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Werner Koch
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247
mpi/mpi-pow.c
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247
mpi/mpi-pow.c
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/* mpi-pow.c - MPI functions
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* Copyright (c) 1997 by Werner Koch (dd9jn)
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*
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* This file is part of G10.
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*
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* G10 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 of the License, or
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* (at your option) any later version.
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*
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* G10 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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*
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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
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* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
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*/
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#include <config.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include "mpi-internal.h"
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#include "longlong.h"
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#include <assert.h>
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/****************
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* RES = BASE ^ EXP mod MOD
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*/
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void
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mpi_powm( MPI res, MPI base, MPI exp, MPI mod)
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{
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mpi_ptr_t rp, ep, mp, bp;
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mpi_size_t esize, msize, bsize, rsize;
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int esign, msign, bsign, rsign;
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mpi_size_t size;
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int mod_shift_cnt;
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int negative_result;
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mpi_ptr_t mp_marker=NULL, bp_marker=NULL, ep_marker=NULL;
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mpi_ptr_t xp_marker=NULL;
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int assign_rp=0;
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esize = exp->nlimbs;
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msize = mod->nlimbs;
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size = 2 * msize;
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esign = exp->sign;
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msign = mod->sign;
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rp = res->d;
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ep = exp->d;
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if( !msize )
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msize = 1 / msize; /* provoke a signal */
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if( !esize ) {
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/* Exponent is zero, result is 1 mod MOD, i.e., 1 or 0
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* depending on if MOD equals 1. */
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rp[0] = 1;
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res->nlimbs = (msize == 1 && mod->d[0] == 1) ? 0 : 1;
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res->sign = 0;
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goto leave;
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}
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/* Normalize MOD (i.e. make its most significant bit set) as required by
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* mpn_divrem. This will make the intermediate values in the calculation
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* slightly larger, but the correct result is obtained after a final
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* reduction using the original MOD value. */
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mp = mp_marker = mpi_alloc_limb_space(msize);
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count_leading_zeros( mod_shift_cnt, mod->d[msize-1] );
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if( mod_shift_cnt )
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mpihelp_lshift( mp, mod->d, msize, mod_shift_cnt );
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else
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MPN_COPY( mp, mod->d, msize );
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bsize = base->nlimbs;
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bsign = base->sign;
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if( bsize > msize ) { /* The base is larger than the module. Reduce it. */
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/* Allocate (BSIZE + 1) with space for remainder and quotient.
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* (The quotient is (bsize - msize + 1) limbs.) */
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bp = bp_marker = mpi_alloc_limb_space( bsize + 1);
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MPN_COPY( bp, base->d, bsize );
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/* We don't care about the quotient, store it above the remainder,
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* at BP + MSIZE. */
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mpihelp_divrem( bp + msize, 0, bp, bsize, mp, msize );
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bsize = msize;
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/* Canonicalize the base, since we are going to multiply with it
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* quite a few times. */
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MPN_NORMALIZE( bp, bsize );
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}
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else
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bp = base->d;
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if( !bsize ) {
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res->nlimbs = 0;
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res->sign = 0;
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goto leave;
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}
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if( res->alloced < size ) {
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/* We have to allocate more space for RES. If any of the input
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* parameters are identical to RES, defer deallocation of the old
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* space. */
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if( rp == ep || rp == mp || rp == bp ) {
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rp = mpi_alloc_limb_space( size );
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assign_rp = 1;
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}
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else {
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mpi_resize( res, size );
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rp = res->d;
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}
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}
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else { /* Make BASE, EXP and MOD not overlap with RES. */
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if( rp == bp ) {
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/* RES and BASE are identical. Allocate temp. space for BASE. */
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assert( !bp_marker );
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bp = bp_marker = mpi_alloc_limb_space( bsize );
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MPN_COPY(bp, rp, bsize);
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}
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if( rp == ep ) {
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/* RES and EXP are identical. Allocate temp. space for EXP. */
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ep = ep_marker = mpi_alloc_limb_space( esize );
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MPN_COPY(ep, rp, esize);
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}
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if( rp == mp ) {
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/* RES and MOD are identical. Allocate temporary space for MOD.*/
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assert( !mp_marker );
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mp = mp_marker = mpi_alloc_limb_space( msize );
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MPN_COPY(mp, rp, msize);
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}
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}
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MPN_COPY( rp, bp, bsize );
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rsize = bsize;
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rsign = bsign;
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{
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mpi_size_t i;
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mpi_ptr_t xp = xp_marker = mpi_alloc_limb_space( 2 * (msize + 1) );
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int c;
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mpi_limb_t e;
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mpi_limb_t carry_limb;
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negative_result = (ep[0] & 1) && base->sign;
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i = esize - 1;
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e = ep[i];
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count_leading_zeros (c, e);
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e = (e << c) << 1; /* shift the exp bits to the left, lose msb */
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c = BITS_PER_MPI_LIMB - 1 - c;
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/* Main loop.
