893c6259 |
1 | /* -*-c-*- |
2 | * |
ab9fc001 |
3 | * $Id: mprand.c,v 1.2 1999/12/22 15:55:33 mdw Exp $ |
893c6259 |
4 | * |
5 | * Generate a random multiprecision integer |
6 | * |
7 | * (c) 1999 Straylight/Edgeware |
8 | */ |
9 | |
10 | /*----- Licensing notice --------------------------------------------------* |
11 | * |
12 | * This file is part of Catacomb. |
13 | * |
14 | * Catacomb is free software; you can redistribute it and/or modify |
15 | * it under the terms of the GNU Library General Public License as |
16 | * published by the Free Software Foundation; either version 2 of the |
17 | * License, or (at your option) any later version. |
18 | * |
19 | * Catacomb is distributed in the hope that it will be useful, |
20 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
21 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
22 | * GNU Library General Public License for more details. |
23 | * |
24 | * You should have received a copy of the GNU Library General Public |
25 | * License along with Catacomb; if not, write to the Free |
26 | * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, |
27 | * MA 02111-1307, USA. |
28 | */ |
29 | |
30 | /*----- Revision history --------------------------------------------------* |
31 | * |
32 | * $Log: mprand.c,v $ |
ab9fc001 |
33 | * Revision 1.2 1999/12/22 15:55:33 mdw |
34 | * Modify `mprand' slightly. Add `mprand_range'. |
35 | * |
893c6259 |
36 | * Revision 1.1 1999/12/10 23:23:05 mdw |
37 | * Support for generating random large integers. |
38 | * |
39 | */ |
40 | |
41 | /*----- Header files ------------------------------------------------------*/ |
42 | |
43 | #include <mLib/alloc.h> |
44 | |
45 | #include "grand.h" |
46 | #include "mp.h" |
47 | #include "mprand.h" |
48 | |
49 | /*----- Main code ---------------------------------------------------------*/ |
50 | |
51 | /* --- @mprand@ --- * |
52 | * |
53 | * Arguments: @mp *d@ = destination integer |
54 | * @unsigned b@ = number of bits |
55 | * @grand *r@ = pointer to random number source |
56 | * @mpw or@ = mask to OR with low-order bits |
57 | * |
58 | * Returns: A random integer with the requested number of bits. |
59 | * |
60 | * Use: Constructs an arbitrarily large pseudorandom integer. |
61 | * Assuming that the generator @r@ is good, the result is |
62 | * uniformly distributed in the interval %$[2^{b - 1}, 2^b)$%. |
63 | * The result is then ORred with the given @or@ value. This |
64 | * will often be 1, to make the result odd. |
65 | */ |
66 | |
67 | mp *mprand(mp *d, unsigned b, grand *r, mpw or) |
68 | { |
ab9fc001 |
69 | size_t sz = (b + 7) >> 3; |
893c6259 |
70 | octet *v = xmalloc(sz); |
71 | unsigned m; |
72 | |
73 | /* --- Fill buffer with random data --- */ |
74 | |
75 | r->ops->fill(r, v, sz); |
76 | |
77 | /* --- Force into the correct range --- * |
78 | * |
79 | * This is slightly tricky. Oh, well. |
80 | */ |
81 | |
ab9fc001 |
82 | b = (b - 1) & 7; |
893c6259 |
83 | m = (1 << b); |
84 | v[0] = (v[0] & (m - 1)) | m; |
85 | |
86 | /* --- Mask, load and return --- */ |
87 | |
88 | d = mp_loadb(d, v, sz); |
89 | d->v[0] |= or; |
90 | free(v); |
91 | return (d); |
92 | } |
93 | |
ab9fc001 |
94 | /* --- @mprand_range@ --- * |
95 | * |
96 | * Arguments: @mp *d@ = destination integer |
97 | * @mp *l@ = limit for random number |
98 | * @grand *r@ = random number source |
99 | * @mpw or@ = mask for low-order bits |
100 | * |
101 | * Returns: A pseudorandom integer, unformly distributed over the |
102 | * interval %$[0, l)$%. |
103 | * |
104 | * Use: Generates a uniformly-distributed pseudorandom number in the |
105 | * appropriate range. |
106 | */ |
107 | |
108 | mp *mprand_range(mp *d, mp *l, grand *r, mpw or) |
109 | { |
110 | size_t b = mp_bits(l); |
111 | size_t sz = (b + 7) >> 3; |
112 | octet *v = xmalloc(sz); |
113 | unsigned m; |
114 | |
115 | /* --- The algorithm --- * |
116 | * |
117 | * Rather simpler than most. Find the number of bits in the number %$l$% |
118 | * (i.e., the integer %$b$% such that %$2^{b - 1} \le l < 2^b$%), and |
119 | * generate pseudorandom integers with %$n$% bits (but not, unlike in the |
120 | * function above, with the top bit forced to 1). If the integer is |
121 | * greater than or equal to %$l$%, try again. |
122 | * |
123 | * This is similar to the algorithms used in @lcrand_range@ and friends, |
124 | * except that I've forced the `raw' range of the random numbers such that |
125 | * %$l$% itself is the largest multiple of %$l$% in the range (since, by |
126 | * the inequality above, %$2^b \le 2l$%). This removes the need for costly |
127 | * division and remainder operations. |
128 | * |
129 | * As usual, the number of iterations expected is two. |
130 | */ |
131 | |
132 | b = (b - 1) & 7; |
133 | m = (1 << b) - 1; |
134 | do { |
135 | r->ops->fill(r, v, sz); |
136 | v[0] &= m; |
137 | d = mp_loadb(d, v, sz); |
138 | d->v[0] |= or; |
139 | } while (MP_CMP(d, >=, l)); |
140 | |
141 | /* --- Done --- */ |
142 | |
143 | free(v); |
144 | return (d); |
145 | } |
146 | |
893c6259 |
147 | /*----- That's all, folks -------------------------------------------------*/ |