fix: correct Lunacek Bi-Rastrigin func

bench/cec2020/benchFunctions.go

@@ -36,23 +36,14 @@ func LunacekBiRastrigin(x []float64) float64 {
 	d := 1.0
 	mu0 := 2.5
 	mu1 := -math.Sqrt((math.Pow(mu0, 2) - d) / s)
-	xhat := make([]float64, nx)
-	xopt := newXopt(nx, mu0)
+	xhat := getMean(x)
 
 	for i := range x {
-		xhat = append(xhat, 2*x[i])
+		sum0 += math.Pow(xhat-mu0, 2)
 
-		if xopt[i] < 0 {
-			xhat[i] *= -1
-		}
-	}
+		sum1 += math.Pow(xhat-mu1, 2)
 
-	for i := range x {
-		sum0 += math.Pow(xhat[i]-mu0, 2)
-
-		sum1 += math.Pow(xhat[i]-mu1, 2)
-
-		zi := math.Pow(100, 0.5*((float64(i)-1)/fnx-1)) * xhat[i]
+		zi := math.Pow(100, 0.5*((float64(i)-1)/fnx-1)) * xhat
 
 		sum2 += (1 - math.Cos(2*math.Pi*zi))
 	}

bench/cec2020/helperFunctions.go

@@ -9,6 +9,7 @@ import (
 
 	"golang.org/x/exp/rand"
 
+	"gonum.org/v1/gonum/stat"
 	"gonum.org/v1/gonum/stat/distuv"
 )
 
@@ -110,6 +111,7 @@ func GetMaxFES(dim int) int {
 	return -1
 }
 
+// nolint: unused
 func newXopt(n int, mu0 float64) []float64 {
 	gaussDist := &distuv.Normal{
 		Src: rand.NewSource(uint64(time.Now().UnixNano())),
@@ -182,6 +184,12 @@ func getMz(x []float64, z []float64) []float64 {
 	return mz
 }
 
+func getMean(x []float64) float64 {
+	mean := stat.Mean(x, nil)
+
+	return mean
+}
+
 func getWeight(x, o []float64, sigma, nx float64) float64 {
 	var sum float64