118 lines
3.1 KiB
C
118 lines
3.1 KiB
C
/* mpn_gcd_1 -- mpn and limb greatest common divisor.
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Copyright 1994, 1996, 2000, 2001 Free Software Foundation, Inc.
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This file is part of the GNU MP Library.
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The GNU MP Library is free software; you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or (at your
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option) any later version.
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The GNU MP Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
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License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with the GNU MP Library; see the file COPYING.LIB. If not, write to
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the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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MA 02110-1301, USA. */
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#include "gmp.h"
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#include "gmp-impl.h"
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#include "longlong.h"
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/* Does not work for U == 0 or V == 0. It would be tough to make it work for
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V == 0 since gcd(x,0) = x, and U does not generally fit in an mp_limb_t.
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The threshold for doing u%v when size==1 will vary by CPU according to
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the speed of a division and the code generated for the main loop. Any
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tuning for this is left to a CPU specific implementation. */
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mp_limb_t
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mpn_gcd_1 (mp_srcptr up, mp_size_t size, mp_limb_t vlimb)
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{
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mp_limb_t ulimb;
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unsigned long zero_bits, u_low_zero_bits;
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ASSERT (size >= 1);
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ASSERT (vlimb != 0);
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ASSERT_MPN_NONZERO_P (up, size);
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ulimb = up[0];
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/* Need vlimb odd for modexact, want it odd to get common zeros. */
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count_trailing_zeros (zero_bits, vlimb);
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vlimb >>= zero_bits;
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if (size > 1)
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{
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/* Must get common zeros before the mod reduction. If ulimb==0 then
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vlimb already gives the common zeros. */
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if (ulimb != 0)
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{
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count_trailing_zeros (u_low_zero_bits, ulimb);
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zero_bits = MIN (zero_bits, u_low_zero_bits);
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}
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ulimb = MPN_MOD_OR_MODEXACT_1_ODD (up, size, vlimb);
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if (ulimb == 0)
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goto done;
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goto strip_u_maybe;
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}
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/* size==1, so up[0]!=0 */
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count_trailing_zeros (u_low_zero_bits, ulimb);
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ulimb >>= u_low_zero_bits;
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zero_bits = MIN (zero_bits, u_low_zero_bits);
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/* make u bigger */
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if (vlimb > ulimb)
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MP_LIMB_T_SWAP (ulimb, vlimb);
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/* if u is much bigger than v, reduce using a division rather than
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chipping away at it bit-by-bit */
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if ((ulimb >> 16) > vlimb)
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{
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ulimb %= vlimb;
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if (ulimb == 0)
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goto done;
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goto strip_u_maybe;
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}
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while (ulimb != vlimb)
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{
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ASSERT (ulimb & 1);
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ASSERT (vlimb & 1);
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if (ulimb > vlimb)
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{
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ulimb -= vlimb;
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do
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{
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ulimb >>= 1;
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ASSERT (ulimb != 0);
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strip_u_maybe:
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;
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}
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while ((ulimb & 1) == 0);
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}
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else /* vlimb > ulimb. */
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{
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vlimb -= ulimb;
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do
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{
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vlimb >>= 1;
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ASSERT (vlimb != 0);
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}
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while ((vlimb & 1) == 0);
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}
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}
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done:
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return vlimb << zero_bits;
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}
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