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 #pragma once
 #include <mutex>
 
 #include <JANA/JEventProcessor.h>
 
 /// This class can be used to safely implement sequential code while ensuring
 /// the factory algorithms are run in parallel.
 ///
 /// n.b. If you need to run sequentially to serialize operation with ROOT
 /// then please use the JEventProcessorSequentialROOT class instead.
 ///
 /// This is a specialized version of JEventProcessor that allows data types
 /// that will be needed to be specified in the subclass definition. The
 /// types will be prefetched via event->Get() calls and then a mutex locked
 /// prior to calling the ProcessSequential method. The types are specified
 /// by adding PrefetchT data members to the class as illustrated in the
 /// example below.
 ///
 /// class DaveTestProcessor: public JEventProcessorSequential {
 ///
 ///  public:
 ///
 ///     // These declare object types that should be automatically fetched
 ///     // from the event before ProcessSequential is called.
 ///     PrefetchT<Hit>     hits     = {this};
 ///     PrefetchT<Cluster> clusters = {this, "MyTag"};
 ///
 ///     // This will be run sequentially
 ///     void ProcessSequential(const std::shared_ptr<const JEvent>& event) override {
 ///
 ///         // Do NOT call event->Get() here!
 ///
 ///         // The hits and clusters objects will already be filled by calls
 ///         // to event->Get(). Just use them here via their operator().
 ///         for( auto h : hits() ){
 ///             // h is const Hit*
 ///         }
 ///     }
 ///
 ///     // Boilerplate stuff
 ///     DaveTestProcessor() { SetTypeName(NAME_OF_THIS); }
 ///     void Init() override {}
 ///     void Finish() override {}
 ///  };
 ///
 class JEventProcessorSequential : public JEventProcessor {
     // This works in the following way:
     //
     // Two utility classes are defined here: Prefetch and PrefetchT with the
     // latter being a template. Subclasses can add any number of data members
     // of type PrefetchT. Using initializers, a list of the Prefetch members
     // is stored in the JEventProcessorSequential object itself.
     //
     // The Process method here loops over the Prefetch members calling
     // event->Get() for each and storing the resulting vector<const *T>
     // in the Prefetch member object. It then locks its own mutex before
     // calling ProcessSequential().
     //
     // The Process method is defined here and marked as "final" so subclasses
     // may not overide it. Instead, a new virtual method, ProcessSequential,
     // is added that the user may override.
     //
     // The tricky business here is capturing a list of the Prefetch data
     // members so they can be looped over at run time. Thus, the strange syntax
     // of passing the "this" argument to the PrefetchT constructors.
     // Similarly, each Prefetch object passes its "this" pointer back to the
     // JEventProcessorSequential object that owns it.
 
 public:
 
     JEventProcessorSequential(){}
     virtual ~JEventProcessorSequential() = default;
 
     // non-templated base class
     class Prefetch{
     public:
         virtual void Get(const std::shared_ptr<const JEvent>& event) = 0;
         virtual void Fill(const std::shared_ptr<const JEvent>& event) = 0;
     };
 
     // typed class
     template<typename T>
     class PrefetchT:public Prefetch{
     public:
         // This constructor gets called by the {this} initializer for the data member.
         PrefetchT(JEventProcessorSequential *jeps, const std::string &tag=""):mTag(tag){jeps->mPrefetch.push_back(this);}
 
         std::vector<const T*>& operator()(){ return mObjs; }
         void Get(const std::shared_ptr<const JEvent>& event){ event->Get<T>(mTag); }
         void Fill(const std::shared_ptr<const JEvent>& event){ mObjs = event->Get<T>(mTag); }
 
     private:
         PrefetchT()=default; // user must pass "this" so member can be added to our mPrefetch
 
         std::string mTag;
         std::vector<const T*> mObjs;
     };
 
     // JEventProcessorSequential methods
     void Init() override {};
     void Finish() override {};
 
     void Process(const std::shared_ptr<const JEvent>& event) override final{
         for( auto p : mPrefetch ) p->Get(event);  // make sure all factories have been activated
         std::lock_guard<std::mutex> lck(mMutex);
         for( auto p : mPrefetch ) p->Fill(event); // Copy object pointers into members
         ProcessSequential( event );
     }
     virtual void ProcessSequential(const std::shared_ptr<const JEvent>& /*event*/){};
 
 private:
 
     std::vector<Prefetch*> mPrefetch;
     std::mutex mMutex;
 };
 
 
 

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