;; 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-abbr-reader.ss" "lang")((modname week4_m_full) (read-case-sensitive #t) (teachpacks ((lib "image.ss" "teachpack" "2htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "image.ss" "teachpack" "2htdp")))))
; a person is
;   - (make-person number symbol)
(define-struct person (ssn name))

;; example people:
(define Adam (make-person 1 'adam))
(define George (make-person 2 'george))
(define Jon (make-person 3 'jon))

; database: revision 0

; a database is either:
; - empty
; - (cons person database)


;; lookup : number database -> person or false
;; to see if a person is in the database
(define (lookup n db) 
  (cond [(empty? db) false]
        [else 
         (cond [(person-matches n (first db)) (first db)]
               [else (lookup n (rest db))])])
  )

(check-expect (lookup 2 (list Adam Jon)) false)
(check-expect (lookup 3 (list Adam Jon)) Jon)
(check-expect (lookup 1 (list Adam Jon)) Adam)

; person-matches : number person -> boolean
; returns whether person has given ssn
(define (person-matches n per) 
  (= n (person-ssn per)))

(check-expect (person-matches 1 Adam) true)
(check-expect (person-matches 2 Adam) false)

;; add : person database -> database
;; to add the person to the database
(define (add person database) (cons person database))

(check-expect (add Adam empty) (cons Adam empty))
(check-expect (add Adam (list Jon)) (list Adam Jon))

;; start : database
;; an empty database
(define start empty)

;performance:

; 100 adds?
; 1 + 2 + 3 + 4 +....

; 100 lookups? in database with 100 things
; best case: 1 + 1 +... + 1 = 100
; worst case: 101 + 101 + ... + 101 = 10100

; n lookups in database with n things
; worst case: about n*n