protected function parseExth()
    {
        if (!$this->mobiHeader) {
            return;
        }
        $file = $this->file;
        $file->fseek($this->mobiHeaderStart + $this->mobiHeader->getLength() + 4);
        $this->exthHeader = new ExthHeader($this->readData($file, 4));
        $records = $this->readData($file, 4);
        for ($i = 0; $i < $records; ++$i) {
            $type = $this->readData($file, 4);
            $length = $this->readData($file, 4);
            $data = $length > 0 ? $file->fread($length - 8) : '';
            $this->exthHeader->addRecord(new ExthRecord($type, $length, $data));
        }
    }