# Tic-Tac-Toe

# global constants
X = "X"
O = "O"
EMPTY = ""
TIE = "TIE"
NUM_SQUARES = 9

def display_instruct():
    """ Display game instructions."""
    print \
          """
    Welcome to the greatest intellectual challenge of all time: Tic-Tac-Toe.
    This showdown will be a battle between your human brain and my processor.

    You will make your move known by entering a number, 0-8. The number will
    correspond to the board position as follows:


            0 | 1 | 2
           -----------
            3 | 4 | 5
           -----------
            6 | 7 | 8

    Prepare yourself. The ultimate battle is about to begin. \n
    """

def bquery(question):
    answer = None
    while answer not in ("y","n"):
        answer = raw_input(question).lower()
    return answer

def ask_number():
    """ Gets a number from the player."""
    move = None
    while move not in range(0,9):
        move = raw_input("Where do you want to move? ")
        move = int(move)
    return move

def go_first():
    """ Asks player to go first or not."""
    answer = bquery("Do you want to go first? ")
    if answer == "y":
        human = X
        computer = O
    else:
        human = O
        computer = X
    return human, computer

def new_board():
    """ Creates a new blank game board."""
    board = []
    for square in range(0,9):
        board.append(EMPTY)
    return board

def display_board(board):
    """ Display game board on screen."""
    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 winning_board(board):
    """ Checks to see if a player has won."""
    win_rows = [(0,1,2),(3,4,5),(6,7,8),(0,3,6),(1,4,7),(2,5,8),(0,4,8),(2,4,6)]
    for row in win_rows:
        if board[row[0]] == board[row[1]] == board[row[2]] != EMPTY:
            winner = board[row[0]]
            return winner
    if EMPTY not in board:
        return TIE
    else:
        return None

def legal_moves(board):
    """ Creates series of legal moves."""
    legal = []
    for square in range(0,9):
        if board[square] == EMPTY:
            legal.append(square)
    print "[debug] legal moves: ", legal
    return legal

def human_move(board, human):
    """ Takes player's move + place on board."""
    move = None
    legal = legal_moves(board)
    while move not in legal:
        move = ask_number()
    board[move] = human
    return board

def cpu_move(board, computer, human):
    """ Takes computer's move + places on board."""
    # make a copy of the board
    board = board[:]
    bestmoves = (0,2,6,8,4,3,5,1,7)
    # if computer can win, play that square:
    for move in legal_moves(board):
        board[move] = computer
        if winning_board(board) == computer:
            print "[debug] cpu win:", move
            return move
        # undo the move because it was empty before
        board[move] = EMPTY
    # if player can win, block that square:
    for move in legal_moves(board):
        board[move] = human
        if winning_board(board) == human:
            print "[debug] block human:", move
            return move
        board[move] = EMPTY
        # chose first best move that is legal
    for move in bestmoves:
        if move in legal_moves(board):
            board[move] = computer
            print "[debug] next best move:", move
            return move


def change_turn(turn):
    if turn == X:
        return O
    else:
        return X

def main():
    human, computer = go_first()
    board = new_board()
    display_board(board)
    turn = X
    while winning_board(board) == None:
        if human == turn:
            board = human_move(board, human)
            turn = change_turn(turn)
            display_board(board)
        else:
            board = cpu_move(board, computer, human)
            turn = change_turn(turn)
            display_board(board)

main()

raw_input("Press Enter to exit.")