::nrand48 is a function in the global namespace supplied with some STL's to draw a positive (that is: non-negative) random number uniformly distributed between 0 and 2^31.

::nrand48 needs to have a buffer supplied. Nrand48 is identical to ::nrand48.


#include <cassert>

#include <cstdlib>

#include <iostream>

#include <stdint.h>



//From http://www.richelbilderbeek.nl/CppNrand48.htm

long int Nrand48(unsigned short int xsubi[3])

{

  //Factor in congruential formula

  static unsigned long long int a = 0x5deece66dull;

  // Additive constant in congruential formula

  static unsigned short int c = 0xb;

  const uint64_t x

    = static_cast<uint64_t>(xsubi[2]) << 32

    | static_cast<uint32_t>(xsubi[1]) << 16

    | xsubi[0];

  const uint64_t y = x * a + c;



  xsubi[0] = y & 0xffff;

  xsubi[1] = (y >> 16) & 0xffff;

  xsubi[2] = (y >> 32) & 0xffff;



  if (sizeof(unsigned short int) == 2)

  {

    return xsubi[2] << 15 | xsubi[1] >> 1;

  }

  else

  {

    return xsubi[2] >> 1;

  }

}



int main()

{

  unsigned short int buffer1[3] = {0,0,0};

  unsigned short int buffer2[3] = {0,0,0};



  for (int i=0; i!=10; ++i)

  {

    const long int a = ::nrand48(buffer1);

    const long int b = Nrand48(buffer2);

    assert(a==b);

    assert(buffer1[0]==buffer2[0]);

    assert(buffer1[1]==buffer2[1]);

    assert(buffer1[2]==buffer2[2]);

    std::cout

      << a << '\t'

      << buffer1[0] << '\t'

      << buffer1[1] << '\t'

      << buffer1[2] << '\t'

      << '\n';

  }

}

Screen output:


0		0		11	0	0	

2116118		2116118		59066	37933	64	

89401895	89401895	22845	21582	2728	

379337186	379337186	52484	29636	11576	

782977366	782977366	58047	37548	23894	

196130996	196130996	9566	29032	5985	

198207689	198207689	23825	53650	6048	

1046291021	1046291021	58952	17562	31930	

1131187612	1131187612	48307	6968	34521	

975888346	975888346	10818	49077	29781