How to design a sequential hash-like function

Question

I want to develop something similar to jsfiddle in where the user can input some data and then \"save\" it and get a unique random looking url that loads that data.

Answer 1

This can't really be reversible. The only way (the one used by url shorteners and jsfiddle) is to store the generated hash (actually it's a digest) in a table/data structure of some sort and *look it up on retrieval.

Why this?

Passing from, e.g. 128 chars of data → a 4 visible char digest, you lose a lot of data.
You cannot store the remaining data in the magical cracks betweeen those 4 bytes, there are none.