/* 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.h" | |

#include "xf86drmRandom.h" | |

#define RANDOM_MAGIC 0xfeedbeef | |

void *drmRandomCreate(unsigned long seed) | |

{ | |

RandomState *state; | |

state = drmMalloc(sizeof(*state)); | |

if (!state) return NULL; | |

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; | |

} | |

double drmRandomDouble(void *state) | |

{ | |

RandomState *s = (RandomState *)state; | |

return (double)drmRandom(state)/(double)s->m; | |

} |