old-racket-sicp-solutions/exercise-1.21-find-smallest-divisor.rkt

#lang racket

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

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

(smallest-divisor 199)
(smallest-divisor 1999)
(smallest-divisor 19999)

(prime? 199)
(prime? 1999)
(prime? 19999)