;; The first three lines of this file were inserted by DrRacket. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-beginner-reader.ss" "lang")((modname pens-conditionals) (read-case-sensitive #t) (teachpacks ((lib "universe.ss" "teachpack" "2htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "universe.ss" "teachpack" "2htdp")))))
;; A store sells pens for 75 cents each for fewer than 10
;;                     or 50 cents each for 10 or more
;; Model the cost per pen

;; cost-per-pen number -> number
;; calculate the cost for a single pen given bulk discounts
(define (cost-per-pen count) 
  (cond [(< count 10) .75]
        [(>= count 10) .50]))

(check-expect (cost-per-pen 10) .5)

;; The store allows printing slogans on the pens.  
;; Slogans add 2 cents per letter and a $3 charge for the full order

;; order-cost number string -> number
;; calculates the price (in dollars) of an order of pens with a certain slogan
;(define (order-cost count slogan)
;  (+ (* count (+ (cost-per-pen count) 
;                 (* (string-length slogan) .02)))
;     3))
;
;(check-expect (order-cost 10 "hi!") 8.6)
;(check-expect (order-cost 10 "") 5.0)

;; The example above fails for an empty slogan because the $3 surcharge is still applied!
  
;; order-cost number string -> number
;; calculates the price (in dollars) of an order of pens with a certain slogan
;(define (order-cost count slogan)
;  (cond [(string=? slogan "") (* count (cost-per-pen count))]
;        [else (+ (* count (+ (cost-per-pen count) 
;                             (* (string-length slogan) .02)))
;                 3)]))
;
;(check-expect (order-cost 10 "hi!") 8.6)
;(check-expect (order-cost 10 "") 5.0)

;; The example above works fine but it's redundant

;; pens-cost number string -> number
;; calculates the price (in dollars) of count pens with a certain slogan
(define (pens-cost count slogan)
  (* count (+ (cost-per-pen count)  
              (* (string-length slogan) .02))))

;; surcharge string -> number
;; calculates the order surcharge (in dollars) for the given slogan
(define (surcharge slogan)
  (cond [(string=? slogan "") 0]
        [else 3]))

;; order-cost number string -> number
;; calculates the price (in dollars) of an order of pens with a certain slogan
(define (order-cost count slogan)
  (+ (pens-cost count slogan) 
     (surcharge slogan)))

(check-expect (order-cost 10 "hi!") 8.6)
(check-expect (order-cost 10 "") 5.0)

;; We've refined our program so it's easier to read