Zip Writer

One of the annoying omissions from the Mozilla platform has always been the inability to create a zip file. It’s been bugging me for some time since it’s the only way for Nightly Tester Tools to properly manage overriding compatibility for an individual extension install, without doing dangerous things like it used to. There’s been a Google Summer of Code project on creating such functionality but it didn’t get all that far.

Well Saturday night I finally bit the bullet, after talking about it for a long time, and took a crack at it myself. I didn’t like the API’s presented in the SoC project so I decided to just start from scratch andknock up something really really simple. And when I say simple, I mean that initially all this thing was going to do was create a zip file (no editing existing ones), and add files and folders to it, with no compression. That makes it simple see, just add a few file headers here and there and all you’re doing is copying data around.

Rather surprisingly it almost worked. Lots of problems handling CRC calculations in JavaScript, but got there in the end. Well to a point. It managed to write out zip files when the files added were not too binary in nature. I made the mistake of attempting all this in JavaScript which it turns out doesn’t work with binary data too well. Particularly not when the Mozilla interfaces I used pass then around as char* data types which js then turns into a unicode string.

So onto the second implementation in C++ this time. I guess it was probably inevitable that it would come to it, but JS at least is real quick to prototype things like this in. The C++ implementation now works as well as I’d initially hoped, so since then I’ve of course got bored and started looking into having real compression in there and maybe editing zip files in the future. Like most of my code all this is open source, you can view it in my subversion repository. What’s more, since I moved on, Mook has taken the JS implementation, plumbed in some cleverness and made it work for binary files too!

Obviously the code is to be used at your own risk and right now it’s not all that thoroughly tested. Hopefully though if I can convince bsmedberg of my API’s, this code might end up making it onto trunk so everyone can use it.