;; a cd is a (make-cd string string number number string)
(define-struct cd(title artist price copies category))

;; a list-of-cd is either
;; empty, or
;; (cons cd list-of-cd)

(define CDs (list (make-cd "greatest hits" "abba" 12.95 42 "rock")
                  (make-cd "beach boys live" "beach boys" 10.00 120 "rock")
                  (make-cd "yanni at the acropolis" "yanni" 5.00 0 "new age")
                  (make-cd "marriage of figaro" "mozart" 20.00 50 "classical")))

;; all-titles:  list-of-cd -> list-of-string
;; consumes a list of cds and produces a list of all the CD titles
(define (all-titles alocd)
  (map cd-title alocd))


;; titles-in-stock:  list-of-cd -> list-of-string
;; consumes a list-of-cd and produces the titles of all those cds with at least 1 copy in stock
(define (titles-in-stock alocd)
  (local [(define (more-than-zero? acd) (> (cd-copies acd) 0))]
    (map cd-title (filter more-than-zero? alocd))))



;; restock:  string number list-of-cd -> list-of-cd
;; consumes the name of a cd, a number of new copies, and a list of cds and produces a list the same as the original except that
;; the named cd has the given number of additional copies
;; (This could also be done with map...try it both ways)
(define (restock name extras alocd)
  (cond [(empty? alocd) empty]
        [(cons? alocd) (cons (add-if-named name extras (first alocd))
                             (restock (rest alocd)))]))

;; add-if-named:  string number cd -> cd
;; consumes the name of a cd, the number of additional copies, and a cd.
;; If the cd has the given name, the cd is returned with the additional number of copies added.
;; Otherwise, the cd is returned unchanged.
(define (add-if-named name extras acd)
  (cond [(string=? name (cd-title acd)) (make-cd (cd-title acd)
                                                 (cd-artist acd)
                                                 (cd-price acd)
                                                 (+ extras (cd-copies acd))
                                                 (cd-category acd))]
        [else acd]))



;; titles-by:  string list-of-cd -> list-of-string
;; consumes an artist name and inventory and produces a list of titles of CDs by that artist
(define (titles-by name alocd)
  (local [(define (name-match? acd) (string=? name (cd-artist acd)))]
    (map cd-title (filter name-match? alocd))))


;; copies-in-stock:  string string list-of-cd -> number
;; consumes a CD title, an artist name, and an inventory and produces the number of copies of that item that are in stock. 
;; Return 0 if the named CD isn't in the inventory.
(define (copies-in-stock title name alocd)
  (cond [(empty? alocd) 0]
        [(cons? alocd) 
         (cond [(and (string=? title (cd-title (first alocd))) (string=? name (cd-artist (first alocd)))) (cd-copies (first alocd))]
               [else (copies-in-stock title name (rest alocd))])]))