It's easy to come up with much more compact schemes that are still completely generic. For example, to represent a variable length number use xxx0, 0xxxxxx1, 0

It's easy to come up with much more compact schemes that are still completely generic. For example, to represent a variable length number use xxx0, 0xxxxxx1, 0xxxxxx1xxxxxx1, etc. Normally you need a pair of these to specify (version,function). But if we assume functions are packed, you can have a run-length scheme (run-length,starting-function-number). So "function 1 is on version 5, function 2 on version 3, function 3 on version 7, and function 4 on version 1" is four functions starting with 1 = (1000,0010),1010,0110,1110,0010 = 24 bits. It gets better quick with larger numbers of functions. BTW, the size of the prime scheme is log2(nthprime(function))*version bits for each function. If you don't know ahead of time which functions there might be, then you have to do a prime factorization, which is a hard problem. I guess if you used really large numbers you could have a cryptographically secure way of indicating which versions of which functions are in your library.