1 /* mpi-bit.c - MPI bit level functions
2 * Copyright (C) 1998, 1999 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
21 #include "mpi-internal.h"
22 #include "longlong.h"
23
24 #define A_LIMB_1 ((mpi_limb_t) 1)
25
26 /****************
27 * Sometimes we have MSL (most significant limbs) which are 0;
28 * this is for some reasons not good, so this function removes them.
29 */
mpi_normalize(MPI a)30 void mpi_normalize(MPI a)
31 {
32 for (; a->nlimbs && !a->d[a->nlimbs - 1]; a->nlimbs--)
33 ;
34 }
35
36 /****************
37 * Return the number of bits in A.
38 */
mpi_get_nbits(MPI a)39 unsigned mpi_get_nbits(MPI a)
40 {
41 unsigned n;
42
43 mpi_normalize(a);
44
45 if (a->nlimbs) {
46 mpi_limb_t alimb = a->d[a->nlimbs - 1];
47 if (alimb)
48 n = count_leading_zeros(alimb);
49 else
50 n = BITS_PER_MPI_LIMB;
51 n = BITS_PER_MPI_LIMB - n + (a->nlimbs - 1) * BITS_PER_MPI_LIMB;
52 } else
53 n = 0;
54 return n;
55 }
56 EXPORT_SYMBOL_GPL(mpi_get_nbits);
57
58 /****************
59 * Test whether bit N is set.
60 */
mpi_test_bit(MPI a,unsigned int n)61 int mpi_test_bit(MPI a, unsigned int n)
62 {
63 unsigned int limbno, bitno;
64 mpi_limb_t limb;
65
66 limbno = n / BITS_PER_MPI_LIMB;
67 bitno = n % BITS_PER_MPI_LIMB;
68
69 if (limbno >= a->nlimbs)
70 return 0; /* too far left: this is a 0 */
71 limb = a->d[limbno];
72 return (limb & (A_LIMB_1 << bitno)) ? 1 : 0;
73 }
74 EXPORT_SYMBOL_GPL(mpi_test_bit);
75
76 /****************
77 * Set bit N of A.
78 */
mpi_set_bit(MPI a,unsigned int n)79 int mpi_set_bit(MPI a, unsigned int n)
80 {
81 unsigned int i, limbno, bitno;
82 int err;
83
84 limbno = n / BITS_PER_MPI_LIMB;
85 bitno = n % BITS_PER_MPI_LIMB;
86
87 if (limbno >= a->nlimbs) {
88 for (i = a->nlimbs; i < a->alloced; i++)
89 a->d[i] = 0;
90 err = mpi_resize(a, limbno+1);
91 if (err)
92 return err;
93 a->nlimbs = limbno+1;
94 }
95 a->d[limbno] |= (A_LIMB_1<<bitno);
96 return 0;
97 }
98
99 /*
100 * Shift A by N bits to the right.
101 */
mpi_rshift(MPI x,MPI a,unsigned int n)102 int mpi_rshift(MPI x, MPI a, unsigned int n)
103 {
104 mpi_size_t xsize;
105 unsigned int i;
106 unsigned int nlimbs = (n/BITS_PER_MPI_LIMB);
107 unsigned int nbits = (n%BITS_PER_MPI_LIMB);
108 int err;
109
110 if (x == a) {
111 /* In-place operation. */
112 if (nlimbs >= x->nlimbs) {
113 x->nlimbs = 0;
114 return 0;
115 }
116
117 if (nlimbs) {
118 for (i = 0; i < x->nlimbs - nlimbs; i++)
119 x->d[i] = x->d[i+nlimbs];
120 x->d[i] = 0;
121 x->nlimbs -= nlimbs;
122 }
123 if (x->nlimbs && nbits)
124 mpihelp_rshift(x->d, x->d, x->nlimbs, nbits);
125 } else if (nlimbs) {
126 /* Copy and shift by more or equal bits than in a limb. */
127 xsize = a->nlimbs;
128 x->sign = a->sign;
129 err = RESIZE_IF_NEEDED(x, xsize);
130 if (err)
131 return err;
132 x->nlimbs = xsize;
133 for (i = 0; i < a->nlimbs; i++)
134 x->d[i] = a->d[i];
135 x->nlimbs = i;
136
137 if (nlimbs >= x->nlimbs) {
138 x->nlimbs = 0;
139 return 0;
140 }
141
142 for (i = 0; i < x->nlimbs - nlimbs; i++)
143 x->d[i] = x->d[i+nlimbs];
144 x->d[i] = 0;
145 x->nlimbs -= nlimbs;
146
147 if (x->nlimbs && nbits)
148 mpihelp_rshift(x->d, x->d, x->nlimbs, nbits);
149 } else {
150 /* Copy and shift by less than bits in a limb. */
151 xsize = a->nlimbs;
152 x->sign = a->sign;
153 err = RESIZE_IF_NEEDED(x, xsize);
154 if (err)
155 return err;
156 x->nlimbs = xsize;
157
158 if (xsize) {
159 if (nbits)
160 mpihelp_rshift(x->d, a->d, x->nlimbs, nbits);
161 else {
162 /* The rshift helper function is not specified for
163 * NBITS==0, thus we do a plain copy here.
164 */
165 for (i = 0; i < x->nlimbs; i++)
166 x->d[i] = a->d[i];
167 }
168 }
169 }
170 MPN_NORMALIZE(x->d, x->nlimbs);
171
172 return 0;
173 }
174 EXPORT_SYMBOL_GPL(mpi_rshift);
175