# include <cmath>
# include <ctime>
# include <cstdlib>
# include <iostream>
# include <iomanip>

using namespace std;

int main ( );
int absorb ( int &seed );
double cross ( double e );
double dist2c ( double e, int &seed );
double energy ( int &seed );
void output ( int na, double ea, double sa, int nr, double er, double sr, 
  int nt, double et, double st, int ntot );
double r8_uniform_01 ( int &seed );
void scatter ( int &seed, double &e, double &mu, double &azm );
void source ( int &seed, double &e, double &mu, double &azm, double &x, 
  double &y, double &z );
void timestamp ( );
void update ( double mu, double azm, double d, double &x, double &y, double &z );

//****************************************************************************80

int main ( )

//****************************************************************************80
//
//  Purpose:
//
//    MAIN is the main program for the reactor shielding simulation.
//
//  Discussion:
//
//    This is a Monte Carlo simulation, using
//    uniform random numbers, which investigates the
//    effectiveness of a shield intended to absorb the
//    neutrons emitted from a nuclear reactor.
// 
//    The reactor is modeled as a point source,
//    located at (0,0,0).
//   
//    A particle emitted from the reactor has a random
//    initial direction, and an energy selected from
//    [Emin,Emax] with a 1/Sqrt(E) distribution.
//   
//    The shield is modeled as a wall of thickness THICK,
//    extending from 0 to THICK in the X direction, and
//    extending forever in the Y and Z directions.
//   
//    Based on the particle energy, a distance D is computed
//    which measures how far the particle could travel through
//    the shield before colliding.
//   
//    Based on the particle direction, the position is updated
//    by D units.
//   
//    If the particle is now to the left of the shield, it is
//    counted as being REFLECTED.
//   
//    If the particle is to the right of the shield, it is 
//    counted as being ABSORBED.
//   
//    If the particle is inside the shield, it has COLLIDED.
//    A particle that collides is either absorbed (end of story)
//    or SCATTERED with a new random direction and a new (lower)
//    energy.
//   
//    Every particle is followed from origin to its final fate,
//    which is reflection, transmission, or absorption.
//    At the end, a summary is printed, giving the number of
//    particles with each fate, and the average energy of each
//    group of particles.
//   
//    Increasing NTOT, the number of particles used, will improve the
//    expected reliability of the results.
//   
//    Increasing THICK, the thickness of the shield, should 
//    result in more absorptions and reflections.
//
//  Licensing:
//
//    This code is distributed under the MIT license.
//
//  Modified:
//
//    03 September 2012
//
//  Author:
//
//    Original FORTRAN77 version by Kahaner, Moler, Nash.
//    C++ version by John Burkardt.
//
//  Reference:
//
//    David Kahaner, Cleve Moler, Steven Nash,
//    Numerical Methods and Software,
//    Prentice Hall, 1989,
//    ISBN: 0-13-627258-4,
//    LC: TA345.K34.
//
//  Local Parameters:
//
//    Local, double AZM, the azimuthal angle of the particle's
//    direction.
//
//    Local, double D, the distance that the particle can
//    travel through the slab, given its current energy.
//
//    Local, double E, the energy of the particle.
//
//    Local, double EA, energy absorbed by the slab.
//
//    Local, double ER, energy reflected by the slab.
//
//    Local, double ET, energy transmitted through the slab.
//
//    Local, double MU, the cosine of the angle between the
//    particle's direction and the X axis.
//
//    Local, int NA, number of particles absorbed by the slab.
//
//    Local, int NPART, the index of the current particle.
//
//    Local, int NR, number of particles reflected by the slab.
//
//    Local, int NT, number of particles transmitted by the slab.
//
//    Local, int NTOT, the total number of particles to be
//    emitted from the neutron source.
//
//    Local, double SA, standard deviation of absorbed energy.
//
//    Local, double SR, standard deviation of reflected energy.
//
//    Local, double ST, standard deviation of transmitted energy.
//
//    Local, double THICK, the thickness of the slab that is
//    intended to absorb most of the particles.
//
//    Local, double X, Y, Z, the current position of the particle.
//
{
  double azm;
  double d;
  double e;
  double ea;
  double er;
  double et;
  double mu;
  int na;
  int nr;
  int nt;
  int ntot = 100000;
  int part;
  double sa;
  int seed;
  double sr;
  double st;
  int test;
  int test_num = 5;
  double thick = 2.0;
  double x;
  double y;
  double z;

