;; This is an iterative breadth-first search example from the book

(setf (get 's 'neighbors) '(a d)
      (get 'a 'neighbors) '(s b d)
      (get 'b 'neighbors) '(a c e)
      (get 'c 'neighbors) '(b)
      (get 'd 'neighbors) '(s a e)
      (get 'e 'neighbors) '(b d f)
      (get 'f 'neighbors) '(e))

(setf goal 'f)

(defun bsolve (initial)
	(do ((queue (list (list initial initial))   ; first parameter
                    (append (cdr queue) new-paths))
	     (new-paths nil nil))                   ; second parameter
	    ((null queue))                          ; termination test
	  (when (goal-recognizer (caar queue))
	        (return (values (caar queue) (cdar queue))))
	  (setq new-paths (expand-path (car queue)))))

(defun goal-recognizer (state)
	(if (eql state goal) t nil))

(defun expand-path (path)
	(print path)
	(mapcar #'(lambda (new-node) (cons new-node path))
        (remove-if #'(lambda (neighbor) (member neighbor path))
                        (get (first path) 'neighbors))))