src/gmxlib/rando.c

   1 /*
   2  * $Id$
   3  *
   4  *       This source code is part of
   5  *
   6  *        G   R   O   M   A   C   S
   7  *
   8  * GROningen MAchine for Chemical Simulations
   9  *
  10  *               VERSION 2.0
  11  *
  12  * Copyright (c) 1991-1999
  13  * BIOSON Research Institute, Dept. of Biophysical Chemistry
  14  * University of Groningen, The Netherlands
  15  *
  16  * Please refer to:
  17  * GROMACS: A message-passing parallel molecular dynamics implementation
  18  * H.J.C. Berendsen, D. van der Spoel and R. van Drunen
  19  * Comp. Phys. Comm. 91, 43-56 (1995)
  20  *
  21  * Also check out our WWW page:
  22  * http://md.chem.rug.nl/~gmx
  23  * or e-mail to:
  24  * gromacs@chem.rug.nl
  25  *
  26  * And Hey:
  27  * Green Red Orange Magenta Azure Cyan Skyblue
  28  */
  29 static char *SRCID_rando_c = "$Id$";
  30
  31 #include <time.h>
  32 #include <unistd.h>
  33 #include "typedefs.h"
  34
  35 int make_seed(void)
  36 {
  37   return (int)(((long)time(NULL)+(long)getpid()) % (long)65536);
  38 }
  39
  40 real rando(int *ig)
  41      /* generate a random number. */
  42 {
  43   int  irand;
  44
  45   int  m    = 100000000;
  46   real rm   = 100000000.0;  /* same number as m, but real format */
  47   int  m1   = 10000;
  48   int  mult = 31415821;
  49
  50   real r;
  51   int  irandh,irandl,multh,multl;
  52
  53   irand = abs(*ig) % m;
  54
  55   /* multiply irand by mult, but take into account that overflow
  56    * must be discarded, and do not generate an error.
  57    */
  58   irandh = irand / m1;
  59   irandl = irand % m1;
  60   multh  = mult / m1;
  61   multl  = mult % m1;
  62   irand  = ((irandh*multl+irandl*multh) % m1) * m1 + irandl*multl;
  63   irand  = (irand + 1) % m;
  64
  65   /* convert irand to a real random number between 0 and 1. */
  66   r = (irand / 10);
  67   r = r * 10 / rm;
  68   if ((r <= 0) || (r > 1))
  69     r = 0.0;
  70   *ig = irand;
  71
  72   return r;
  73 }
  74
  75
  76