// -*- C++ -*-
#ifndef RIVET_InvMassFinalState_HH
#define RIVET_InvMassFinalState_HH

#include "Rivet/Projections/FinalState.hh"

namespace Rivet {


  /// @brief Identify particles which can be paired to fit within a given invariant mass window
  class InvMassFinalState : public FinalState {
  public:

    /// Constructor for a single inv-mass pair.
    InvMassFinalState(const FinalState& fsp,
                      const std::pair<PdgId, PdgId>& idpair, // pair of decay products
                      double minmass, // min inv mass
                      double maxmass, // max inv mass
                      double masstarget=-1.0);


    /// Constructor for multiple inv-mass pairs.
    InvMassFinalState(const FinalState& fsp,
                      const std::vector<std::pair<PdgId, PdgId> >& idpairs,  // vector of pairs of decay products
                      double minmass, // min inv mass
                      double maxmass, // max inv mass
                      double masstarget=-1.0);


    /// Same thing as above, but we want to pass the particles directly to the calc method
    InvMassFinalState(const std::pair<PdgId, PdgId>& idpair, // pair of decay products
                      double minmass, // min inv mass
                      double maxmass, // max inv mass
                      double masstarget=-1.0);
    InvMassFinalState(const std::vector<std::pair<PdgId, PdgId> >& idpairs,  // vector of pairs of decay products
                      double minmass, // min inv mass
                      double maxmass, // max inv mass
                      double masstarget=-1.0);


    /// Clone on the heap.
    RIVET_DEFAULT_PROJ_CLONE(InvMassFinalState);

    /// Import to avoid warnings about overload-hiding
    using Projection::operator =;


    /// Constituent pairs.
    const std::vector<std::pair<Particle, Particle> >& particlePairs() const;


    /// Choose whether to use the full inv mass or just the transverse mass.
    void useTransverseMass(bool usetrans=true) {
      _useTransverseMass = usetrans;
    }

    /// Operate on a given particle vector directly instead of through project (no caching)
    void calc(const Particles& inparticles);


    /// Apply the projection on the supplied event.
    void project(const Event& e);

    /// Compare projections.
    CmpState compare(const Projection& p) const;


  protected:

    /// IDs of the decay products.
    vector<PdgIdPair> _decayids;

    /// Constituent pairs.
    vector<pair<Particle, Particle> > _particlePairs;

    /// Min inv mass.
    double _minmass;

    /// Max inv mass.
    double _maxmass;

    /// Target mass if only one pair should be returned.
    double _masstarget;

    /// Flag to decide whether to use the full inv mass or just the transverse mass.
    bool _useTransverseMass;

  };


}

#endif
