cec2020: implement Happycat func

bench/cec2020/basicFunctions.go

@@ -80,7 +80,28 @@ func Griewank(x []float64) float64 {
 func Ackley(x []float64) float64 { return 0 }
 
 // Happycat is the "Happycat Function" of CEC2020.
-func Happycat(x []float64) float64 { return 0 }
+func Happycat(x []float64) float64 {
+	var sum1 float64
+
+	var sum2 float64
+
+	var sum3 float64
+
+	// float64 version of the length of x.
+	fnx := float64(len(x))
+
+	for i := range x {
+		xipow2 := math.Pow(x[i], 2)
+
+		sum1 += xipow2 - fnx
+
+		sum2 += xipow2
+
+		sum3 += x[i]
+	}
+
+	return math.Pow(math.Abs(sum1), 0.25) + (((0.5 * sum2) + sum3) / (fnx + 0.5))
+}
 
 // Discus is the "Discus Function" of CEC2020.
 func Discus(x []float64) float64 {