;; a position (posn) represents a dot (pixel) by its x and y 
;; coordinates. Use a function make-posn to create a new
;; position and posn-x, posn-y to get x and y coordinates of
;; an already created position

(make-posn 5 6) ; make a position with x = 5, y = 6
(define pos (make-posn 3 4)) ; make a position with x = 3, y = 4 and call it pos
pos ; printing the value of pos
(posn-x pos) ; getting x coordinate of pos

;; distance-to-0 : posn  ->  number
;; to compute the distance of a-posn to the origin 
(define (distance-to-0 a-posn)
  (sqrt
    (+ (sqr (posn-x a-posn))
       (sqr (posn-y a-posn)))))

;; Tests 
(check-expect (distance-to-0 (make-posn 3 4)) 5)
(check-expect (distance-to-0 (make-posn 8 6)) 10)
(check-expect (distance-to-0 (make-posn 5 12)) 13)

;; drawing with positions -- teachpack draw.ss required
(define pixel-10-10 (make-posn 10 10))
(define pixel-100-10 (make-posn 100 10))
(define pixel-150-150 (make-posn 150 150))
(define pixel-10-20 (make-posn 10 20))

;; open the canvas
(start 300 300)
(draw-solid-line pixel-10-10 pixel-100-10 'green) 
(draw-solid-disk pixel-150-150 50 'orange)
(draw-solid-rect pixel-10-20 100 1500 'blue)

;; sleep for a while
(sleep-for-a-while 1)

;; clear drawings 
(clear-solid-line pixel-10-10 pixel-100-10)
(sleep-for-a-while 1)
(clear-solid-disk pixel-150-150 50)
(sleep-for-a-while 1)
(clear-solid-rect pixel-10-20 100 1500)

 
;; add more drawings: concentric circles