mpir/mpn/generic/divebyfobm1.c

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/* mpn_divexact_by exact division by (B-1)/f
Copyright 2009 Jason Moxham
This file is part of the MPIR Library.
The MPIR Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published
by the Free Software Foundation; either version 2.1 of the License, or (at
your option) any later version.
The MPIR Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
License for more details.
You should have received a copy of the GNU Lesser General Public License
along with the MPIR Library; see the file COPYING.LIB. If not, write
to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
Boston, MA 02110-1301, USA.
*/
#include "mpir.h"
#include "gmp-impl.h"
#include "longlong.h"
mp_limb_t mpn_divexact_fobm1(mp_ptr qp,mp_srcptr xp,mp_size_t n,mp_limb_t f,mp_limb_t Bm1of)
{mp_size_t j;mp_limb_t c,acc,ax,dx;
ASSERT(n>0);
ASSERT_MPN(xp,n);
ASSERT(MPN_SAME_OR_SEPARATE_P(qp,xp,n));
ASSERT(Bm1of*f+1==0);
acc=0*Bm1of;// carry in is 0
for(j=0;j<=n-1;j++)
{umul_ppmm(dx,ax,xp[j],Bm1of);
SUBC_LIMB(c,acc,acc,ax);
qp[j]=acc;
acc-=dx+c;}
// return next quotient*-f
return acc*-f;} // so (xp,n) = (qp,n)*f -ret*B^n and 0 <= ret < f
// Note the divexact_by3 code is just a special case of this