1 /* xf86drmRandom.c -- "Minimal Standard" PRNG Implementation
2 * Created: Mon Apr 19 08:28:13 1999 by faith@precisioninsight.com
3 *
4 * Copyright 1999 Precision Insight, Inc., Cedar Park, Texas.
5 * All Rights Reserved.
6 *
7 * Permission is hereby granted, free of charge, to any person obtaining a
8 * copy of this software and associated documentation files (the "Software"),
9 * to deal in the Software without restriction, including without limitation
10 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
11 * and/or sell copies of the Software, and to permit persons to whom the
12 * Software is furnished to do so, subject to the following conditions:
13 *
14 * The above copyright notice and this permission notice (including the next
15 * paragraph) shall be included in all copies or substantial portions of the
16 * Software.
17 *
18 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
19 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
20 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
21 * PRECISION INSIGHT AND/OR ITS SUPPLIERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
22 * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
23 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
24 * DEALINGS IN THE SOFTWARE.
25 *
26 * Authors: Rickard E. (Rik) Faith <faith@valinux.com>
27 *
28 * DESCRIPTION
29 *
30 * This file contains a simple, straightforward implementation of the Park
31 * & Miller "Minimal Standard" PRNG [PM88, PMS93], which is a Lehmer
32 * multiplicative linear congruential generator (MLCG) with a period of
33 * 2^31-1.
34 *
35 * This implementation is intended to provide a reliable, portable PRNG
36 * that is suitable for testing a hash table implementation and for
37 * implementing skip lists.
38 *
39 * FUTURE ENHANCEMENTS
40 *
41 * If initial seeds are not selected randomly, two instances of the PRNG
42 * can be correlated. [Knuth81, pp. 32-33] describes a shuffling technique
43 * that can eliminate this problem.
44 *
45 * If PRNGs are used for simulation, the period of the current
46 * implementation may be too short. [LE88] discusses methods of combining
47 * MLCGs to produce much longer periods, and suggests some alternative
48 * values for A and M. [LE90 and Sch92] also provide information on
49 * long-period PRNGs.
50 *
51 * REFERENCES
52 *
53 * [Knuth81] Donald E. Knuth. The Art of Computer Programming. Volume 2:
54 * Seminumerical Algorithms. Reading, Massachusetts: Addison-Wesley, 1981.
55 *
56 * [LE88] Pierre L'Ecuyer. "Efficient and Portable Combined Random Number
57 * Generators". CACM 31(6), June 1988, pp. 742-774.
58 *
59 * [LE90] Pierre L'Ecuyer. "Random Numbers for Simulation". CACM 33(10,
60 * October 1990, pp. 85-97.
61 *
62 * [PM88] Stephen K. Park and Keith W. Miller. "Random Number Generators:
63 * Good Ones are Hard to Find". CACM 31(10), October 1988, pp. 1192-1201.
64 *
65 * [Sch92] Bruce Schneier. "Pseudo-Ransom Sequence Generator for 32-Bit
66 * CPUs". Dr. Dobb's Journal 17(2), February 1992, pp. 34, 37-38, 40.
67 *
68 * [PMS93] Stephen K. Park, Keith W. Miller, and Paul K. Stockmeyer. In
69 * "Technical Correspondence: Remarks on Choosing and Implementing Random
70 * Number Generators". CACM 36(7), July 1993, pp. 105-110.
71 *
72 */
73
74 #include <stdio.h>
75 #include <stdlib.h>
76
77 #include "xf86drm.h"
78 #include "xf86drmRandom.h"
79
80 #define RANDOM_MAGIC 0xfeedbeef
81
drmRandomCreate(unsigned long seed)82 void *drmRandomCreate(unsigned long seed)
83 {
84 RandomState *state;
85
86 state = drmMalloc(sizeof(*state));
87 if (!state) return NULL;
88 state->magic = RANDOM_MAGIC;
89 #if 0
90 /* Park & Miller, October 1988 */
91 state->a = 16807;
92 state->m = 2147483647;
93 state->check = 1043618065; /* After 10000 iterations */
94 #else
95 /* Park, Miller, and Stockmeyer, July 1993 */
96 state->a = 48271;
97 state->m = 2147483647;
98 state->check = 399268537; /* After 10000 iterations */
99 #endif
100 state->q = state->m / state->a;
101 state->r = state->m % state->a;
102
103 state->seed = seed;
104 /* Check for illegal boundary conditions,
105 and choose closest legal value. */
106 if (state->seed <= 0) state->seed = 1;
107 if (state->seed >= state->m) state->seed = state->m - 1;
108
109 return state;
110 }
111
drmRandomDestroy(void * state)112 int drmRandomDestroy(void *state)
113 {
114 drmFree(state);
115 return 0;
116 }
117
drmRandom(void * state)118 unsigned long drmRandom(void *state)
119 {
120 RandomState *s = (RandomState *)state;
121 unsigned long hi;
122 unsigned long lo;
123
124 hi = s->seed / s->q;
125 lo = s->seed % s->q;
126 s->seed = s->a * lo - s->r * hi;
127 if ((s->a * lo) <= (s->r * hi)) s->seed += s->m;
128
129 return s->seed;
130 }
131
drmRandomDouble(void * state)132 double drmRandomDouble(void *state)
133 {
134 RandomState *s = (RandomState *)state;
135
136 return (double)drmRandom(state)/(double)s->m;
137 }
138