45 lines
1.5 KiB
C
45 lines
1.5 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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mp_limb_t mpn_divexact_byBm1of(mp_ptr qp,mp_srcptr xp,mp_size_t n,mp_limb_t f,mp_limb_t Bm1of)
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{mp_size_t j;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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{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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// return next quotient*-f
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// we could save this mul here , we dont really need it , just left in for mathematical clarity
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return acc*-f;} // so (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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