stdlib/drand48-iter.c

   1 /* Copyright (C) 1995-2024 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 #include <errno.h>
  19 #include <stdlib.h>
  20 #include <limits.h>
  21 #include <stdint.h>
  22 #include <sys/types.h>
  23
  24 /* Global state for non-reentrant functions.  */
  25 struct drand48_data __libc_drand48_data;
  26
  27
  28 int
  29 __drand48_iterate (unsigned short int xsubi[3], struct drand48_data *buffer)
  30 {
  31   uint64_t X;
  32   uint64_t result;
  33
  34   /* Initialize buffer, if not yet done.  */
  35   if (__glibc_unlikely (!buffer->__init))
  36     {
  37       buffer->__a = 0x5deece66dull;
  38       buffer->__c = 0xb;
  39       buffer->__init = 1;
  40     }
  41
  42   /* Do the real work.  We choose a data type which contains at least
  43      48 bits.  Because we compute the modulus it does not care how
  44      many bits really are computed.  */
  45
  46   X = (uint64_t) xsubi[2] << 32 | (uint32_t) xsubi[1] << 16 | xsubi[0];
  47
  48   result = X * buffer->__a + buffer->__c;
  49
  50   xsubi[0] = result & 0xffff;
  51   xsubi[1] = (result >> 16) & 0xffff;
  52   xsubi[2] = (result >> 32) & 0xffff;
  53
  54   return 0;
  55 }