// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"

namespace Rivet {


  /// @brief e+ e- > n nbar
  class BESIII_2021_I1966612 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2021_I1966612);


    /// @name Analysis methods
    ///@{

    /// Book histograms and initialise projections before the run
    void init() {

      // Initialise and register projections
      declare(FinalState(), "FS");

      // Book histograms
      book(_nneutron, 1, 1, 1);

      for (const string& en : _nneutron.binning().edges<0>()) {
        const double end = std::stod(en)*GeV;
        if (isCompatibleWithSqrtS(end)) {
          _ecms = en;
          break;
        }
      }
      if(_ecms.empty()) MSG_ERROR("Beam energy incompatible with analysis.");
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      const FinalState& fs = apply<FinalState>(event, "FS");
      if(fs.particles().size()!=2) vetoEvent;
      for (const Particle& p :  fs.particles()) {
	if(abs(p.pid())!=PID::NEUTRON) vetoEvent;
      }
      _nneutron->fill(_ecms);
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      scale(_nneutron, crossSection()/ sumOfWeights() /picobarn);
    }

    ///@}


    /// @name Histograms
    ///@{
    BinnedHistoPtr<string> _nneutron;
    string _ecms;
    ///@}


  };


  RIVET_DECLARE_PLUGIN(BESIII_2021_I1966612);

}
