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 // This file is part of FastJet.
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 #ifndef __FASTJET_MASS_DROP_TAGGER_HH__
 #define __FASTJET_MASS_DROP_TAGGER_HH__
 
 #include "fastjet/tools/Transformer.hh"
 #include "fastjet/LimitedWarning.hh"
 #include "fastjet/WrappedStructure.hh"
 
 FASTJET_BEGIN_NAMESPACE
 
 class MassDropTagger;
 class MassDropTaggerStructure;
 
 //----------------------------------------------------------------------
 /// @ingroup tools_taggers
 /// \class MassDropTagger
 /// Class that helps perform 2-pronged boosted tagging using
 /// the "mass-drop" technique (with asymmetry cut) introduced by Jonathan
 /// Butterworth, Adam Davison, Mathieu Rubin and Gavin Salam in
 /// arXiv:0802.2470 in the context of a boosted Higgs search.
 ///
 /// The tagger proceeds as follows:
 ///
 ///  0. start from a jet obtained from with the Cambridge/Aachen
 ///     algorithm
 ///
 ///  1. undo the last step of the clustering step j -> j1 + j2 (label
 ///     them such as j1 is the most massive).
 ///  
 ///  2. if there is a mass drop, i.e. m_j1/m_j < mu_cut, and the
 ///     splitting is sufficiently symmetric, \f${\rm
 ///     min}(p_{tj1}^2,p_{tj2}^2)\Delta R_{j1,j2}^2 > y_{\rm cut}
 ///     m_j^2\f$, keep j as the result of the tagger (with j1 and j2
 ///     its 2 subjets)
 ///
 ///  3. otherwise, redefine j to be equal to j1 and return to step 1.
 ///
 /// Note that in the original proposal, j1 and j2 are both required
 /// to be b-tagged and a filter (with Rfilt=min(0.3,Rbb/2) and
 /// n_filt=3) is also applied to j to obtain the final "Higgs candidate".
 /// See the example \subpage Example12 for details.
 ///
 /// \section desc Options
 /// 
 /// The constructor has the following arguments:
 ///  - The first argument is the minimal mass drop that is required (mu_cut) [0.67
 ///    by default]
 ///  - The second argument is the asymmetry cut (y_cut) [0.09 by default]
 ///
 /// \section input Input conditions
 /// 
 ///  - one must be able to successively "uncluster" the original jet
 ///    using "has_parents"
 ///
 /// \section output Output/structure
 /// 
 ///  - the 2 subjets are kept as pieces if some substructure is found,
 ///    otherwise a single 0-momentum piece is returned
 ///  - the 'mu' and 'y' values corresponding to the unclustering step
 ///    that passed the tagger's cuts
 ///
 /// See also \subpage Example12  for a usage example.
 class MassDropTagger : public Transformer{
 public:
   /// default ctor
   MassDropTagger(const double mu=0.67, const double ycut=0.09) : _mu(mu), _ycut(ycut){};
 
   /// 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()).
   ///  \param jet   the PseudoJet to tag
   virtual PseudoJet result(const PseudoJet & jet) const;
 
   /// the type of the associated structure
   typedef MassDropTaggerStructure StructureType;
 
 protected:
   double _mu, _ycut;
   static LimitedWarning _warnings_nonca;
   static LimitedWarning _negative_mass_warning;
 };
 
 
 //------------------------------------------------------------------------
 /// @ingroup tools_taggers
 /// \class MassDropTaggerStructure
 /// the structure returned by the MassDropTagger transformer.
 ///
 /// See the MassDropTagger class description for the details of what
 /// is inside this structure
 ///
 class MassDropTaggerStructure : public WrappedStructure{
 public:
   /// ctor with initialisation
   ///  \param pieces  the pieces of the created jet
   ///  \param rec     the recombiner from the underlying cluster sequence
   MassDropTaggerStructure(const PseudoJet & result_jet) :
     WrappedStructure(result_jet.structure_shared_ptr()), _mu(0.0), _y(0.0){}
 
   /// returns the mass-drop ratio, pieces[0].m()/jet.m(), for the splitting
   /// that triggered the mass-drop condition
   inline double mu() const{return _mu;}
 
   /// returns the value of y = (squared kt distance) / (squared mass) for the
   /// splitting that triggered the mass-drop condition
   inline double y() const {return _y;}
 
 //  /// returns the original jet (before tagging)
 //  const PseudoJet & original() const {return _original_jet;}
 
 protected:
   double _mu;              ///< the value of the mass-drop parameter
   double _y;               ///< the value of the asymmetry parameter
 //  PseudoJet _original_jet; ///< the original jet (before tagging)
 
   // allow the tagger to set these
   friend class MassDropTagger;
 };
 
 
 
 FASTJET_END_NAMESPACE
 
 #endif  //  __FASTJET_MASS_DROP_TAGGER_HH__
 

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