xf86drmRandom.c

   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 "libdrm_macros.h"
  78 #include "xf86drm.h"
  79 #include "xf86drmRandom.h"
  80
  81 #define RANDOM_MAGIC 0xfeedbeef
  82
  83 drm_public void *drmRandomCreate(unsigned long seed)
  84 {
  85     RandomState  *state;
  86
  87     state           = drmMalloc(sizeof(*state));
  88     if (!state) return NULL;
  89     state->magic    = RANDOM_MAGIC;
  90 #if 0
  91                                 /* Park & Miller, October 1988 */
  92     state->a        = 16807;
  93     state->m        = 2147483647;
  94     state->check    = 1043618065; /* After 10000 iterations */
  95 #else
  96                                 /* Park, Miller, and Stockmeyer, July 1993 */
  97     state->a        = 48271;
  98     state->m        = 2147483647;
  99     state->check    = 399268537; /* After 10000 iterations */
 100 #endif
 101     state->q        = state->m / state->a;
 102     state->r        = state->m % state->a;
 103
 104     state->seed     = seed;
 105                                 /* Check for illegal boundary conditions,
 106                                    and choose closest legal value. */
 107     if (state->seed <= 0)        state->seed = 1;
 108     if (state->seed >= state->m) state->seed = state->m - 1;
 109
 110     return state;
 111 }
 112
 113 drm_public int drmRandomDestroy(void *state)
 114 {
 115     drmFree(state);
 116     return 0;
 117 }
 118
 119 drm_public unsigned long drmRandom(void *state)
 120 {
 121     RandomState   *s = (RandomState *)state;
 122     unsigned long hi;
 123     unsigned long lo;
 124
 125     hi      = s->seed / s->q;
 126     lo      = s->seed % s->q;
 127     s->seed = s->a * lo - s->r * hi;
 128     if ((s->a * lo) <= (s->r * hi)) s->seed += s->m;
 129
 130     return s->seed;
 131 }
 132
 133 drm_public double drmRandomDouble(void *state)
 134 {
 135     RandomState *s = (RandomState *)state;
 136
 137     return (double)drmRandom(state)/(double)s->m;
 138 }