;; ****************************************
;; *  Csu 211 : 1/28/2008
;; *  Lecture #8 Code
;; *  Structure Review
;; ****************************************


;;**** NOTE: This code is broken!!  It's your task
;;           to fix it  :(

;;     Read through the whole thing... it will explain
;;       lots of things I didn't have time to go over
;;       in class.  I removed all the Magic, and renamed
;;       to more concrete things.

;; Radius Constant
(define R 50)

;;     meaning            Where    Direction 'vector'
;; A World is: (make-world Pnt        Pnt)
(define-struct world (p d))

;; A Pnt is (make-pnt Number Number)
;; Represents a pair of coordinates *OR* a vector direction
(define-struct pnt (x y))


;; image: World -> Scene
;; Draw a Ball into an empty Scene
(define (image w)
  (place-image (circle R "solid" "red")
               (pnt-x (world-p w))
               (pnt-y (world-p w)) (empty-scene 400 400)))


;; xy-length: Number Number -> Number
;; The length of this x/y pair interpreted as a Vector
(define (xy-length x y)
  (sqrt (+ (sqr x) (sqr y))))


;; vec-length: Pnt -> Number
;; The length of this Pnt interpreted as a Vector
(define (vec-length p)
  (xy-length (pnt-x p) (pnt-y p)))


;; vec-scale: Pnt -> Pnt
;; Scale this Pnt pair interpreted as a Vector, to have
;;   unit length (i.e., 1)
(define (vec-scale p)
  (make-pnt (/ (pnt-x p) (vec-length p)) 
            (/ (pnt-y p) (vec-length p))))


;; ** Tests
;; Here's some tests that should explain what all these
;;   functions do...
"Vector Tests"
(= 50 (xy-length 40 30))
(= 100 (xy-length 60 80))
(= 1 (vec-length (vec-scale (make-pnt 19 -21))))
(= 1 (vec-length (vec-scale (make-pnt -14 23))))


;; slow-speed: World -> World
;; Reduce the speed but keep the direction of the Ball
;;   in the World
(define (slow-speed w) 
  (make-world (vec-scale (world-p w)) (world-d w)))


;; tick: World -> World
;; On the tick, move the ball in the right direction
(define (tick w)
  (make-world (make-pnt (+ (pnt-x (world-p w))
                           (pnt-x (world-d w)))
                        (+ (pnt-y (world-p w))
                           (pnt-y (world-d w))))
              (world-d w)))


;; mouse: World Number Number Symbol -> World
;; Redirect the Ball on a mouse-press
(define (mouse w x y s)
  (cond [(symbol=? 'button-down s)
         (slow-speed (make-world (world-p w)
                                 (make-pnt (- x (pnt-x (world-p w)))
                                           (- y (pnt-y (world-p w))))))]
         [else w]))

;; Setup for the window and drawing of the World
(big-bang 400 400 .2 (make-world (make-pnt 200 200)
                                 (make-pnt 0 0)))
(on-redraw image)
(on-tick-event tick)
(on-mouse-event mouse)