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package minhash
import "math"
// MinWise is a collection of minimum hashes for a set
type MinWise struct {
minimums []uint64
h1 Hash64
h2 Hash64
}
type Hash64 func([]byte) uint64
// NewMinWise returns a new MinWise Hashing implementation
func NewMinWise(h1, h2 Hash64, size int) *MinWise {
minimums := make([]uint64, size)
for i := range minimums {
minimums[i] = math.MaxUint64
}
return &MinWise{
h1: h1,
h2: h2,
minimums: minimums,
}
}
// NewMinWiseFromSignatures returns a new MinWise Hashing implementation
// using a user-provided set of signatures
func NewMinWiseFromSignatures(h1, h2 Hash64, signatures []uint64) *MinWise {
minimums := make([]uint64, len(signatures))
copy(minimums, signatures)
return &MinWise{
h1: h1,
h2: h2,
minimums: signatures,
}
}
// Push adds an element to the set.
func (m *MinWise) Push(b []byte) {
v1 := m.h1(b)
v2 := m.h2(b)
for i, v := range m.minimums {
hv := v1 + uint64(i)*v2
if hv < v {
m.minimums[i] = hv
}
}
}
// Merge combines the signatures of the second set, creating the signature of their union.
func (m *MinWise) Merge(m2 *MinWise) {
for i, v := range m2.minimums {
if v < m.minimums[i] {
m.minimums[i] = v
}
}
}
// Cardinality estimates the cardinality of the set
func (m *MinWise) Cardinality() int {
// http://www.cohenwang.com/edith/Papers/tcest.pdf
sum := 0.0
for _, v := range m.minimums {
sum += -math.Log(float64(math.MaxUint64-v) / float64(math.MaxUint64))
}
return int(float64(len(m.minimums)-1) / sum)
}
// Signature returns a signature for the set.
func (m *MinWise) Signature() []uint64 {
return m.minimums
}
// Similarity computes an estimate for the similarity between the two sets.
func (m *MinWise) Similarity(m2 *MinWise) float64 {
if len(m.minimums) != len(m2.minimums) {
panic("minhash minimums size mismatch")
}
intersect := 0
for i := range m.minimums {
if m.minimums[i] == m2.minimums[i] {
intersect++
}
}
return float64(intersect) / float64(len(m.minimums))
}
// SignatureBbit returns a b-bit reduction of the signature. This will result in unused bits at the high-end of the words if b does not divide 64 evenly.
func (m *MinWise) SignatureBbit(b uint) []uint64 {
var sig []uint64 // full signature
var w uint64 // current word
bits := uint(64) // bits free in current word
mask := uint64(1<<b) - 1
for _, v := range m.minimums {
if bits >= b {
w <<= b
w |= v & mask
bits -= b
} else {
sig = append(sig, w)
w = 0
bits = 64
}
}
if bits != 64 {
sig = append(sig, w)
}
return sig
}
// SimilarityBbit computes an estimate for the similarity between two b-bit signatures
func SimilarityBbit(sig1, sig2 []uint64, b uint) float64 {
if len(sig1) != len(sig2) {
panic("signature size mismatch")
}
intersect := 0
count := 0
mask := uint64(1<<b) - 1
for i := range sig1 {
w1 := sig1[i]
w2 := sig2[i]
bits := uint(64)
for bits >= b {
v1 := (w1 & mask)
v2 := (w2 & mask)
count++
if v1 == v2 {
intersect++
}
bits -= b
w1 >>= b
w2 >>= b
}
}
return float64(intersect) / float64(count)
}
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