cec2020: implement Composition3 func

bench/cec2020/benchFunctions.go

@@ -265,4 +265,43 @@ func Composition2(x []float64) float64 {
 }
 
 // Composition3 is the "Composition Function 3" of CEC2020.
-func Composition3(x []float64) float64 { return 0 }
+func Composition3(x []float64) float64 {
+	nx := len(x)
+	fnx := float64(nx)
+
+	// optimum positions.
+	o := newOpt(nx)
+	sigma := []float64{10, 20, 30, 40, 50}
+	lambda := []float64{10, 1, 10, 1e-6, 1}
+	bias := []float64{0, 100, 200, 300, 400}
+	omega := make([]float64, nx)
+	funcs := []func([]float64) float64{
+		Rastrigin,
+		Happycat,
+		Ackley,
+		Discus,
+		Rosenbrock,
+	}
+
+	var sum float64
+
+	var weights []float64
+
+	for i := range funcs {
+		wi := getWeight(x, o, sigma[i], fnx)
+
+		weights = append(weights, wi)
+
+		var wsum float64
+
+		for j := range weights {
+			wsum += weights[j]
+		}
+
+		omega = append(omega, wi/wsum)
+
+		sum += (omega[i] * (lambda[i]*funcs[i](x) + bias[i])) + funcs[i](x)
+	}
+
+	return sum
+}