Add connection pool implementation (part 3)

Signed-off-by: Pavel Korotkov <pavel@nspcc.ru>

connections/sampler.go

@@ -0,0 +1,76 @@
+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]
+}