Add LUHN validation checker and tests to v2. (#138)

# Describe Request

Add LUHN validation checker and tests to v2.

# Change Type

New code.

v2/luhn.go

@@ -0,0 +1,61 @@
+// Copyright (c) 2023-2024 Onur Cinar.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+// https://github.com/cinar/checker
+
+package v2
+
+import (
+	"reflect"
+	"unicode"
+)
+
+const (
+	// nameLUHN is the name of the LUHN check.
+	nameLUHN = "luhn"
+)
+
+var (
+	// ErrNotLUHN indicates that the given value is not a valid LUHN number.
+	ErrNotLUHN = NewCheckError("LUHN")
+)
+
+// IsLUHN checks if the value is a valid LUHN number.
+func IsLUHN(value string) (string, error) {
+	var sum int
+	var alt bool
+
+	for i := len(value) - 1; i >= 0; i-- {
+		r := rune(value[i])
+		if !unicode.IsDigit(r) {
+			return value, ErrNotLUHN
+		}
+
+		n := int(r - '0')
+		if alt {
+			n *= 2
+			if n > 9 {
+				n -= 9
+			}
+		}
+		sum += n
+		alt = !alt
+	}
+
+	if sum%10 != 0 {
+		return value, ErrNotLUHN
+	}
+
+	return value, nil
+}
+
+// checkLUHN checks if the value is a valid LUHN number.
+func checkLUHN(value reflect.Value) (reflect.Value, error) {
+	_, err := IsLUHN(value.Interface().(string))
+	return value, err
+}
+
+// makeLUHN makes a checker function for the LUHN checker.
+func makeLUHN(_ string) CheckFunc[reflect.Value] {
+	return checkLUHN
+}

v2/luhn_test.go

@@ -0,0 +1,76 @@
+// Copyright (c) 2023-2024 Onur Cinar.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+// https://github.com/cinar/checker
+
+package v2_test
+
+import (
+	"fmt"
+	"testing"
+
+	v2 "github.com/cinar/checker/v2"
+)
+
+func ExampleIsLUHN() {
+	_, err := v2.IsLUHN("79927398713")
+	if err != nil {
+		fmt.Println(err)
+	}
+}
+
+func TestIsLUHNInvalid(t *testing.T) {
+	_, err := v2.IsLUHN("123456789")
+	if err == nil {
+		t.Fatal("expected error")
+	}
+}
+
+func TestIsLUHNValid(t *testing.T) {
+	_, err := v2.IsLUHN("79927398713")
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestCheckLUHNNonString(t *testing.T) {
+	defer FailIfNoPanic(t, "expected panic")
+
+	type Card struct {
+		Number int `checkers:"luhn"`
+	}
+
+	card := &Card{}
+
+	v2.CheckStruct(card)
+}
+
+func TestCheckLUHNInvalid(t *testing.T) {
+	type Card struct {
+		Number string `checkers:"luhn"`
+	}
+
+	card := &Card{
+		Number: "123456789",
+	}
+
+	_, ok := v2.CheckStruct(card)
+	if ok {
+		t.Fatal("expected error")
+	}
+}
+
+func TestCheckLUHNValid(t *testing.T) {
+	type Card struct {
+		Number string `checkers:"luhn"`
+	}
+
+	card := &Card{
+		Number: "79927398713",
+	}
+
+	_, ok := v2.CheckStruct(card)
+	if !ok {
+		t.Fatal("expected valid")
+	}
+}

v2/maker.go

@@ -26,6 +26,7 @@ var makers = map[string]MakeCheckFunc{
 	nameIPv4:         makeIPv4,
 	nameIPv6:         makeIPv6,
 	nameISBN:         makeISBN,
+	nameLUHN:         makeLUHN,
 	nameMaxLen:       makeMaxLen,
 	nameMinLen:       makeMinLen,
 	nameRequired:     makeRequired,