SICP 1.18.part1

(define (double a) (* a 2))

(define (halve a) (/ a 2))

(define (even? n) (= (remainder n 2) 0))

(define (fast-mult a b )
  (fast-mult-iter 0 a b))

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