;; find-route : node node graph  ->  (listof node) or false
;; to create a path from origination to destination in G
;; if there is no path, the function produces false
(define (find-route origination destination G)
  (cond
    [(symbol=? origination destination) (list destination)]
    [else (local ((define possible-route 
		    (find-route/list (neighbors origination G) destination G)))
	    (cond
	      [(boolean? possible-route) false]
	      [else (cons origination possible-route)]))]))

#| neighbors: node graph -> (listof node)
   (define (neighbors a-node a-graph) ...)

   Purpose: compute a-node's neighbors in a-graph

   Example:
     in figure~xyz,
       G has no neighbors: empty
       A has the list of neighbors (list 'B 'E)
|#
(define (neighbors a-node a-graph)
  (cond
    ((empty? a-graph) (error 'neighbors "can't happen"))
    (else (cond
	    ((eq? (first (first a-graph)) a-node)
	     (second (first a-graph)))
	    (else (neighbors a-node (rest a-graph)))))))

;; find-route/list : (listof node) node graph  ->  (listof node) or false
;; to create a path from some node on lo-Os to D
;; if there is no path, the function produces false
(define (find-route/list lo-Os D G)
  (cond
    [(empty? lo-Os) false]
    [else (local ((define possible-route (find-route (first lo-Os) D G)))
	    (cond
	      [(boolean? possible-route) (find-route/list (rest lo-Os) D G)]
	      [else possible-route]))]))


(define GRAPH
  '((A (B C))
    (B (C D))
    (C (D))
    (D ())))

(equal? (find-route 'A 'D GRAPH) '(A B C D))
(equal? (find-route 'A 'A GRAPH) '(A))
(equal? (find-route 'C 'B GRAPH) false)


;(define CYCLIC
;  '((A (A))))
;
;(find-route 'A 'C CYCLIC)