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+#lang racket
+(require 2htdp/universe 2htdp/image)
+
+;;; world structure
+(struct ufo (x y fart))
+
+;;; constants
+(define WORLD-WIDTH 300)
+(define WORLD-HEIGHT 325)
+(define MOVE-LEN 3)
+(define UFO (bitmap/file "resources/zarking-ufo.png"))
+(define UFO-FART (bitmap/file "resources/ufo-fart.png"))
+(define UFO-WIDTH (image-width UFO))
+(define UFO-HEIGHT (image-height UFO))
+(define UFO-FART-HEIGHT (image-height UFO-FART))
+
+;;; ufo movement functions
+(define (ufo-move-up current-state)
+  (let ((x (ufo-x current-state))
+        (y-up (- (ufo-y current-state) MOVE-LEN))
+        (fart #t))
+    (cond [(>= y-up (+ (/ UFO-HEIGHT 2) (/ UFO-FART-HEIGHT 2)))
+           (ufo x y-up fart)]
+          [else current-state])))
+
+(define (ufo-move-down current-state)
+  (let ((x (ufo-x current-state))
+        (y-down (+ (ufo-y current-state) MOVE-LEN)))
+    (cond [(<= y-down (- (+ WORLD-HEIGHT (/ UFO-FART-HEIGHT 2))
+                         (/ UFO-HEIGHT 2)))
+           (ufo x y-down #t)]
+          [else current-state])))
+
+
+(define (ufo-move-left current-state)
+  (let ((x-left (- (ufo-x current-state) MOVE-LEN))
+        (y (ufo-y current-state))
+        (fart #t))
+    (cond [(>= x-left (/ UFO-WIDTH 2))
+           (ufo x-left y fart)]
+          [else current-state])))
+
+(define (ufo-move-right current-state)
+  (let ((x-right (+ (ufo-x current-state) MOVE-LEN))
+        (y (ufo-y current-state))
+        (fart #t))
+    (cond [(<= x-right (- WORLD-WIDTH (/ UFO-WIDTH 2)))
+           (ufo x-right y fart)]
+          [else current-state])))
+
+
+;;; big bang functions
+(define (draw-a-ufo current-state)
+  (place-image (overlay/align/offset
+                "middle" "bottom" UFO 0 35
+                (if (ufo-fart current-state)
+                    UFO-FART
+                    (circle 0 "outline" "white")))
+               (ufo-x current-state)
+               (ufo-y current-state)
+               (empty-scene WORLD-WIDTH WORLD-HEIGHT)))
+
+(define (add-3-to-posy current-state)
+  (ufo (ufo-x current-state)
+           (+ (ufo-y current-state) 3)))
+
+(define (posy-is-300 current-state)
+  (>= (ufo-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]))
+
+(define (ufo-stopped current-state key)
+  (let ((fart #f))
+    (ufo (ufo-x current-state) (ufo-y current-state) fart)))
+
+;;; the big bang
+(big-bang (ufo (/ WORLD-WIDTH 2) (/ WORLD-HEIGHT 2) #f)
+          (to-draw draw-a-ufo)
+          (on-key move-ufo)
+          (on-release ufo-stopped))
+