Further "compression" -- you only want the index of the first occurrence of a given string. Thus, you create a table of offsets for each string length, where index 0 is the first unique string of length n, index 1 is the second, etc. This allows you to elide the issue that indices can be arbitrarily large. Of course, the table is hard to compute for large n, and the resulting "compression" has indices at worst the size of the original data... so it's gonna be super efficient at compressing the bits of pi, and no worse than unity on average.