;; solutions to Guided Practice 6.1: pizza problem

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;;; QUESTION 1
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(define-struct plain-pizza ())
(define-struct topped-pizza (topping base))

;; A Topping is a String.

;; A Pizza is either
;; -- (make-plain-pizza)
;; -- (make-topped-pizza Topping Pizza)
;; INTERP:
;; (make-plain-pizza)      represents a pizza with no toppings
;; (make-topped-pizza t p) represents the pizza p with topping t added
;;                         on top.
    
;; replace-all-anchovies-with-onions 
;;   : Pizza -> Pizza
;; RETURNS: a pizza like the given pizza, but with
;; anchovies in place of each layer of onions
;; STRATEGY: Use template for Pizza on p

(define (replace-all-anchovies-with-onions p)
  (cond
    [(plain-pizza? p) empty]
    [else (if (string=? (topped-pizza-topping p) "anchovies")
            (make-topped-pizza "onions"
              (replace-all-anchovies-with-onions 
               (topped-pizza-base p)))
            (make-topped-pizza (topped-pizza-topping p)
              (replace-all-anchovies-with-onions 
               (topped-pizza-base p))))]))


;; replace-all-anchovies : Pizza Topping -> Pizza
;; RETURNS: a pizza like the given pizza, but with 
;; all anchovies replaced by the given topping.
;; STRATEGY: Use template for Pizza on p

(define (replace-all-anchovies p replacement)
  (cond
    [(plain-pizza? p) empty]
    [else (if (string=? (topped-pizza-topping p) "anchovies")
            (make-topped-pizza replacement
              (replace-all-anchovies
                (topped-pizza-base p)
                replacement))
            (make-topped-pizza (topped-pizza-topping p)
              (replace-all-anchovies 
               (topped-pizza-base p)
               replacement)))]))


;; replace-topping : Pizza Topping Topping -> Pizza
;; RETURNS: a pizza like the given one, but with 
;; all instances of the first topping replaced by
;; the second one.
;; STRATEGY: Use template for Pizza on p

(define (replace-topping p topping replacement)
  (cond
    [(plain-pizza? p) empty]
    [else (if (string=? (topped-pizza-topping p) topping)
            (make-topped-pizza replacement
              (replace-topping 
               (topped-pizza-base p)
               topping
               replacement))
            (make-topped-pizza (topped-pizza-topping p)
              (replace-topping 
               (topped-pizza-base p)
               topping
               replacement)))]))

;; another solution (also Use template for Pizza on p):

(define (replace-topping p topping replacement)
  (cond
    [(plain-pizza? p) empty]
    [else (make-topped-pizza
            (if (string=? (topped-pizza-topping p) topping)
              replacement
              (topped-pizza-topping p))
            (replace-topping 
               (topped-pizza-base p)
               topping
               replacement))]))