luhn.go

// Information about the algorithm is available on Wikipedia
//
// https://en.wikipedia.org/wiki/Luhn_algorithm
//
package luhn

import "fmt"

var m = [10]uint{0, 2, 4, 6, 8, 1, 3, 5, 7, 9}
var zero = uint('0')

// Digit returns luhn digit for given numeric string
func Digit(cc string) (uint, error) {
	var (
		i int = len(cc) - 1
		x uint
		d uint
	)

loop:
	if i < 0 {
		x = (10 - (x - (x/10)*10))
		if x == 10 {
			return 0, nil
		}
		return x, nil
	}
	d = uint(cc[i]) - zero
	if d > 9 {
		return 1, fmt.Errorf("string must contain only digits")
	}
	switch i & 1 {
	case 1:
		x += d
	default:
		x += m[d]
	}
	i--
	goto loop
}

// Validate returns true if numeric string is signed with valid luhn digit
func Validate(cc string) (ok bool) {
	digit, err := Digit(cc)
	return err == nil && digit == 0
}

// Generate signs string with luhn digit
func Generate(cc string) (string, error) {
	digit, err := Digit(cc)
	if err != nil {
		return cc, err
	}
	return cc + string(rune(digit+zero)), nil
}