package luhn

func IsValid(digits []int) bool {
	sum := 0
	alt := false
	for i := len(digits) - 1; 0 <= i; i-- {
		d := digits[i]
		if alt {
			d *= 2
			if d > 9 {
				d -= 9
			}
		}
		sum += d
		alt = !alt
	}
	return sum%10 == 0
}