python minimax function

minimax.py

def minimax(
	board: chess.Board, 
	depth: int, 
	player: int) -> Tuple[Union[int, chess.Move]]:
	"""
	This functions receives a board, depth and a player; and it returns
	the best move for the current board based on how many depths we're looking ahead
	and which player is playing. For every move we will do the recursion until
	leaf nodes and evaluate all leaf lodes.

	OBS:
		- We only need to evaluate the value for leaf nodes because they are our final states
		of the board and therefore we need to use their scores to decide the best move.
	Arguments: 
		- board: chess board state
		- depth: how many depths we want to calculate for this board
		- player: player that is currently moving pieces. 1 for AI (max), -1 for human (min).
	Returns:
		- best_score, best_move: returns best move that it found and its value.
	"""

	# recursion base case
	if depth == 0:
		# evaluate current board
		score = board_value(board)
		return score, None

	best_move = None

	# initializing best_move_value found depending on the player
	if player:
		best_move_value = float("-inf")
	else:
		best_move_value = float("inf")

	for move in board.legal_moves:
		
 		# make the move
		board.push(move)
		
		# if threefold repetition we do not analyze this position
		if board.can_claim_threefold_repetition():
			board.pop()  # unmake the last move
			continue

		score = minimax(board, depth-1, not player)[0]

		if player:
			# look for moves that maximize position, max player (AI moves)
			if score > best_move_value:
				# if it was the highest score move so far, we make this move
				best_move_value = score
				best_move = move
		else:
			# look for best moves that minimize position, min player (Human moves)
			if score < best_move_value:
				# we assume human is making the best move for himself
				best_move_value = score
				best_move = move

		# take move back
		board.pop()

	# if it returned no best move, we make a random one
	if not best_move:
		if board.legal_moves:
			best_move = random.choice([move for move in board.legal_moves])
		else:
			best_move = (None, None)

	return best_move_value, best_move