protected function _decodeInt24($source)
    {
        if (strlen($source) != 4) {
            throw new InvalidArgumentException('Source must be exactly 4 bytes');
        }

        $integer  = strpos($this->_itoa64, $source{0});
        $integer += (strpos($this->_itoa64, $source{1}) << 0x06);
        $integer += (strpos($this->_itoa64, $source{2}) << 0x0c);
        $integer += (strpos($this->_itoa64, $source{3}) << 0x12);

        return $integer;
    }