  timestamp ( );

  cout << "\n";
  cout << "REACTOR_SIMULATION\n";
  cout << "  C++ version\n";
  cout << "  The reactor shielding simulation.\n";
  cout << "\n";
  cout << "  Shield thickness is THICK = " << thick << "\n";
  cout << "  Number of simulated particles is NTOT = " << ntot << "\n";
  cout << "  Number of tests TEST_NUM = " << test_num << "\n";

  seed = 123456789;

  for ( test = 1; test <= test_num; test++ )
  {
    cout << "\n";
    cout << "  Test # " << test << "\n";
    cout << "  SEED = " << seed << "\n";
//
//  Initialize.
//
    ea = 0.0;
    er = 0.0;
    et = 0.0;
    na = 0;
    nr = 0;
    nt = 0;
    sa = 0.0;
    sr = 0.0;
    st = 0.0;
//
//  Loop over the particles.
//
    for ( part = 1; part <= ntot; part++ )
    {
//
//  Generate a new particle.
//
      source ( seed, e, mu, azm, x, y, z );

      while ( 1 )
      {
//
//  Compute the distance that the particle can travel through the slab,
//  based on its current energy.
//
        d = dist2c ( e, seed );
//
//  Update the particle's position by D units.
//
        update ( mu, azm, d, x, y, z );
//
//  The particle was reflected by the shield, and this path is complete.
//
        if ( x < 0.0 )
        {
          nr = nr + 1;
          er = er + e;
          sr = sr + e * e;
          break;
        }
//
//  The particle was transmitted through the shield, and this path is complete.
//
        else if ( thick < x )
        {
          nt = nt + 1;
          et = et + e;
          st = st + e * e;
          break;
        }
//
//  The particle collided with the shield, and was absorbed.  This path is done.
//
        else if ( absorb ( seed ) )
        {
          na = na + 1;
          ea = ea + e;
          sa = sa + e * e;
          break;
        }
//
//  The particle collided with the shield and was scattered.
//  Find the scattering angle and energy, and continue along the new path.
//
        else
        {
          scatter ( seed, e, mu, azm );
        }
      }
    }
//
//  Print the results of the simulation.
//
    output ( na, ea, sa, nr, er, sr, nt, et, st, ntot );
  }
//
//  Terminate.
//
  cout << "\n";
  cout << "REACTOR_SIMULATION:\n";
  cout << "  Normal end of execution.\n";

  cout << "\n";
  timestamp ( );

  return 0;
}
//****************************************************************************80

int absorb ( int &seed )

//****************************************************************************80
//
//  Purpose:
//
//    ABSORB determines if a colliding particle is absorbed.
//
//  Licensing:
//
//    This code is distributed under the MIT license.
//
//  Modified:
//
//    03 September 2012
//
//  Author:
//
//    Original FORTRAN77 version by Kahaner, Moler, Nash.
//    C++ version by John Burkardt.
//
//  Reference:
//
//    David Kahaner, Cleve Moler, Steven Nash,
//    Numerical Methods and Software,
//    Prentice Hall, 1989,
//    ISBN: 0-13-627258-4,
//    LC: TA345.K34.
//
//  Parameters:
//
//    Input/output, int &SEED, a seed for the random
//    number generator.
//
//    Output, logical ABSORB, is TRUE if the particle is absorbed.
//
//  Local parameters:
//
//    Local, double PA, the probability of absorption.
//
{
  double pa = 0.1;
  double u;
  int value;

  u = r8_uniform_01 ( seed );

  if ( u <= pa )
  {
    value = 1;
  }
  else
  {
    value = 0;
  }
  return value;
}
//****************************************************************************80

double cross ( double e )

