;;; MINIMAX strategy of computer player
;;; Only for educational reasons, use ALPHA-BETA instead.


(defun minimax (player board)
  "Make perfect move with minimax strategy."
  (let ((moves (legal-moves board)))
    (if (or (null moves) (has-won-p player board) (has-won-p (opponent player) board))
	(evaluate player board)
	(let (best-move best-val)
	  (loop for move in moves
		with val do
		(place-piece move player board)
		(setf val (- (minimax (opponent player) board)))
		(place-piece move empty board)
		(when (or (null best-val) (> val best-val))
		  (setf best-val val best-move move)))
	  (values best-val best-move)))))


(defun minimax-searcher ()
  "Wrapper for MINIMAX."
  #'(lambda (player board)
      (multiple-value-bind (value move)
	  (minimax player board)
	(declare (ignore value))
	move)))