(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 (square x)
  (* x x))

;;;

(define runtime current-inexact-milliseconds)

(define (timed-function-test fun)
  (let ((start-time (runtime))
        (result (fun))
        (end-time (runtime)))
    (values result (- end-time start-time))))