EgammaRandomSeeds.h

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#ifndef RecoEgamma_EgammaTools_EgammaRandomSeeds_h
#define RecoEgamma_EgammaTools_EgammaRandomSeeds_h

//author: Sam Harper (RAL)
//description:
//  provides a seed for a random number generator based on object / event properties
//  the supercluster method is prefered as it will be the same for electrons & photons
//  the SC is based on seed ID and #crystals in SC, the #crystals in SC is for added
//  entropy although this means any re-recos will change the returned number as #crystals
//  will likely change with any re-reco while seed ID is fairly stable

#include "FWCore/Framework/interface/Event.h"
#include "DataFormats/EgammaReco/interface/SuperClusterFwd.h"

#include <random>

namespace egamma {

  uint32_t getRandomSeedFromSC(const edm::Event& iEvent, const reco::SuperClusterRef scRef);
  template <typename T>
  uint32_t getRandomSeedFromObj(const edm::Event& iEvent, const T& obj, size_t nrObjs, size_t objNr) {
    std::seed_seq seeder = {int(iEvent.id().event()),
                            int(iEvent.id().luminosityBlock()),
                            int(iEvent.id().run()),
                            int(nrObjs),
                            int(std::numeric_limits<int>::max() * obj.phi() / M_PI) & 0xFFF,
                            int(objNr)};
    uint32_t seed = 0, tries = 10;
    do {
      seeder.generate(&seed, &seed + 1);
      tries++;
    } while (seed == 0 && tries < 10);
    return seed ? seed : iEvent.id().event() + 10000 * objNr;
  }
}  // namespace egamma

#endif