;; The recursive search example with association lists for 
;; representing the graph.

(setf s '((neighbors (a d)) (coordinates (0 3))))

(setf a '((neighbors (s b d)) (coordinates (4 6))))

(setf b '((neighbors (a c e)) (coordinates (7 6))))

(setf c '((neighbors (b)) (coordinates (11 6))))

(setf d '((neighbors (s a e)) (coordinates (3 0))))

(setf e '((neighbors (b d f)) (coordinates (6 0))))

(setf f '((neighbors (e)) (coordinates (11 3))))

(defun breadth-first (start finish &optional
                       (queue (list (list start))))
     (cond ((endp queue) nil)
	   ((equal finish (first (first queue)))
	       (reverse (first queue)))
	   (t (breadth-first start finish
                             (append (rest queue)
			             (extend (first queue)))))))

(defun extend (path)
	(print (reverse path))
	(mapcar #'(lambda (new-node) (cons new-node path))
             (remove-if #'(lambda (neighbor) (member neighbor path))
                      (second (assoc 'neighbors (eval (first path)))))))