cec2020: implement HGBat func

bench/cec2020/basicFunctions.go

@@ -32,7 +32,22 @@ func HighConditionedElliptic(x []float64) float64 {
 }
 
 // HGBat is the "HGBat Function" of CEC2020.
-func HGBat(x []float64) float64 { return 0 }
+func HGBat(x []float64) float64 {
+	var sum1 float64
+
+	var sum2 float64
+
+	for i := range x {
+		sum1 += math.Pow(x[i], 2)
+
+		sum2 += x[i]
+	}
+
+	// float64 version of the length of x.
+	fnx := float64(len(x))
+
+	return math.Pow(math.Abs(math.Pow(sum1, 2)-math.Pow(sum2, 2)), 0.5) + (((0.5 * sum1) + sum2) / (fnx + 0.5))
+}
 
 // Rosenbrock is the "Rosenbrock's Function" of CEC2020.
 // ref: https://infinity77.net/global_optimization/test_functions_nd_R.html#go_benchmark.Rosenbrock