'''全局变量：
 X 和 O 表示两方的棋子;
 EMPTY 表示棋位为空；
 TIE 表示平局；
 NUM_SQUARES 表示有 9 个棋位
 '''
 X = "X"
 O = "O"
 EMPTY = " "
 TIE = "TIE"
 NUM_SQUARES = 9

def ask_yes_no(question):
  '''问一个是或否的问题，用 y 或 n 回答。'''
  response = None
  while response not in ('y', 'n'):
    response = input(question).lower()
  return response


def ask_number(question, low, high):
  '''讯问一个规定范围的数字。'''
  response = None
  while response not in range(low, high):
    response = int(input(question))
  return response


def pieces():
  '''决定在人和机器之间谁先行棋。'''
  go_first = ask_yes_no('你先走？ (y/n): ')
  if go_first == 'y':
    print('\n好，你先请。')
    human = X
    computer = O
  else:
    print('\n你放弃先手，我先走。')
    computer = X
    human = O
  return computer, human


def new_board():
  '''创建一个棋盘。'''
  board = []
  for square in range(NUM_SQUARES):
    board.append(EMPTY)
  return board


def display_board(board):
  '''显示棋盘。'''
  print('\n\t', board[0], '|', board[1], '|', board[2])
  print('\t', '---------')
  print('\t', board[3], '|', board[4], '|', board[5])
  print('\t', '---------')
  print('\t', board[6], '|', board[7], '|', board[8], '\n')


def legal_moves(board):
  '''创建合法的行棋位置清单。'''
  moves = []
  for square in range(NUM_SQUARES):
    if board[square] == EMPTY: # 该棋位为空
      moves.append(square)
  return moves


def winner(board):
  '''判定游戏获胜者。'''
  WAYS_TO_WIN = ((0, 1, 2), # 横
          (3, 4, 5),
          (6, 7, 8),
          (0, 3, 6), # 竖
          (1, 4, 7),
          (2, 5, 8),
          (0, 4, 8), # 主对角线
          (2, 4, 6)) # 副对角线
  # 谁先有三个棋子在一条直线上谁就获胜。
  for row in WAYS_TO_WIN:
    if board[row[0]] == board[row[1]] == board[row[2]] != EMPTY:
      winner = board[row[0]]
      return winner
  
  # 没有获胜方，但棋盘已经下满，判为平局
  if EMPTY not in board:
    return TIE

  return None # 没有获胜方，且非平局


def human_move(board, human):
  '''获取玩家的行棋位置。''' 
  legal = legal_moves(board) # 合法的行棋位置清单
  move = None
  while move not in legal:
    move = ask_number('你走哪？ (0 - 8):', 0, NUM_SQUARES)
    if move not in legal:
      print('\n你选的棋位已落子，重新选择。\n')
  print('落子无悔...')
  return move


def computer_move(board, computer, human):
  '''获取电脑的行棋位置。'''
  
  board = board[:] # 通过切片复制棋盘, 创建局部变量
  BEST_MOVES = (4, 0, 2, 6, 8, 1, 3, 5, 7) # 优先的行棋位置

  print('我走到：', end=' ')
  
  # 在局部空间确定可以获胜的走法
  for move in legal_moves(board): # 合法的行棋位置列表
    board[move] = computer # 更新棋盘副本
    if winner(board) == computer:
      # 计算机能获胜
      print(move)
      return move
    # 如果计算机在该棋位行棋不能获胜，
    board[move] = EMPTY # 悔棋，更换下一个合法棋位
  
  # 阻止玩家获胜
  for move in legal_moves(board): # 合法的行棋位置列表
    board[move] = human # 更新棋盘
    if winner(board) == human:
      # 在该棋位玩家行棋后将获胜
      print(move)
      return move
    # 玩家不能获胜，不行棋至此
    board[move] = EMPTY

  # 没有能使行棋双方立决胜负的棋位，从优先棋位选择合法行棋位置
  for move in BEST_MOVES:
    if move in legal_moves(board):
      print(move)
      return move


def next_turn(turn):
  '''切换行棋方'''
  if turn == X:
    return O
  else:
    return X

  
def congrat_winner(the_winner, computer, human):
  '''向获胜方表示祝贺或声明平局'''
  if the_winner != TIE:
    print(the_winner, '祝贺你！\n')
  else:
    print('平局。。。\n')

  if the_winner == computer:
    print('如我所料，我又胜了。\n这是否能说明计算机在给方面都优于人类呢？')

  elif the_winner == human:
    print('真是匪夷所思，你没捣鬼吧？人类怎么会胜？ \n不会有下次了。' )

  elif the_winner == TIE:
    print('你很厉害嘛，能跟计算机打成平手。')

def main():
  display_instruct() # 显示游戏操作指南
  computer, human = pieces() # 决定谁先行棋
  turn = X # 先行棋方执子 X
  board = new_board() # 创建空棋盘
  display_board(board) # 显示棋盘

  while not winner(board): 
    # 判定结果，在没有获胜方且不是平局时进入循环
    if turn == human:
      # 轮到玩家行棋，玩家行棋
      move = human_move(board, human) # 玩家的行棋位置
      board[move] = human # 更新棋盘
    else:
      # 轮到电脑行棋
      move = computer_move(board, computer, human) # 电脑的行棋位置
      board[move] = computer # 更新棋盘
    display_board(board) # 显示更新后的棋盘
    turn = next_turn(turn) # 切换行棋方

  the_winner = winner(board) # 判定获胜者，返回获胜方的执子，平局返回 None
  congrat_winner(the_winner, computer, human) # 向获胜方表示祝贺或声明是平局

main()
input('\n\n按回车键退出程序。') # 等待用户