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81 lines
2.0 KiB
C
81 lines
2.0 KiB
C
/* Arithmetic.
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Copyright (C) 2001-2002 Free Software Foundation, Inc.
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Written by Bruno Haible <bruno@clisp.org>, 2001.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2, or (at your option)
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any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software Foundation,
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Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */
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/* Specification. */
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#include "gcd.h"
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#include <stdlib.h>
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/* Return the greatest common divisor of a > 0 and b > 0. */
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unsigned long
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gcd (unsigned long a, unsigned long b)
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{
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/* Why no division, as in Euclid's algorithm? Because in Euclid's algorithm
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the division result floor(a/b) or floor(b/a) is very often = 1 or = 2,
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and nearly always < 8. A sequence of a few subtractions and tests is
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faster than a division. */
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/* Why not Euclid's algorithm? Because the two integers can be shifted by 1
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bit in a single instruction, and the algorithm uses fewer variables than
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Euclid's algorithm. */
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unsigned long c = a | b;
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c = c ^ (c - 1);
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/* c = largest power of 2 that divides a and b. */
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if (a & c)
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{
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if (b & c)
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goto odd_odd;
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else
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goto odd_even;
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}
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else
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{
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if (b & c)
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goto even_odd;
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else
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abort ();
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}
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for (;;)
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{
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odd_odd: /* a/c and b/c both odd */
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if (a == b)
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break;
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if (a > b)
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{
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a = a - b;
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even_odd: /* a/c even, b/c odd */
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do
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a = a >> 1;
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while ((a & c) == 0);
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}
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else
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{
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b = b - a;
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odd_even: /* a/c odd, b/c even */
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do
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b = b >> 1;
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while ((b & c) == 0);
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}
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}
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/* a = b */
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return a;
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}
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