Brochure pages calculation

from argparse import ArgumentParser, Action
import re


class RangeAction(Action):
    def __call__(self, parser, namespace, values, option_string=None):
        xs = []
        for v in values:
            try:
                m = re.match(r'(\d+)(?:\.\.(\d+))?$', v)
                if m:
                    start = m.group(1)
                    end = m.group(2) or start
                    xs.extend(list(range(int(start, 10), int(end, 10) + 1)))
                else:
                    xs.append(int(v))
            except ValueError:
                continue
        flat_xs = sorted(list(set(xs)))
        flat_xs.reverse()
        setattr(namespace, self.dest, flat_xs)


def run():
    parser = ArgumentParser(description='Calculate brochure pages.')
    parser.add_argument('--xs', metavar='N', action=RangeAction, nargs='*',
                        help='integers for the accumulator')
    args = parser.parse_args()
    print(calc(args.xs))


# duplex short side!
# 3, 51..101
def calc(pages):
    n0 = len(pages)
    last = pages[-1]
    # ensure pages is divisible by 4
    n = n0 if n0 % 4 == 0 else (n0 // 4 + 1) * 4
    xs = pages + [last] * (n - n0)
    return split(xs, True)


# xs is guaranteed to have 4k elements
def split(xs, duplex=False):
    n = (len(xs) + 1) // 2
    if not duplex:
        # xs[1::2]  starting from 1 iterating by 2
        (front, back) = split_fb(xs)
        zs = flatten([[x, y] for (x, y) in zip(front, back)])
        return zs[:n], zs[n:]
    else:
        # [1,2,3,4,5,6,7,8] -> ([8,7,6,5],[1,2,3,4])
        (front, back) = reversed(xs[n:]), xs[:n]
        (front2, back2) = list(pairwise(front)), list(pairwise(back))
        f = lambda zs1, zs2: [zs1[0], zs2[0], zs2[1], zs1[1]]
        blocks = [f(zs1, zs2) for (zs1, zs2) in zip(front2, back2)]
        zs = flatten(blocks)
        return zs


def split_fb(xs):
    (x2s, y2s) = xs[1::2], xs[::2]
    return list(reversed(y2s)), list(x2s)


def pairwise(xs):
    p = []
    for x in xs:
        if len(p) < 2:
            p.append(x)
        else:
            yield p[:]
            p = [x]
    yield p


def flatten(xs):
    return [x for x0s in xs for x in x0s]


if __name__ == "__main__":
    run()