[SICP] 연습문제 1.18

연습문제 1.17을 풀다가 얼떨결에 풀어버린.. 코드만;;

(define (double a)
  (+ a a))
(define (halve a)
  (/ a 2))
(define (even? n)
  (= (remainder n 2) 0))

(define (fast-*-iter p a b)
  (cond ((= b 0) p)
        ((even? b) (fast-*-iter p (double a) (halve b)))
        (else (fast-*-iter (+ p a) a (- b 1)))))