cec2020: implement Weierstrass func

bench/cec2020/basicFunctions.go

@@ -167,4 +167,30 @@ func Schaffer(x []float64) float64 { return 0 }
 
 // Weierstrass is the "Weierstrass Function" of CEC2020 with a=0.5, b=3 and
 // kmax=20.
-func Weierstrass(x []float64) float64 { return 0 }
+func Weierstrass(x []float64) float64 {
+	a := 0.5
+	b := 3.0
+	kmax := 20.0
+	// float64 version of the length of x.
+	fnx := float64(len(x))
+
+	var sum1 float64
+
+	var sum2 float64
+
+	for i := range x {
+		var ksum float64
+
+		for k := 0.0; k < kmax; k++ {
+			ksum += math.Pow(a, k) * math.Cos(2*math.Pi*math.Pow(b, k)*(x[i]+0.5))
+		}
+
+		sum1 += ksum
+	}
+
+	for k := 0.0; k < kmax; k++ {
+		sum2 += math.Pow(a, k) * math.Cos(2*math.Pi*math.Pow(b, k)*0.5)
+	}
+
+	return sum1 - (fnx * sum2)
+}