Three Rail Decrypt when rails are different lengths

Question

Im having a ton of trouble trying to figure out how to get my decrypt function to work. It works in ideal cases where the msg is divisible by 3, but after that i am totally lost

Answer 1

You can slice strings (or any other sequence) using a stride; you can select every 3rd character with that:

def threeRailEncrypt(plaintext):
    return plaintext[::3] + plaintext[1::3] + plaintext[2::3]

To reverse the trend, slice up the ciphertext into three chunks and use itertools.zip_longest() to recombine them again:

from itertools import zip_longest, chain

def threeRailDecrypt(ciphertext):
    stride, remainder = divmod(len(ciphertext), 3)
    # how large was each of the three sections?
    sizes = [stride] * 3
    for i in range(remainder):
        sizes[i] += 1
    # slice ciphertext up into 3 sections again
    pos = 0
    pieces = []
    for s in sizes:
        pieces.append(ciphertext[pos:pos + s])
        pos += s
    # recombine the triplets
    return ''.join(chain.from_iterable(zip_longest(*pieces, fillvalue='')))

Demo:

>>> threeRailEncrypt('Foo bar baz and all')
'F raa lob znaoab dl'
>>> threeRailDecrypt(threeRailEncrypt('Foo bar baz and all'))
'Foo bar baz and all'

The last part without zip_longest would be:

# recombine the triplets
plaintext = []
for i in range(len(parts[0])):
    for part in parts:
        try:
            plaintext.append(part[i])
        except IndexError:  # part too short
            pass
return ''.join(plaintext)