C:/xampp/htdocs/moodle/lib/zend/Zend/Validate/Barcode/Issn.php

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<?php
require_once 'Zend/Validate/Barcode/AdapterAbstract.php';

class Zend_Validate_Barcode_Issn extends Zend_Validate_Barcode_AdapterAbstract
{
    protected $_length = array(8, 13);

    protected $_characters = '0123456789X';

    protected $_checksum = '_gtin';

    public function checkChars($value)
    {
        if (strlen($value) != 8) {
            if (strpos($value, 'X') !== false) {
                return false;
            }
        }

        return parent::checkChars($value);
    }

    public function checksum($value)
    {
        if (strlen($value) == 8) {
            $this->_checksum = '_issn';
        } else {
            $this->_checksum = '_gtin';
        }

        return parent::checksum($value);
    }

    protected function _issn($value)
    {
        $checksum = substr($value, -1, 1);
        $values   = str_split(substr($value, 0, -1));
        $check    = 0;
        $multi    = 8;
        foreach($values as $token) {
            if ($token == 'X') {
                $token = 10;
            }

            $check += ($token * $multi);
            --$multi;
        }

        $check %= 11;
        $check  = 11 - $check;
        if ($check == $checksum) {
            return true;
        } else if (($check == 10) && ($checksum == 'X')) {
            return true;
        }

        return false;
    }
}