generic_mpih-rshift.c

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/* mpih-rshift.c  -  MPI helper functions
 * Copyright (C) 1994, 1996, 1998, 1999,
 *               2000, 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"

/* Shift U (pointed to by UP and USIZE limbs long) CNT bits to the right
 * and store the USIZE least significant limbs of the result at WP.
 * The bits shifted out to the right are returned.
 *
 * Argument constraints:
 * 1. 0 < CNT < BITS_PER_MP_LIMB
 * 2. If the result is to be written over the input, WP must be <= UP.
 */

mpi_limb_t
mpihelp_rshift(mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize, unsigned cnt)
{
    mpi_limb_t high_limb, low_limb;
    unsigned sh_1, sh_2;
    mpi_size_t i;
    mpi_limb_t retval;

    sh_1 = cnt;
    wp -= 1;
    sh_2 = BITS_PER_MPI_LIMB - sh_1;
    high_limb = up[0];
    retval = high_limb << sh_2;
    low_limb = high_limb;
    for (i = 1; i < usize; i++) {
        high_limb = up[i];
        wp[i] = (low_limb >> sh_1) | (high_limb << sh_2);
        low_limb = high_limb;
    }
    wp[i] = low_limb >> sh_1;

    return retval;
}