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    #lang sicp

(#%require rackunit)


(define (square x) (* x x))

(define (smallest-divisor n)
  (find-divisor n 2))

(define (find-divisor n test-divisor)
  (cond ((> (square test-divisor) n) n)
        ((divides? test-divisor n) test-divisor)
        (else (find-divisor n (+ test-divisor 1)))))

(define (divides? a b)
  (= (remainder b a) 0))

(define (prime? n)
  (= n (smallest-divisor n)))

(define (timed-prime-test n)
  (newline)
  (display n)
  (start-prime-test n (runtime)))

(define (start-prime-test n start-time)
  (if (prime? n)
      (report-prime (- (runtime) start-time))))

(define (report-prime elapsed-time)
  (display " *** ")
  (display elapsed-time))

(define (even? n)
  (= (remainder n 2) 0))

(define (search-for-primes start end)
  (if (not (even? start))
      (timed-prime-test start))
  (if (not (= start end))
      (search-for-primes (+ start 1) end)))


(check-equal? (divides? 1 2) #t)
(check-equal? (divides? 3 2) #f)
(check-equal? (divides? 5 7) #f)

(check-equal? (smallest-divisor 7) 7)
(check-equal? (smallest-divisor 10) 2)
(check-equal? (smallest-divisor 15) 3)
(check-equal? (smallest-divisor 163) 163)
(check-equal? (smallest-divisor 199) 199)
(check-equal? (smallest-divisor 1999) 1999)
(check-equal? (smallest-divisor 19999) 7)

(check-equal? (prime? 1) #t)
(check-equal? (prime? 2) #t)
(check-equal? (prime? 4) #f)
(check-equal? (prime? 6) #f)
(check-equal? (prime? 163) #t)
(check-equal? (prime? 164) #f)