/* mpihelp-add_2.c - MPI helper functions
* Copyright (C) 1994, 1996, 1997, 1998, 2001 Free Software Foundation, Inc.
*
* This file is part of GnuPG.
*
* GnuPG is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* GnuPG 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 General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
*
* Note: This code is heavily based on the GNU MP Library.
* Actually it's the same code with only minor changes in the
* way the data is stored; this is to support the abstraction
* of an optional secure memory allocation which may be used
* to avoid revealing of sensitive data due to paging etc.
* The GNU MP Library itself is published under the LGPL;
* however I decided to publish this code under the plain GPL.
*/
#include "mpi-internal.h"
#include "longlong.h"
mpi_limb_t
mpihelp_sub_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
mpi_ptr_t s2_ptr, mpi_size_t size)
{
mpi_limb_t x, y, cy;
mpi_size_t j;
/* The loop counter and index J goes from -SIZE to -1. This way
the loop becomes faster. */
j = -size;
/* Offset the base pointers to compensate for the negative indices. */
s1_ptr -= j;
s2_ptr -= j;
res_ptr -= j;
cy = 0;
do {
y = s2_ptr[j];
x = s1_ptr[j];
y += cy; /* add previous carry to subtrahend */
cy = y < cy; /* get out carry from that addition */
y = x - y; /* main subtract */
cy += y > x; /* get out carry from the subtract, combine */
res_ptr[j] = y;
} while (++j);
return cy;
}
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