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// Copyright © 2020 rsiddharth <s@ricketyspace.net>
// SPDX-License-Identifier: ISC
package lib
func HammingDistance(a, b []byte) int {
if len(a) != len(b) {
return -1 // Fail.
}
d := 0
for i := 0; i < len(a); i++ {
c := a[i] ^ b[i]
d += setBits(c)
}
return d
}
// Returns number of set bits.
func setBits(b byte) int {
var c byte = 0
for i := 0; i < 8; i++ {
c += b & 0x1
b = b >> 1
}
return int(c)
}
// Returns key size with minimum normalized hamming distance
// 'keyMin' is the minimum key size
// 'keyMax' is the maximum key size
func KeySizeWithMinDistance(keyMin, keyMax int, bs []byte) int {
var mk int = 0 // Key size with min distance.
var md float64 = 100.0 // Distance for key size 'mk'.
for k := keyMin; k <= keyMax; k++ {
p := bs[0 : k*8] // K bytes.
q := bs[k*8 : k*8*2] // Next K bytes.
// Compute distance.
d := HammingDistance(p, q)
nd := float64(d) / float64(k)
if nd < md {
mk = k
md = nd
}
}
return mk
}
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