1 /* raisemod.c: bcmath library file. */
2 /*
3 Copyright (C) 1991, 1992, 1993, 1994, 1997 Free Software Foundation, Inc.
4 Copyright (C) 2000 Philip A. Nelson
5
6 This library is free software; you can redistribute it and/or
7 modify it under the terms of the GNU Lesser General Public
8 License as published by the Free Software Foundation; either
9 version 2 of the License, or (at your option) any later version.
10
11 This library 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 GNU
14 Lesser General Public License for more details. (LICENSE)
15
16 You should have received a copy of the GNU Lesser General Public
17 License along with this library; if not, write to:
18
19 The Free Software Foundation, Inc.
20 59 Temple Place, Suite 330
21 Boston, MA 02111-1307 USA.
22
23 You may contact the author by:
24 e-mail: philnelson@acm.org
25 us-mail: Philip A. Nelson
26 Computer Science Department, 9062
27 Western Washington University
28 Bellingham, WA 98226-9062
29
30 *************************************************************************/
31
32 #include "bcmath.h"
33 #include <stddef.h>
34
35 /* Raise BASE to the EXPO power, reduced modulo MOD. The result is placed in RESULT. */
bc_raisemod(bc_num base,bc_num expo,bc_num mod,bc_num * result,size_t scale)36 raise_mod_status bc_raisemod(bc_num base, bc_num expo, bc_num mod, bc_num *result, size_t scale)
37 {
38 bc_num power, exponent, modulus, parity, temp;
39
40 /* Check the base for scale digits. */
41 if (base->n_scale != 0) {
42 return BASE_HAS_FRACTIONAL;
43 }
44 /* Check the exponent for scale digits. */
45 if (expo->n_scale != 0) {
46 return EXPO_HAS_FRACTIONAL;
47 }
48 if (bc_is_neg(expo)) {
49 return EXPO_IS_NEGATIVE;
50 }
51 /* Check the modulus for scale digits. */
52 if (mod->n_scale != 0) {
53 return MOD_HAS_FRACTIONAL;
54 }
55 /* Modulus cannot be 0 */
56 if (bc_is_zero(mod)) {
57 return MOD_IS_ZERO;
58 }
59
60 /* Set initial values. */
61 power = bc_copy_num(base);
62 exponent = bc_copy_num(expo);
63 modulus = bc_copy_num(mod);
64 temp = bc_copy_num(BCG(_one_));
65 bc_init_num(&parity);
66
67 /* Do the calculation. */
68 if (!bc_compare(modulus, BCG(_one_))) {
69 bc_free_num (&temp);
70 temp = bc_new_num (1, scale);
71 } else {
72 while (!bc_is_zero(exponent)) {
73 (void) bc_divmod(exponent, BCG(_two_), &exponent, &parity, 0);
74 if (!bc_is_zero(parity)) {
75 bc_multiply(temp, power, &temp, scale);
76 (void) bc_modulo(temp, modulus, &temp, scale);
77 }
78 bc_multiply(power, power, &power, scale);
79 (void) bc_modulo(power, modulus, &power, scale);
80 }
81 }
82
83 /* Assign the value. */
84 bc_free_num (&power);
85 bc_free_num (&exponent);
86 bc_free_num (&modulus);
87 bc_free_num (result);
88 bc_free_num (&parity);
89 *result = temp;
90 return OK;
91 }
92
