generic_mpih-sub1.c

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/* 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;
}