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;;;; Under Creative Commons Attribution-ShareAlike 4.0
;;;; International. See
;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
;;;
;;; __________________
;;; < +, - everything. >
;;; ------------------
;;; \ . . .
;;; \ . . . ` ,
;;; \ .; . : .' : : : .
;;; \ i..`: i` i.i.,i i .
;;; \ `,--.|i |i|ii|ii|i:
;;; U**U\.'@@@@@@`.||'
;;; \__/(@@@@@@@@@@)'
;;; (@@@@@@@@)
;;; `YY~~~~YY'
;;; || ||
;;;
(define-module (net ricketyspace sicp one seventeen)
#:export (double
halve
fast-*)
#:replace (even?))
(define (double a)
"Return A + A."
(+ a a))
(define (halve b)
"Return B/2.
B must be an even number."
(cond ((= b 0) 0)
(else (+ 1 (halve (- b 2))))))
(define (even? b)
"Return #t if B is divisible by 2; #f otherwise."
(cond ((= b 1) #f)
((= b 0) #t)
(else (even? (- b 2)))))
(define (fast-* a b)
"Return A * B."
(cond ((= b 0) 0)
((even? b) (double (fast-* a (halve b))))
(else (+ a (fast-* a (- b 1))))))
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