geant4/source/global/HEPRandom/include/G4RandomTools.hh

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//
//
//
//
// ---------------------------------------------------------------------------
//      GEANT 4 class header file
// ---------------------------------------------------------------------------
// Class description:
//
// Utility functions

// History:
//
// 24.08.17 - E.Tcherniaev, added G4RandomRadiusInRing, G4RandomPointInEllipse
//                          G4RandomPointOnEllipse, G4RandomPointOnEllipsoid
// 07.11.08 - P.Gumplinger, based on implementation in G4OpBoundaryProcess
//
// ---------------------------------------------------------------------------

#ifndef G4RANDOMTOOLS_HH
#define G4RANDOMTOOLS_HH

#include <CLHEP/Units/PhysicalConstants.h>

#include "G4RandomDirection.hh"
#include "G4ThreeVector.hh"
#include "G4TwoVector.hh"
#include "Randomize.hh"
#include "globals.hh"

// ---------------------------------------------------------------------------
// Returns a random lambertian unit vector (rejection sampling)
//
inline G4ThreeVector G4LambertianRand(const G4ThreeVector& normal)
{
  G4ThreeVector vect;
  G4double ndotv;
  G4int count            = 0;
  const G4int max_trials = 1024;

  do
  {
    ++count;
    vect  = G4RandomDirection();
    ndotv = normal * vect;

    if(ndotv < 0.0)
    {
      vect  = -vect;
      ndotv = -ndotv;
    }

  } while(!(G4UniformRand() < ndotv) && (count < max_trials));

  return vect;
}

// ---------------------------------------------------------------------------
// Chooses a random vector within a plane given by the unit normal
//
inline G4ThreeVector G4PlaneVectorRand(const G4ThreeVector& normal)
{
  G4ThreeVector vec1 = normal.orthogonal();
  G4ThreeVector vec2 = vec1.cross(normal);

  G4double phi    = CLHEP::twopi * G4UniformRand();
  G4double cosphi = std::cos(phi);
  G4double sinphi = std::sin(phi);

  return cosphi * vec1 + sinphi * vec2;
}

// ---------------------------------------------------------------------------
// Returns a random radius in annular ring
//
inline G4double G4RandomRadiusInRing(G4double rmin, G4double rmax)
{
  if(rmin == rmax)
  {
    return rmin;
  }
  G4double k = G4UniformRand();
  return (rmin <= 0) ? rmax * std::sqrt(k)
                     : std::sqrt(k * rmax * rmax + (1. - k) * rmin * rmin);
}

// ---------------------------------------------------------------------------
// Returns a random point in ellipse (x/a)^2 + (y/b)^2 = 1
// (rejection sampling)
//
inline G4TwoVector G4RandomPointInEllipse(G4double a, G4double b)
{
  G4double aa = (a * a == 0) ? 0 : 1 / (a * a);
  G4double bb = (b * b == 0) ? 0 : 1 / (b * b);
  for(G4int i = 0; i < 1000; ++i)
  {
    G4double x = a * (2 * G4UniformRand() - 1);
    G4double y = b * (2 * G4UniformRand() - 1);
    if(x * x * aa + y * y * bb <= 1)
      return G4TwoVector(x, y);
  }
  return G4TwoVector(0, 0);
}

// ---------------------------------------------------------------------------
// Returns a random point on ellipse (x/a)^2 + (y/b)^2 = 1
// (rejection sampling)
//
inline G4TwoVector G4RandomPointOnEllipse(G4double a, G4double b)
{
  G4double A      = std::abs(a);
  G4double B      = std::abs(b);
  G4double mu_max = std::max(A, B);

  G4double x, y;
  for(G4int i = 0; i < 1000; ++i)
  {
    G4double phi = CLHEP::twopi * G4UniformRand();
    x            = std::cos(phi);
    y            = std::sin(phi);
    G4double mu  = std::sqrt((B * x) * (B * x) + (A * y) * (A * y));
    if(mu_max * G4UniformRand() <= mu)
      break;
  }
  return G4TwoVector(A * x, B * y);
}

// ---------------------------------------------------------------------------
// Returns a random point on ellipsoid (x/a)^2 + (y/b)^2 + (z/c)^2 = 1
// (rejection sampling)
//
inline G4ThreeVector G4RandomPointOnEllipsoid(G4double a, G4double b,
                                              G4double c)
{
  G4double A      = std::abs(a);
  G4double B      = std::abs(b);
  G4double C      = std::abs(c);
  G4double mu_max = std::max(std::max(A * B, A * C), B * C);

  G4ThreeVector p;
  for(G4int i = 0; i < 1000; ++i)
  {
    p            = G4RandomDirection();
    G4double xbc = p.x() * B * C;
    G4double yac = p.y() * A * C;
    G4double zab = p.z() * A * B;
    G4double mu  = std::sqrt(xbc * xbc + yac * yac + zab * zab);
    if(mu_max * G4UniformRand() <= mu)
      break;
  }
  return G4ThreeVector(A * p.x(), B * p.y(), C * p.z());
}

#endif /* G4RANDOMTOOLS_HH */