Efficient factorial

fact.rkt

#lang racket
(define (fact n)
  (mult 1 n 1))

(define (mult m n k)
  ; m*(m+k)*(m+2k)*...
  ; last factor <= n
  (if (< (- n m) k)
      m
      (* (mult    m    n (* 2 k))
         (mult (+ m k) n (* 2 k)))))

(time (begin (fact 1000000) (void)))