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      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 #include "xf86drm.h"
     78 #include "xf86drmRandom.h"
     79 
     80 #define RANDOM_MAGIC 0xfeedbeef
     81 
     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 
    112 int drmRandomDestroy(void *state)
    113 {
    114     drmFree(state);
    115     return 0;
    116 }
    117 
    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 
    132 double drmRandomDouble(void *state)
    133 {
    134     RandomState *s = (RandomState *)state;
    135 
    136     return (double)drmRandom(state)/(double)s->m;
    137 }
    138