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mpi-genesis             1 /* mpihelp-add_2.c  -  MPI helper functions
mpi-genesis             2  * Copyright (C) 1994, 1996, 1997, 1998, 2001 Free Software Foundation, Inc.
mpi-genesis             3  *
mpi-genesis             4  * This file is part of GnuPG.
mpi-genesis             5  *
mpi-genesis             6  * GnuPG is free software; you can redistribute it and/or modify
mpi-genesis             7  * it under the terms of the GNU General Public License as published by
mpi-genesis             8  * the Free Software Foundation; either version 3 of the License, or
mpi-genesis             9  * (at your option) any later version.
mpi-genesis            10  *
mpi-genesis            11  * GnuPG is distributed in the hope that it will be useful,
mpi-genesis            12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
mpi-genesis            13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
mpi-genesis            14  * GNU General Public License for more details.
mpi-genesis            15  *
mpi-genesis            16  * You should have received a copy of the GNU General Public License
mpi-genesis            17  * along with this program; if not, see <http://www.gnu.org/licenses/>.
mpi-genesis            18  *
mpi-genesis            19  * Note: This code is heavily based on the GNU MP Library.
mpi-genesis            20  *	 Actually it's the same code with only minor changes in the
mpi-genesis            21  *	 way the data is stored; this is to support the abstraction
mpi-genesis            22  *	 of an optional secure memory allocation which may be used
mpi-genesis            23  *	 to avoid revealing of sensitive data due to paging etc.
mpi-genesis            24  *	 The GNU MP Library itself is published under the LGPL;
mpi-genesis            25  *	 however I decided to publish this code under the plain GPL.
mpi-genesis            26  */
mpi-genesis            27 
mpi-genesis            28 #include <config.h>
mpi-genesis            29 #include <stdio.h>
mpi-genesis            30 #include <stdlib.h>
mpi-genesis            31 #include "mpi-internal.h"
mpi-genesis            32 #include "longlong.h"
mpi-genesis            33 
mpi-genesis            34 mpi_limb_t
mpi-genesis            35 mpihelp_sub_n( mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
mpi-genesis            36 				  mpi_ptr_t s2_ptr, mpi_size_t size)
mpi-genesis            37 {
mpi-genesis            38     mpi_limb_t x, y, cy;
mpi-genesis            39     mpi_size_t j;
mpi-genesis            40 
mpi-genesis            41     /* The loop counter and index J goes from -SIZE to -1.  This way
mpi-genesis            42        the loop becomes faster.  */
mpi-genesis            43     j = -size;
mpi-genesis            44 
mpi-genesis            45     /* Offset the base pointers to compensate for the negative indices.  */
mpi-genesis            46     s1_ptr -= j;
mpi-genesis            47     s2_ptr -= j;
mpi-genesis            48     res_ptr -= j;
mpi-genesis            49 
mpi-genesis            50     cy = 0;
mpi-genesis            51     do {
mpi-genesis            52 	y = s2_ptr[j];
mpi-genesis            53 	x = s1_ptr[j];
mpi-genesis            54 	y += cy;		  /* add previous carry to subtrahend */
mpi-genesis            55 	cy = y < cy;		  /* get out carry from that addition */
mpi-genesis            56 	y = x - y;		  /* main subtract */
mpi-genesis            57 	cy += y > x;		  /* get out carry from the subtract, combine */
mpi-genesis            58 	res_ptr[j] = y;
mpi-genesis            59     } while( ++j );
mpi-genesis            60 
mpi-genesis            61     return cy;
mpi-genesis            62 }
mpi-genesis            63 
mpi-genesis            64