Info - HowTo write to specific location in a file using XPCOM

I'm writing torrents to disk, so a lot data is written to files out-of-order. During setup, I pre-allocate the file space by opening a file and moving to the position the last byte would be, using the nsISeekableStream interface. This technique I picked up from DownThemAll's manager class.
When I receive a piece of a file, I need to put it in the right place. What I was doing was moving to the right place, then writing:

seekstream.seek(0x00, offset);
stream.write(data, data.length);

This failed, somehow I was overwriting/truncating what was in the file. The solution was to first move to the end of the file, then move to the offset.

function insertInFile(filename, offset, data) {
var file = Components.classes["@mozilla.org/file/local;1"]
           .createInstance(Components.interfaces.nsILocalFile);
file.initWithPath( filename );
var stream = Components.classes["@mozilla.org/network/file-output-stream;1"]
            .createInstance(Components.interfaces.nsIFileOutputStream);
stream.init(file, 0x04 | 0x08, 0664, 0); // readwrite | create
var seekstream = stream.QueryInterface(Components.interfaces.nsISeekableStream);
seekstream.seek(0x02, 0); // move to end of file
seekstream.seek(0x01, offset - seekstream.tell() ); // move to where we actually want to write
stream.write(data, data.length);
stream.close();
}

I'll leave with a note that in my tests I've been seeing a byte with the value 0x0A at the end of my files. If I find that's to do with something I've done here, I'll try to remember to update this post. Just watch out for it if you're using the above method.

Upcoming later today - 5th Release of Bittorrent Firefox Extension with fixes Not happening. Latest version has a critical bug with reconnecting to it's own server.