Picqer\Barcode\BarcodeGenerator::checksum_s25 PHP Method

    protected function checksum_s25($code)
    {
        $len = strlen($code);
        $sum = 0;
        for ($i = 0; $i < $len; $i += 2) {
            $sum += $code[$i];
        }
        $sum *= 3;
        for ($i = 1; $i < $len; $i += 2) {
            $sum += $code[$i];
        }
        $r = $sum % 10;
        if ($r > 0) {
            $r = 10 - $r;
        }
        return $r;
    }