;; solutions to Guided Practice 6.1: pizza problem


(define-struct topped-pizza (topping base))
;; A Topping is a String.
;; A Pizza is either
;; -- the string "plain crust"
;; -- (make-topped-pizza Topping Pizza)
;; INTERP:
;; "plain crust" means a pizza with no toppings
;; (make-topped-pizza t p) represents the pizza p with topping t added
;; on top.

;; empty-pizza? : Pizza -> Boolean
;; RETURNS: true iff the pizza is empty
;; STRATEGY: function composition
;; Note: string=? only works on strings, so we need to _guard_ call the
;; string-equal? with a string? test.
(define (empty-pizza? p)
  (if (string? p)
    (string-equal? p "plain crust")
    false))
    

;; 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: Structural decomposition on p : Pizza
(define (replace-all-anchovies-with-onions p)
  (cond
    [(empty-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: Structural decomposition on p : Pizza
(define (replace-all-anchovies p replacement)
  (cond
    [(empty-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: Structural decomposition on p : Pizza
(define (replace-topping p topping replacement)
  (cond
    [(empty-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)))]))