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chd_builder.go
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chd_builder.go
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package mph
import (
"bytes"
"errors"
"fmt"
"math/rand"
"sort"
"time"
)
type chdHasher struct {
r []uint64
size uint64
buckets uint64
rand *rand.Rand
}
type bucket struct {
index uint64
keys [][]byte
values [][]byte
}
func (b *bucket) String() string {
a := "bucket{"
for _, k := range b.keys {
a += string(k) + ", "
}
return a + "}"
}
// Intermediate data structure storing buckets + outer hash index.
type bucketVector []bucket
func (b bucketVector) Len() int { return len(b) }
func (b bucketVector) Less(i, j int) bool { return len(b[i].keys) > len(b[j].keys) }
func (b bucketVector) Swap(i, j int) { b[i], b[j] = b[j], b[i] }
// Build a new CDH MPH.
type CHDBuilder struct {
keys [][]byte
values [][]byte
}
// Create a new CHD hash table builder.
func Builder() *CHDBuilder {
return &CHDBuilder{}
}
// Add a key and value to the hash table.
func (b *CHDBuilder) Add(key []byte, value []byte) {
b.keys = append(b.keys, key)
b.values = append(b.values, value)
}
// Try to find a hash function that does not cause collisions with table, when
// applied to the keys in the bucket.
func tryHash(hasher *chdHasher, seen map[uint64]bool, keys [][]byte, values [][]byte, indices []uint16, bucket *bucket, ri uint16, r uint64) bool {
// Track duplicates within this bucket.
duplicate := make(map[uint64]bool)
// Make hashes for each entry in the bucket.
hashes := make([]uint64, len(bucket.keys))
for i, k := range bucket.keys {
h := hasher.Table(r, k)
hashes[i] = h
if seen[h] {
return false
}
if duplicate[h] {
return false
}
duplicate[h] = true
}
// Update seen hashes
for _, h := range hashes {
seen[h] = true
}
// Add the hash index.
indices[bucket.index] = ri
// Update the the hash table.
for i, h := range hashes {
keys[h] = bucket.keys[i]
values[h] = bucket.values[i]
}
return true
}
func (b *CHDBuilder) Build() (*CHD, error) {
n := uint64(len(b.keys))
m := n / 2
if m == 0 {
m = 1
}
keys := make([][]byte, n)
values := make([][]byte, n)
hasher := newCHDHasher(n, m)
buckets := make(bucketVector, m)
indices := make([]uint16, m)
// An extra check to make sure we don't use an invalid index
for i := range indices {
indices[i] = ^uint16(0)
}
// Have we seen a hash before?
seen := make(map[uint64]bool)
// Used to ensure there are no duplicate keys.
duplicates := make(map[string]bool)
for i := range b.keys {
key := b.keys[i]
value := b.values[i]
k := string(key)
if duplicates[k] {
return nil, errors.New("duplicate key " + k)
}
duplicates[k] = true
oh := hasher.HashIndexFromKey(key)
buckets[oh].index = oh
buckets[oh].keys = append(buckets[oh].keys, key)
buckets[oh].values = append(buckets[oh].values, value)
}
// Order buckets by size (retaining the hash index)
collisions := 0
sort.Sort(buckets)
nextBucket:
for i, bucket := range buckets {
if len(bucket.keys) == 0 {
continue
}
// Check existing hash functions.
for ri, r := range hasher.r {
if tryHash(hasher, seen, keys, values, indices, &bucket, uint16(ri), r) {
continue nextBucket
}
}
// Keep trying new functions until we get one that does not collide.
// The number of retries here is very high to allow a very high
// probability of not getting collisions.
for i := 0; i < 10000000; i++ {
if i > collisions {
collisions = i
}
ri, r := hasher.Generate()
if tryHash(hasher, seen, keys, values, indices, &bucket, ri, r) {
hasher.Add(r)
continue nextBucket
}
}
// Failed to find a hash function with no collisions.
return nil, fmt.Errorf(
"failed to find a collision-free hash function after ~10000000 attempts, for bucket %d/%d with %d entries: %s",
i, len(buckets), len(bucket.keys), &bucket)
}
// println("max bucket collisions:", collisions)
// println("keys:", len(table))
// println("hash functions:", len(hasher.r))
keylist := make([]dataSlice, len(b.keys))
valuelist := make([]dataSlice, len(b.values))
var buf bytes.Buffer
for i, k := range keys {
keylist[i].start = uint64(buf.Len())
buf.Write(k)
keylist[i].end = uint64(buf.Len())
valuelist[i].start = uint64(buf.Len())
buf.Write(values[i])
valuelist[i].end = uint64(buf.Len())
}
return &CHD{
r: hasher.r,
indices: indices,
mmap: buf.Bytes(),
keys: keylist,
values: valuelist,
}, nil
}
func newCHDHasher(size uint64, buckets uint64) *chdHasher {
rs := rand.NewSource(time.Now().UnixNano())
c := &chdHasher{size: size, buckets: buckets, rand: rand.New(rs)}
c.Add(c.random())
return c
}
func (c *chdHasher) random() uint64 {
return (uint64(c.rand.Uint32()) << 32) | uint64(c.rand.Uint32())
}
// Hash index from key.
func (h *chdHasher) HashIndexFromKey(b []byte) uint64 {
return (hasher(b) ^ h.r[0]) % h.buckets
}
// Table hash from random value and key. Generate() returns these random values.
func (h *chdHasher) Table(r uint64, b []byte) uint64 {
return (hasher(b) ^ h.r[0] ^ r) % h.size
}
func (c *chdHasher) Generate() (uint16, uint64) {
return c.Len(), c.random()
}
// Add a random value generated by Generate().
func (c *chdHasher) Add(r uint64) {
c.r = append(c.r, r)
}
func (c *chdHasher) Len() uint16 {
return uint16(len(c.r))
}
func (h *chdHasher) String() string {
return fmt.Sprintf("chdHasher{size: %d, buckets: %d, r: %v}", h.size, h.buckets, h.r)
}