Another way is to keep the file secret and just output a score, but if you want to use a public file, you have to include the program in the calculated size.
I don't think GP was asking about the decompressor, but specifically the compressor.
The FAQ has a fairly detailed answer for this though: http://prize.hutter1.net/hfaq.htm#addcomp
Another way is to keep the file secret and just output a score, but if you want to use a public file, you have to include the program in the calculated size.