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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)