text: /* 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>
  
  #include "xf86drm_mock.h"
  #include "xf86drmRandom_mock.h"
  
  #define RANDOM_MAGIC 0xfeedbeef
  
  void *drmRandomCreate(unsigned long seed)
  {
      RandomState  *state;
  
      state           = (RandomState *)drmMalloc(sizeof(*state));
      if (!state) return nullptr;
      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)
  {
      drmFree(state);
      return 0;
  }
  
  unsigned long drmRandom(void *state)
  {
      RandomState   *s = (RandomState *)state;
      unsigned long hi;
      unsigned long lo;
  
      hi      = s->seed / s->q;
      lo      = s->seed % s->q;
      s->seed = s->a * lo - s->r * hi;
      if ((s->a * lo) <= (s->r * hi)) s->seed += s->m;
  
      return s->seed;
  }
  