forked from greatroar/blobloom
-
Notifications
You must be signed in to change notification settings - Fork 1
/
bloomfilter.go
411 lines (356 loc) · 10.7 KB
/
bloomfilter.go
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
// Copyright 2020-2022 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 implements blocked Bloom filters.
//
// Blocked Bloom filters are an approximate set data structure: if a key has
// been added to a filter, a lookup of that key returns true, but if the key
// has not been added, there is a non-zero probability that the lookup still
// returns true (a false positive). False negatives are impossible: if the
// lookup for a key returns false, that key has not been added.
//
// In this package, keys are represented exclusively as hashes. Client code
// is responsible for supplying a 64-bit hash value.
//
// Compared to standard Bloom filters, blocked Bloom filters use the CPU
// cache more efficiently. A blocked Bloom filter is an array of ordinary
// Bloom filters of fixed size BlockBits (the blocks). The lower half of the
// hash selects the block to use.
//
// To achieve the same false positive rate (FPR) as a standard Bloom filter,
// a blocked Bloom filter requires more memory. For an FPR of at most 2e-6
// (two in a million), it uses ~20% more memory. At 1e-10, the space required
// is double that of standard Bloom filter.
//
// For more details, see the 2010 paper by Putze, Sanders and Singler,
// https://algo2.iti.kit.edu/documents/cacheefficientbloomfilters-jea.pdf.
package blobloom
import (
"bytes"
"encoding/base64"
"encoding/binary"
"encoding/json"
"errors"
"io"
"math"
"sync/atomic"
)
// BlockBits is the number of bits per block and the minimum number of bits
// in a Filter.
//
// The value of this constant is chosen to match the L1 cache line size
// of popular architectures (386, amd64, arm64).
const BlockBits = 512
// MaxBits is the maximum number of bits supported by a Filter.
const MaxBits = BlockBits << 32 // 256GiB.
// A Filter is a blocked Bloom filter.
type Filter struct {
b []block // Shards.
k int // Number of hash functions required.
}
// New 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 New(nbits uint64, nhashes int) *Filter {
nbits, nhashes = fixBitsAndHashes(nbits, nhashes)
return &Filter{
b: make([]block, nbits/BlockBits),
k: nhashes,
}
}
func fixBitsAndHashes(nbits uint64, nhashes int) (uint64, int) {
if nbits < 1 {
nbits = BlockBits
}
if nhashes < 2 {
nhashes = 2
}
if nbits > MaxBits {
panic("nbits exceeds MaxBits")
}
// Round nbits up to a multiple of BlockBits.
if nbits%BlockBits != 0 {
nbits += BlockBits - nbits%BlockBits
}
return nbits, nhashes
}
// Add insert a key with hash value h into f.
func (f *Filter) Add(h uint64) {
h1, h2 := uint32(h>>32), uint32(h)
b := getblock(f.b, h2)
for i := 1; i < f.k; i++ {
h1, h2 = doublehash(h1, h2, i)
b.setbit(h1)
}
}
// log(1 - 1/BlockBits) computed with 128 bits precision.
// Note that this is extremely close to -1/BlockBits,
// which is what Wikipedia would have us use:
// https://en.wikipedia.org/wiki/Bloom_filter#Approximating_the_number_of_items_in_a_Bloom_filter.
const log1minus1divBlockbits = -0.0019550348358033505576274922418668121377
// 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).
func (f *Filter) Cardinality() float64 {
return cardinality(f.k, f.b, onescount)
}
func cardinality(nhashes int, b []block, onescount func(*block) int) float64 {
k := float64(nhashes - 1)
// The probability of some bit not being set in a single insertion is
// p0 = (1-1/BlockBits)^k.
//
// logProb0Inv = 1 / log(p0) = 1 / (k*log(1-1/BlockBits)).
logProb0Inv := 1 / (k * log1minus1divBlockbits)
var n float64
for i := range b {
ones := onescount(&b[i])
if ones == 0 {
continue
}
n += math.Log1p(-float64(ones) / BlockBits)
}
return n * logProb0Inv
}
// Clear resets f to its empty state.
func (f *Filter) Clear() {
for i := 0; i < len(f.b); i++ {
f.b[i] = block{}
}
}
// Empty reports whether f contains no keys.
func (f *Filter) Empty() bool {
for i := 0; i < len(f.b); i++ {
if f.b[i] != (block{}) {
return false
}
}
return true
}
// Equals returns true if f and g contain the same keys (in terms of Has)
// when used with the same hash function.
func (f *Filter) Equals(g *Filter) bool {
if g.k != f.k || len(g.b) != len(f.b) {
return false
}
for i := range g.b {
if f.b[i] != g.b[i] {
return false
}
}
return true
}
// Fill set f to a completely full filter.
