59 lines
1.6 KiB
C
59 lines
1.6 KiB
C
/* mpn_divexact_by exact division by (B-1)/f
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Copyright 2009 Jason Moxham
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This file is part of the MPIR Library.
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The MPIR 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
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by the Free Software Foundation; either version 2.1 of the License, or (at
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your option) any later version.
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The MPIR 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 MPIR Library; see the file COPYING.LIB. If not, write
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to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
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Boston, MA 02110-1301, USA.
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*/
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#include "mpir.h"
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#include "gmp-impl.h"
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#include "longlong.h"
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/*
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(xp, n) = (qp, n)*f - ret*B^n and 0 <= ret < f
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Note the divexact_by3 code is just a special case of this
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*/
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mp_limb_t mpn_divexact_byfobm1(mp_ptr qp, mp_srcptr xp, mp_size_t n,
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mp_limb_t f, mp_limb_t Bm1of)
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{
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mp_size_t j;
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mp_limb_t c, acc, ax, dx;
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ASSERT(n > 0);
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ASSERT_MPN(xp, n);
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ASSERT(MPN_SAME_OR_SEPARATE_P(qp, xp, n));
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ASSERT(Bm1of*f + 1 == 0);
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acc = 0*Bm1of; /* carry in is 0 */
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for (j = 0; j <= n - 1; j++)
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{
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umul_ppmm(dx, ax, xp[j], Bm1of);
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SUBC_LIMB(c, acc, acc, ax);
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qp[j] = acc;
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acc -= dx + c;
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}
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/* return next quotient*(-f) */
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return acc*(-f);
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}
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