[s369610]

what if instead of using your own dictionary, you use an index into an existing dictionary? such as an index into a subsequence of pi. Couldn't you then find a sequence of bytes in the file in which the index into pi takes less bytes and then replace them all with the index? If you couldn't find any in pi use e or another such number? What am I missing

[raverbashing]

In this case your dictionary either doesn't have everything or to adequately point to it you take as much space as not using it.

While Pi has all pairs of 2 digits, your index would take more space than storing the pairs itself (because you might need to go beyond position 99)

For one situation you might "get lucky" and find a coincidence, but this won't scale generically

[gbl08ma]

See also: https://github.com/philipl/pifs and https://github.com/philipl/pifs/issues/23

I admit this fooled me for a bit. Good news is, I won't be fooled again by something similar :)