cec2020: implement SchwefelModified func

bench/cec2020/basicFunctions.go

@@ -3,6 +3,8 @@
 
 package cec2020
 
+import "math"
+
 // Rastrigin computes the value of the Rastrigin function for x.
 func Rastrigin(x []float64) float64 { return 0 }
 
@@ -33,7 +35,31 @@ func Discus(x []float64) float64 { return 0 }
 // g(zi) = zi * sin(|zi|^(1/2)) ... if |zi| <= 500,
 // g(zi) = (500-mod(zi,500)) * sin(sqrt(|500-mod(zi,500)|)) - (zi-500^2)/10000D ... if zi > 500,
 // g(zi) = (mod(|zi|,500)-500) * sin(sqrt(|mod(|zi|,500)-500|)) - (zi-500^2)/10000D ... if zi < -500.
-func SchwefelModified(x []float64) float64 { return 0 }
+func SchwefelModified(x []float64) float64 {
+	var sum float64
+
+	// float64 version of the length of x.
+	fnx := float64(len(x))
+
+	for i := range x {
+		zi := x[i] + 4.209687462275036e+002
+
+		switch {
+		case zi > 500:
+			// g(zi)
+			sum += (500-math.Mod(zi, 500))*math.Sin(math.Sqrt(math.Abs(500-math.Mod(zi, 500)))) - (math.Pow(zi-500.0, 2.0) - 10000*fnx)
+
+		case zi < -500:
+			// g(zi)
+
+		case math.Abs(zi) <= 500:
+			// g(zi)
+			sum += zi * math.Sin(math.Pow(math.Abs(zi), 0.5))
+		}
+	}
+
+	return 418.9829*fnx - sum
+}
 
 // Schaffer is the "Expanded Schaffer's Function" of CEC2020.
 func Schaffer(x []float64) float64 { return 0 }