;Exercise 51.

; world constants
(define scaleTL 100)             ; Single control point, scale of traffic light, default 100
(define wHeight (* scaleTL 2.8))
(define wLength (* scaleTL 1))
(define inactiveLightRadius (* scaleTL 0.45))
(define activeLightRadius (* scaleTL 0.40))

(define backgroundColor "black")
(define inactiveLightColor "dimGray")
(define Background (place-image (rectangle wLength wHeight "solid" backgroundColor)
                                (/ wLength 2)
                                (/ wHeight 2)
                                (empty-scene wLength wHeight)))


;---------------------

; A TrafficLight (TL) is one of the following Strings:
; – "red"
; – "green"
; – "yellow"
; interpretation the three strings represent the three
; possible states that a traffic light may assume

;---------------------

; traffic-light-next
; TrafficLight -> TrafficLight
; yields the next state given current state s
(check-expect (traffic-light-next "green") "yellow")
(check-expect (traffic-light-next "yellow") "red")
(check-expect (traffic-light-next "red") "green")
(define (traffic-light-next TL)
  (cond
    [(string=? "red" TL) "green"]
    [(string=? "green" TL) "yellow"]
    [(string=? "yellow" TL) "red"]))

; inactiveLight
; draws inactive light
(define inactiveLight (circle inactiveLightRadius "solid" inactiveLightColor))

; activeLight
; TL -> Image
; draws current active light
(define (activeLight TL)
(circle activeLightRadius "solid" TL))

;light-positions - used by place-images function
(define topPos    (make-posn ( / wLength 2)( * 1/6 wHeight)))
(define centerPos (make-posn ( / wLength 2)( * 3/6 wHeight)))
(define bottomPos (make-posn ( / wLength 2)( * 5/6 wHeight)))

; activeLightPosition
; TL -> Number
 (define (activeLightPosition TL)
   (cond ((string=? TL "green") topPos)
         ((string=? TL "yellow") centerPos)
         ((string=? TL "red") bottomPos)))

; render
; TrafficLight -> Image
; draws TrafficLight depending on current light
(define (render TL)
  (place-images  (list (activeLight TL)
                       inactiveLight
                       inactiveLight
                       inactiveLight)
                 (list (activeLightPosition TL)
                       topPos
                       centerPos
                       bottomPos)
                 Background))

;tock
; TL -> TL
; changes TL to next state
(define (tock TL)
  (traffic-light-next TL))


; main big-bang function
; TL -> TL
(define (main TL)
  (big-bang TL
    [to-draw render]
    [on-tick tock]
    ))

;Launch
(main "red")