(See if you can spot the evil trick.)
Just throw out the rest of the file after the first byte.
...but it turned out to be this.
</sarcasm>
The hard part was finding a way to store the remainder of the division if the data was odd...
Uncompressing simply requires knowing how many compression steps I took. :)
(http://www.patrickcraig.co.uk/other/compression.htm)
EDIT:
(http://www.maximumcompression.com/compression_fun.php)
> Does there exist a file which is extremely compressable by one archiver, but almost incompressable by another one?. I thought the answer was no, but Nimda Admin did sent me a file with these very strange properties. Please download and extract the rarred file strange.rar and try to compress it with (win)rar and (win)zip. You will see compression is RAR is extremely good, but compression in ZIP is almost 0%!. Just when you think you understand compression, someone sends you this file. Thanks Nimda :-)
I'm not so sure, but it leads to interesting ideas.
A genetic algorithm, taking sample chunks from the expanded data, creating dictionaries, compressing, and comparing scores might be a useful approach. (But a poor fit for the hutter prize's restrictions.)
Beautiful.