//****************************************************************************80
//
//  Purpose:
//
//    CROSS returns the "cross section" of a particle based on its energy.
//
//  Discussion:
//
//    The particle's cross section is a measure of its likelihood to collide
//    with the material of the slab.  This quantity typically depends on both
//    the particle's energy and the kind of medium through which it is traveling.
//
//  Licensing:
//
//    This code is distributed under the MIT license.
//
//  Modified:
//
//    03 September 2012
//
//  Author:
//
//    Original FORTRAN77 version by Kahaner, Moler, Nash.
//    C++ version by John Burkardt.
//
//  Reference:
//
//    David Kahaner, Cleve Moler, Steven Nash,
//    Numerical Methods and Software,
//    Prentice Hall, 1989,
//    ISBN: 0-13-627258-4,
//    LC: TA345.K34.
//
//  Parameters:
//
//    Input, double E, the energy of the particle.
//
//    Output, double CROSS, the cross section.
//
{
  double s;
  double value;
  double y;

  s = fabs ( sin ( 100.0 * ( exp ( e ) - 1.0 ) ) 
    + sin ( 18.81 * ( exp ( e ) - 1.0 ) ) );

  y = fmax ( 0.02, s );

  value = 10.0 * exp ( -0.1 / y );

  return value;
}
//****************************************************************************80

double dist2c ( double e, int &seed )

//****************************************************************************80
//
//  Purpose:
//
//    DIST2C returns the distance to collision.
//
//  Discussion:
//
//    Assuming the particle has a given energy, and assuming it is currently
//    somewhere inside the shield, it is possible to determine a typical distance
//    which the particle can travel before it collides with the material of
//    the shield.
//
//    The computation of the collision distance is made by estimating a
//    "cross section" (as though having more energy made the particle "bigger"
//    and hence more likely to collide) and then randomly selecting a distance
//    that is logarithmically distributed.
//
//  Licensing:
//
//    This code is distributed under the MIT license.
//
//  Modified:
//
//    03 September 2012
//
//  Author:
//
//    Original FORTRAN77 version by Kahaner, Moler, Nash.
//    C++ version by John Burkardt.
//
//  Reference:
//
//    David Kahaner, Cleve Moler, Steven Nash,
//    Numerical Methods and Software,
//    Prentice Hall, 1989,
//    ISBN: 0-13-627258-4,
//    LC: TA345.K34.
//
//  Parameters:
//
//    Input, double E, the energy of the particle.
//
//    Input/output, int &SEED, a seed for the random
//    number generator.
//
//    Output, double DIST2C, the distance the particle can travel
//    through the slab before colliding.
//
{
  double u;
  double value;

  u = r8_uniform_01 ( seed );

  value = - log ( u ) / cross ( e );

  return value;
}
//****************************************************************************80

double energy ( int &seed )

//****************************************************************************80
//
//  Purpose:
//
//    ENERGY assigns an energy to an emitted particle.
//
//  Discussion:
//
//    The energy E is in the range [EMIN,EMAX], with distribution
//    const/sqrt(energy).
//
//    An inverse function approach is used to compute this.
//
//    The energies are measured in MeV.
//
//  Licensing:
//
//    This code is distributed under the MIT license.
//
//  Modified:
//
//    03 September 2012
//
//  Author:
//
//    Original FORTRAN77 version by Kahaner, Moler, Nash.
//    C++ version by John Burkardt.
//
//  Reference:
//
//    David Kahaner, Cleve Moler, Steven Nash,
//    Numerical Methods and Software,
//    Prentice Hall, 1989,
//    ISBN: 0-13-627258-4,
//    LC: TA345.K34.
//
//  Parameters:
//
//    Input/output, int &SEED, a seed for the random
//    number generator.
//
//    Output, double ENERGY, a randomly chosen energy that is
//    distributed as described above.
//
//  Local parameters:
//
//    Local, double EMIN, EMAX, the minimum and maximum
//    energies.
//
{
  double c;
  double emax = 2.5;
  double emin = 1.0E-03;
  double u;
  double value;

  u = r8_uniform_01 ( seed );

  c = 1.0 / ( 2.0 * ( sqrt ( emax ) - sqrt ( emin ) ) );

  value = ( u / ( 2.0 * c ) + sqrt ( emin ) );
  value = value * value;

  return value;
}
//****************************************************************************80

void output ( int na, double ea, double sa, int nr, double er, double sr, 
  int nt, double et, double st, int ntot )

