OpenXE/vendor/laminas/laminas-validator/src/Isbn/Isbn10.php

<?php

/**
 * @see       https://github.com/laminas/laminas-validator for the canonical source repository
 * @copyright https://github.com/laminas/laminas-validator/blob/master/COPYRIGHT.md
 * @license   https://github.com/laminas/laminas-validator/blob/master/LICENSE.md New BSD License
 */

namespace Laminas\Validator\Isbn;

class Isbn10
{
    /**
     * @param int|string $value
     * @return int|string
     */
    public function getChecksum($value)
    {
        $sum = $this->sum($value);
        return $this->checksum($sum);
    }

    /**
     * Calculate the value sum.
     *
     * @param int|string $value
     * @return int
     */
    private function sum($value)
    {
        $sum = 0;

        for ($i = 0; $i < 9; $i++) {
            $sum += (10 - $i) * $value[$i];
        }

        return $sum;
    }

    /**
     * Calculate the checksum for the value's sum.
     *
     * @param int $sum
     * @return int|string
     */
    private function checksum($sum)
    {
        $checksum = 11 - ($sum % 11);

        if ($checksum == 11) {
            return '0';
        }

        if ($checksum == 10) {
            return 'X';
        }

        return $checksum;
    }
}