#lang racket
(require htdp/testing)

;; this file:  unicalc.rkt
;; name:
;; time spent:
;; other comments?


;; this next module provides the symbols unicalc-db and UDB
;; both refer to the same association list of units...
(require "unicalc-db.rkt")


;;
;; The unit-calculator application - in Racket
;;

;; Provided functions - for comparing and sorting symbols
;;
;; comparing symbols alphabetically requires conv. to strings
(define (sym<? sym1 sym2)
  (string<? (symbol->string sym1) (symbol->string sym2)))

;; a function for sorting lists of symbols: sortsym
(define (sortsym L) (sort L sym<?))

;; here's how to use sortsym (no third argument needed!)
(check-expect (sortsym '(b c a)) '(a b c))


;; An "information structure" abstraction for quantity lists
;;
; Function: make-QL
; Input:
;    quant - a number
;    num - a list of symbols representing the units in the numerator
;    den - a list of symbols representing the units in the denominator
;    unc - a number: the absolute 1-sigma uncertainty associated with quant
; Output:
;    a representation of a quantity list
(define (make-QL quant num den unc)
  (list quant (sortsym num) (sortsym den) unc))         

; Accessor functions for our Quantity list abstraction
(define get-quant first)
(define get-num second)
(define get-den third)
(define get-unc fourth)


;; A predicate for checking if two quantity lists are "equal"
;; We will need this for our check-expect tests
;;
;; equal-QLs: a predicate to check if two quantity lists
;;   are close enough to be considered the same, for testing
;;   We use 42/100000 as our threshold, naturally... .
(define (equal-QLs? QL1 QL2)
  (let* ((q1 (get-quant QL1))  (q2 (get-quant QL2))
         (N1 (get-num QL1))    (N2 (get-num QL2))
         (D1 (get-den QL1))    (D2 (get-den QL2))
         (u1 (get-unc QL1))    (u2 (get-unc QL2)))
    (and ;; and can take any number of arguments!
     (< (abs (- q1 q2)) 4.2e-5)
     (< (abs (- u1 u2)) 4.2e-5)
     (equal? N1 N2)
     (equal? D1 D2))))

(check-expect (equal-QLs? (make-QL 42.0 '(m) '(s) 0.0)
                          (make-QL 42.0 '(m) '(s) 0.0))
              #t)

;; Add your unicalc functions here... 

  
(generate-report)