;; traffic light exercise
; http://www.htdp.org/2003-09-26/Book/curriculum-Z-H-9.html#node_sec_6.2

;; dimensions of traffic light    
(define WIDTH 50)
(define HEIGHT 160)
(define BULB-RADIUS 20)
(define BULB-DISTANCE 10)

;; the positions of the bulbs 
(define X-BULBS (quotient WIDTH 2))
(define Y-RED (+ BULB-DISTANCE BULB-RADIUS))
(define Y-YELLOW (+ Y-RED BULB-DISTANCE (* 2 BULB-RADIUS)))
(define Y-GREEN (+ Y-YELLOW BULB-DISTANCE (* 2 BULB-RADIUS)))

;; draw the light with the red bulb turned on
(start WIDTH HEIGHT)
(draw-solid-disk (make-posn X-BULBS Y-RED) BULB-RADIUS 'red)
(draw-circle (make-posn X-BULBS Y-YELLOW) BULB-RADIUS 'yellow)
(draw-circle (make-posn X-BULBS Y-GREEN) BULB-RADIUS 'green)

;; clear the light bulb of a specified color

;; Contract: symbol -> boolean

;; Purpose: takes a symbol representing one of the
;; three traffic light colors (red, green, and yellow)
;; and clears the corresponding light bulb
;; Returns true if the operation is successful

;; Examples: (clear-bulb 'red) replaces the red bulb drawing
;; with a red circle
(define (clear-bulb color) 
  (cond 
    [(symbol=? color 'red) 
     (and 
      (clear-solid-disk (make-posn X-BULBS Y-RED) BULB-RADIUS) 
      (draw-circle (make-posn X-BULBS Y-RED) BULB-RADIUS 'red)
     )]

  ;; fill in the rest of the function code
    )
)
 
(check-expect (clear-bulb 'red) true)


;; Contract: symbol -> boolean

;; Purpose: takes a symbol representing one of the
;; three traffic light colors (red, green, and yellow)
;; and draws the corresponding light bulb
;; Returns true if the operation is successful

;; Examples: (draw-bulb 'red) replaces the red bulb drawing
;; with a red disk


;(draw-bulb 'green) 

;; write a function (switch from to) as defined in exercise 6.2.4


;(sleep-for-a-while 1)
;(switch 'green 'yellow)

;; use the function and the next function as stated in exercise 6.2.5
;; inlcude the pause by using sleep-for-a-while function

;; next : symbol  ->  symbol
;; to switch a traffic light's current color and to return the next one



;(sleep-for-a-while 1)
;
;(next 'red)
;(sleep-for-a-while 1)
;(next 'green)
;(sleep-for-a-while 1)
;(next 'yellow)
;(sleep-for-a-while 1)
;(next 'red)