;; #1 ---------------------------------------

;; in-category? : stock symbol -> boolean
;; is a stock item in a particular category?
(define (in-category? a-stock category)
  (symbol=? category (album-category (stock-album a-stock))))

;; #2 ---------------------------------------

(define-struct update (artist title type change-in-copies change-in-price))

(define u1 (make-update "Beatles" "Abbey Road" 'tape 0 -2))
(define u2 (make-update "Pink Floyd" "The Wall" 'cd 2 0))
(define u3 (make-update "Beatles" "Yellow Submarine" 'cd 0 -2))
(define u4 (make-update "Pink Floyd" "The Wall" 'tape 2 0))
(define u5 (make-update "Beatles" "Abbey Road" 'cd 4 5))

;; need one that checks update for a stock item
;; apply-update? : an-update a-stock -> boolean
;; determines whether an update applies to a stock item
(define (apply-update? an-update a-stock)
  (and (string=? (update-artist an-update)
		 (album-artist (stock-album a-stock)))
       (string=? (update-title an-update)
		 (album-title (stock-album a-stock)))))

;; update-format : symbol update format -> format
;; update a format with new inventory information
(define (update-format type an-update a-format)
  (cond [(boolean? a-format) a-format]
	[else
	  (make-format type
	               (+ (format-copies a-format)
			  (update-change-in-copies an-update))
		       (+ (format-price a-format)
			  (update-change-in-price an-update)))]))

;; update-stock : update stock -> stock
;; update a stock item with new inventory information
(define (update-stock an-update a-stock)
  (cond [(symbol=? (update-type an-update) 'cd)
	 (make-stock (stock-album a-stock)
	             (update-format 'cd an-update (stock-cd a-stock))
		     (stock-tape a-stock))]
	[(symbol=? (update-type an-update) 'tape)
	 (make-stock (stock-album a-stock)
		     (stock-cd a-stock)
		     (update-format 'tape an-update (stock-tape a-stock)))]))

;; update-stock : update stock -> stock
;; update a stock item with new inventory information
(define (update-stock an-update a-stock)
  (cond [(symbol=? (update-type an-update) 'cd)
	 (cond [(boolean? (stock-cd a-stock)) a-stock]
	       [else
		 (make-stock (stock-album a-stock)
		             (make-format 'cd (+ (format-copies (stock-cd a-stock))
						 (update-change-in-copies an-update))
			                      (+ (format-price (stock-cd a-stock))
						 (update-change-in-price an-update)))
			     (stock-tape a-stock))])]
	[(symbol=? (update-type an-update) 'tape)
	 (cond [(boolean? (stock-tape a-stock)) a-stock]
	       [else
		 (make-stock (stock-album a-stock)
		             (stock-cd a-stock)
			     (make-format 'tape (+ (format-copies (stock-tape a-stock))
						   (update-change-in-copies an-update))
			                        (+ (format-price (stock-tape a-stock))
						   (update-change-in-price an-update))))])]))