def generate_move_minimax(
board: Board, player: Player, saved_state: Optional[SavedState]
) -> Tuple[PlayerAction, Optional[SavedState]]:
"""
Generates a move for player on the current board.
Optionally consumes and updates a saved state.
:param board: The board state
:param player: The player to generate a move for
:param saved_state: (optional) Saved state
:return: Returns the calculated move and the new saved state
"""
# first, check if a winning move is possible. This can fix a problem where MiniMax
# would not finish the game since all moves will have
for move in board.actions():
nb = board.drop_piece_copy(move)
if nb.check_winner(player):
return move, saved_state
# retrieve our MiniMaxCache from saved state if possible, otherwise create the cache object
cache = saved_state if type(
saved_state) == MiniMaxCache2 else MiniMaxCache2()
# calculate the MiniMax result
minimax_result = mini_max(board,
alpha=-np.inf,
beta=np.inf,
depth=3,
max_player=player,
cache=cache)
print(
f"generate_move_minimax() returned move {minimax_result.best_move}, h={minimax_result.value}"
)
return minimax_result.best_move, cache
def mini_max(board: Board, alpha: float, beta: float, depth: int, max_player: Player, cache: MiniMaxCache2) \
-> MiniMaxResult:
"""
Execute the MiniMax algorithm with alpha-beta-pruning on a board.
:param board: The board
:param alpha: Lower boundary
:param beta: Upper boundary
:param depth: Maximum depth
:param max_player: The maximizing player ID
:param cache: The MiniMax cache to use
:return: Returns the best possible result after traversing the tree with the given depth.
"""
# first, let's see if we already cached this value
cached_value = cache.get(board, max_player)
if cached_value is not None:
return cached_value
# If we hit max depth or the board is in a final state, calculate and return the heuristic.
# We cannot decide about the best move here, so leave it empty.
if depth == 0 or board.get_state() != GameState.ONGOING:
return MiniMaxResult(None,
calculate_heuristic(board, max_player).value)
# collect a list of available moves from the board and order them according to moveOrder
# which orders moves based on their distance to the middle column
available_moves = board.actions()
available_moves = [move for move in moveOrder if move in available_moves]
# check if we are maximizing right now
is_maximizing = (board.player == max_player)
# store min and max value, initialize with boundaries
max_value, min_value = alpha, beta
# initialize the best move randomly
best_move: Column = random.choice(available_moves)
# enumerate all available moves. Recurse for each while respecting is_maximizing and adjusting
# min_value and max_value accordingly.
for move in available_moves:
# create a board with the new state
child_board = board.drop_piece_copy(move)
if is_maximizing:
min_result = mini_max(child_board, max_value, beta, depth - 1,
max_player, cache)
if min_result == np.inf:
# we cannot perform better than +inf.
return MiniMaxResult(move, np.inf)
elif min_result.value > max_value:
max_value = min_result.value
best_move = move
if max_value >= beta:
# beta pruning
break
else:
max_result = mini_max(child_board, alpha, min_value, depth - 1,
max_player, cache)
if max_result.value == -np.inf:
# we cannot perform better than -inf.
return MiniMaxResult(move, -np.inf)
elif max_result.value < min_value:
min_value = max_result.value
best_move = move
if min_value <= alpha:
# alpha pruning
break
# return the best move and according heuristic
return MiniMaxResult(best_move, max_value if is_maximizing else min_value)
