src/gmxlib/rando.c

   1 /*
   2  *
   3  *                This source code is part of
   4  *
   5  *                 G   R   O   M   A   C   S
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   7  *          GROningen MAchine for Chemical Simulations
   8  *
   9  *                        VERSION 3.2.0
  10  * Written by David van der Spoel, Erik Lindahl, Berk Hess, and others.
  11  * Copyright (c) 1991-2000, University of Groningen, The Netherlands.
  12  * Copyright (c) 2001-2004, The GROMACS development team,
  13  * check out http://www.gromacs.org for more information.
  14
  15  * This program is free software; you can redistribute it and/or
  16  * modify it under the terms of the GNU General Public License
  17  * as published by the Free Software Foundation; either version 2
  18  * of the License, or (at your option) any later version.
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  34  */
  35 #ifdef HAVE_CONFIG_H
  36 #include <config.h>
  37 #endif
  38
  39 #include <time.h>
  40 #if ((defined WIN32 || defined _WIN32 || defined WIN64 || defined _WIN64) && !defined __CYGWIN__ && !defined __CYGWIN32__)
  41 #include <process.h>
  42 #endif
  43 #include "sysstuff.h"
  44 #include "typedefs.h"
  45 #include "random.h"
  46
  47 int make_seed(void)
  48 {
  49 #if ((defined WIN32 || defined _WIN32 || defined WIN64 || defined _WIN64) && !defined __CYGWIN__ && !defined __CYGWIN32__)
  50   return (int)_getpid();
  51 #else
  52   return (int)getpid();
  53 #endif
  54 }
  55
  56 real rando(int *ig)
  57      /* generate a random number. */
  58 {
  59   int  irand;
  60
  61   int  m    = 100000000;
  62   real rm   = 100000000.0;  /* same number as m, but real format */
  63   int  m1   = 10000;
  64   int  mult = 31415821;
  65
  66   real r;
  67   int  irandh,irandl,multh,multl;
  68
  69   irand = abs(*ig) % m;
  70
  71   /* multiply irand by mult, but take into account that overflow
  72    * must be discarded, and do not generate an error.
  73    */
  74   irandh = irand / m1;
  75   irandl = irand % m1;
  76   multh  = mult / m1;
  77   multl  = mult % m1;
  78   irand  = ((irandh*multl+irandl*multh) % m1) * m1 + irandl*multl;
  79   irand  = (irand + 1) % m;
  80
  81   /* convert irand to a real random number between 0 and 1. */
  82   r = (irand / 10);
  83   r = r * 10 / rm;
  84   if ((r <= 0) || (r > 1))
  85     r = 0.0;
  86   *ig = irand;
  87
  88   return r;
  89 }
  90
  91
  92