frostfs-http-gw/connections/sampler.go

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package connections
import "math/rand"
// https://www.keithschwarz.com/darts-dice-coins/
type Sampler struct {
randomGenerator *rand.Rand
probabilities []float64
alias []int
}
type workList []int
func (wl *workList) push(e int) {
*wl = append(*wl, e)
}
func (wl *workList) pop() int {
l := len(*wl) - 1
n := (*wl)[l]
*wl = (*wl)[:l]
return n
}
func NewSampler(probabilities []float64, source rand.Source) *Sampler {
sampler := &Sampler{}
var (
small workList
large workList
)
n := len(probabilities)
sampler.randomGenerator = rand.New(source)
sampler.probabilities = make([]float64, n)
sampler.alias = make([]int, n)
// Compute scaled probabilities.
p := make([]float64, n)
for i := 0; i < n; i++ {
p[i] = probabilities[i] * float64(n)
}
for i, pi := range p {
if pi < 1 {
small = append(small, i)
} else {
large = append(large, i)
}
}
for len(large) > 0 && len(small) > 0 {
l, g := small.pop(), large.pop()
sampler.probabilities[l] = p[l]
sampler.alias[l] = g
p[g] = (p[g] + p[l]) - 1
if p[g] < 1 {
small.push(g)
} else {
large.push(g)
}
}
for len(large) > 0 {
g := large.pop()
sampler.probabilities[g] = 1
}
for len(small) > 0 {
l := small.pop()
sampler.probabilities[l] = 1
}
return sampler
}
func (g *Sampler) Next() int {
n := len(g.alias)
i := g.randomGenerator.Intn(n)
if g.randomGenerator.Float64() < g.probabilities[i] {
return i
}
return g.alias[i]
}