;; 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 lect05) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ())))




; `cond` -- like if-else-if.
; syntax:
;  (cond [q1 a1]
;        [q2 a2]
;        ...
;        [qn an])
; where each q1..qn is a boolean-expression.


(define sl 'green)  ; sl for stop-light

; turn : symbol -> symbol
; Give the next stop-light color, after turning.
(define (turn sl)
  (cond [(symbol=? sl 'red)    'green]
        [(symbol=? sl 'yellow) 'red]
        [(symbol=? sl 'green)  'yellow]))

(check-expect (turn 'red) 'green)
(check-expect (turn 'green) 'yellow)
(check-expect (turn 'yellow) 'red)

#| The Design Recipe (take 1 -- primitive types only)

  ------- per function:
    4. tests
    5. stub : signature, header, description, stub-body
    6. If your data-type was a union, have a cond with one branch per variant.
    7. complete the body-expression
    8. watch your tests pass
|#

; Task: Write a function to return the tax-due, given taxable-income.
;  (for single tax-payers; we'll stop after 4 brackets)
;  ($9075 => 10%; $36900 => 15%;  else 25% )
; http://www.forbes.com/sites/kellyphillipserb/2013/10/31/irs-announces-2014-tax-brackets-standard-deduction-amounts-and-more/

(check-expect (tax 0) 0)
(check-expect (tax 10) 1)
(check-expect (tax 9000) 900)
(check-expect (tax 9076) (+ 907.5 0.15))
(check-expect (tax 36900) (+ 907.5 (* 0.15 (- 36900 9075))))
(check-expect (tax 50000) (+ 907.5 (* 0.15 (- 36900 9075)) (* 0.25 (- 50000 36900))))


; a taxable-income is either:
;  - a number <= 9075, OR it's
;  - a number >  9075 and is <= 36900,  Or it's
;  - a number > 36900.

; tax : number -> number
;  return the tax-due, given taxable-income.
;  (for single tax-payers; we'll stop after 4 brackets)

(define (tax income)
  (cond [(<= income  9075) (* income 0.10)]
        [(<= income 36900) (+ 907.5 (* 0.15 (- income 9075)))]
        [else              (+ 907.5
                              (* 0.15 (- 36900 9075))
                              (* 0.25 (- income 36900)))]))


; and : boolean boolean -> boolean
; Recall: what is the signature of "<" --
;   < : number number -> boolean