// The Hamming distances between the nth values of one RNG seeded with two
// values with Hamming distance 1 should be binomially distributed. This test
// derives its name from the bit avalanche effect.
//
// NOTE: ReaderSource RNGs (e.g. crypto-reader) will be incorrect.
func Avalanche(r rand.Source) float64 {
// hax teh copypasta from popcount
counts := [64]int64{}
v := r.Int63()
var a, b int64
for i := 0; i < avalancheN; i++ {
for j := uint(0); j < 63; j++ {
r.Seed(v)
for k := 0; k < avalancheFuse; k++ {
r.Int63()
}
a = r.Int63()
r.Seed(v ^ (1 << j))
for k := 0; k < avalancheFuse; k++ {
r.Int63()
}
b = r.Int63()
// Hamming distance from a to b is equal to popcount(a^b)
x := a ^ b
// popcount_3() from http://en.wikipedia.org/wiki/Hamming_weight
x -= x >> 1 & 0x5555555555555555
x = x&0x3333333333333333 + x>>2&0x3333333333333333
x = (x + x>>4) & 0x0f0f0f0f0f0f0f0f
x = x * 0x0101010101010101 >> 56 // popcount
counts[x]++
}
v = b // silly to use a^b because that depends upon what we're testing
}
var chi2 float64
for i, v := range counts {
d := float64(v)*binomialBitScale - binomialBitPMF[i]
chi2 += d * d / binomialBitPMF[i]
}
return 1 - LowerGamma(30, chi2/2)*1.13099628864477e-31
}