// After Fill, Has returns true for any key.
func (f *Filter) Fill() {
for i := 0; i < len(f.b); i++ {
for j := 0; j < blockWords; j++ {
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 *Filter) Has(h uint64) bool {
h1, h2 := uint32(h>>32), uint32(h)
b := getblock(f.b, h2)
for i := 1; i < f.k; i++ {
h1, h2 = doublehash(h1, h2, i)
if !b.getbit(h1) {
return false
}
}
return true
}
func (f *Filter) K() uint {
// to do: modify f.k's type to uint
return uint(f.k)
}
// TestLocations returns true if all locations are set in the Filter, false otherwise.
func (f *Filter) TestLocations(locs []uint64) bool {
if len(locs) < f.k {
// return false positive
return true
}
b := getblock(f.b, uint32(locs[0]))
for i := 1; i < f.k; i++ {
if !b.getbit(uint32(locs[i])) {
return false
}
}
return true
}
// Locations returns a list of hash locations representing a data item.
func Locations(h uint64, k uint) []uint64 {
locs := make([]uint64, k)
h1, h2 := uint32(h>>32), uint32(h)
locs[0] = uint64(h2)
for i := 1; i < int(k); i++ {
h1, h2 = doublehash(h1, h2, i)
locs[i] = uint64(h1)
}
return locs
}
type filterInJson struct {
K uint `json:"k"`
B []block `json:"b"`
}
// MarshalJSON marshals a blcok as a JSON structure
func (b Filter) MarshalJSON() ([]byte, error) {
return json.Marshal(&filterInJson{
K: uint(b.k),
B: b.b,
})
}
// UnmarshalJSON unmarshals a block from JSON created using MarshalJSON
func (b *Filter) UnmarshalJSON(data []byte) error {
filterInJson := &filterInJson{}
err := json.Unmarshal(data, filterInJson)
if err != nil {
return nil
}
b.b = filterInJson.B
b.k = int(filterInJson.K)
return nil
}
// doublehash generates the hash values to use in iteration i of
// enhanced double hashing from the values h1, h2 of the previous iteration.
// See https://www.ccs.neu.edu/home/pete/pub/bloom-filters-verification.pdf.
func doublehash(h1, h2 uint32, i int) (uint32, uint32) {
h1 = h1 + h2
h2 = h2 + uint32(i)
return h1, h2
}
// NumBits returns the number of bits of f.
func (f *Filter) NumBits() uint64 {
return BlockBits * uint64(len(f.b))
}
func checkBinop(f, g *Filter) {
if len(f.b) != len(g.b) {
panic("Bloom filters do not have the same number of bits")
}
if f.k != g.k {
panic("Bloom filters do not have the same number of hash functions")
}
}
// Intersect sets f to the intersection of f and g.
//
// Intersect panics when f and g do not have the same number of bits and
// hash functions. Both Filters must be using the same hash function(s),
// but Intersect cannot check this.
//
// Since Bloom filters may return false positives, Has may return true for
// a key that was not in both f and g.
//
// After Intersect, the estimates from f.Cardinality and f.FPRate should be
// considered unreliable.
func (f *Filter) Intersect(g *Filter) {
checkBinop(f, g)
f.intersect(g)
}
// Union sets f to the union of f and g.
//
// Union panics when f and g do not have the same number of bits and
// hash functions. Both Filters must be using the same hash function(s),
// but Union cannot check this.
func (f *Filter) Union(g *Filter) {
checkBinop(f, g)
f.union(g)
}
const (
wordSize = 32
blockWords = BlockBits / wordSize
)
// A block is a fixed-size Bloom filter, used as a shard of a Filter.
type block [blockWords]uint32
func getblock(b []block, h2 uint32) *block {
i := reducerange(h2, uint32(len(b)))
return &b[i]
}
// reducerange maps i to an integer in the range [0,n).
// https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
func reducerange(i, n uint32) uint32 {
return uint32((uint64(i) * uint64(n)) >> 32)
}
// getbit reports whether bit (i modulo BlockBits) is set.
func (b *block) getbit(i uint32) bool {
bit := uint32(1) << (i % wordSize)
x := (*b)[(i/wordSize)%blockWords] & bit
return x != 0
}
// setbit sets bit (i modulo BlockBits) of b.
func (b *block) setbit(i uint32) {
bit := uint32(1) << (i % wordSize)
(*b)[(i/wordSize)%blockWords] |= bit
}
func (b *block) WriteTo(w io.Writer) (int64, error) {
buf := make([]byte, len(b)*4)
for j := range b {
x := atomic.LoadUint32(&b[j])
binary.LittleEndian.PutUint32(buf[4*j:], x)
}
k, err := w.Write(buf[:])
if err != nil {
return 0, nil
}
return int64(k), err
}
func (b *block) ReadFrom(r io.Reader) (int64, error) {
buf := make([]byte, len(b)*4)
k, err := io.ReadFull(r, buf[:])
if err != nil {
if errors.Is(err, io.EOF) {
err = io.ErrUnexpectedEOF
}
return 0, err
}
for i := range b {
b[i] |= binary.LittleEndian.Uint32(buf[4*i:])
}
return int64(k), nil
}
// MarshalJSON marshals a blcok as a JSON structure
func (b block) MarshalJSON() ([]byte, error) {
buffer := bytes.NewBuffer(make([]byte, 0))
_, err := b.WriteTo(buffer)
if err != nil {
return nil, err
}
// URLEncode all bytes
return json.Marshal(base64.URLEncoding.EncodeToString(buffer.Bytes()))
}
// UnmarshalJSON unmarshals a block from JSON created using MarshalJSON
func (b *block) UnmarshalJSON(data []byte) error {
var s string
err := json.Unmarshal(data, &s)
if err != nil {
return err
}
// URLDecode string
buf, err := base64.URLEncoding.DecodeString(s)
if err != nil {
return err
}
_, err = b.ReadFrom(bytes.NewReader(buf))
return err
}