;;;; This example illustrates the use of the design recipe for
;;;; functions consuming compound (structured) data.
;;;; This one is a little more complex than some because there
;;;; are actually 2 kinds of structured data involved (Circle
;;;; and Posn), and the right way to deal with this is to develop
;;;; an additional helper function.

;; (Step 1) Data definition
(define-struct circle (center radius color))
;; A Circle is (make-circle Posn Number Symbol)

;; (Step 1 continued) Circle examples
(define c1 (make-circle (make-posn 3 4) 2 'red))
(define c2 (make-circle (make-posn 0 0) 5 'black))

;; Note that we're taking for granted the following data definition:
;; A Posn is (make-posn Number Number)

;; (Step 2) Contract/purpose/header (for main function)
;; move-circle : Circle Number -> Circle
;; moves a circle horizontally by a given distance
;; (define (move-circle a-circle dist) ...)

#| (Step 4) Template (for main function)
(define (move-circle a-circle dist)
  ... (circle-center a-circle) ...
  ... (circle-radius a-circle) ...
  ... (circle-color a-circle) ... )
|#

;; (Step 5) Body (of main function)
(define (move-circle a-circle dist)
  (make-circle
   (move-posn (circle-center a-circle) dist)
   (circle-radius a-circle)
   (circle-color a-circle)))

;; (Step 2) Contract/purpose/header (for helper)
;; move-posn : Posn Number -> Posn
;; moves a posn horizontally
;; (define (move-posn a-posn delta-x) ...)

#| (Step 4) Template (for helper)
(define (move-posn a-posn delta-x)
  ... (posn-x a-posn) ...
  ... (posn-y a-posn) ... )
|#

;; (Step 4) Body (for helper)
(define (move-posn a-posn delta-x)
  (make-posn
   (+ (posn-x a-posn) delta-x)
   (posn-y a-posn)))

;; (Step 3) Examples and (Step 6) Tests (for helper)
(move-posn (make-posn 3 4) 5) "should be" (make-posn 8 4)
(move-posn (make-posn 0 0) -2) "should be" (make-posn -2 0)

;; (Step 3) Examples and (Step 4) Tests (for main function)
(move-circle c1 -2)
"should be"
(make-circle (make-posn 1 4) 2 'red)

(move-circle c2 10)
"should be"
(make-circle (make-posn 10 0) 5 'black)