;; next : {'up, 'down} number list-of-number -> number
;; Purpose: select the next floor where the elevator should stop
(define (next dir cf demands)
  (printf "~s ~s ~s ~n" dir cf demands)
  (cond
    ((eq? dir 'up)
     (up-controller cf 
		    (demands-going-down cf demands)
		    (demands-going-up cf demands)))
		    
    ((eq? dir 'down)
     (down-controller cf 
		      (demands-going-down cf demands)
		      (demands-going-up cf demands)))
    (else (error 'next "impossible"))))


;; up-controller : number list-of-number list-of-number -> number
;; Purpose: select the next floor where the elevator should stop
;;   first consider upwards, then downwards direction 
(define (up-controller cf downs ups)
  (cond
    ((cons? ups) (smallest ups))
    ((cons? downs) (largest downs))
    (else cf)))

;; down-controller : number list-of-number list-of-number -> number
;; Purpose: select the next floor where the elevator should stop
;;   first consider downwards, then upwards direction 
(define (down-controller cf downs ups)
  (cond
    ((cons? downs) (largest downs))
    ((cons? ups) (smallest ups))
    (else cf)))

;; demands-going-up : number list-of-number -> list-of-number
;; Purpose: filter out all those numbers from demands that are larger than cf
(define (demands-going-up cf demands)
  (filter (lambda (n) (> fst n)) demands))

;; demands-going-down : number list-of-number -> list-of-number
;; Purpose: filter out all those numbers from demands that are smaller than cf
(define (demands-going-down cf demands)
  (filter (lambda (n) (< fst n)) demands))

;; smallest : ne-list-of-number -> number
;; Purpose: find the smallest number in a non-empty list of numbers
(define (smallest nelon)
  (foldr min (first nelon) (rest nelon)))

;; largest : ne-list-of-number -> number
;; Purpose: find the largest number in a non-empty list of numbers
(define (largest nelon)
  (foldr max (first nelon) (rest nelon)))