1 /* xf86drmRandom.c -- "Minimal Standard" PRNG Implementation 2 * Created: Mon Apr 19 08:28:13 1999 by faith (at) 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 (at) 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 #define RANDOM_MAIN 0 78 79 #if !RANDOM_MAIN 80 # include "xf86drm.h" 81 #endif 82 83 #define RANDOM_MAGIC 0xfeedbeef 84 #define RANDOM_DEBUG 0 85 86 #if RANDOM_MAIN 87 #define RANDOM_ALLOC malloc 88 #define RANDOM_FREE free 89 #else 90 #define RANDOM_ALLOC drmMalloc 91 #define RANDOM_FREE drmFree 92 #endif 93 94 typedef struct RandomState { 95 unsigned long magic; 96 unsigned long a; 97 unsigned long m; 98 unsigned long q; /* m div a */ 99 unsigned long r; /* m mod a */ 100 unsigned long check; 101 long seed; 102 } RandomState; 103 104 #if RANDOM_MAIN 105 extern void *drmRandomCreate(unsigned long seed); 106 extern int drmRandomDestroy(void *state); 107 extern unsigned long drmRandom(void *state); 108 extern double drmRandomDouble(void *state); 109 #endif 110 111 void *drmRandomCreate(unsigned long seed) 112 { 113 RandomState *state; 114 115 state = RANDOM_ALLOC(sizeof(*state)); 116 if (!state) return NULL; 117 state->magic = RANDOM_MAGIC; 118 #if 0 119 /* Park & Miller, October 1988 */ 120 state->a = 16807; 121 state->m = 2147483647; 122 state->check = 1043618065; /* After 10000 iterations */ 123 #else 124 /* Park, Miller, and Stockmeyer, July 1993 */ 125 state->a = 48271; 126 state->m = 2147483647; 127 state->check = 399268537; /* After 10000 iterations */ 128 #endif 129 state->q = state->m / state->a; 130 state->r = state->m % state->a; 131 132 state->seed = seed; 133 /* Check for illegal boundary conditions, 134 and choose closest legal value. */ 135 if (state->seed <= 0) state->seed = 1; 136 if (state->seed >= state->m) state->seed = state->m - 1; 137 138 return state; 139 } 140 141 int drmRandomDestroy(void *state) 142 { 143 RANDOM_FREE(state); 144 return 0; 145 } 146 147 unsigned long drmRandom(void *state) 148 { 149 RandomState *s = (RandomState *)state; 150 long hi; 151 long lo; 152 153 hi = s->seed / s->q; 154 lo = s->seed % s->q; 155 s->seed = s->a * lo - s->r * hi; 156 if (s->seed <= 0) s->seed += s->m; 157 158 return s->seed; 159 } 160 161 double drmRandomDouble(void *state) 162 { 163 RandomState *s = (RandomState *)state; 164 165 return (double)drmRandom(state)/(double)s->m; 166 } 167 168 #if RANDOM_MAIN 169 static void check_period(long seed) 170 { 171 unsigned long count = 0; 172 unsigned long initial; 173 void *state; 174 175 state = drmRandomCreate(seed); 176 initial = drmRandom(state); 177 ++count; 178 while (initial != drmRandom(state)) { 179 if (!++count) break; 180 } 181 printf("With seed of %10ld, period = %10lu (0x%08lx)\n", 182 seed, count, count); 183 drmRandomDestroy(state); 184 } 185 186 int main(void) 187 { 188 RandomState *state; 189 int i; 190 unsigned long rand; 191 192 state = drmRandomCreate(1); 193 for (i = 0; i < 10000; i++) { 194 rand = drmRandom(state); 195 } 196 printf("After 10000 iterations: %lu (%lu expected): %s\n", 197 rand, state->check, 198 rand - state->check ? "*INCORRECT*" : "CORRECT"); 199 drmRandomDestroy(state); 200 201 printf("Checking periods...\n"); 202 check_period(1); 203 check_period(2); 204 check_period(31415926); 205 206 return 0; 207 } 208 #endif 209
