;; ---------------------------------------------------------
;; Basic constanst: 
(define WIDTH  50)
(define HIGHT 160)
(define RADIUS 20)

(define DELTA 10)

(define X (quotient WIDTH 2))

;; off-set : num -> num
(define (off-set y) (+ y DELTA (* 2 RADIUS)))

(define Y:RED (off-set (- RADIUS)))
(define Y:YEL (off-set Y:RED))
(define Y:GRN (off-set Y:YEL))

;; A light is (make-light posn color)
(define-struct light (center color))

(define RED:LIGHT (make-light (make-posn X Y:RED) RED))
(define YEL:LIGHT (make-light (make-posn X Y:YEL) YELLOW))
(define GRN:LIGHT (make-light (make-posn X Y:GRN) GREEN))

;; ---------------------------------------------------------
;; successor : light -> light 
(define (successor l)
  (cond
    [(eq? RED:LIGHT l) GRN:LIGHT]
    [(eq? GRN:LIGHT l) YEL:LIGHT]
    [(eq? YEL:LIGHT l) RED:LIGHT]))

;; ---------------------------------------------------------
;; current-state : light
(define current-state GRN:LIGHT)

;; init : -> void
;; effect: display all colors, then start with green
(define (init)
  (start WIDTH HIGHT)
  (turn-on RED:LIGHT)
  (turn-on YEL:LIGHT)
  (turn-on GRN:LIGHT)
  (sleep 1)
  (turn-off RED:LIGHT)
  (turn-off YEL:LIGHT)
  (set! current-state GRN:LIGHT))

;; next : -> void
;; effect: change current state to its successor and switch lights
(define (next)
  (turn-off current-state)
  (set! current-state (successor current-state))
  (turn-on current-state))

;; make-on/off : draw clear -> (light -> void)
(define (make-on/off draw clear)
  (lambda (l)
    (let ((p (light-center l))
	  (c (light-color l)))
      (clear p RADIUS c)
      (draw  p RADIUS c))))

;; turn-on : light -> void
;; effect: draw full disk
(define turn-on (make-on/off draw-solid-disk clear-circle))

;; turn-off : light -> void
;; effect: blank out and draw circle
(define turn-off (make-on/off draw-circle clear-solid-disk))

(define (cycle)
  (next)
  (sleep 1)
  (cycle))

(init)
(cycle)