67 lines
1.7 KiB
Scheme
67 lines
1.7 KiB
Scheme
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; Copyright (c) 1993-1999 by Richard Kelsey and Jonathan Rees. See file COPYING.
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;Date: Mon, 24 Jan 94 15:10:30 -0500
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;To: jar@ai.mit.edu
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;Subject: integer->string
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;From: kelsey@research.nj.nec.com
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;Sender: kelsey@research.nj.nec.com
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;
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;
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;I have gotten tired of waiting for bignums to print out.
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;Here is a somewhat more complex and faster version of integer->string.
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;
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;Converting 10**100 to a string:
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; Current: 0.44 seconds
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; This: 0.12 seconds
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; This using integer-divide: 0.06 seconds
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;
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;There is no overwhelming reason to use this, but here it is.
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(define integer->string
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(let ()
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(define (integer->string n radix)
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(define (small-integer->magnitude n l)
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(if (= n 0)
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l
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(small-integer->magnitude (quotient n radix)
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(cons (integer->digit (remainder n radix))
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l))))
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(define (integer->magnitude n)
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(let ((rrrr (* (* radix radix) (* radix radix))))
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(let recur ((n n) (l '()))
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(if (< n rrrr)
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(small-integer->magnitude n l)
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(do ((i 4 (- i 1))
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(n0 (remainder n rrrr) (quotient n0 radix))
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(l l (cons (integer->digit (remainder n0 radix)) l)))
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((= 0 i)
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(recur (quotient n rrrr) l)))))))
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(let ((magnitude (cond ((= n 0) '(#\0))
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((< n 1000000)
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(small-integer->magnitude (abs n) '()))
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(else
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(integer->magnitude (abs n))))))
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(list->string (if (>= n 0)
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magnitude
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(cons #\- magnitude)))))
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(define (integer->digit n)
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(ascii->char (+ n (if (< n 10)
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zero
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a-minus-10))))
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(define zero (char->ascii #\0))
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(define a-minus-10 (- (char->ascii #\a) 10))
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integer->string))
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