luhn

go.mod

@@ -0,0 +1,3 @@
+module git.tertyshy.dev/cards
+
+go 1.23.2

lib/validate.go

@@ -0,0 +1,62 @@
+package validate
+
+import (
+	"slices"
+	"strconv"
+	"strings"
+)
+
+type Card struct {
+	Pan   string
+	Month string
+	Year  string
+}
+
+type ValidationResult struct {
+	Code  int
+	Error error
+}
+
+func Luhn(pan string) bool {
+	if pan == "" {
+		return false
+	}
+
+	lastIndex := len(pan) - 1
+
+	asDigits := []int{}
+
+	for _, char := range strings.Split(pan, "") {
+		digit, err := strconv.Atoi(char)
+
+		if err != nil {
+			return false
+		}
+
+		asDigits = append(asDigits, digit)
+	}
+
+	sum := 0
+
+	// do we really need reversed array?
+	for i, digit := range slices.Backward(asDigits[:lastIndex]) {
+
+		if i%2 == len(pan)%2 {
+			if digit > 4 {
+				sum += 2*digit - 9
+			} else {
+				sum += 2 * digit
+			}
+		} else {
+			sum += digit
+		}
+
+	}
+
+	return asDigits[lastIndex] == (10-(sum%10))%10
+}
+
+func Validate(card Card) (bool, ValidationResult) {
+
+	return true, ValidationResult{}
+}

lib/validate_test.go

@@ -0,0 +1,60 @@
+package validate
+
+import "testing"
+
+func TestLuhn(t *testing.T) {
+	tests := []struct {
+		name string
+		pan  string
+		want bool
+	}{
+		{
+			"Mastercard",
+			"5555555555554444",
+			true,
+		},
+		{
+			"Visa",
+			"4111111111111111",
+			true,
+		},
+		{
+			"example from wiki",
+			"17893729974",
+			true,
+		},
+		{
+			"another example",
+			"79927398713",
+			true,
+		},
+		{
+			"worong check digit",
+			"17893729975",
+			false,
+		},
+		{
+			"off by one error",
+			"27893729974",
+			false,
+		},
+		{
+			"empty",
+			"",
+			false,
+		},
+		{
+			"not a digit",
+			"27893a29974",
+			false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := Luhn(tt.pan)
+			if got != tt.want {
+				t.Errorf("Luhn(%s) = %v, want %v", tt.name, got, tt.want)
+			}
+		})
+	}
+}