"""
display the game instructions
determine who goes first
create an empty tic-tac-toe board
display the board
while nobody's won and it's not a tie
    if it's the human turn
        get the human move
        update the board with move
    otherwise
        calculate the computer move
        update the board with move
    display the board
    switch turns
congratulate the winner or declare a tie
"""

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

# 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 will be a showdown between your human brain and my sillicon 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 = input(question).lower()
    return response

def ask_number(question, low, high):
    """Ask for a number within a range."""
    response = None
    while response not in (low, high):
        response = int(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("\n Then take the first move. You will need it.")
        human = X
        computer = O
    else:
        print("\n Your bravery will be your undoing... I will go first.")
        computer = X
        human = O
    return computer, human

def new_board():
    """Create a 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("\t", board[3], "|", board[4], "|", board[5])
    print("\t", "---------")
    print("\t", board[6], "|", board[7], "|", board[8], "\n")

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

def winner(board):
    """Determines 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("\n That 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 a changing list
    board = board[:]
    BEST_MOVES = (4, 0, 2, 6, 8, 1, 3, 5, 7)
    print("I shall take square number", end=" ")
    # 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):
    """Congratulate 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("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()
input("\n\n Enter to exit.")