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 // Copyright (c) 2005-2021, Matteo Cacciari, Gavin P. Salam and Gregory Soyez
 //
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 // 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.
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 //  The algorithms that underlie FastJet have required considerable
 //  development. They are described in the original FastJet paper,
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 #ifndef __FASTJET_TOOLS_SUBTRACTOR_HH__
 #define __FASTJET_TOOLS_SUBTRACTOR_HH__
 
 #include "fastjet/internal/base.hh"     // namespace macros (include explicitly to help Doxygen)
 #include "fastjet/tools/Transformer.hh" // to derive Subtractor from Transformer
 #include "fastjet/tools/BackgroundEstimatorBase.hh" // used as a ctor argument
 
 FASTJET_BEGIN_NAMESPACE     // defined in fastjet/internal/base.hh
 
 
 //----------------------------------------------------------------------
 /// @ingroup tools_background
 /// \class Subtractor
 /// Class that helps perform jet background subtraction.
 ///
 /// This class derives from Transformer and makes use of a pointer to
 /// a BackgroundEstimatorBase object in order to determine the background
 /// in the vicinity of a given jet and then subtract area*background from
 /// the jet. It can also be initialised with a specific fixed value for the 
 /// background pt density.
 ///
 /// \section input Input conditions
 /// 
 /// The original jet must have area support (4-vector)
 ///
 /// \section output Output/interface
 /// 
 /// The underlying structure of the returned, subtracted jet
 /// (i.e. constituents, pieces, etc.) is identical to that of the
 /// original jet.
 ///
 class Subtractor : public Transformer{
 public:
   /// define a subtractor based on a BackgroundEstimator
   Subtractor(const BackgroundEstimatorBase * bge) : 
     _bge(bge), _rho(-1.0) { set_defaults(); }
 
   /// define a subtractor that uses a fixed value of rho, the background
   /// pt density per unit area (which must be positive)
   Subtractor(double rho);
 
   /// define a subtractor that uses a fixed value of rho and rho_m;
   /// both must be >= 0;
   Subtractor(double rho, double rho_m);
 
   /// default constructor
   Subtractor() : _bge(0), _rho(_invalid_rho) { set_defaults(); }
 
   /// default dtor
   virtual ~Subtractor(){};
 
   /// @name configuring the behaviour
   //\{
   //----------------------------------------------------------------
 
   /// reset all parameters to default values
   ///
   /// Note: by default, the rho_m term is not included and the safety
   /// test for the mass is not done. This is mostly for backwards
   /// compatibility with FastJet 3.0 and is highly likely to change in
   /// a future release of FastJet
   void set_defaults();
 
   void set_background_estimator(const BackgroundEstimatorBase * bge) {
     _bge = bge;
     _rho = _invalid_rho;
   }
 
   /// when 'use_rho_m' is true, include in the subtraction the
   /// correction from rho_m, the purely longitudinal,
   /// particle-mass-induced component of the background density per
   /// unit area
   ///
   /// Note: this will be switched off by default (for backwards
   /// compatibility with FastJet 3.0) but is highly likely to change
   /// in a future release of FastJet
   void set_use_rho_m(bool use_rho_m_in = true){
     if (_bge == 0  && _rho_m < 0) {
       throw Error("Subtractor: rho_m support works only for Subtractors constructed with a background estimator or an explicit rho_m value");
     }
     _use_rho_m=use_rho_m_in;
   }
   
   /// returns whether or not the rho_m component is used
   bool use_rho_m() const{ return _use_rho_m;}
 
   /// when 'safe_mass' is true, ensure that the mass of the subtracted
   /// 4-vector remain positive
   ///
   /// when true, if the subtracted mass is negative, we return a
   /// 4-vector with 0 mass, pt and phi from the subtracted 4-vector
   /// and the rapidity of the original, unsubtracted jet.
   ///
   /// Note: this will be switched off by default (for backwards
   /// compatibility with FastJet 3.0) but is highly likely to change
   /// in a future release of FastJet
   void set_safe_mass(bool safe_mass_in=true){ _safe_mass=safe_mass_in;}
 
   /// returns whether or not safety tests on the mass are included
   bool safe_mass() const{ return _safe_mass;}
 
   /// This is mostly intended for cherge-hadron-subtracted type of
   /// events where we wich to use vertex information to improve the
   /// subtraction.
   ///
   /// Given the following parameters:
   ///   \param sel_known_vertex    selects the particles with a
   ///                              known vertex origin
   ///   \param sel_leading_vertex  amongst the particles with a
   ///                              known vertex origin, select those
   ///                              coming from the leading vertex
   /// Momentum identified as coming from the leading vertex will be
   /// kept, momentum identified as coming from a non-leading vertex
   /// will be eliminated and a regular area-median subtraction will be
   /// applied on the 4-vector sum of the particles with unknown vertex
   /// origin.
   ///
   /// When this is set, we shall ensure that the pt of the subtracted
   /// 4-vector is at least the pt of the particles that are known to
   /// come from the leading vertex (if it fails, subtraction returns
   /// the component that is known to come from the leading vertex ---
   /// or, the original unsubtracted jet if it contains no particles
   /// from the leading vertex).  Furthermore, when safe_mass() is on, we
   /// also impose a similar constraint on the mass of the subtracted
   /// 4-vector (if the test fails, the longitudinal part of the
   /// subtracted 4-vector is taken from the component that is known to
   /// come from the leading vertex).
   void set_known_selectors(const Selector &sel_known_vertex,
                const Selector &sel_leading_vertex){
     _sel_known_vertex   = sel_known_vertex;
     _sel_leading_vertex = sel_leading_vertex;
   }
 
   //\}
 
   /// @name description and action
   //\{
   //----------------------------------------------------------------
 
   /// returns a jet that's subtracted
   ///
   /// \param jet    the jet that is to be subtracted
   /// \return       the subtracted jet
   virtual PseudoJet result(const PseudoJet & jet) const;
 
   /// class description
   virtual std::string description() const;
 
   //\}
 protected:
   /// compute the 4-vector that should be subtracted from the given
   /// jet
   PseudoJet _amount_to_subtract(const PseudoJet &jet) const;
 
   /// the tool used to estimate the background
   /// if has to be mutable in case its underlying selector takes a reference jet
   const BackgroundEstimatorBase * _bge;
   /// the fixed value of rho and/or rho_m to use if the user has selected that option
   double _rho, _rho_m;
 
   // configuration parameters/flags
   bool _use_rho_m;   ///< include the rho_m correction
   bool _safe_mass;   ///< ensures that the subtracted mass is +ve
 
   Selector _sel_known_vertex;   ///< selects the particles with a
                 ///< known vertex origin
   Selector _sel_leading_vertex; ///< amongst the particles with a
                 ///< known vertex origin, select those
                 ///< coming from the leading vertex
 
   /// a value of rho that is used as a default to label that the stored
   /// rho is not valid for subtraction. 
   //
   // NB: there are two reasons for not having the value written here:
   // 1) that it caused problems on karnak with g++ 4.0.1 and 2) that
   // we anyway like -infinity as a default, and since that's a function,
   // that's not allowed in an include file.
   static const double _invalid_rho;
 
   static LimitedWarning _unused_rho_m_warning;
 };
 
 FASTJET_END_NAMESPACE
 
 #endif  // __FASTJET_TOOLS_SUBTRACTOR_HH__
 

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