EngineCreator.h

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#ifndef art_Framework_Core_detail_EngineCreator_h
#define art_Framework_Core_detail_EngineCreator_h

// ======================================================================
// EngineCreator - enable a derived class to access the
//                 RandomNumberGenerator::createEngine()
//
// MT-TODO:
//
//   Once we adopt multiple module types (legacy, one, per-schedule,
//   and global), then the steps that are necessary for creating
//   engines will depend on the given module type.  If a user
//   specifies:
//
//     createEngine(1237);
//
//   then N instances of the specified engine should be created, one
//   for each of N schedules, regardless of the module type.
//
//   A decision must be made if each instance should receive the same
//   seed, or different ones.  I suspect they should be different so
//   as to avoid undesirable over-sampling of fluctuations.
// ======================================================================

#include "art/Framework/Services/Optional/RandomNumberGenerator.h"
#include "art/Framework/Services/Registry/ServiceHandle.h"
#include "art/Utilities/ScheduleID.h"

namespace art::detail {
  class EngineCreator {
    using RNGservice = RandomNumberGenerator;

  public:
    // --- CLHEP engine characteristics:
    using base_engine_t = CLHEP::HepRandomEngine;
    using label_t = RNGsnapshot::label_t;
    using seed_t = long;
    using engine_state_t = RNGsnapshot::engine_state_t;

    explicit EngineCreator(std::string const& label, ScheduleID sid);

    base_engine_t& createEngine(seed_t seed);
    base_engine_t& createEngine(seed_t seed,
                                std::string const& kind_of_engine_to_make);
    base_engine_t& createEngine(seed_t seed,
                                std::string const& kind_of_engine_to_make,
                                label_t const& engine_label);

  private:
    static ServiceHandle<RandomNumberGenerator>& rng();
    void requireValid(); // Can remove once default c'tor is gone.
    std::string const moduleLabel_{};
    ScheduleID const sid_{};
  }; // EngineCreator
}

#endif /* art_Framework_Core_detail_EngineCreator_h */

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// mode: c++
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