16.py

def zw(f, *args):
  return [f(a) for a in zip(*args)]

def j(l, joiner=''):
  return joiner.join(l)

def substrs(s, n):
    ls = [s[i::n] for i in xrange(n)]
    return zw(j, *ls)

def converge(f, e):
    last = e
    e = f(e)
    while e != last:
        last = e
        e = f(e)
    return e

def step(a):
  b = a[::-1]
  b = j('1' if x == '0' else '0' for x in b)
  return a + '0' + b

def is_same(n):
  if n[0] == n[1]:
    return '1'
  return '0'

def checksum_step(s):
  if len(s) % 2 != 0:
    return s
  subs = substrs(s, 2)
  return j(is_same(x) for x in subs)

def checksum(s):
  return converge(checksum_step, s)

gen = '10001001100000001'
size = 272
while len(gen) < size:
  gen = step(gen)
g = gen[:size]
print 'part a', checksum(g)
size = 35651584
while len(gen) < size:
  gen = step(gen)
g = gen[:size]