34 lines
877 B
Scheme
34 lines
877 B
Scheme
; Copyright (c) 1993, 1994 Richard Kelsey and Jonathan Rees. See file COPYING.
|
|
|
|
|
|
; 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))
|