Suppose that you hash a string in python using a custom-made hash function named sash().
sash("hello world") returns something like 2769834847158000631.
What code (in python) would implement sash() function and a unsash() function such that unsash(sash("hello world")) returns "hello world"?
If you like, assume that the string contains ASCII characters only.
There are 128 ASCII characters.
Thus, each python string is like a natural number written in base 128.
A hash is fixed in size, whereas a string is not. Therefore there will be more more possible strings than hash values, making it impossible to reverse.
In your example, you have an 11-character string containing 77 bits. Your corresponding integer would fit in 64 bits (actually 62 bits, but I will take 64 bits as what you might have been imagining). If we consider only 11-character strings (obviously there are far more), we have 277 possible strings. Assuming a 64-bit hash, there are only 264 hash values. Each hash value would have, on average, 8192 strings that map to it. So given just the hash value, you would have no idea which of those 8192 strings to decode it to.
If you don't mind a hash of unbounded size, then sure, you can simply consider the string itself to be the hash. Then no decoding required. You can get a little fancier, since you are limiting the characters to 0..127, and pack seven bits for each character into a string of bytes, reducing the size by 1/8th. This is effectively the base-128 number you are referring to. You may be able to get it smaller with compression if your 0..127 characters do not have the same probability. Then on average, the string can be compressed, with some possible strings necessarily getting larger instead of smaller.
