Samuel Merritt.py

#!/usr/bin/env python

from itertools import product, repeat
import json
import sys

PLAYERS = ['X', 'O']
RUN_LENGTH = 4

def value_at(board, coordinate):
    value = board
    for dimension in coordinate:
        value = value[dimension]
    return value

def vector_add(v1, v2):
    return map(lambda x1, x2: x1 + x2,
               v1, v2)

def vector_scale(factor, vector):
    return [factor * x for x in vector]

def is_run(board, start, step):
    try:
        player = value_at(board, start)
        return all(map(lambda c: value_at(board, c) == player,
                       (vector_add(vector_scale(x, step),
                                   start)
                        for x in range(1, RUN_LENGTH))))
    except IndexError:
        return False
            
def coordinates(arr):
    return product(*[range(x) for x in arr])

def find_dimensions(arr):
    if not isinstance(arr[0], list):
        return [len(arr)]
    else:
        dimensions = [find_dimensions(x) for x in arr]
        return ([len(arr)] + map(max, zip(*dimensions)))

def find_winner(board):
    board_dimensions = find_dimensions(board)

    # XXX this could stand to be a lot smarter; by going backwards and
    # forwards, we test each candidate twice.
    steps = [step for step in product(*repeat([-1, 0, 1],
                                              len(board_dimensions)))
             if any(step)] # filter out (0, 0, ..., 0)
    for start in coordinates(board_dimensions):
        for step in steps:
            try:
                if value_at(board, start) in PLAYERS and is_run(board, start, step):
                    return value_at(board, start)
            except IndexError:
                pass
                

if __name__ == '__main__':
    board = json.loads(sys.stdin.read())
    winner = find_winner(board)
    if winner:
        print "Winner: %s" % (winner,)
    else:
        print "No winner."