def negamax(board, player_to_move):
"""Use a variation of the Negamax algorithm to choose the next move.
Recursively determine the best next move for the given player. Return a
tuple containing the score and the board index of the move (chosen randomly
if there are several best moves). The score is the likely outcome of the
game on that line of play: -1 for a loss, 0 for a draw, 1 for a win.
Source: http://en.wikipedia.org/wiki/Negamax.
"""
# Stop recursing if you just lost. That's the worst possible outcome.
if rules.getWinner(board) == rules.getOpposite(player_to_move):
return (-1, None)
# Second stop case: a cat's game, no moves left.
if rules.isFull(board):
return (0, None)
# Make a pretend move in each legal square and see how it turns out.
best_score = -1
best_moves = []
for s in rules.getOpenSquares(board):
new_board = copy.deepcopy(board)
new_board[s] = player_to_move
# Get the best response to this move. Don't care what it was, just
# score it.
score, _ = negamax(new_board, rules.getOpposite(player_to_move))
# The score for you is the opposite of the score for the other player.
score *= -1
if score == 1:
# That move led to a win, so return it
return (1, s)
elif score > best_score:
best_score = score
best_moves = [s]
elif score == best_score:
best_moves.append(s)
assert best_moves
return (best_score, random.choice(best_moves))
player_char = 'o'
comp_char = 'x'
board = [' '] * 9
winner = None
first_move = True
legend = "123456789"
rules.printBoard(legend)
print "\nComputer goes first\n"
while not winner and not rules.isFull(board):
# On the first turn, skip the human player to let the computer go first.
if not first_move:
# Player's turn
board[ai.humanPlayer(board)] = player_char
winner = rules.getWinner(board)
# Computer's turn
if not winner and not rules.isFull(board):
if first_move:
board[ai.firstMove()] = comp_char
else:
# Don't care what the score was, just take the best move.
_, move = ai.negamax(board, comp_char)
assert move is not None
board[move] = comp_char
winner = rules.getWinner(board)
# Either the game just ended or we're on to the next round.
first_move = False
rules.printBoard(board)
