stdlib/drand48-iter.c

   1 /* Copyright (C) 1995, 1996 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu>, August 1995.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Library General Public License as
   7    published by the Free Software Foundation; either version 2 of the
   8    License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Library General Public License for more details.
  14
  15    You should have received a copy of the GNU Library General Public
  16    License along with the GNU C Library; see the file COPYING.LIB.  If not,
  17    write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  18    Boston, MA 02111-1307, USA.  */
  19
  20 #include <errno.h>
  21 #include <stdlib.h>
  22 #include <limits.h>
  23 #include <sys/types.h>
  24
  25 /* Global state for non-reentrant functions.  */
  26 struct drand48_data __libc_drand48_data;
  27
  28
  29 int
  30 __drand48_iterate (xsubi, buffer)
  31      unsigned short int xsubi[3];
  32      struct drand48_data *buffer;
  33 {
  34   u_int64_t X, a, result;
  35
  36   /* Initialize buffer, if not yet done.  */
  37   if (!buffer->init)
  38     {
  39 #if (USHRT_MAX == 0xffffU)
  40       buffer->a[2] = 0x5;
  41       buffer->a[1] = 0xdeec;
  42       buffer->a[0] = 0xe66d;
  43 #else
  44       buffer->a[2] = 0x5deecUL;
  45       buffer->a[1] = 0xe66d0000UL;
  46       buffer->a[0] = 0;
  47 #endif
  48       buffer->c = 0xb;
  49       buffer->init = 1;
  50     }
  51
  52   /* Do the real work.  We choose a data type which contains at least
  53      48 bits.  Because we compute the modulus it does not care how
  54      many bits really are computed.  */
  55
  56   if (sizeof (unsigned short int) == 2)
  57     {
  58       X = (u_int64_t)xsubi[2] << 32 | (u_int64_t)xsubi[1] << 16 | xsubi[0];
  59       a = ((u_int64_t)buffer->a[2] << 32 | (u_int64_t)buffer->a[1] << 16
  60            | buffer->a[0]);
  61
  62       result = X * a + buffer->c;
  63
  64       xsubi[0] = result & 0xffff;
  65       xsubi[1] = (result >> 16) & 0xffff;
  66       xsubi[2] = (result >> 32) & 0xffff;
  67     }
  68   else
  69     {
  70       X = (u_int64_t)xsubi[2] << 16 | xsubi[1] >> 16;
  71       a = (u_int64_t)buffer->a[2] << 16 | buffer->a[1] >> 16;
  72
  73       result = X * a + buffer->c;
  74
  75       xsubi[0] = result >> 16 & 0xffffffffl;
  76       xsubi[1] = result << 16 & 0xffff0000l;
  77     }
  78
  79   return 0;
  80 }