cec2020: implement HighConditionElliptic func

bench/cec2020/basicFunctions.go

@@ -18,7 +18,18 @@ func Rastrigin(x []float64) float64 {
 
 // HighConditionedElliptic is the "High Conditioned Elliptic Function" of
 // CEC2020.
-func HighConditionedElliptic(x []float64) float64 { return 0 }
+func HighConditionedElliptic(x []float64) float64 {
+	var sum float64
+
+	// float64 version of the length of x.
+	fnx := float64(len(x))
+
+	for i := range x {
+		sum += math.Pow(math.Pow(10, 6), float64(i-1)/(fnx-1)) * math.Pow(x[i], 2)
+	}
+
+	return sum
+}
 
 // HGBat is the "HGBat Function" of CEC2020.
 func HGBat(x []float64) float64 { return 0 }