;; The first three lines of this file were inserted by DrRacket. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-beginner-reader.ss" "lang")((modname world-of-traffic) (read-case-sensitive #t) (teachpacks ((lib "image.rkt" "teachpack" "2htdp") (lib "universe.rkt" "teachpack" "2htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "image.rkt" "teachpack" "2htdp") (lib "universe.rkt" "teachpack" "2htdp")))))
;; DEMO: world of traffic

;;data definitions

;;A 'light' is either
;;  - "red"
;;  - "yellow"
;;  - "green"

;; bulb : color -> image
;; to render a traffic light bulb of the given color
(define (bulb color)
  (circle RADIUS "solid" color))
(define RADIUS 12)
(define red-bulb (bulb "red"))
(define dark-bulb (bulb "black"))

(check-expect red-bulb (circle 12 "solid" "red"))

(define spacer (square (/ RADIUS 4) 0 "white"))



;; light->image : light -> image
;; draw the light
(define (light->image light)
   (cond [(string=? light "red") 
              (above red-bulb spacer dark-bulb spacer dark-bulb)]
         [(string=? light "yellow")
               (above dark-bulb spacer (bulb "yellow") spacer dark-bulb)]
         [(string=? light "green")
               (above dark-bulb spacer dark-bulb spacer (bulb "green"))
          ]))
                                

;; change-light : light -> light
;; given the current color, return the next color
(define (change-light light)
  (cond [(string=? light "red") "green"]
        [(string=? light "yellow") "red"]
        [(string=? light "green") "yellow"]))
(check-expect (change-light "green") "yellow")
(check-expect (change-light "yellow") "red")
(check-expect (change-light "red") "green")
(check-error (change-light "purple"))

;; run-simulation : number -> world
(define (run-simiulation rate)
  (big-bang "green"
    (on-tick change-light rate)
    (to-draw light->image)))