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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