'''全局變量：
 X 和 O 表示兩方的棋子;
 EMPTY 表示棋位為空；
 TIE 表示平局；
 NUM_SQUARES 表示有 9 個棋位
 '''
 X = "X"
 O = "O"
 EMPTY = " "
 TIE = "TIE"
 NUM_SQUARES = 9

def ask_yes_no(question):
  '''問一個是或否的問題，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):
  '''訊問一個規定范圍的數字。'''
  response = None
  while response not in range(low, high):
    response = int(input(question))
  return response


def pieces():
  '''決定在人和機器之間誰先行棋。'''
  go_first = ask_yes_no('你先走？ (y/n): ')
  if go_first == 'y':
    print('\n好，你先請。')
    human = X
    computer = O
  else:
    print('\n你放棄先手，我先走。')
    computer = X
    human = O
  return computer, human


def new_board():
  '''創建一個棋盤。'''
  board = []
  for square in range(NUM_SQUARES):
    board.append(EMPTY)
  return board


def display_board(board):
  '''顯示棋盤。'''
  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):
  '''創建合法的行棋位置清單。'''
  moves = []
  for square in range(NUM_SQUARES):
    if board[square] == EMPTY: # 該棋位為空
      moves.append(square)
  return moves


def winner(board):
  '''判定游戲獲勝者。'''
  WAYS_TO_WIN = ((0, 1, 2), # 橫
          (3, 4, 5),
          (6, 7, 8),
          (0, 3, 6), # 豎
          (1, 4, 7),
          (2, 5, 8),
          (0, 4, 8), # 主對角線
          (2, 4, 6)) # 副對角線
  # 誰先有三個棋子在一條直線上誰就獲勝。
  for row in WAYS_TO_WIN:
    if board[row[0]] == board[row[1]] == board[row[2]] != EMPTY:
      winner = board[row[0]]
      return winner
  
  # 沒有獲勝方，但棋盤已經下滿，判為平局
  if EMPTY not in board:
    return TIE

  return None # 沒有獲勝方，且非平局


def human_move(board, human):
  '''獲取玩家的行棋位置。''' 
  legal = legal_moves(board) # 合法的行棋位置清單
  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你選的棋位已落子，重新選擇。\n')
  print('落子無悔...')
  return move


def computer_move(board, computer, human):
  '''獲取電腦的行棋位置。'''
  
  board = board[:] # 通過切片復制棋盤, 創建局部變量
  BEST_MOVES = (4, 0, 2, 6, 8, 1, 3, 5, 7) # 優先的行棋位置

  print('我走到：', end=' ')
  
  # 在局部空間確定可以獲勝的走法
  for move in legal_moves(board): # 合法的行棋位置列表
    board[move] = computer # 更新棋盤副本
    if winner(board) == computer:
      # 計算機能獲勝
      print(move)
      return move
    # 如果計算機在該棋位行棋不能獲勝，
    board[move] = EMPTY # 悔棋，更換下一個合法棋位
  
  # 阻止玩家獲勝
  for move in legal_moves(board): # 合法的行棋位置列表
    board[move] = human # 更新棋盤
    if winner(board) == human:
      # 在該棋位玩家行棋后將獲勝
      print(move)
      return move
    # 玩家不能獲勝，不行棋至此
    board[move] = EMPTY

  # 沒有能使行棋雙方立決勝負的棋位，從優先棋位選擇合法行棋位置
  for move in BEST_MOVES:
    if move in legal_moves(board):
      print(move)
      return move


def next_turn(turn):
  '''切換行棋方'''
  if turn == X:
    return O
  else:
    return X

  
def congrat_winner(the_winner, computer, human):
  '''向獲勝方表示祝賀或聲明平局'''
  if the_winner != TIE:
    print(the_winner, '祝賀你！\n')
  else:
    print('平局。。。\n')

  if the_winner == computer:
    print('如我所料，我又勝了。\n這是否能說明計算機在給方面都優于人類呢？')

  elif the_winner == human:
    print('真是匪夷所思，你沒搗鬼吧？人類怎么會勝？ \n不會有下次了。' )

  elif the_winner == TIE:
    print('你很厲害嘛，能跟計算機打成平手。')

def main():
  display_instruct() # 顯示游戲操作指南
  computer, human = pieces() # 決定誰先行棋
  turn = X # 先行棋方執子 X
  board = new_board() # 創建空棋盤
  display_board(board) # 顯示棋盤

  while not winner(board): 
    # 判定結果，在沒有獲勝方且不是平局時進入循環
    if turn == human:
      # 輪到玩家行棋，玩家行棋
      move = human_move(board, human) # 玩家的行棋位置
      board[move] = human # 更新棋盤
    else:
      # 輪到電腦行棋
      move = computer_move(board, computer, human) # 電腦的行棋位置
      board[move] = computer # 更新棋盤
    display_board(board) # 顯示更新后的棋盤
    turn = next_turn(turn) # 切換行棋方

  the_winner = winner(board) # 判定獲勝者，返回獲勝方的執子，平局返回 None
  congrat_winner(the_winner, computer, human) # 向獲勝方表示祝賀或聲明是平局

main()
input('\n\n按回車鍵退出程序。') # 等待用戶