1.22.scm

(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 (time-prime-test n)
  (start-prime-test n (runtime)))

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

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