; A Document-Summary is one of
;  (make-letter Symbol Symbol Symbol)
;  (make-memo Symbol Symbol Symbol Symbol)
;  (make-resume Symbol Symbol Boolean)
 
(define-struct letter (to date signature))
(define-struct memo (from to date subject))
(define-struct resume (name date sent?))
 
;;from : Document-Summary -> Symbol
;; - return who the document is from
(define (from a-doc)
  (cond
   [(letter? a-doc) (letter-signature a-doc)]
   [(memo? a-doc) (memo-from a-doc)]
   [(resume? a-doc) (resume-name a-doc)]))
 
#|TESTS|#
(from (make-letter 'Mike 'Feb-14 'Kate))
"should be" 'Kate
(from (make-memo 'Chris 'Jean 'Jan-22 'Budget))
"should be" 'Chris
(from (make-resume 'Tara 'May-1 false))
"should be" 'Tara
 
 

; A 3dshape is one of
;  (make-cube Number)
;  (make-prism Number Number Number)
;  (make-sphere Number)
 
(define-struct cube (edge))
(define-struct prism (length width height))
(define-struct sphere (radius))
 
;;volume: 3dshape -> Number
;  return volume of shape
(define (volume a-3dshape)
  (cond
   [(cube? a-3dshape) (expt (cube-edge a-3dshape) 3)]
   [(prism? a-3dshape) (* (prism-length a-3dshape)
			  (prism-width a-3dshape)
			  (prism-height a-3dshape))]
   [(sphere? a-3dshape) 
    (* 4/3 pi (expt (sphere-radius a-3dshape) 3))]))
 
#| TESTS |#
(volume (make-cube 2))
"should be" 8
(volume (make-prism 2 3 4))
"should be" 24
(volume (make-sphere 2))
"should be" (* 4/3 8 pi)
 
 
 

; A train is one of
;  (make-commuter Number Number Boolean)
;  (make-amtrak Number Number Symbol)
;  (make-subway Number Number Symbol)
 
(define-struct commuter (cars ppc allstops?))
(define-struct amtrak (cars ppc type))
(define-struct subway (cars ppc color))
 
;;capacity : Train -> Number
; - return passenger capacity of train
(define (capacity a-train)
  (cond
   [(commuter? a-train)
    (* (commuter-cars a-train) (commuter-ppc a-train))]
   [(amtrak? a-train)
    (* (amtrak-cars a-train) (amtrak-ppc a-train))]
   [(subway? a-train) 
    (* (subway-cars a-train) (subway-ppc a-train))]))
 
;;hold-all? : Train Number -> Boolean
;  - return true if train will hold number of passengers, 
;    false otherwise
(define (hold-all? a-train passengers)
  (<= passengers (capacity a-train)))
 
 
#| TESTS |#
(capacity (make-commuter 10 20 true))
"should be" 200
(capacity (make-amtrak 30 30 'Limited))
"should be" 900
(capacity (make-subway 15 40 'blue))
"should be" 600
 
(hold-all? (make-commuter 10 60 false) 600)
"should be" true
(hold-all? (make-amtrak 15 40 true) 200)
"should be" true
(hold-all? (make-subway 10 35 'red) 351)
"should be" false
 
 

;An employee is one of
;  (make-principal Number Number Symbol)
;  (make-teacher Number Number Symbol)
;  (make-assistant Number Number Symbol)
 
(define-struct principal (salary office degree))
(define-struct teacher (salary classroom degree))
(define-struct assistant (wage hours degree))
 
;tax : Employee Number -> Number
; - compute social security tax for employee
(define (tax an-emp rate)
  (cond
   [(principal? an-emp) (* (principal-salary an-emp) rate)]
   [(teacher? an-emp) (* (teacher-salary an-emp) rate)]
   [(assistant? an-emp) (* (assistant-wage an-emp)
			   (assistant-hours an-emp)
			   rate)]))
 
#| TESTS |#
(tax (make-principal 100000 201 'MS) .075)
"should be" 7500
(tax (make-teacher 50000 222 'BS) .15)
"should be" 7500
(tax (make-assistant 25 40 'Diploma) .10)
"should be" 100
 

; An Account is one of
;   (make-checking Number Number)
;   (make-savings Number Number)
;   (make-credit Number Number Number)
 
; A Response is one of
;  - Account
;  - 'Error
 
(define-struct checking (balance trans))
(define-struct savings (balance trans))
(define-struct credit (balance limit trans))
 
;withdrawal: Account Number -> Response
;  - process a withdrawal request and return an updated account or 'Error
(define (withdrawal acct amt)
  (cond
   [(checking? acct)
    (cond
     [(< (- (checking-balance acct) amt) -1000)
      'Error]
     [else
      (make-checking
       (- (checking-balance acct) amt)
       (+ (checking-trans acct) 1))])]
   [(savings? acct)
    (cond
     [(< (- (savings-balance acct) amt) 0)
      'Error]
     [else
      (make-savings
       (- (savings-balance acct) amt)
       (+ (savings-trans acct) 1))])]
   [(credit? acct)
    (cond
     [(< (credit-limit acct) (+ (credit-balance acct) amt))
      'Error]
     [else
      (make-credit
       (+ (credit-balance acct) amt)
       (credit-limit acct)
       (+ (credit-trans acct) 1))])]))
 
#| TESTS |#
 
(withdrawal (make-savings 10 10) 10)
"should be" (make-savings 0 11)
(withdrawal (make-savings 20 1) 21)
"should be" 'Error
 
(withdrawal (make-checking -999 10) 1)
"should be" (make-checking -1000 11)
(withdrawal (make-checking -999 10) 2)
"should be" 'Error
 
(withdrawal (make-credit 10 1000 1) 990)
"should be" (make-credit 1000 1000 2)
(withdrawal (make-credit 1000 1000 2) 1)
"should be" 'Error