问生命游戏:如何跟踪活跃细胞

EN

from itertools import product

def show(board):
    for row in board:
        print " ".join(row)

def init(N):
    board = []
    for x in range(N):
        board.append([])
        for y in range(N):
            board[x].append(".");
    return board

def create_plane(board):
    board[2][0] = "x"
    board[2][1] = "x"
    board[2][2] = "x"
    board[1][2] = "x"
    board[0][1] = "x"

def neighbors(i, j, N):
    g1 = {x for x in product([1, 0, -1], repeat=2) if x != (0, 0)}
    g2 = {(i + di, j + dj) for di, dj in g1}
    return [(x, y) for x, y in g2 if x >= 0 and x < N and y >= 0 and y < N]

def live(board):
    N = len(board)
    acells = {}
    for i in range(N):
        for j in range(N):
            if board[i][j] == "x":
                for (x, y) in neighbors(i, j, N):
                    if (x, y) not in acells: acells[(x, y)] = board[x][y]

    while True:
        print "-" * 2 * N, len(acells), "cells to check"
        show(board)
        raw_input("Press any key...")
        for c in acells.keys():
            a = len([x for x in neighbors(c[0], c[1], N) if board[x[0]][x[1]] == "x"])
            cur = board[c[0]][c[1]]
            if a == 0:
                del acells[c]                       # if no live cells around, remove from active
            elif cur == "x" and a not in (2, 3):
                acells[c] = "."                     # if alive and not 2 or 3 neighbors - dead
            elif cur == "." and a == 3:
                acells[c] = "x"                     # if dead and 3 neighbors - alive
                for x in neighbors(c[0], c[1], N):  # add all neighbors of new born
                    if x not in acells: acells[x] = board[x[0]][x[1]] 

        for c in acells:
            board[c[0]][c[1]] = acells[c]

N = 7
board = init(N)
create_plane(board)

live(board)