day10-part2.py

import sys

def read(xs, pos, length):
    ys = xs + xs
    pos = pos % len(xs)
    return ys[pos:pos+length]

def write(xs, pos, ys):
    for i, y in enumerate(ys):
        xs[(i + pos) % len(xs)] = y

def hash1(lengths, xs, pos, skip):
    xs = xs + [] # Make a copy.
    for length in lengths:
        assert length < 256
        write(xs, pos, reversed(read(xs, pos, length)))
        pos += length + skip
        skip += 1
    return xs, pos, skip

def groups(xs, n):
    "Ignores leftovers."
    xss = []
    for k in range(len(xs) // n):
        xss.append(xs[k * n : (k + 1) * n])
    return xss

def xor_all(xs):
    r = 0
    for x in xs:
        r ^= x
    return r

lengths = map(ord, sys.stdin.readline().strip()) + [17, 31, 73, 47, 23]

xs = range(256)
pos = 0
skip = 0
for _ in range(64):
    (xs, pos, skip) = hash1(lengths, xs, pos, skip)

dense = map(xor_all, groups(xs, 16))
hash = ""
for value in dense:
    hash += "%02x" % value

print hash