;; a helper file for lab9-library.ss
;; Searching a graph.
;;
;;(define-structure (city name nbhrs seen?))


(define already-seen null)

(define (have-seen2? c) (city-in-list? c already-seen))
(define (mark-seen2! c) (set! already-seen (cons c already-seen)))


(define (reachable2? here there)
  (begin
    (mark-seen2! here)
    (if (same-city? here there)
	#t
	(reachable2-from? (city-nbhrs here) there))))


(define (reachable2-from? loc there)
  (cond ((null? loc) #f)
	((if (have-seen2? (car loc))
	     #f
	     (reachable2? (car loc) there))
	 #t)
	(else (reachable2-from? (cdr loc) there))))







;; (forget2)
;; allow reachable2? to be called again
;;
(define (forget2) (set! already-seen null))

(define (city-in-list? c loc)
  (cond ((null? loc) #f)
    (else (if (same-city? c (car loc))
	      #t
	      (city-in-list? c (cdr loc))))))





;; ------- a printing version --------
;; (print the list already-seen every time
;; we check it)
;;    Buggy to click "stop executing" while (a library function)
;;    is printing?
;;
(define (names-of loc) (map city-name loc))
(define (names-seen) (names-of already-seen))

(define (have-seen2p? c)
  (begin
    (printf "Looking for ~s in ~s~n~n" (city-name c) (names-seen))
    (city-in-list? c already-seen)))

(define (reachable2p? here there)
  (begin
    (mark-seen2! here)
    (if (same-city? here there)
	#t
	(reachable2p-from? (city-nbhrs here) there))))


(define (reachable2p-from? loc there)
  (cond ((null? loc) #f)
	((if (have-seen2p? (car loc))
	     #f
	     (reachable2p? (car loc) there))
	 #t)
	(else (reachable2p-from? (cdr loc) there))))