;; The first three lines of this file were inserted by DrScheme. 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 11.5.1) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor mixed-fraction #f #t none #f ())))
; Worked exercise 11.5.1
(define MILES-PER-GALLON #i28)
(define PRICE-PER-GALLON 2.459)
(define MOTEL-PRICE-PER-NIGHT 40)
(define CAR-RENTAL-FIXED-FEE 10)
(define CAR-RENTAL-PER-DAY 29.95)
(define CAR-RENTAL-PER-MILE 0.10)

; gas-needed : number (miles) -> number
(define (gas-needed miles)
  ; miles              a number
  ; MILES-PER-GALLON   a number
  (/ miles MILES-PER-GALLON)
  )
(check-within (gas-needed 0) 0 .01)
(check-within (gas-needed 28) 1 .01)
(check-within (gas-needed 56) 2 .01)
(check-within (gas-needed 77) 2.75 .01)
(check-within (gas-needed 358) 12.8 .02) ; actually not within .01, but .02 will do.

; cost-of-gallons : number (gallons) -> number
(define (cost-of-gallons gallons)
  ; gallons             number
  ; PRICE-PER-GALLON    number
  (* gallons PRICE-PER-GALLON)
  )
(check-within (cost-of-gallons 0) 0 .01)
(check-within (cost-of-gallons 1) 2.459 .01)
(check-within (cost-of-gallons 2) 4.918 .01)
(check-within (cost-of-gallons 2.75) 6.76225 .01)

; gas-cost : number(miles) -> number

; Note: all the test cases will need to be modified if PRICE-PER-GALLON changes.
(check-within (gas-cost 0) 0 .01)
(check-within (gas-cost 28) 2.459 .01) ; i.e.   one gallon
(check-within (gas-cost 56) 4.918 .01) ; i.e.   two gallons
(check-within (gas-cost 77) 6.76 .01)  ; 2-3/4 gal; use calculator
(check-within (gas-cost 358) 31.44 .01) ; yecch; use calculator

(define (gas-cost miles)
  ; miles           a number
  ; #i28            a fixed number (mpg)
  ; 2.459           a fixed number ($/gal)
  (cost-of-gallons (gas-needed miles))
  )

; nights-in-motel : number (days) -> number
; Assumes the number of days is a positive integer.
(check-expect (nights-in-motel 1) 0)
(check-expect (nights-in-motel 2) 1)
(check-expect (nights-in-motel 38) 37)

(define (nights-in-motel days)
  ; days     number
  (- days 1)
  )

; motel-cost : number (days) -> number
(check-expect (motel-cost 1) 0)
(check-expect (motel-cost 2) 40)
(check-expect (motel-cost 38) 1480)

(define (motel-cost days)
  ; days                    number
  ; MOTEL-PRICE-PER-NIGHT   number
  ; (nights-in-motel days)  number
  (* MOTEL-PRICE-PER-NIGHT (nights-in-motel days))
  )

; rental-cost : number (miles) number (days) -> number
(check-expect (rental-cost 0 1) 39.95)
(check-expect (rental-cost 0 2) 69.90)
(check-expect (rental-cost 100 1) 49.95)
(check-expect (rental-cost 100 2) 79.90)
(check-expect (rental-cost 28 1) 42.75)
(check-expect (rental-cost 77 2) 77.60)
(check-expect (rental-cost 358 3) 135.65)

(define (rental-cost miles days)
  ; miles                number
  ; days                 number
  ; CAR-RENTAL-FIXED-FEE number
  ; CAR-RENTAL-PER-DAY   number
  ; CAR-RENTAL-PER-MILE  number
  ; (* days CAR-RENTAL-PER-DAY)   number
  ; (* miles CAR-RENTAL-PER-MILE) number
  (+ (* days CAR-RENTAL-PER-DAY)
     (* miles CAR-RENTAL-PER-MILE)
     CAR-RENTAL-FIXED-FEE)
  )

; road-trip-cost : number (miles) number (days) -> number
(check-within (road-trip-cost 0 1) 39.95 .01) ; gas and motels are $0
(check-within (road-trip-cost 0 2) 109.90 .01) ; gas still $0, motel $40
(check-within (road-trip-cost 28 1) 45.209 .01)
; $42.75 for car, $0 for motel, $2.459 for gas
(check-within (road-trip-cost 77 2) 124.36 .01)
; $77.60 for car, c. $6.76 for gas, $40 for motel
(check-within (road-trip-cost 358 3) 247.09 .01)
; $135.65 for car, c. $31.44 for gas, $80 for motel

(define (road-trip-cost miles days)
  ; miles            number
  ; days             number
  ; (gas-cost miles) number
  ; (motel-cost days) number
  ; (rental-cost miles days) number
  (+ (gas-cost miles)
     (motel-cost days)
     (rental-cost miles days))
  )