generic_mpih-sub1.c

/* 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;
}