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//
// G4BOptnLeadingParticle
//
// Class Description:
//
// A G4VBiasingOperation that implements the so-called "Leading
// particle biasing scheme". It is of interest in the shield problem
// to estimate the flux leaking from the shield.
// It works as follows:
// - it is intented for hadronic inelastic interaction
// - at each interaction, are kept:
//     - the most energetic particle (the leading particle)
//         - with unmodified weight
//     - randomly one particle of each species
//         - with this particle weight = n * primary_weight where
//           n is the number of particles of this species
//
// Author: Marc Verderi, November 2019.
// --------------------------------------------------------------------

#ifndef G4BOptnLeadingParticle_hh
#define G4BOptnLeadingParticle_hh 1

#include "G4VBiasingOperation.hh"
#include "G4ParticleChange.hh"

class G4BOptnLeadingParticle : public G4VBiasingOperation
{
  public:

    // -- Constructor :
    G4BOptnLeadingParticle(const G4String& name);
    // -- destructor:
    virtual ~G4BOptnLeadingParticle();
  
    // -- Methods from G4VBiasingOperation interface:
    // ----------------------------------------------
    // -- Unused:
    virtual const G4VBiasingInteractionLaw*
    ProvideOccurenceBiasingInteractionLaw( const G4BiasingProcessInterface*,
                                           G4ForceCondition& ) { return nullptr; }
    // -- Used:
    virtual G4VParticleChange*
    ApplyFinalStateBiasing( const G4BiasingProcessInterface*, // -- Method used for this biasing. The related biasing operator
                            const G4Track*,                   // -- returns this biasing operation at the post step do it level
                            const G4Step*,                    // -- when the wrapped process has won the interaction length race.
                            G4bool& );                        // -- The wrapped process final state is then trimmed.
    // -- Unused:
    virtual G4double
    DistanceToApplyOperation( const G4Track*, G4double, G4ForceCondition* ) { return 0.0; }
    virtual G4VParticleChange*
    GenerateBiasingFinalState( const G4Track*, const G4Step* ) { return nullptr; }

    // -- The possibility is given to further apply a Russian roulette on tracks that are accompagnying the leading particle
    // -- after the classical leading particle biasing algorithm has been applied.
    // -- This is of interest when applying the technique to e+ -> gamma gamma for example. Given one gamma is leading,
    // -- the second one is alone in its category, hence selected. With the Russian roulette it is then possible to keep
    // -- this one randomly. This is also of interest for pi0 decays, or for brem. e- -> e- gamma where the e- or gamma
    // -- are alone in their category.
    void SetFurtherKillingProbability( G4double p ) // -- if p <= 0.0 the killing is ignored.
    {
      fRussianRouletteKillingProbability = p;
    }
    G4double GetFurtherKillingProbability() const
    {
      return fRussianRouletteKillingProbability;
    }

  private:

    // -- Particle change used to return the trimmed final state:
    G4ParticleChange fParticleChange;
    G4double fRussianRouletteKillingProbability = -1.0;
};

#endif