xf86drmRandom.c 4.7 KB

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