stdlib/drand48-iter.c

   1 /* Copyright (C) 1995-2017 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 Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the 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    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 #include <errno.h>
  20 #include <stdlib.h>
  21 #include <limits.h>
  22 #include <stdint.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 (unsigned short int xsubi[3], struct drand48_data *buffer)
  31 {
  32   uint64_t X;
  33   uint64_t result;
  34
  35   /* Initialize buffer, if not yet done.  */
  36   if (__glibc_unlikely (!buffer->__init))
  37     {
  38       buffer->__a = 0x5deece66dull;
  39       buffer->__c = 0xb;
  40       buffer->__init = 1;
  41     }
  42
  43   /* Do the real work.  We choose a data type which contains at least
  44      48 bits.  Because we compute the modulus it does not care how
  45      many bits really are computed.  */
  46
  47   X = (uint64_t) xsubi[2] << 32 | (uint32_t) xsubi[1] << 16 | xsubi[0];
  48
  49   result = X * buffer->__a + buffer->__c;
  50
  51   xsubi[0] = result & 0xffff;
  52   xsubi[1] = (result >> 16) & 0xffff;
  53   xsubi[2] = (result >> 32) & 0xffff;
  54
  55   return 0;
  56 }