Merger.h

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#ifndef UtilAlgos_Merger_h
#define UtilAlgos_Merger_h

#include "FWCore/Framework/interface/global/EDProducer.h"
#include "FWCore/Framework/interface/Event.h"
#include "FWCore/ParameterSet/interface/ConfigurationDescriptions.h"
#include "FWCore/ParameterSet/interface/ParameterSetDescription.h"
#include "FWCore/ParameterSet/interface/ParameterSet.h"
#include "FWCore/Utilities/interface/transform.h"
#include "FWCore/Utilities/interface/InputTag.h"
#include "DataFormats/Common/interface/CloneTrait.h"
#include <vector>

template <typename InputCollection,
          typename OutputCollection = InputCollection,
          typename P = typename edm::clonehelper::CloneTrait<InputCollection>::type>
class Merger : public edm::global::EDProducer<> {
public:
  explicit Merger(const edm::ParameterSet&);
  ~Merger() override;
  static void fillDescriptions(edm::ConfigurationDescriptions& descriptions);

private:
  void produce(edm::StreamID, edm::Event&, const edm::EventSetup&) const override;
  typedef std::vector<edm::EDGetTokenT<InputCollection>> vtoken;
  vtoken srcToken_;
};

template <typename InputCollection, typename OutputCollection, typename P>
Merger<InputCollection, OutputCollection, P>::Merger(const edm::ParameterSet& par)
    : srcToken_(edm::vector_transform(par.template getParameter<std::vector<edm::InputTag>>("src"),
                                      [this](edm::InputTag const& tag) { return consumes<InputCollection>(tag); })) {
  produces<OutputCollection>();
}

template <typename InputCollection, typename OutputCollection, typename P>
Merger<InputCollection, OutputCollection, P>::~Merger() {}

template <typename InputCollection, typename OutputCollection, typename P>
void Merger<InputCollection, OutputCollection, P>::produce(edm::StreamID,
                                                           edm::Event& evt,
                                                           const edm::EventSetup&) const {
  std::unique_ptr<OutputCollection> coll(new OutputCollection);
  for (typename vtoken::const_iterator s = srcToken_.begin(); s != srcToken_.end(); ++s) {
    edm::Handle<InputCollection> h;
    evt.getByToken(*s, h);
    for (typename InputCollection::const_iterator c = h->begin(); c != h->end(); ++c) {
      coll->push_back(P::clone(*c));
    }
  }
  evt.put(std::move(coll));
}

template <typename InputCollection, typename OutputCollection, typename P>
void Merger<InputCollection, OutputCollection, P>::fillDescriptions(edm::ConfigurationDescriptions& descriptions) {
  edm::ParameterSetDescription desc;
  desc.add<std::vector<edm::InputTag>>("src",
                                       {
                                           edm::InputTag("collection1"),
                                           edm::InputTag("collection2"),
                                       });
  descriptions.addWithDefaultLabel(desc);
}

#endif