generic_mpih-rshift.c

/* 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;
}