;; 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))

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

; imagine a sequence of 100 adds:
; (define db1 (add Jon empty))
; (define db2 (add George db1))
; ...
; (define db100 (add Frank db99))

; imagine a sequence of 100 lookups:
; ...(lookup db100 ...)
; ...(lookup db100 ...)
; best case: lookup Jon, takes 2-3 steps 100 times =~ 300 steps
; worst case: lookup UNKNOWN, takes 2-3 steps 100*100 =~ 30,000

(check-expect (lookup 2 (list Adam George Jon))
              George)


(check-expect (lookup 2 empty)
              false)



; person-matches?: number person -> boolean
; says whether person has given ssn
(define (person-matches? ssn person)
  (= ssn (person-ssn person)))

(check-expect (person-matches? 2 George) true)
(check-expect (person-matches? 1 Jon) false)

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

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

; database: revision 0

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