In general, do that then run some compression on it. That will do better if the data wasn't random. Or if you're expecting random data, invent a biased compres

In general, do that then run some compression on it. That will do better if the data wasn't random. Or if you're expecting random data, invent a biased compression algorithm and use that (e.g. one that has shortcuts for storing any multiples of 3). Most of the time it will do nothing and a few times you'll randomly get data it's good at. Actually on second thought I'm not sure if that works or not. You'll need a header to identify if the compression was used or not. So the compression has to save more on average than this header info costs. This header problem becomes clearer if you try to chain thousands of special-case compression algorithms that map a single input to one bit and otherwise are unused. Seems to save space at first (at cost of CPU time, and space to store these algorithms), but actually identifying which are used or not will be a problem. Since they all need unique headers, you need just as many header possibilities as there are numbers in the range, so you must as well just use the numbers for the headers, at which case you're actually not doing anything since the header is the data.