Encoding a numeric string into a shortened alphanumeric string, and back again

Question

Quick question. I'm trying to find or write an encoder in Python to shorten a string of numbers by using upper and lower case letters. The numeric strings look something like this:

20120425161608678259146181504021022591461815040210220120425161608667

The length is always the same.

My initial thought was to write some simple encoder to utilize upper and lower case letters and numbers to shorten this string into something that looks more like this:

a26Dkd38JK

That was completely arbitrary, just trying to be as clear as possible. I'm certain that there is a really slick way to do this, probably already built in. Maybe this is an embarrassing question to even be asking.

Also, I need to be able to take the shortened string and convert it back to the longer numeric value. Should I write something and post the code, or is this a one line built in function of Python that I should already know about?

Thanks!

Answer 1

This is a pretty good compression:

import base64

def num_to_alpha(num):
    num = hex(num)[2:].rstrip("L")

    if len(num) % 2:
        num = "0" + num

    return base64.b64encode(num.decode('hex'))

It first turns the integer into a bytestring and then base64 encodes it. Here's the decoder:

def alpha_to_num(alpha):
    num_bytes = base64.b64decode(alpha)
    return int(num_bytes.encode('hex'), 16)

Example:

>>> num_to_alpha(20120425161608678259146181504021022591461815040210220120425161608667)
'vw4LUVm4Ea3fMnoTkHzNOlP6Z7eUAkHNdZjN2w=='
>>> alpha_to_num('vw4LUVm4Ea3fMnoTkHzNOlP6Z7eUAkHNdZjN2w==')
20120425161608678259146181504021022591461815040210220120425161608667