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; Part of Scheme 48 1.9. See file COPYING for notices and license.
; Authors: Richard Kelsey, Jonathan Rees
; Integer-length, a la Common Lisp, written in portable Scheme.
(define-syntax cons-stream
(syntax-rules ()
((cons-stream head tail)
(cons head (delay tail)))))
(define head car)
(define (tail s) (force (cdr s)))
(define integer-length
(let ()
(define useful
(let loop ((p 256) (n 4))
(cons-stream (cons p n)
(loop (* p p) (* n 2)))))
(define (recur n)
(if (< n 16)
(vector-ref '#(0 1 2 2 3 3 3 3 4 4 4 4 4 4 4 4) n)
(let loop ((s useful) (prev 16))
(let ((z (head s)))
(if (< n (car z))
(+ (cdr z) (recur (quotient n prev)))
(loop (tail s) (car z)))))))
(define (integer-length n)
(if (exact? n)
(if (< n 0)
(recur (- -1 n))
(recur n))
(integer-length (inexact->exact n))))
integer-length))
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