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139 lines
4.7 KiB
139 lines
4.7 KiB
/* xf86drmRandom.c -- "Minimal Standard" PRNG Implementation
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* Created: Mon Apr 19 08:28:13 1999 by faith@precisioninsight.com
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*
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* Copyright 1999 Precision Insight, Inc., Cedar Park, Texas.
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* All Rights Reserved.
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*
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* Permission is hereby granted, free of charge, to any person obtaining a
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* copy of this software and associated documentation files (the "Software"),
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* to deal in the Software without restriction, including without limitation
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* the rights to use, copy, modify, merge, publish, distribute, sublicense,
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* and/or sell copies of the Software, and to permit persons to whom the
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* Software is furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice (including the next
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* paragraph) shall be included in all copies or substantial portions of the
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* Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
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* PRECISION INSIGHT AND/OR ITS SUPPLIERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
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* OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
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* ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
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* DEALINGS IN THE SOFTWARE.
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*
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* Authors: Rickard E. (Rik) Faith <faith@valinux.com>
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*
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* DESCRIPTION
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*
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* This file contains a simple, straightforward implementation of the Park
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* & Miller "Minimal Standard" PRNG [PM88, PMS93], which is a Lehmer
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* multiplicative linear congruential generator (MLCG) with a period of
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* 2^31-1.
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*
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* This implementation is intended to provide a reliable, portable PRNG
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* that is suitable for testing a hash table implementation and for
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* implementing skip lists.
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*
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* FUTURE ENHANCEMENTS
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*
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* If initial seeds are not selected randomly, two instances of the PRNG
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* can be correlated. [Knuth81, pp. 32-33] describes a shuffling technique
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* that can eliminate this problem.
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*
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* If PRNGs are used for simulation, the period of the current
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* implementation may be too short. [LE88] discusses methods of combining
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* MLCGs to produce much longer periods, and suggests some alternative
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* values for A and M. [LE90 and Sch92] also provide information on
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* long-period PRNGs.
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*
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* REFERENCES
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*
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* [Knuth81] Donald E. Knuth. The Art of Computer Programming. Volume 2:
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* Seminumerical Algorithms. Reading, Massachusetts: Addison-Wesley, 1981.
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*
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* [LE88] Pierre L'Ecuyer. "Efficient and Portable Combined Random Number
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* Generators". CACM 31(6), June 1988, pp. 742-774.
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*
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* [LE90] Pierre L'Ecuyer. "Random Numbers for Simulation". CACM 33(10,
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* October 1990, pp. 85-97.
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*
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* [PM88] Stephen K. Park and Keith W. Miller. "Random Number Generators:
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* Good Ones are Hard to Find". CACM 31(10), October 1988, pp. 1192-1201.
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*
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* [Sch92] Bruce Schneier. "Pseudo-Ransom Sequence Generator for 32-Bit
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* CPUs". Dr. Dobb's Journal 17(2), February 1992, pp. 34, 37-38, 40.
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*
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* [PMS93] Stephen K. Park, Keith W. Miller, and Paul K. Stockmeyer. In
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* "Technical Correspondence: Remarks on Choosing and Implementing Random
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* Number Generators". CACM 36(7), July 1993, pp. 105-110.
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*
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include "libdrm_macros.h"
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#include "xf86drm.h"
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#include "xf86drmRandom.h"
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#define RANDOM_MAGIC 0xfeedbeef
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drm_public void *drmRandomCreate(unsigned long seed)
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{
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RandomState *state;
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state = drmMalloc(sizeof(*state));
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if (!state) return NULL;
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state->magic = RANDOM_MAGIC;
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#if 0
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/* Park & Miller, October 1988 */
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state->a = 16807;
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state->m = 2147483647;
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state->check = 1043618065; /* After 10000 iterations */
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#else
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/* Park, Miller, and Stockmeyer, July 1993 */
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state->a = 48271;
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state->m = 2147483647;
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state->check = 399268537; /* After 10000 iterations */
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#endif
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state->q = state->m / state->a;
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state->r = state->m % state->a;
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state->seed = seed;
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/* Check for illegal boundary conditions,
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and choose closest legal value. */
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if (state->seed <= 0) state->seed = 1;
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if (state->seed >= state->m) state->seed = state->m - 1;
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return state;
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}
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drm_public int drmRandomDestroy(void *state)
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{
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drmFree(state);
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return 0;
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}
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drm_public unsigned long drmRandom(void *state)
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{
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RandomState *s = (RandomState *)state;
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unsigned long hi;
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unsigned long lo;
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hi = s->seed / s->q;
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lo = s->seed % s->q;
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s->seed = s->a * lo - s->r * hi;
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if ((s->a * lo) <= (s->r * hi)) s->seed += s->m;
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return s->seed;
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}
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drm_public double drmRandomDouble(void *state)
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{
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RandomState *s = (RandomState *)state;
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return (double)drmRandom(state)/(double)s->m;
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}
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