1/* mpihelp-lshift.c  -	MPI helper functions
2 * Copyright (C) 1994, 1996, 1998, 2001 Free Software Foundation, Inc.
3 *
4 * This file is part of GnuPG.
5 *
6 * GnuPG is free software; you can redistribute it and/or modify
7 * it under the terms of the GNU General Public License as published by
8 * the Free Software Foundation; either version 2 of the License, or
9 * (at your option) any later version.
10 *
11 * GnuPG is distributed in the hope that it will be useful,
12 * but WITHOUT ANY WARRANTY; without even the implied warranty of
13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
14 * GNU General Public License for more details.
15 *
16 * You should have received a copy of the GNU General Public License
17 * along with this program; if not, write to the Free Software
18 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
19 *
20 * Note: This code is heavily based on the GNU MP Library.
21 *	 Actually it's the same code with only minor changes in the
22 *	 way the data is stored; this is to support the abstraction
23 *	 of an optional secure memory allocation which may be used
24 *	 to avoid revealing of sensitive data due to paging etc.
25 *	 The GNU MP Library itself is published under the LGPL;
26 *	 however I decided to publish this code under the plain GPL.
27 */
28
29#include "mpi-internal.h"
30
31/* Shift U (pointed to by UP and USIZE digits long) CNT bits to the left
32 * and store the USIZE least significant digits of the result at WP.
33 * Return the bits shifted out from the most significant digit.
34 *
35 * Argument constraints:
36 * 1. 0 < CNT < BITS_PER_MP_LIMB
37 * 2. If the result is to be written over the input, WP must be >= UP.
38 */
39
40mpi_limb_t
41mpihelp_lshift(mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize, unsigned int cnt)
42{
43	mpi_limb_t high_limb, low_limb;
44	unsigned sh_1, sh_2;
45	mpi_size_t i;
46	mpi_limb_t retval;
47
48	sh_1 = cnt;
49	wp += 1;
50	sh_2 = BITS_PER_MPI_LIMB - sh_1;
51	i = usize - 1;
52	low_limb = up[i];
53	retval = low_limb >> sh_2;
54	high_limb = low_limb;
55	while (--i >= 0) {
56		low_limb = up[i];
57		wp[i] = (high_limb << sh_1) | (low_limb >> sh_2);
58		high_limb = low_limb;
59	}
60	wp[i] = high_limb << sh_1;
61
62	return retval;
63}
64