//****************************************************************************80
//
//  Purpose:
//
//    OUTPUT prints the results of the reactor shielding simulation.
//
//  Licensing:
//
//    This code is distributed under the MIT license.
//
//  Modified:
//
//    03 September 2012
//
//  Author:
//
//    Original FORTRAN77 version by Kahaner, Moler, Nash.
//    C++ version by John Burkardt.
//
//  Reference:
//
//    David Kahaner, Cleve Moler, Steven Nash,
//    Numerical Methods and Software,
//    Prentice Hall, 1989,
//    ISBN: 0-13-627258-4,
//    LC: TA345.K34.
//
//  Parameters:
//
//    Input, int NA, number of particles absorbed by the slab.
//
//    Input, double EA, energy absorbed by the slab.
//
//    Input, double SA, the sum of the squares of the 
//    absorbed energies.
//
//    Input, int NR, number of particles reflected by the slab.
//
//    Input, double ER, energy reflected by the slab.
//
//    Input, double SR, the sum of the squares of the 
//    reflected energies.
//
//    Input, int NT, number of particles transmitted by the slab.
//
//    Input, double ET, energy transmitted through the slab.
//
//    Input, double ST, the sum of the squares of the 
//    transmitted energies.
//
//    Input, int NTOT, the total number of particles.
//
{
  double ea_ave;
  double er_ave;
  double et_ave;
  double etot;
  double pa;
  double pr;
  double pt;
  double ptot;

  cout << "\n";
  cout << "  The Reactor Shielding Problem:\n";
  cout << "\n";
  cout << "                           Total                   Average\n";
  cout << "                    #      Energy      ";
  cout << "Percent     Energy         StDev\n";
  cout << "\n";

  etot = ea + er + et;

  if ( 0 < na )
  {
    ea_ave = ea / ( double ) ( na );
    sa = sqrt ( sa / ( double ) ( na ) - ea_ave * ea_ave );
  }
  else
  {
    ea_ave = 0.0;
  }

  pa = ( double ) ( na * 100 ) / ( double ) ( ntot );

  cout << "Absorbed   "
       << "  " << setw(8) << na
       << "  " << setw(14) << ea
       << "  " << setw(6) << pa
       << "  " << setw(14) << ea_ave
       << "  " << setw(14) << sa << "\n";

  if ( 0 < nr )
  {
    er_ave = er / ( double ) ( nr );
    sr = sqrt ( sr / ( double ) ( nr ) - er_ave * er_ave );
  }
  else
  {
    er_ave = 0.0;
  }

  pr = ( double ) ( nr * 100 ) / ( double ) ( ntot );

  cout << "Reflected  "
       << "  " << setw(8) << nr
       << "  " << setw(14) << er
       << "  " << setw(6) << pr
       << "  " << setw(14) << er_ave
       << "  " << setw(14) << sr << "\n";

  if ( 0 < nt )
  {
    et_ave = et / ( double ) ( nt );
    st = sqrt ( st / ( double ) ( nt ) - et_ave * et_ave );
  }
  else
  {
    et_ave = 0.0;
  }

  pt = ( double ) ( nt * 100 ) / ( double ) ( ntot );

  cout << "Transmitted  "
       << "  " << setw(8) << nt
       << "  " << setw(14) << et
       << "  " << setw(6) << pt
       << "  " << setw(14) << et_ave
       << "  " << setw(14) << st << "\n";

  ptot = 100.0;

  cout << "\n";
  cout << "Total      "
       << "  " << setw(8) << ntot
       << "  " << setw(14) << etot
       << "  " << setw(6) << ptot << "\n";

  return;
}
//****************************************************************************80

double r8_uniform_01 ( int &seed )

