aboutsummaryrefslogtreecommitdiff
path: root/libdrm/xf86drmRandom.c
blob: ecab9e2d60e67464c1965b473c49fd052c72f310 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
/* xf86drmRandom.c -- "Minimal Standard" PRNG Implementation
 * Created: Mon Apr 19 08:28:13 1999 by faith@precisioninsight.com
 *
 * Copyright 1999 Precision Insight, Inc., Cedar Park, Texas.
 * All Rights Reserved.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice (including the next
 * paragraph) shall be included in all copies or substantial portions of the
 * Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * PRECISION INSIGHT AND/OR ITS SUPPLIERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
 * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 * DEALINGS IN THE SOFTWARE.
 * 
 * Authors: Rickard E. (Rik) Faith <faith@valinux.com>
 *
 * DESCRIPTION
 *
 * This file contains a simple, straightforward implementation of the Park
 * & Miller "Minimal Standard" PRNG [PM88, PMS93], which is a Lehmer
 * multiplicative linear congruential generator (MLCG) with a period of
 * 2^31-1.
 *
 * This implementation is intended to provide a reliable, portable PRNG
 * that is suitable for testing a hash table implementation and for
 * implementing skip lists.
 *
 * FUTURE ENHANCEMENTS
 *
 * If initial seeds are not selected randomly, two instances of the PRNG
 * can be correlated.  [Knuth81, pp. 32-33] describes a shuffling technique
 * that can eliminate this problem.
 *
 * If PRNGs are used for simulation, the period of the current
 * implementation may be too short.  [LE88] discusses methods of combining
 * MLCGs to produce much longer periods, and suggests some alternative
 * values for A and M.  [LE90 and Sch92] also provide information on
 * long-period PRNGs.
 *
 * REFERENCES
 *
 * [Knuth81] Donald E. Knuth. The Art of Computer Programming.  Volume 2:
 * Seminumerical Algorithms.  Reading, Massachusetts: Addison-Wesley, 1981.
 *
 * [LE88] Pierre L'Ecuyer. "Efficient and Portable Combined Random Number
 * Generators".  CACM 31(6), June 1988, pp. 742-774.
 *
 * [LE90] Pierre L'Ecuyer. "Random Numbers for Simulation". CACM 33(10,
 * October 1990, pp. 85-97.
 *
 * [PM88] Stephen K. Park and Keith W. Miller. "Random Number Generators:
 * Good Ones are Hard to Find". CACM 31(10), October 1988, pp. 1192-1201.
 *
 * [Sch92] Bruce Schneier. "Pseudo-Ransom Sequence Generator for 32-Bit
 * CPUs".  Dr. Dobb's Journal 17(2), February 1992, pp. 34, 37-38, 40.
 *
 * [PMS93] Stephen K. Park, Keith W. Miller, and Paul K. Stockmeyer.  In
 * "Technical Correspondence: Remarks on Choosing and Implementing Random
 * Number Generators". CACM 36(7), July 1993, pp. 105-110.
 *
 */

#include <stdio.h>
#include <stdlib.h>

#define RANDOM_MAIN 0

#if !RANDOM_MAIN
# include "xf86drm.h"
#endif

#define RANDOM_MAGIC 0xfeedbeef
#define RANDOM_DEBUG 0

#if RANDOM_MAIN
#define RANDOM_ALLOC malloc
#define RANDOM_FREE  free
#else
#define RANDOM_ALLOC drmMalloc
#define RANDOM_FREE  drmFree
#endif

typedef struct RandomState {
    unsigned long magic;
    unsigned long a;
    unsigned long m;
    unsigned long q;		/* m div a */
    unsigned long r;		/* m mod a */
    unsigned long check;
    long          seed;
} RandomState;

#if RANDOM_MAIN
extern void          *drmRandomCreate(unsigned long seed);
extern int           drmRandomDestroy(void *state);
extern unsigned long drmRandom(void *state);
extern double        drmRandomDouble(void *state);
#endif

void *drmRandomCreate(unsigned long seed)
{
    RandomState  *state;

    state           = RANDOM_ALLOC(sizeof(*state));
    if (!state) return NULL;
    state->magic    = RANDOM_MAGIC;
#if 0
				/* Park & Miller, October 1988 */
    state->a        = 16807;
    state->m        = 2147483647;
    state->check    = 1043618065; /* After 10000 iterations */
#else
				/* Park, Miller, and Stockmeyer, July 1993 */
    state->a        = 48271;
    state->m        = 2147483647;
    state->check    = 399268537; /* After 10000 iterations */
#endif
    state->q        = state->m / state->a;
    state->r        = state->m % state->a;

    state->seed     = seed;
				/* Check for illegal boundary conditions,
                                   and choose closest legal value. */
    if (state->seed <= 0)        state->seed = 1;
    if (state->seed >= state->m) state->seed = state->m - 1;

    return state;
}

int drmRandomDestroy(void *state)
{
    RANDOM_FREE(state);
    return 0;
}

unsigned long drmRandom(void *state)
{
    RandomState   *s = (RandomState *)state;
    long          hi;
    long          lo;

    hi      = s->seed / s->q;
    lo      = s->seed % s->q;
    s->seed = s->a * lo - s->r * hi;
    if (s->seed <= 0) s->seed += s->m;

    return s->seed;
}

double drmRandomDouble(void *state)
{
    RandomState *s = (RandomState *)state;
    
    return (double)drmRandom(state)/(double)s->m;
}

#if RANDOM_MAIN
static void check_period(long seed)
{
    unsigned long count = 0;
    unsigned long initial;
    void          *state;
    
    state = drmRandomCreate(seed);
    initial = drmRandom(state);
    ++count;
    while (initial != drmRandom(state)) {
	if (!++count) break;
    }
    printf("With seed of %10ld, period = %10lu (0x%08lx)\n",
	   seed, count, count);
    drmRandomDestroy(state);
}

int main(void)
{
    RandomState   *state;
    int           i;
    unsigned long rand;

    state = drmRandomCreate(1);
    for (i = 0; i < 10000; i++) {
	rand = drmRandom(state);
    }
    printf("After 10000 iterations: %lu (%lu expected): %s\n",
	   rand, state->check,
	   rand - state->check ? "*INCORRECT*" : "CORRECT");
    drmRandomDestroy(state);

    printf("Checking periods...\n");
    check_period(1);
    check_period(2);
    check_period(31415926);
    
    return 0;
}
#endif