glibc/stdlib/drand48-iter.c

/* Copyright (C) 1995-2023 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

#include <errno.h>
#include <stdlib.h>
#include <limits.h>
#include <stdint.h>
#include <sys/types.h>

/* Global state for non-reentrant functions.  */
struct drand48_data __libc_drand48_data;


int
__drand48_iterate (unsigned short int xsubi[3], struct drand48_data *buffer)
{
  uint64_t X;
  uint64_t result;

  /* Initialize buffer, if not yet done.  */
  if (__glibc_unlikely (!buffer->__init))
    {
      buffer->__a = 0x5deece66dull;
      buffer->__c = 0xb;
      buffer->__init = 1;
    }

  /* Do the real work.  We choose a data type which contains at least
     48 bits.  Because we compute the modulus it does not care how
     many bits really are computed.  */

  X = (uint64_t) xsubi[2] << 32 | (uint32_t) xsubi[1] << 16 | xsubi[0];

  result = X * buffer->__a + buffer->__c;

  xsubi[0] = result & 0xffff;
  xsubi[1] = (result >> 16) & 0xffff;
  xsubi[2] = (result >> 32) & 0xffff;

  return 0;
}