(define WINDOW-WIDTH 150)
(define WINDOW-HEIGHT 180)
(define DIAMETER 10)
(define DX 5)
(define DY 5)

; A world is a posn.
; Its display is a blue ball at those coordinates
; show-world : world -> image
(define (show-world where) 
  (overlay/xy (empty-scene WINDOW-WIDTH WINDOW-HEIGHT)
              (posn-x where) (posn-y where)
              (circle DIAMETER "solid" "blue")))
"Example of show-world:"
(show-world (make-posn 30 50))
"should be"
(overlay/xy (empty-scene WINDOW-WIDTH WINDOW-HEIGHT) 30 50
            (circle DIAMETER "solid" "blue"))

; move-right : posn -> posn
(define (move-right p)
  (make-posn (+ (posn-x p) DX) (posn-y p)))
"examples of move-right:"
(move-right (make-posn 5 7)) "should be" (make-posn 10 7)

; move-left : posn -> posn
(define (move-left p)
  (make-posn (- (posn-x p) DX) (posn-y p)))
"example of move-left:"
(move-left (make-posn 29 47)) "should be" (make-posn 24 47)

; move-up : posn -> posn
(define (move-up p)
  (make-posn (posn-x p) (- (posn-y p) DY)))
"example of move-up:"
(move-up (make-posn 29 47)) "should be" (make-posn 29 42)

; move-down : posn -> posn
(define (move-down p)
  (make-posn (posn-x p) (+ (posn-y p) DY)))
"example of move-down:"
(move-down (make-posn 29 47)) "should be" (make-posn 29 52)


; The next world is moved up, down, left, right, or nowhere depending on what key the user hit.
; next-world : world -> world
(define (next-world key where) 
  (cond [(not (symbol? key)) where] ; handle letters, numbers, etc.
        [(symbol=? key 'right)
         (move-right where)]
        [(symbol=? key 'left)
         (move-left where)]
        [(symbol=? key 'up)
         (move-up where)]
        [(symbol=? key 'down)
         (move-down where)]
        [else where]))
"Examples of next-world:"
(next-world 'right (make-posn 29 47)) "should be" (make-posn 34 47)
(next-world 'left (make-posn 29 47)) "should be" (make-posn 24 47)
(next-world 'up (make-posn 29 47)) "should be"  (make-posn 29 42)
(next-world 'down (make-posn 29 47)) "should be" (make-posn 29 52)

(big-bang WINDOW-WIDTH WINDOW-HEIGHT 1 (make-posn 50 50))
(on-update-event show-world)
(on-key-event next-world)