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generic_mpih-lshift.c
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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 
42 {
43  mpi_limb_t high_limb, low_limb;
44  unsigned sh_1, sh_2;
45  mpi_size_t i;
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 }