(define WIDTH 150)
(define HEIGHT 180)
(define RADIUS 10)
(define DX 5)
(define DY 5)

; A colored-posn comprises two numbers (x and y) and a string (color).
(define-struct colored-posn [x y color])
; Functions that come "for free" with this:
; make-colored-posn : num num string -> colored-posn
; colored-posn-x : colored-posn -> num
; colored-posn-y : colored-posn -> num
; colored-posn-color : colored-posn -> string
; colored-posn? : object -> boolean


; Useful auxiliary functions
; draw-ball-at-colored-posn : colored-posn -> image
(define (draw-ball-at-colored-posn where)
  (overlay/xy (empty-scene WIDTH HEIGHT)
              (colored-posn-x where) (colored-posn-y where)
              (circle RADIUS "solid" (colored-posn-color where))))
"Examples of draw-ball-at-colored-posn:"
(draw-ball-at-colored-posn (make-colored-posn 50 10 "blue"))
"should be an empty scene with a blue ball at (50,10)"

; move-colored-posn : colored-posn num num -> posn
(define (move-colored-posn old dx dy)
  (make-colored-posn (+ (colored-posn-x old) dx)
                     (+ (colored-posn-y old) dy)
                     (colored-posn-color old)))
"Example of move-posn:"
(move-colored-posn (make-colored-posn 30 20 "blue") -2 9) "should be"
(make-colored-posn 28 29 "blue")

; switch-color : colored-posn -> colored-posn
; switches between green & red; other colors are left unchanged.
(define (switch-color old)
  (make-colored-posn
   (colored-posn-x old)
   (colored-posn-y old)
   (cond [(string=? (colored-posn-color old) "red") "green"]
         [(string=? (colored-posn-color old) "green") "red"]
         [else (colored-posn-color old)])))
"Examples of switch-color:"
(switch-color (make-colored-posn 5 10 "green"))
"should be" (make-colored-posn 5 10 "red")
(switch-color (make-colored-posn 5 10 "red"))
"should be" (make-colored-posn 5 10 "green")
(switch-color (make-colored-posn 5 10 "purple"))
"should be" (make-colored-posn 5 10 "purple")

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

; move-right : colored-posn -> colored-posn
(define (move-right p)
  (move-colored-posn p DX 0))
"examples of move-right:"
(move-right (make-colored-posn 5 7 "orange")) "should be" (make-colored-posn 10 7 "orange")

; move-left : colored-posn -> colored-posn
(define (move-left p)
  (move-colored-posn p (- DX) 0))
"example of move-left:"
(move-left (make-colored-posn 29 4 "purple")) "should be" (make-colored-posn 24 47 "purple")

; move-up : colored-posn -> colored-posn
(define (move-up p)
  (move-colored-posn p 0 (- DY)))
"example of move-up:"
(move-up (make-colored-posn 29 47 "green")) "should be" (make-colored-posn 29 42 "green")

; move-down : colored-posn -> colored-posn
(define (move-down p)
  (move-colored-posn p 0 DY))
"example of move-down:"
(move-down (make-colored-posn 29 47 "black")) "should be" (make-colored-posn 29 52 "black")


; The next world is moved up, down, left, right, or nowhere depending on what key the user hit.
; handle-key : world -> world
(define (handle-key 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 handle-key:"
(handle-key 'right (make-colored-posn 29 47 "pink")) "should be" (make-colored-posn 34 47 "pink")
(handle-key 'left (make-colored-posn 29 47 "red")) "should be" (make-colored-posn 24 47 "red")
(handle-key 'up (make-colored-posn 29 47 "yellow")) "should be"  (make-colored-posn 29 42 "yellow")
(handle-key 'down (make-colored-posn 29 47 "blue")) "should be" (make-colored-posn 29 52 "blue")

(big-bang WIDTH HEIGHT 1 (make-colored-posn 50 50 "red"))
(on-redraw-event show-world)
(on-key-event handle-key)
(on-tick-event switch-color)
; note that switch-color has the right contract (world -> world)
; to be a tick-handler, even though it doesn't happen to be named "handle-tick".