luhn.go

package checkdigit

type luhn struct{}

// Verify implements checkdigit.Verifier interface.
func (l luhn) Verify(code string) bool {
	if len(code) < 2 {
		return false
	}
	i, err := l.Generate(code[:len(code)-1])

	return err == nil && i == int(code[len(code)-1]-'0')
}

// Generate implements checkdigit.Generator interface.
func (l *luhn) Generate(seed string) (int, error) {
	if seed == "" {
		return 0, ErrInvalidArgument
	}

	sum, parity := 0, (len(seed)+1)%2
	for i, n := range seed {
		if isNotNumber(n) {
			return 0, ErrInvalidArgument
		}
		d := int(n - '0')
		if i%2 == parity {
			d *= 2
			if d > 9 {
				d -= 9
			}
		}
		sum += d
	}

	return sum * 9 % 10, nil
}