;; A Color is a (make-color red green blue)
;; where red, green, and blue are Numbers

;; Examples
(define RED   (make-color 255 0 0))
(define GREEN (make-color 0 255 0))
(define BLUE  (make-color 0 0 255))
(define MID   (make-color 85 85 85))

#| TEMPLATE for color
(define (f c)
  ... (color-red   c) ...
  ... (color-green c) ...
  ... (color-blue  c) ... )
|#




;; A ListOfColor is either
;; -- empty
;; (cons Color ListOfColor)

#| TEMPLATE for ListOfColor
(define (c loc)
  (cons
   [(empty? loc) ...]
   [(cons?  los) ... (first loc) ... (f (rest loc)) ...]))
|#

;; Examples
empty
(cons RED empty)
(cons BLUE (cons RED empty))




;; Number Number Number -> Number
;; Find the average value of the color portions
(define  (color-avg r g b)
  (round (/ (+ r g b) 3)))

;; Tests
(= (color-avg 255 0 0) 85)
(= (color-avg 0 255 0) 85)
(= (color-avg 0 0 255) 85)




;; ColorList -> ColorList
;; Convert all color to BW colors
(define (all-bw loc)
  (cond
    [(empty? loc) empty]
    [(cons?  loc) (cons (make-color (color-avg (color-red (first loc))
                                               (color-green (first loc))
                                               (color-blue (first loc)))
                                    (color-avg (color-red (first loc))
                                               (color-green (first loc))
                                               (color-blue (first loc)))
                                    (color-avg (color-red (first loc))
                                               (color-green (first loc))
                                               (color-blue (first loc))))
                        (all-bw    (rest loc)))]))

;; Tests
(equal? (all-bw empty) empty)
(equal? (all-bw (cons RED empty)) (cons MID empty))



;; Image -> Image
;; Convert img to black and white
(define (image->bw img)
  (color-list->image (all-bw (image->color-list img))
                     (image-width img)
                     (image-height img)
                     (/ (image-width img) 2)
                     (/ (image-height img) 2)))