Question

I’m working on an NMDC client (p2p, DC++ and friends) with Qt. The protocol itself is pretty straightforward:

$command parameters|

Except for compression:

“ZPipe works by sending a command $ZOn| to the client. After $ZOn a ZLib compressed stream containing commands will follow. This stream will end with an EOF that ZLib defines. (there is no $ZOff in the compressed stream!)”

Here’s the relevant code:

QTcpSocket *conn;
bool compressed;
QByteArray zbuffer;
QByteArray buffer;

// ...

void NMDCConnection::on_conn_readyRead() {
    // this gets called whenever we get new data from the hub

    if(compressed) {            // gets set when we receive $ZOn
        zbuffer.append(conn->readAll());


        // Magic happens here


        if( stream_is_complete ) {
            buffer.append(uncompressed_stream);
            buffer.append(remainder_of_data);
            compressed = false;
        }
    } else { 
        buffer.append(conn->readAll());
    };
    parse(buffer);
}

So, how do I get the values for stream_is_complete, uncompressed_stream, and remainder_of_data? I can’t look for the next ‘$’ because the stream can contain it. I tried looking for something resembling an EOF in the zlib documentation, but there is no such thing, in fact, every stream ends with a seemingly random character.

I also played around with qUncompress(), but that wants a complete stream, nothing less, nothing more.

Answer 1

Are you using zlib directly?

Totally untested…

z_stream zstrm;
QByteArray zout;
// when you see a $ZOn|, initialize the z_stream struct
parse() {
    ...
    if (I see a $ZOn|) {
        zstrm.next_in = Z_NULL;
        zstrm.avail_in = 0;
        zstrm.zalloc = Z_NULL;
        zstrm.zfree = Z_NULL;
        zstrm.opaque = 0;
        inflateInit(&zstrm);
        compressed = true;
    }
}
void NMDCConnection::on_conn_readyRead() {
    if (compressed) {
        zbuffer.append(conn->readAll());
        int rc;
        do {
            zstrm.next_in = zbuffer.data();
            zstrm.avail_in = zbuffer.size();
            zout.resize(zstrm.total_out + BLOCK_SIZE);
            zstrm.next_out = zout.data() + zstrm.total_out;
            zstrm.avail_out = BLOCK_SIZE;
            rc = inflate(&zstrm, Z_SYNC_FLUSH);
            zbuffer.remove(0, zstrm.next_in - zbuffer.data());
        } while (rc == Z_OK && zstrm->avail_out == 0);
        if (rc == Z_STREAM_END) {
            zout.truncate(zstrm.total_out);
            buffer.append(zout);
            zout.clear();
            buffer.append(zbuffer);
            zbuffer.clear();
            compress = false;
            inflateEnd(&zstrm);
        }
        else if (rc != Z_OK) {
            // ERROR!  look at zstrm.msg
        }
    }
    else // whatever
}

This incrementally decompresses (inflates) from qbuffer to qout, and stops when inflate says “no more”.

Maybe it would be better to borrow from QuaZip instead.