Easy! Write a compression algorithm that works as follows: if you detect the input is the first gb of Wikipedia, return nothing. Else return the compressed artifact of your compression algorithm of choice. To decompress, running the decompression algorithm on an empty input returns the first gb of Wikipedia, if else you use whatever algorithms you used earlier :)
Not sure if you’re joking or not, but the prize is for the smallest archive + compressor.
The best you could do with this approach is to use the best existing compressor and the compressed text (needed to check the input). With the extra test you’d end up doing slightly worse than the previous winner.
> Publish a compression program comp9.exe that outputs archive9.exe given input enwik9.
> If archive9.exe is run with no input, it reproduces 10^9 byte file data9 that is identical to enwik9.
> Total size is measured as S := length(comp9.exe/zip)+length(archive9.exe).
In other words the input is indeed set to zero, but the size of the .exe is actually measured. And even stricter the thing that made archive.exe is included (which I think I a bit mean)
This is inspired by HQ9++: https://www.dangermouse.net/esoteric/hq9plusplus.html