Implement the stochastic roulette selector

Author: jjr90

selection.go

@@ -117,3 +117,42 @@ func (sel SelRoulette) Apply(n uint, indis Individuals, rng *rand.Rand) (Individ
 func (sel SelRoulette) Validate() error {
 	return nil
 }
+
+// SelStochRoulette performs a stochastic variant of the roulette wheel with
+// faster O(1) time complexity, at the expense of some noise.
+// https://arxiv.org/abs/1109.3627
+// https://en.wikipedia.org/wiki/Fitness_proportionate_selection @ "stochastic acceptance"
+type SelStochRoulette struct{}
+
+// Apply SelStochRoulette
+func (sel SelStochRoulette) Apply(n uint, indis Individuals, rng *rand.Rand) (Individuals, []int, error) {
+	var (
+		selected    = make(Individuals, n)
+		indexes     = make([]int, n)
+		bestFitness = indis.FitMin()
+	)
+
+	for i := range selected {
+		var (
+			index    int
+			accepted bool
+		)
+		for !accepted {
+			index = rng.Intn(len(indis))
+			if rng.Float64() < indis[index].Fitness/bestFitness {
+				accepted = true
+			}
+		}
+
+		var winner = indis[index]
+
+		indexes[i] = index
+		selected[i] = winner
+	}
+	return selected.Clone(rng), indexes, nil
+}
+
+// Validate SelStochRoulette fields.
+func (sel SelStochRoulette) Validate() error {
+	return nil
+}

selection_test.go

@@ -10,6 +10,7 @@ var (
 		SelElitism{},
 		SelTournament{3},
 		SelRoulette{},
+		SelStochRoulette{},
 	}
 	invalidSelectors = []Selector{
 		SelTournament{0},
@@ -39,23 +40,23 @@ func TestSelectionSize(t *testing.T) {
 
 func TestSelectionUniqueness(t *testing.T) {
 	var (
-		rng = newRand()
-		indis = newIndividuals(3, NewVector, rng)
+		rng       = newRand()
+		indis     = newIndividuals(3, NewVector, rng)
 		selectors = []Selector{
 			SelTournament{
 				NContestants: 2,
 			},
 			SelElitism{},
 		}
 		numIterations = 500
-		anyNotUnique = false
+		anyNotUnique  = false
 	)
 	for _, selector := range selectors {
 		for i := 0; i < numIterations; i++ {
-			var selected, _, _= selector.Apply(2, indis, rng)
-			var unique= false
-			var first= selected[0].Genome.(Vector)
-			var second= selected[1].Genome.(Vector)
+			var selected, _, _ = selector.Apply(2, indis, rng)
+			var unique = false
+			var first = selected[0].Genome.(Vector)
+			var second = selected[1].Genome.(Vector)
 			for index := range first {
 				if first[index] != second[index] {
 					unique = true
@@ -136,6 +137,21 @@ func TestSelRoulette(t *testing.T) {
 	}
 }
 
+func TestSelStochRoulette(t *testing.T) {
+	var (
+		rng   = newRand()
+		indis = newIndividuals(30, NewVector, rng)
+		sel   = SelStochRoulette{}
+	)
+	indis.Evaluate(false)
+	for _, n := range []uint{0, 1, 10, 30} {
+		var selected, _, _ = sel.Apply(n, indis, rng)
+		if len(selected) != int(n) {
+			t.Error("SelStochRoulette didn't select the right number of individuals")
+		}
+	}
+}
+
 // TestSelectorsValidate checks that each selector's Validate method doesn't
 // return an error in case of a valid model and that it does for invalid models.
 func TestSelectorsValidate(t *testing.T) {