;; ------------------------------------------------------------------------------------
;; dan's cannon simulation
;; TODO: add realistic sound effects
;; ------------------------------------------------------------------------------------

(define-struct obj (x y vx vy))

(define WORLDX 700)
(define WORLDY 350)

(define g -10) ;; a gross estimate

;; ------------------------------------------------------------------------------------
;; draw-obj : (make-obj x y vx vy) symbol -> boolean
;; paint a cannonball
(define (draw-obj o color)
  (draw-solid-disk 
   (make-posn (obj-x o) 
              (- WORLDY (obj-y o))) ;; upside down
   8 color))

;; ------------------------------------------------------------------------------------
;; launch : number number symbol -> boolean
;; provides a convenient way to launch a cannonball at a given angle (in degrees) and 
;; speed (in meters per second) from (0,0)
(define (launch angle speed color)
  (local
      ((define x0 0)
       (define y0 0)
       (define rads (* 2 pi (/ angle 360.0)))
       (define vx0 (* speed (cos rads)))
       (define vy0 (* speed (sin rads))))
    (and
     (draw-solid-line (make-posn x0 (- WORLDY y0))
                      (make-posn (+ x0 (* 5 vx0))
                                 (- WORLDY (+ y0 (* 5 vy0))))
                      color)
     (sim (make-obj x0 y0 vx0 vy0) color))))

;; ------------------------------------------------------------------------------------
;; sim : (make-obj x y vx vy) symbol -> boolean
;; simulates the motion of the given object in the xy-plane, given the 
;; constant nagging influence of gravity in the y-direction. 
;; simulation will stop when the object hits the ground.
(define (sim o color)
  (cond 
    [(< (obj-y o) 0) 
     ; we're done. print to the display where the cannonball hit the ground (roughly)
     (draw-solid-string 
      (make-posn (obj-x o) (- WORLDY 10)) 
      (format "x = ~a m" (inexact->exact (round (obj-x o)))))]
    [else 
     ; keep simulating.
     (and
      (sleep-for-a-while 0.1)
      (draw-obj o color)
      (sim (make-obj
            ; x1 = x0 + vx0
            ; y1 = y0 + vy0
            (+ (obj-x o) (obj-vx o))
            (+ (obj-y o) (obj-vy o))
            ; vx1 = vx0 + accel_x
            ; vy1 = vy0 + accel_y
            (obj-vx o)
            (+ (obj-vy o) g)) 
           color))]))

;; ------------------------------------------------------------------------------------
;; fire at will!

(start WORLDX WORLDY)
(wait-for-mouse-click)
(launch 45 70 'black)
(wait-for-mouse-click)
(launch 60 70 'blue)
;(wait-for-mouse-click)
;(launch 45 35 'red)