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#lang racket
(require 2htdp/universe 2htdp/image)
;;; world structure
(struct ufo-pos (x y))
;;; constants
(define WORLD-WIDTH 300)
(define WORLD-HEIGHT 325)
(define MOVE-LEN 3)
(define UFO (bitmap/file "resources/zarking-ufo.png"))
(define UFO-WIDTH (image-width UFO))
(define UFO-HEIGHT (image-height UFO))
;;; ufo movement functions
(define (ufo-move-up current-state)
(let ((x (ufo-pos-x current-state))
(y-up (- (ufo-pos-y current-state) MOVE-LEN)))
(cond [(>= y-up (/ UFO-HEIGHT 2)) (ufo-pos x y-up)]
[else current-state])))
(define (ufo-move-down current-state)
(let ((x (ufo-pos-x current-state))
(y-down (+ (ufo-pos-y current-state) MOVE-LEN)))
(cond [(<= y-down (- WORLD-HEIGHT (/ UFO-HEIGHT 2)))
(ufo-pos x y-down)]
[else current-state])))
(define (ufo-move-left current-state)
(let ((x-left (- (ufo-pos-x current-state) MOVE-LEN))
(y (ufo-pos-y current-state)))
(cond [(>= x-left (/ UFO-WIDTH 2))
(ufo-pos x-left y)]
[else current-state])))
(define (ufo-move-right current-state)
(let ((x-right (+ (ufo-pos-x current-state) MOVE-LEN))
(y (ufo-pos-y current-state)))
(cond [(<= x-right (- WORLD-WIDTH (/ UFO-WIDTH 2)))
(ufo-pos x-right y)]
[else current-state])))
;;; big bang functions
(define (draw-a-ufo-onto-an-empty-scene current-state)
(place-image UFO
(ufo-pos-x current-state)
(ufo-pos-y current-state)
(empty-scene WORLD-WIDTH WORLD-HEIGHT)))
(define (add-3-to-posy current-state)
(ufo-pos (ufo-pos-x current-state)
(+ (ufo-pos-y current-state) 3)))
(define (posy-is-300 current-state)
(>= (ufo-pos-y current-state) 300))
(define (move-ufo current-state key)
(cond [(key=? key "up") (ufo-move-up current-state)]
[(key=? key "down") (ufo-move-down current-state)]
[(key=? key "left") (ufo-move-left current-state)]
[(key=? key "right") (ufo-move-right current-state)]
[else current-state]))
;;; the big bang
(big-bang (ufo-pos (/ WORLD-WIDTH 2) (/ WORLD-HEIGHT 2))
(to-draw draw-a-ufo-onto-an-empty-scene)
(on-key move-ufo))
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