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*
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* Make the result be pointed to alternately by XP and RP. This
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* helps us avoid block copying, which would otherwise be necessary
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* with the overlap restrictions of mpihelp_divmod. With 50% probability
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* the result after this loop will be in the area originally pointed
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* by RP (==RES->d), and with 50% probability in the area originally
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* pointed to by XP.
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*/
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for(;;) {
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while( c ) {
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mpi_ptr_t tp;
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mpi_size_t xsize;
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mpihelp_mul_n(xp, rp, rp, rsize);
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xsize = 2 * rsize;
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if( xsize > msize ) {
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mpihelp_divrem(xp + msize, 0, xp, xsize, mp, msize);
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xsize = msize;
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}
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tp = rp; rp = xp; xp = tp;
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rsize = xsize;
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if( (mpi_limb_signed_t)e < 0 ) {
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mpihelp_mul( xp, rp, rsize, bp, bsize );
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xsize = rsize + bsize;
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if( xsize > msize ) {
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mpihelp_divrem(xp + msize, 0, xp, xsize, mp, msize);
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xsize = msize;
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}
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tp = rp; rp = xp; xp = tp;
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rsize = xsize;
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}
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e <<= 1;
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c--;
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}
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i--;
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if( i < 0 )
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break;
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e = ep[i];
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c = BITS_PER_MPI_LIMB;
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}
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/* We shifted MOD, the modulo reduction argument, left MOD_SHIFT_CNT
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* steps. Adjust the result by reducing it with the original MOD.
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*
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* Also make sure the result is put in RES->d (where it already
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* might be, see above).
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*/
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if( mod_shift_cnt ) {
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carry_limb = mpihelp_lshift( res->d, rp, rsize, mod_shift_cnt);
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rp = res->d;
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if( carry_limb ) {
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rp[rsize] = carry_limb;
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rsize++;
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}
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}
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else {
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MPN_COPY( res->d, rp, rsize);
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rp = res->d;
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}
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if( rsize >= msize ) {
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mpihelp_divrem(rp + msize, 0, rp, rsize, mp, msize);
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rsize = msize;
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}
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/* Remove any leading zero words from the result. */
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if( mod_shift_cnt )
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mpihelp_rshift( rp, rp, rsize, mod_shift_cnt);
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MPN_NORMALIZE (rp, rsize);
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}
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if( negative_result && rsize ) {
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if( mod_shift_cnt )
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mpihelp_rshift( mp, mp, msize, mod_shift_cnt);
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mpihelp_sub( rp, mp, msize, rp, rsize);
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rsize = msize;
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rsign = msign;
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MPN_NORMALIZE(rp, rsize);
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}
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res->nlimbs = rsize;
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res->sign = rsign;
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leave:
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if( assign_rp ) mpi_assign_limb_space( res, rp, size );
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if( mp_marker ) mpi_free_limb_space( mp_marker );
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if( bp_marker ) mpi_free_limb_space( bp_marker );
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if( ep_marker ) mpi_free_limb_space( ep_marker );
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if( xp_marker ) mpi_free_limb_space( xp_marker );
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}
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