHashes = 2
+	}
+	if f.nrHashes > maxNrHashes {
+		return nil, fmt.Errorf("requested parameters need too many hashes")
+	}
+	f.active1 = make([]byte, m/8)
+	f.active2 = make([]byte, m/8)
+	return f, nil
+// TestAndSet tests the A2Filter for a given value's membership, adds the
+// value to the filter and returns if it was present at the time of the call.
+func (f *A2Filter) TestAndSet(b []byte) bool {
+	hashes := f.getHashes(b)
+	f.Lock()
+	defer f.Unlock()
+	// If the member is present in Active1, just return.
+	if f.testCache(f.active1, hashes) {
+		return true
+	}
+	// Test Active2 for membership, and add the value to Active1.
+	ret := f.testCache(f.active2, hashes)
+	if f.nrEntries++; f.nrEntries > f.nrEntriesMax {
+		// Active1 is full, clear Active2 and swap the buffers, this leaves
+		// Active1 empty, and Active2 populated to saturation, immediately
+		// after the tested entry will be added to Active1.
+		f.active2 = make([]byte, len(f.active2))
+		f.active1, f.active2 = f.active2, f.active1
+		f.nrEntries = 1
+	}
+	f.addActive1(hashes)
+	return ret
+// MaxEntries returns the maximum capacity of the A2Filter.  This value is
+// usually an underestimate as the filter is double buffered, however entry
+// count accounting is only done for Active1, so Active2 should be ignored in
+// calculations.
+func (f *A2Filter) MaxEntries() int {
+	return f.nrEntriesMax
+func (f *A2Filter) testCache(cache []byte, hashes []uint32) bool {
+	for i := 0; i < f.nrHashes; i++ {
+		idx := hashes[i] & f.hashMask
+		if 0 == cache[idx/8]&(1<<(idx&7)) {
+			// Break out early if there is a miss.
+			return false
+		}
+	}
+	return true
+func (f *A2Filter) addActive1(hashes []uint32) {
+	for i := 0; i < f.nrHashes; i++ {
+		idx := hashes[i] & f.hashMask
+		f.active1[idx/8] |= (1 << (idx & 7))
+	}
+func (f *A2Filter) getHashes(b []byte) []uint32 {
+	// Per "Less Hashing, Same Performance: Building a Better Bloom Filter"
+	// (Kirsch and Miteznmacher), with a suitably good PRF, only two calls to
+	// the hash algorithm are needed.  As SipHash-2-4 returns a 64 bit digest,
+	// and we use 32 bit hashes for the filter, this results in only one
+	// invocation of SipHash-2-4.
+	hashes := make([]uint32, f.nrHashes)
+	baseHash := siphash.Hash(f.k1, f.k2, b)
+	hashes[0] = uint32(baseHash & math.MaxUint32)
+	hashes[1] = uint32(baseHash >> 32)
+	for i := 2; i < f.nrHashes; i++ {
+		hashes[i] = hashes[0] + uint32(i)*hashes[1]
+	}
+	return hashes