Decompression and extraction of files from streaming archive on the fly

Question

I'm writing a browser plugin, similiar to Flash and Java in that it starts downloading a file (.jar or .swf) as soon as it gets displayed. Java waits (I believe) until the entire jar files is loaded, but Flash does not. I want the same ability, but with a compressed archive file. I would like to access files in the archive as soon as the bytes necessary for their decompression are downloaded.

For example I'm downloading the archive into a memory buffer, and as soon as the first file is possible to decompress, I want to be able to decompress it (also to a memory buffer).

Are there any formats/libraries that support this?

EDIT: If possible, I'd prefer a single file format instead of separate ones for compression and archiving, like gz/bzip2 and tar.

Answer 1

Sure, zlib for example uses z_stream for incremental compression and decompression via functions inflateInit, inflate, deflateInit, deflate. libzip2 has similar abilities.

For incremental extraction from the archive (as it gets deflated), look e.g. to the good old tar format.

Answer 2

Check out the boost::zlib filters. They make using zlib a snap.

Here's the sample from the boost docs that will decompress a file and write it to the console:

#include <fstream>
#include <iostream>
#include <boost/iostreams/filtering_streambuf.hpp>
#include <boost/iostreams/copy.hpp>
#include <boost/iostreams/filter/zlib.hpp>

int main() 
{
    using namespace std;

    ifstream file("hello.z", ios_base::in | ios_base::binary);
    filtering_streambuf<input> in;
    in.push(zlib_decompressor());
    in.push(file);
    boost::iostreams::copy(in, cout);
}

Answer 3

There are 2 issues here

1) How to write the code.

2) What format to use.

On the file format, You can't use the .ZIP format because .ZIP puts the table of contents at the end of the file. That means you'd have to download the entire file before you can know what's in it

Gzipped tar files don't have this problem. Tar files are stored header, file, header file, and the compression is on top of that so it's possible to decompress as the file is downloaded and use the files as they become available. You can create gzipped tar files easily in windows using winrar (commerical) or 7-zip (free) and on linux, osx and cygwin use the tar command.

On the code to write,

O3D does this and is open source so you can look at the code http://o3d.googlecode.com

The decompression code is in o3d/import/cross/...

It targets the NPAPI using some glue which can be found in o3d/plugin/cross