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//
// G4ImportanceAlgorithm implementation
//
// Author: Michael Dressel (CERN), 2002
// ----------------------------------------------------------------------

#include "G4Types.hh"
#include <sstream>
#include "Randomize.hh"
#include "G4Threading.hh"

#include "G4ImportanceAlgorithm.hh"

#ifdef G4MULTITHREADED
G4Mutex G4ImportanceAlgorithm::ImportanceMutex = G4MUTEX_INITIALIZER;
#endif

G4ImportanceAlgorithm::G4ImportanceAlgorithm()
{
}

G4ImportanceAlgorithm::~G4ImportanceAlgorithm()
{
}

G4Nsplit_Weight
G4ImportanceAlgorithm::Calculate(G4double ipre,
                                 G4double ipost,
                                 G4double init_w) const
{

#ifdef G4MULTITHREADED
  G4MUTEXLOCK(&G4ImportanceAlgorithm::ImportanceMutex);
#endif

  G4Nsplit_Weight nw;
  if (ipost>0.)
  {
    if (!(ipre>0.))
    {
      Error("Calculate() - ipre==0.");
    }
    G4double ipre_over_ipost = ipre/ipost;
    if ((ipre_over_ipost<0.25 || ipre_over_ipost> 4) && !fWarned)
    {
      std::ostringstream os;
      os << "Calculate() - ipre_over_ipost ! in [0.25, 4]." << G4endl
         << "ipre_over_ipost = " << ipre_over_ipost << ".";
      Warning(os.str());
      fWarned = true;
      if (ipre_over_ipost<=0)
      {
        Error("Calculate() - ipre_over_ipost<=0.");
      }
    }
    if (init_w<=0.)
    {
      Error("Calculate() - iniitweight<= 0. found!");
    }

    // default geometrical splitting 
    // in integer mode 
    // for ipre_over_ipost <= 1
    G4double inv = 1./ipre_over_ipost;
    nw.fN = static_cast<G4int>(inv);
    nw.fW = init_w * ipre_over_ipost;
    
    // geometrical splitting for double mode
    if (ipre_over_ipost<1)
    {
      if ( static_cast<G4double>(nw.fN) != inv)
      {
        // double mode
        // probability p for splitting into n+1 tracks
        G4double p = inv - nw.fN;
        // get a random number out of [0,1)
        G4double r = G4UniformRand();
        if (r<p)
        {
          ++nw.fN;
        } 
      }  
    }
    else if (ipre_over_ipost>1)   // russian roulette
    {
      // probabiity for killing track
      G4double p = 1-inv;
      // get a random number out of [0,1)
      G4double r = G4UniformRand();
      if (r<p)
      {
        // kill track
        nw.fN = 0;
        nw.fW = 0;
      }
      else
      {
        nw.fN = 1;     
      }
    }
  }
#ifdef G4MULTITHREADED
  G4MUTEXUNLOCK(&G4ImportanceAlgorithm::ImportanceMutex);
#endif
  return nw;
}

void G4ImportanceAlgorithm::Error(const G4String& msg) const
{
  G4Exception("G4ImportanceAlgorithm::Error()",
              "GeomBias0002", FatalException, msg);
}

void G4ImportanceAlgorithm::Warning(const G4String& msg) const
{
  G4Exception("G4ImportanceAlgorithm::Warning()",
              "GeomBias1001", JustWarning, msg);
}