FIND_DECAY and FIND_MCDECAY functor

Create two functor named FIND_DECAY and FIND_MC_DECAY that implements respectively the algorithms DecayFinder (new, see !3091 (merged)) and MCDecayFinder.

To be used in composition with other functors to get information about particles belonging to the decay chain.

FIND_DECAY (FIND_MC_DECAY) is MAP of _FIND_DECAY (_FIND_MC_DECAY), functor accepting const LHCb::Particle* (const LHCb::MCParticle*) or const LHCb::Particle& (const LHCb::MCParticle&) and returning a const LHCb::Particle* (const LHCb::MCParticle*).

TODO:

• Fix test;
• Adapt the functor to support MCParticles.

Perhaps related to issue: #356

Example in the DV MR DaVinci!781 (merged)

cc: @amathad

Phys/FunctorCore/include/Functors/Decay.h

-/*****************************************************************************\
+/***************************************************************************** \
 * (c) Copyright 2019-20 CERN for the benefit of the LHCb Collaboration        *
 *                                                                             *
 * This software is distributed under the terms of the GNU General Public      *
@@ -9,20 +9,25 @@
 * or submit itself to any jurisdiction.                                       *
 \*****************************************************************************/
 #pragma once
-
 #include "Functors/Function.h"
 #include "Functors/Utilities.h"
 #include "GaudiAlg/GaudiTupleAlg.h"
 #include "GaudiKernel/GaudiException.h"
 #include "GaudiKernel/System.h"
 #include "Kernel/IDecayFinder.h"
+#include "MCInterfaces/IMCDecayFinder.h"
 
 /** @file  Decay.h
  *  @brief Definitions of functors related to the decay chain.
  */
 namespace Functors::Decay {
+  template <bool MC = false>
+  struct FindDecay;
+  
+
   /** @brief Use the DecayFinder algorithm to get the information related to the decay chain. */
-  struct FindDecay : public Function {
+  template<>
+  struct FindDecay<false> : public Function {
     FindDecay( std::string decay_desc, std::string tool_name = "DecayFinder" )
         : m_decay_desc{std::move( decay_desc )}, m_tool_name{std::move( tool_name )} {}
 
@@ -43,4 +48,34 @@ namespace Functors::Decay {
     std::string                             m_tool_name;
     std::optional<ToolHandle<IDecayFinder>> m_find_decay;
   };
+
+  /** @brief Use the DecayFinder algorithm to get the information related to the decay chain. */
+  template<>
+  struct FindDecay<true> : public Function {
+    FindDecay( std::string decay_desc, std::string tool_name = "MCDecayFinder" )
+        : m_decay_desc{std::move( decay_desc )}, m_tool_name{std::move( tool_name )} {}
+
+    void bind( TopLevelInfo& top_level ) { m_find_decay.emplace( m_tool_name, top_level.algorithm() ); }
+
+    auto prepare( EventContext const&, TopLevelInfo const& ) const {
+      assert( m_find_decay.has_value() );
+      if ( !m_find_decay->get() ) m_find_decay->retrieve().ignore();
+      return [dd = m_decay_desc, tool = m_find_decay->get()]( auto mask, const auto& in ) {
+        m_find_decay -> setDecay(dd).ignore();
+        mask = mask && in.size() && m_find_decay -> hasDecay( in );
+        const LHCb::MCParticle* out = nullptr;
+        std::vector<const LHCb::MCParticle*> out_v;
+        while ( m_find_Decay -> findDecay( in, out ) ) 
+          out_v.push_back( out );
+        if ( out_v.size() > 1 )
+          warning() << "More than one signal decay found." << endmsg;
+        return mask ? Functors::Optional{out} : std::nullopt;
+      };
+    }
+
+  private:
+    std::string                               m_decay_desc;
+    std::string                               m_tool_name;
+    std::optional<ToolHandle<IMCDecayFinder>> m_find_decay;
+  };
 } // namespace Functors::Decay