# Tic-Tac-Toe
# Plays the game of Tic-Tac-Toe against a human opponent

# Global constants
X = "X"
O = "O"
EMPTY = " "
NUM_SQUARES = 9
TIE = "TIE"
def display_instruct():
    """Display game instructions."""
    print \
    """
    Welcome to the greatest intellectual challenge of all time: Tic-Tac-Toe.
    This will be a showdown between your human brain and my silicon processor.

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


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

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

def ask_yes_no(question):
    """Ask a yes or no question."""
    response = None
    while response not in ("y", "n"):
        response = raw_input(question).lower()
    return response

def ask_number(question, low, high):
    """Ask for a number within a range."""
    response = None
    while response not in range(low, high):
        response = int(raw_input(question))
    return response

def pieces():
    """Determine if player or computer goes first."""
    go_first = ask_yes_no("Do you require the first move? (y/n): ")
    if go_first == "y":
        print "\nThen take the first move. You will need it."
        human = X
        computer = O
    else:
        print "\nYour bravery will be your undoing... I will go first."
        computer = X
        human = O
    return computer, human

def new_board():
    """Create new game board."""
    board = []
    for square in range(NUM_SQUARES):
        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 "\n\t", board[3], "|", board[4], "|", board[5]
    print "\t", "---------"
    print "\n\t", board[6], "|", board[7], "|", board[8]

def legal_moves(board):
    """Creates list of legal moves."""
    moves = []
    for square in range(NUM_SQUARES):
        if board[square] == EMPTY:
            moves.append(square)
    return moves

def winner(board):
    """Determine the game winner."""
    WAYS_TO_WIN = ((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 WAYS_TO_WIN:
        if board[row[0]] == board[row[1]] == board[row[2]] != EMPTY:
            winner = board[row[0]]
            return winner
        if EMPTY not in board:
            return TIE
        return None

def human_move(board, human):
    """Get human move."""
    legal = legal_moves(board)
    move = None
    while move not in legal:
        move = ask_number("Where will you move? (0-8): ", 0, NUM_SQUARES)
        if move not in legal:
            print "\nThat square is already occupied, foolish human. Choose another.\n"
        print "Fine..."
        return move
def computer_move(board, computer, human):
    """Make computer move."""
    # Make a copy to work with since function will be changing list
    board = board[:]
    # The best positions to have, in order
    BEST_MOVES = (4, 0, 2, 6, 8, 1, 3, 5, 7)
    print "I shall take square number",
    # If computer can win, take that move
    for move in legal_moves(board):
        board[move] = computer
        if winner(board) == computer:
            print move
            return move
        # Done checking this move, undo it
        board[move] = EMPTY
    # If human can win, block that move
    for move in legal_moves(board):
        board[move] = human
        if winner(board) == human:
            print move
            return move
        # Done checking this move, undo it
        board[move] = EMPTY
    # Since no one can win on next move, pick best open square
    for move in BEST_MOVES:
        if move in legal_moves(board):
            print move
            return move

def next_turn(turn):
    """Switch turns."""
    if turn == X:
        return O
    else:
        return X

def congrat_winner(the_winner, computer, human):
    """Congratule the winner."""
    if the_winner != TIE:
        print the_winner, "won!\n"
    else:
        print "It's a tie!\n"
    if the_winner == computer:
        print "As I predicted, human, I am triumphant once more. \n" \
              " Proof that computers are superior to humans in all regards."

    elif the_winner == human:
        print "No, no! It cannot be! Somehow you tricked me, human. \n" \
              "But never again! I, the computer, so swears it!"

    elif the_winner == TIE:
        print "You were most lucky, human, and somehow managed to tie me. \n" \
              "Celebrate today... for this is the best you will ever achieve."

def main():
    display_instruct()
    computer, human = pieces()
    turn = X
    board = new_board()
    display_board(board)

    while not winner(board):
        if turn == human:
            move = human_move(board, human)
            board[move] = human
        else:
            move = computer_move(board, computer, human)
            board[move] = computer
        display_board(board)
        turn = next_turn(turn)
        the_winner = winner(board)
        congrat_winner(the_winner, computer, human)

# Start the program
main()
raw_input("\n\nPress the enter key to quit.")