SICP Exercise 1.18

SICP Exercise 1.18.scm

(define (fast-multi a b)
  (define (double n) (+ n n))
  (define (halve n) (/ n 2))
  (define (even? n) (= (remainder n 2) 0))
  (define (iter a b x)
    (cond ((or (= a 0) (= b 0)) 0)
          ((= b 1) (+ a x))
          ((even? b) (iter (double a) (halve b) x))
          (else (iter a (- b 1) (+ a x)))))
  (iter a b 0))