// Package luhn is for validating per the Luhn formula.
package luhn
import (
	"log"
	"strconv"
	"strings"
	"unicode"
)
// Valid returns true if input is valid according to Luhn formula, false otherwise.
func Valid(input string) bool {
	input = strings.ReplaceAll(input, " ", "")
	if len(input) <= 1 {
		return false
	}
	for _, ch := range input {
		if !unicode.IsDigit(ch) {
			return false
		}
	}
	return luhn(input)%10 == 0
}
func luhn(input string) int {
	var sum int
	var second bool
	for i := len(input) - 1; i >= 0; i-- {
		digit, err := strconv.Atoi(string(input[i]))
		if err != nil {
			log.Panicf("error converting %q to int: %s", input[i], err)
		}
		if !second {
			sum += digit
			second = true
			continue
		}
		digit *= 2
		if digit > 9 {
			digit -= 9
		}
		sum += digit
		second = false
	}
	return sum % 10
}