cec2020: implement Rastrigin func + test

bench/cec2020/basicFunctions.go

@@ -5,8 +5,16 @@ package cec2020
 
 import "math"
 
-// Rastrigin computes the value of the Rastrigin function for x.
-func Rastrigin(x []float64) float64 { return 0 }
+// Rastrigin calculates the value of the Rastrigin function for x.
+func Rastrigin(x []float64) float64 {
+	var sum float64
+
+	for i := range x {
+		sum += math.Pow(x[i], 2) - (10 * math.Cos(2*math.Pi*x[i])) + 10
+	}
+
+	return sum
+}
 
 // HighConditionedElliptic is the "High Conditioned Elliptic Function" of
 // CEC2020.

bench/cec2020/basicFunctions_test.go

@@ -0,0 +1,37 @@
+// Copyright 2023 wanderer <a_mirre at utb dot cz>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package cec2020
+
+import "testing"
+
+// TestRastrigin checks that the Rastrigin func performs as expected.
+func TestRastrigin(t *testing.T) {
+	testInput := []float64{
+		-2.7137778065280593,
+		-1.9789783390895765,
+		-4.38267902085263,
+		-4.617877608383319,
+		-2.1529909200665474,
+		-3.801849324611571,
+		2.4968697503272486,
+		-0.27358100201730196,
+		2.0444319451255977,
+		3.8798952350630653,
+		-0.6150024279011337,
+		2.8643010641852413,
+		-0.6692444051928748,
+		4.4649633199687475,
+		3.2510298731558507,
+		-4.2095036422081495,
+		-0.130882052243404,
+		1.2001716295708604,
+		-0.29427650577602193,
+	}
+	want := 343.97127081044846
+	got := Rastrigin(testInput)
+
+	if want != got {
+		t.Errorf("incorrect Rastrigin output, want: %f, got: %f", want, got)
+	}
+}