;; UFO game
(define CWIDTH 600)
(define CHEIGHT 400)

(define CANVAS (empty-scene CWIDTH CHEIGHT))
(define TANK (rectangle (* CWIDTH 0.1) (* CHEIGHT 0.1) "solid" "darkgreen"))
(define MISSILE (triangle 10 "solid" "red"))
(define UFO (ellipse (* CWIDTH 0.15) (* CHEIGHT .1) "solid" "blue"))

(define-struct aim (ufo tank))
(define-struct fired (ufo tank missile))
; A UFO is Posn.
; interp.: (make-posn x y) is the UFO's current location

; A Missile is Posn.S
; interp.: (make-posn x y) is the missile's current location

(define-struct tank (loc vel))
; A Tank is (make-tank Number Number).
; interp.: (make-tank x dx) means the tank is at (x ,HEIGHT)
;   and that it moves dx pixels per clock tick

(define w0 (make-aim (make-posn (random CWIDTH) 5)
                     (make-tank (floor (/ CWIDTH 2)) 0)))

;; on-tick : world -> world
;; process movements

(big-bang w0
          (on-tick tock 0.5)
          (on-key typed)
          (stop-when end?))