private static function getPool($type = 'alnum')
    {
        switch ($type) {
            case 'alnum':
                $pool = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
                break;
            case 'alpha':
                $pool = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
                break;
            case 'hexdec':
                $pool = '0123456789abcdef';
                break;
            case 'numeric':
                $pool = '0123456789';
                break;
            case 'nozero':
                $pool = '123456789';
                break;
            case 'distinct':
                $pool = '2345679ACDEFHJKLMNPRSTUVWXYZ';
                break;
            default:
                $pool = (string) $type;
                break;
        }

        return $pool;
    }