cec2020: implement Hybrid2 func

bench/cec2020/benchFunctions.go

@@ -129,7 +129,31 @@ func Hybrid1(x []float64) float64 {
 }
 
 // Hybrid2 is the "Hybrid Function 2" of CEC2020.
-func Hybrid2(x []float64) float64 { return 0 }
+func Hybrid2(x []float64) float64 {
+	nx := len(x)
+	fnx := float64(nx)
+
+	funcs := []func([]float64) float64{
+		SchafferExpanded,
+		HGBat,
+		Rosenbrock,
+		SchwefelModified,
+	}
+	// percentages used to control the amount of contribution of each func.
+	p := []float64{0.2, 0.2, 0.3, 0.3}
+	gnx := getGnx(p, fnx)
+
+	z := newOpt(nx)
+	mz := getMz(x, z)
+
+	var sum float64
+
+	for i := range funcs {
+		sum += funcs[i](mz) * gnx[i]
+	}
+
+	return sum
+}
 
 // Hybrid3 is the "Hybrid Function 3" of CEC2020.
 func Hybrid3(x []float64) float64 { return 0 }