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 #ifndef __FASTJET_RESTFRAMENSUBJETTINESS_TAGGER_HH__
 #define __FASTJET_RESTFRAMENSUBJETTINESS_TAGGER_HH__
 
 //FJSTARTHEADER
 // $Id$
 //
 // Copyright (c) 2005-2021, Matteo Cacciari, Gavin P. Salam and Gregory Soyez
 //
 //----------------------------------------------------------------------
 // This file is part of FastJet.
 //
 //  FastJet is free software; you can redistribute it and/or modify
 //  it under the terms of the GNU General Public License as published by
 //  the Free Software Foundation; either version 2 of the License, or
 //  (at your option) any later version.
 //
 //  The algorithms that underlie FastJet have required considerable
 //  development. They are described in the original FastJet paper,
 //  hep-ph/0512210 and in the manual, arXiv:1111.6097. If you use
 //  FastJet as part of work towards a scientific publication, please
 //  quote the version you use and include a citation to the manual and
 //  optionally also to hep-ph/0512210.
 //
 //  FastJet is distributed in the hope that it will be useful,
 //  but WITHOUT ANY WARRANTY; without even the implied warranty of
 //  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 //  GNU General Public License for more details.
 //
 //  You should have received a copy of the GNU General Public License
 //  along with FastJet. If not, see <http://www.gnu.org/licenses/>.
 //----------------------------------------------------------------------
 //FJENDHEADER
 
 #include "fastjet/PseudoJet.hh"
 #include "fastjet/JetDefinition.hh"
 #include "fastjet/CompositeJetStructure.hh"
 #include "fastjet/tools/Transformer.hh"
 
 FASTJET_BEGIN_NAMESPACE
 
 class RestFrameNSubjettinessTagger;
 class RestFrameNSubjettinessTaggerStructure;
 
 //----------------------------------------------------------------------
 /// @ingroup tools_taggers
 /// \class RestFrameNSubjettinessTagger
 /// Class that helps perform 2-pronged boosted tagging using
 /// a reclustering in the jet's rest frame, supplemented with a cut on N-subjettiness
 /// (and a decay angle), as discussed by Ji-Hun Kim in arXiv:1011.1493.
 ///
 /// To tag a fat jet, the tagger proceeds as follows:
 ///
 ///  - boost its constituents into the rest frame of the jet
 ///
 ///  - recluster them using another jet definition (the original
 ///    choice was SISCone in spherical coordinates with R=0.6 and
 ///    f=0.75.
 ///
 ///  - keep the 2 most energetic subjets (\f$q_{1,2}\f$) and compute
 ///    the 2-subjettiness
 ///    \f[ 
 ///      \tau_2^j = \frac{2}{m_{\rm jet}^2}\,
 ///                 \sum_{k\in {\rm jet}} {\rm min}(q_1.p_k,q_2.p_k)
 ///    \f]
 ///    where the sum runs over the constituents of the jet. 
 ///
 ///  - require \f$\tau_2^j < \tau_2^{\rm cut}\f$ [0.08 by default]
 ///
 ///  - impose that (in the rest frame of the fat jet), the angles
 ///    between the 2 most energetic subjets and the boost axis are
 ///    both large enough: \f$\cos(\theta_s)<c_\theta^{\rm cut}\f$ 
 ///    [0.8 by default]
 ///
 /// Note that in the original version, the jets to be tagged were reconstructed
 /// using SISCone with R=0.8 and f=0.75. Also, b-tagging was imposed
 /// on the 2 subjets found in the rest-frame tagging procedure.
 ///
 /// \section desc Options
 /// 
 /// The constructor has the following arguments:
 ///  - The first argument is the jet definition to be used to
 ///    recluster the constituents of the jet to be filtered (in the
 ///    rest frame of the tagged jet).
 ///  - The second argument is the cut on tau_2 [0.08 by default]
 ///  - The 3rd argument is the cut on cos(theta_s) [0.8 by default]
 ///  - If the 4th argument is true, 2 exclusive rest-frame jets will
 ///    be considered in place of the 2 most energetic inclusive jets
 ///
 /// \section input Input conditions
 /// 
 ///  - the original jet must have constituents
 ///
 /// \section output Output/structure
 /// 
 ///  - the 2 subjets are kept as pieces if some substructure is found,
 ///    otherwise a single 0-momentum piece
 ///  - the tau2 and maximal cos(theta_s) values computed during the
 ///    tagging can be obtained via the resulting jet's structure_of<...>() 
 ///    function
 ///
 class RestFrameNSubjettinessTagger : public Transformer{
 public:
   /// ctor with arguments (see the class description above)
   RestFrameNSubjettinessTagger(const JetDefinition subjet_def, 
                       const double tau2cut=0.08, 
                       const double costhetascut=0.8,
                       const bool use_exclusive = false)
     : _subjet_def(subjet_def), _t2cut(tau2cut), _costscut(costhetascut),
       _use_exclusive(use_exclusive){};
 
   /// returns a textual description of the tagger 
   virtual std::string description() const;
 
   /// runs the tagger on the given jet and
   /// returns the tagged PseudoJet if successful, a PseudoJet==0 otherwise
   /// (standard access is through operator()).
   virtual PseudoJet result(const PseudoJet & jet) const;
 
   /// the type of Structure returned
   typedef RestFrameNSubjettinessTaggerStructure StructureType;
 
 protected:
   JetDefinition _subjet_def;
   double _t2cut, _costscut;
   bool _use_exclusive;
 };
 
 
 //------------------------------------------------------------------------
 /// @ingroup tools_taggers
 /// \class RestFrameNSubjettinessTaggerStructure
 /// the structure returned by the RestFrameNSubjettinessTagger transformer.
 ///
 /// See the RestFrameNSubjettinessTagger class description for the details of
 /// what is inside this structure
 ///
 class RestFrameNSubjettinessTaggerStructure : public CompositeJetStructure{
 public:
   /// ctor with pieces initialisation
   RestFrameNSubjettinessTaggerStructure(const std::vector<PseudoJet> & pieces_in) :
     CompositeJetStructure(pieces_in), _tau2(0.0), _costhetas(1.0){}
 
   /// returns the associated N-subjettiness
   inline double tau2() const{return _tau2;}
 
   /// returns the associated angle with the boosted axis
   inline double costhetas() const {return _costhetas;}
 
 //  /// returns the original jet (before tagging)
 //  const PseudoJet & original() const {return _original_jet;}
 
 protected:
   double _tau2;      ///< the value of the N-subjettiness
   double _costhetas; ///< the minimal angle between the dijets
                      ///< and the boost axis
 //  PseudoJet _original_jet;  ///< the original jet (before tagging)
 
   // allow the tagger to set these
   friend class RestFrameNSubjettinessTagger;
 };
 
 FASTJET_END_NAMESPACE
 #endif  //  __FASTJET_RESTFRAMENSUBJETTINESS_TAGGER_HH__
 

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