1 (define (* a b)
2   (if (= b 0)
3       0
4       (+ a (* a (- b 1)))))
5 
6 ;; a * b = {
7 ;; 	 0 if b = 0,
8 ;; 	 2 * a * (b/2) if b is even,
9 ;; 	 a + a * (b-1) if b is odd
10 ;; }
11 
12 (define (fast-mult a b)
13   (cond ((= b 0) 0)
14 	((even? b) (double (* a (halve b))))
15 	(else (+ a (* a (- b 1))))))
16 (define (double x)
17   (+ x x))
18 (define (halve x)
19   (/ x 2))
20 
21 (define (test-case actual expected)
22   (load-option 'format)
23   (newline)
24   (format #t "Actual: ~A Expected: ~A" actual expected))
25 (test-case (fast-mult 0 0) 0)
26 (test-case (fast-mult 0 1) 0)
27 (test-case (fast-mult 0 8) 0)
28 (test-case (fast-mult 5 0) 0)
29 (test-case (fast-mult 2 1) 2)
30 (test-case (fast-mult 3 3) 9)
31 (test-case (fast-mult 5 4) 20)
32 (test-case (fast-mult 12 13) 156)
33 (test-case (fast-mult 12 24) 288)
34