RNGsnapshot.h

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#ifndef canvas_Persistency_Common_RNGsnapshot_h
#define canvas_Persistency_Common_RNGsnapshot_h

// ======================================================================
//
// RNGsnapshot - a data product holding saved state from/for the
//               RandomNumberGenerator
//
// ======================================================================
//
// Notes:
// ------
//
// CLHEP specifies that the state of any of its engines will be returned
// as a vector<unsigned long>.  However, the size of such a long is
// machine-dependent.  If unsigned long is an 8 byte variable, only the
// least significant 4 bytes are filled and the most significant 4 bytes
// are zero.  We choose to store only the useful 32 bits, making the
// assumption (verified at compile-time) that unsigned int is always a
// 32-bit type.
//
// We would prefer to be more explicit about this by using a standard
// typedef such as saved_t32_t.  However, ROOT would have issues with this
// since ROOT sees only the underlying type, and not the typedef proper.
// Thus values written on one system and read on another would be
// problematic whenever the two systems did not agree on the underlying
// type.
//
// ======================================================================

#include <limits>
#include <string>
#include <type_traits>
#include <vector>

// ======================================================================

namespace art {

  class RNGsnapshot {
  public:
    // --- CLHEP engine state characteristics:
    using CLHEP_t = unsigned long;
    using engine_state_t = std::vector<CLHEP_t>;

    // --- Our state characteristics:
    using saved_t = unsigned int;
    using snapshot_state_t = std::vector<saved_t>;
    using label_t = std::string;

    static_assert(std::numeric_limits<saved_t>::digits == 32,
                  "std::numeric_limits<saved_t>::digits != 32");
    static_assert(sizeof(saved_t) <= sizeof(CLHEP_t),
                  "sizeof(saved_t) > sizeof(CLHEP_t)");

    RNGsnapshot() = default;
    explicit RNGsnapshot(std::string const& ekind,
                         label_t const& label,
                         engine_state_t const& est);

    // --- Access:
    std::string const&
    ekind() const
    {
      return engine_kind_;
    }
    label_t const&
    label() const
    {
      return label_;
    }
    snapshot_state_t const&
    state() const
    {
      return state_;
    }

    // --- Save/restore:
    void saveFrom(std::string const&, label_t const&, engine_state_t const&);
    engine_state_t restoreState() const;

  private:
    std::string engine_kind_{};
    label_t label_{};
    snapshot_state_t state_{};

  }; // RNGsnapshot

} // art

  // ======================================================================

#endif /* canvas_Persistency_Common_RNGsnapshot_h */

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