//****************************************************************************80
//
//  Purpose:
//
//    R8_UNIFORM_01 returns a unit pseudorandom R8.
//
//  Discussion:
//
//    This routine implements the recursion
//
//      seed = ( 16807 * seed ) mod ( 2^31 - 1 )
//      u = seed / ( 2^31 - 1 )
//
//    The integer arithmetic never requires more than 32 bits,
//    including a sign bit.
//
//    If the initial seed is 12345, then the first three computations are
//
//      Input     Output      R8_UNIFORM_01
//      SEED      SEED
//
//         12345   207482415  0.096616
//     207482415  1790989824  0.833995
//    1790989824  2035175616  0.947702
//
//  Licensing:
//
//    This code is distributed under the MIT license. 
//
//  Modified:
//
//    09 April 2012
//
//  Author:
//
//    John Burkardt
//
//  Reference:
//
//    Paul Bratley, Bennett Fox, Linus Schrage,
//    A Guide to Simulation,
//    Second Edition,
//    Springer, 1987,
//    ISBN: 0387964673,
//    LC: QA76.9.C65.B73.
//
//    Bennett Fox,
//    Algorithm 647:
//    Implementation and Relative Efficiency of Quasirandom
//    Sequence Generators,
//    ACM Transactions on Mathematical Software,
//    Volume 12, Number 4, December 1986, pages 362-376.
//
//    Pierre L'Ecuyer,
//    Random Number Generation,
//    in Handbook of Simulation,
//    edited by Jerry Banks,
//    Wiley, 1998,
//    ISBN: 0471134031,
//    LC: T57.62.H37.
//
//    Peter Lewis, Allen Goodman, James Miller,
//    A Pseudo-Random Number Generator for the System/360,
//    IBM Systems Journal,
//    Volume 8, Number 2, 1969, pages 136-143.
//
//  Parameters:
//
//    Input/output, int &SEED, the "seed" value.  Normally, this
//    value should not be 0.  On output, SEED has been updated.
//
//    Output, double R8_UNIFORM_01, a new pseudorandom variate, 
//    strictly between 0 and 1.
//
{
  int i4_huge = 2147483647;
  int k;
  double r;

  if ( seed == 0 )
  {
    cerr << "\n";
    cerr << "R8_UNIFORM_01 - Fatal error!\n";
    cerr << "  Input value of SEED = 0.\n";
    exit ( 1 );
  }

  k = seed / 127773;

  seed = 16807 * ( seed - k * 127773 ) - k * 2836;

  if ( seed < 0 )
  {
    seed = seed + i4_huge;
  }
  r = ( double ) ( seed ) * 4.656612875E-10;

  return r;
}
//****************************************************************************80

void scatter ( int &seed, double &e, double &mu, double &azm )

//****************************************************************************80
//
//  Purpose:
//
//    SCATTER returns the new direction and energy of a particle that is scattered.
//
//  Discussion:
//
//    The scattering direction is chosen uniformly on the sphere.
//
//    The energy of the scattered particle is chosen uniformly in
//    [ 0.3*E, E ].
//
//  Licensing:
//
//    This code is distributed under the MIT license.
//
//  Modified:
//
//    03 September 2012
//
//  Author:
//
//    Original FORTRAN77 version by Kahaner, Moler, Nash.
//    C++ version by John Burkardt.
//
//  Reference:
//
//    David Kahaner, Cleve Moler, Steven Nash,
//    Numerical Methods and Software,
//    Prentice Hall, 1989,
//    ISBN: 0-13-627258-4,
//    LC: TA345.K34.
//
//  Parameters:
//
//    Input/output, int &SEED, a seed for the random
//    number generator.
//
//    Input/output, double &E.  On input, the particle energy
//    before collision.  On output, the particle energy after collision
//    and scattering.
//
//    Output, double &MU, the cosine of the angle between the
//    particle's direction and the X axis.
//
//    Output, double &AZM, the azimuthal angle of the particle's
//    direction.
//
{
  double pi = 3.141592653589793;
  double u;

  u = r8_uniform_01 ( seed );
  mu = - 1.0 + 2.0 * u;

  u = r8_uniform_01 ( seed );
  azm = u * 2.0 * pi;

  u = r8_uniform_01 ( seed );
  e = ( u * 0.7 + 0.3 ) * e;

  return;
}
//****************************************************************************80

void source ( int &seed, double &e, double &mu, double &azm, double &x, 
  double &y, double &z )

