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   1 /* -*-c-*-
   2  *
   3  * $Id: lcrand.h,v 1.3 2004/04/08 01:36:15 mdw Exp $
   4  *
   5  * Simple linear congruential generator
   6  *
   7  * (c) 1999 Straylight/Edgeware
   8  */
   9
  10 /*----- Licensing notice --------------------------------------------------*
  11  *
  12  * This file is part of Catacomb.
  13  *
  14  * Catacomb is free software; you can redistribute it and/or modify
  15  * it under the terms of the GNU Library General Public License as
  16  * published by the Free Software Foundation; either version 2 of the
  17  * License, or (at your option) any later version.
  18  *
  19  * Catacomb is distributed in the hope that it will be useful,
  20  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  21  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  22  * GNU Library General Public License for more details.
  23  *
  24  * You should have received a copy of the GNU Library General Public
  25  * License along with Catacomb; if not, write to the Free
  26  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  27  * MA 02111-1307, USA.
  28  */
  29
  30 /*----- Notes on the linear congruential generator ------------------------*
  31  *
  32  * This pseudorandom number generator is simple, but has absolutely no
  33  * cryptographic strength whatever.  It may be used whenever random numbers
  34  * are required but cryptographic strength is not, for example when
  35  * generating numbers for use in primality tests.  To be honest, it's not
  36  * even particularly fast, although a certain amount of effort has been
  37  * expended on making it better than awfully slow.  To put things in
  38  * perspective, it can't quite spit bytes out as fast as OFB DES.  (Then
  39  * again, bytes aren't its natural output format.)  Its main use is probably
  40  * seeding a Fibonacci generator.
  41  *
  42  * There exists a fixed-point input @LCRAND_FIXEDPT@ -- when fed to the
  43  * generator it comes straight back out again.  All other inputs less than
  44  * the modulus are part of the same sequence of period %$p - 1$%.
  45  *
  46  * The generator has been tested for its statistical properties.  George
  47  * Marsaglia's Diehard tests give it a reasonably clean bill of health.
  48  *
  49  * The modulus %$p$% is chosen as the largest prime number less than
  50  * %$2^{32}$%.  The multiplier %$a$% and additive constant %$c$% are based on
  51  * the decimal expansions of %$\pi$% and %$e$%, with the additional
  52  * restriction that the multiplier must be a primitive element modulo %$p$%.
  53  * The fixed point value is determined as %$c / (1 - a) \bmod p$%.
  54  */
  55
  56 #ifndef CATACOMB_LCRAND_H
  57 #define CATACOMB_LCRAND_H
  58
  59 #ifdef __cplusplus
  60   extern "C" {
  61 #endif
  62
  63 /*----- Header files ------------------------------------------------------*/
  64
  65 #include <mLib/bits.h>
  66
  67 #ifndef CATACOMB_GRAND_H
  68 #  include "grand.h"
  69 #endif
  70
  71 /*----- Constants ---------------------------------------------------------*/
  72
  73 #define LCRAND_P 4294967291u            /* Modulus for the generator */
  74 #define LCRAND_A 314159265u             /* Multiplier (primitive mod @p@) */
  75 #define LCRAND_C 271828183u             /* Additive constant */
  76
  77 #define LCRAND_FIXEDPT 3223959250u      /* Fixed point (only bad input) */
  78
  79 /*----- Functions provided ------------------------------------------------*/
  80
  81 /* --- @lcrand@ --- *
  82  *
  83  * Arguments:   @uint32 x@ = seed value
  84  *
  85  * Returns:     New state of the generator.
  86  *
  87  * Use:         Steps the generator.  Returns %$ax + c \bmod p$%.
  88  */
  89
  90 extern uint32 lcrand(uint32 /*x*/);
  91
  92 /* --- @lcrand_range@ --- *
  93  *
  94  * Arguments:   @uint32 *x@ = pointer to seed value (updated)
  95  *              @uint32 m@ = limit allowable
  96  *
  97  * Returns:     A uniformly distributed pseudorandom integer in the interval
  98  *              %$[0, m)$%.
  99  */
 100
 101 extern uint32 lcrand_range(uint32 */*x*/, uint32 /*m*/);
 102
 103 /* --- @lcrand_create@ --- *
 104  *
 105  * Arguments:   @uint32 x@ = initial seed
 106  *
 107  * Returns:     Pointer to a generic generator.
 108  *
 109  * Use:         Constructs a generic generator interface over a linear
 110  *              congruential generator.
 111  */
 112
 113 extern grand *lcrand_create(uint32 /*x*/);
 114
 115 /*----- That's all, folks -------------------------------------------------*/
 116
 117 #ifdef __cplusplus
 118   }
 119 #endif
 120
 121 #endif