// Package isbn validates ISBN strings.
package isbn
import (
	"log"
	"strconv"
	"strings"
	"unicode"
)
const validLength = 10
// IsValidISBN returns true for valid ISBN numbers, false otherwise.
func IsValidISBN(input string) bool {
	input = strings.ReplaceAll(input, "-", "")
	if len(input) != validLength {
		return false
	}
	sum := 0
	for i, d := range input {
		if d == 'X' && i == validLength-1 {
			sum += 10
			break
		}
		if !unicode.IsDigit(d) {
			return false
		}
		tmp, err := strconv.Atoi(string(d))
		if err != nil {
			log.Panicln(err)
		}
		sum += (validLength - i) * tmp
	}
	return sum%11 == 0
}