34 lines
878 B
Scheme
34 lines
878 B
Scheme
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; Copyright (c) 1993-1999 by Richard Kelsey and Jonathan Rees. See file COPYING.
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; Integer-length, a la Common Lisp, written in portable Scheme.
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(define-syntax cons-stream
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(syntax-rules ()
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((cons-stream head tail)
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(cons head (delay tail)))))
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(define head car)
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(define (tail s) (force (cdr s)))
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(define integer-length
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(let ()
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(define useful
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(let loop ((p 256) (n 4))
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(cons-stream (cons p n)
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(loop (* p p) (* n 2)))))
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(define (recur n)
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(if (< n 16)
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(vector-ref '#(0 1 2 2 3 3 3 3 4 4 4 4 4 4 4 4) n)
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(let loop ((s useful) (prev 16))
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(let ((z (head s)))
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(if (< n (car z))
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(+ (cdr z) (recur (quotient n prev)))
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(loop (tail s) (car z)))))))
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(define (integer-length n)
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(if (exact? n)
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(if (< n 0)
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(recur (- -1 n))
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(recur n))
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(integer-length (inexact->exact n))))
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integer-length))
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