SICP exercise 1.22 - A surprising performance discovery

sicp_exercise_1_22.scm

; Slow!  The (next) "optimisation" slows it down 
(define (find-divisor n test-divisor)
  (define (next n)
    (if (= n 2)
      3
      (+ n 2)))
  (cond ((> (square test-divisor) n) n)
        ((divides? test-divisor n) test-divisor)
        (else (find-divisor n (next test-divisor)))))

; Fast!  Approx 1.6x as fast as when testing every possible divisor
(define (next n)
  (if (= n 2)
    3
    (+ n 2)))
(define (find-divisor n test-divisor)
  (cond ((> (square test-divisor) n) n)
        ((divides? test-divisor n) test-divisor)
        (else (find-divisor n (next test-divisor)))))