Inverse of random.shuffle()?

Question

I have a function, for simplicity I'll call it shuffler and it takes an list, gives random a seed 17 and then prints that list shuffled.

def shuffler( n ):
   import random
   random.seed( 17 )
   print( random.shuffle( n ) )

How would I create another function called unshuffler that "unshuffles" that list that is returned by shuffler(), bringing it back to the list I inputted into shuffler() assuming that I know the seed?

Answer 1

Reseed the random generator with the seed in question and then shuffle the list 1, 2, ..., n. This tells you exactly what ended up where in the shuffle.

Answer 2

Here are two functions that do what you need:

import random
import numpy as np

def shuffle_forward(l):
    order = range(len(l)); random.shuffle(order)
    return list(np.array(l)[order]), order

def shuffle_backward(l, order):
    l_out = [0] * len(l)
    for i, j in enumerate(order):
        l_out[j] = l[i]
    return l_out

Example

l = range(10000); random.shuffle(l)
l_shuf, order = shuffle_forward(l)
l_unshuffled  = shuffle_backward(l_shuf, order)

print l == l_unshuffled
#True