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#lang racket
(require 2htdp/universe 2htdp/image)
(define WIDTH 300)
(define HEIGHT 300)
(define CAROILLE (bitmap/file "resources/caroille.png"))
(define CAROILLE-WIDTH (image-width CAROILLE))
;;;
;;; If the car is placed at X = 1/2 its width, its back will be
;;; touching the left edge of the World.
;;;
;;; If the car is place at X = - 1/2 its width, its front will be touching
;;; the left edge of the World.
;;;
(define CAROILLE-WIDTH-HALF (/ CAROILLE-WIDTH 2.0))
;;; Structure to represent the X position of two cars in animation.
(struct cars (one two))
(define (caroille-past-right-edge? pos)
(> pos (- WIDTH CAROILLE-WIDTH-HALF)))
(define (caroille-fully-past-right-edge? pos)
(>= pos (+ WIDTH CAROILLE-WIDTH-HALF)))
(define (caroille-fully-past-left-edge? pos)
(>= pos CAROILLE-WIDTH-HALF))
(define (caroille-fully-inside? pos)
(and (caroille-fully-past-left-edge? pos)
(not (caroille-past-right-edge? pos))))
(define (move caroilles)
(let ((caroille-one (cars-one caroilles))
(caroille-two (cars-two caroilles)))
(cond
;; Case set I - one of the cars is fully inside.
((caroille-fully-inside? caroille-one)
(cars (+ 1 caroille-one) caroille-two))
((caroille-fully-inside? caroille-two)
(cars caroille-one (+ 1 caroille-two)))
;; Case set II - one of the cars disappeared into the right edge.
((caroille-fully-past-right-edge? caroille-one)
(cars (- CAROILLE-WIDTH-HALF) (+ 1 caroille-two)))
((caroille-fully-past-right-edge? caroille-two)
(cars (+ 1 caroille-one) (- CAROILLE-WIDTH-HALF)))
;; Case else - Both cars are partially out.
(else (cars (+ 1 caroille-one) (+ 1 caroille-two))))))
(define (draw-cars caroilles)
(place-image CAROILLE (cars-one caroilles) (/ HEIGHT 2)
(place-image CAROILLE (cars-two caroilles) (/ HEIGHT 2)
(empty-scene WIDTH HEIGHT))))
(define (start)
(big-bang (cars CAROILLE-WIDTH-HALF (- CAROILLE-WIDTH-HALF))
(on-tick move)
(to-draw draw-cars)))
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