//****************************************************************************80
//
//  Purpose:
//
//    SOURCE generates a new particle from the neutron source.
//
//  Licensing:
//
//    This code is distributed under the MIT license.
//
//  Modified:
//
//    03 September 2012
//
//  Author:
//
//    Original FORTRAN77 version by Kahaner, Moler, Nash.
//    C++ version by John Burkardt.
//
//  Reference:
//
//    David Kahaner, Cleve Moler, Steven Nash,
//    Numerical Methods and Software,
//    Prentice Hall, 1989,
//    ISBN: 0-13-627258-4,
//    LC: TA345.K34.
//
//  Parameters:
//
//    Input/output, int &SEED, a seed for the random
//    number generator.
//
//    Output, double &E, the initial energy of the particle.
//
//    Output, double &MU, the cosine of the angle between the
//    particle's direction and the X axis.
//
//    Output, double &AZM, the azimuthal angle of the particle's
//    direction.
//
//    Output, double &X, &Y, &Z, the initial coordinates of the particle.
//
{
  double pi = 3.141592653589793;
  double u;

  u = r8_uniform_01 ( seed );
  mu = u;

  u = r8_uniform_01 ( seed );
  azm = u * 2.0 * pi;

  x = 0.0;
  y = 0.0;
  z = 0.0;

  e = energy ( seed );

  return;
}
//****************************************************************************80

void timestamp ( )

//****************************************************************************80
//
//  Purpose:
//
//    TIMESTAMP prints the current YMDHMS date as a time stamp.
//
//  Example:
//
//    31 May 2001 09:45:54 AM
//
//  Licensing:
//
//    This code is distributed under the MIT license.
//
//  Modified:
//
//    08 July 2009
//
//  Author:
//
//    John Burkardt
//
//  Parameters:
//
//    None
//
{
# define TIME_SIZE 40

  static char time_buffer[TIME_SIZE];
  const struct std::tm *tm_ptr;
  std::time_t now;

  now = std::time ( NULL );
  tm_ptr = std::localtime ( &now );

  std::strftime ( time_buffer, TIME_SIZE, "%d %B %Y %I:%M:%S %p", tm_ptr );

  std::cout << time_buffer << "\n";

  return;
# undef TIME_SIZE
}
//****************************************************************************80

void update ( double mu, double azm, double d, double &x, double &y, double &z )

//****************************************************************************80
//
//  Purpose:
//
//    UPDATE determines the position of the particle after it has traveled D units.
//
//  Licensing:
//
//    This code is distributed under the MIT license.
//
//  Modified:
//
//    03 September 2012
//
//  Author:
//
//    Original FORTRAN77 version by Kahaner, Moler, Nash.
//    C++ version by John Burkardt.
//
//  Reference:
//
//    David Kahaner, Cleve Moler, Steven Nash,
//    Numerical Methods and Software,
//    Prentice Hall, 1989,
//    ISBN: 0-13-627258-4,
//    LC: TA345.K34.
//
//  Parameters:
//
//    Input, double MU, the cosine of the angle between the
//    particle's direction and the X axis.
//
//    Input, double AZM, the azimuthal angle of the particle's
//    direction.
//
//    Input, double D, the distance the particle traveled.
//
//    Input/output, double &X, &Y, &Z.  On input, the previous
//    coordinates of the particle.  On output, the updated coordinates of the
//    particle.
//
{
  double s;

  s = sqrt ( 1.0 - mu * mu );

  x = x + d * mu;
  y = y + d * s * cos ( azm );
  z = z + d * s * sin ( azm );

  return;
}