ColumnFillers.h

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#ifndef FWCore_SOA_ColumnFillers_h
#define FWCore_SOA_ColumnFillers_h
// -*- C++ -*-
//
// Package:     FWCore/SOA
// Class  :     ColumnFillers
//
//
// Original Author:  Chris Jones
//         Created:  Thu, 24 Aug 2017 22:22:42 GMT
//

// system include files

// user include files

// forward declarations
#include <tuple>

namespace edm {
  namespace soa {

    template <typename... Args>
    class ColumnFillers {
      using Layout = std::tuple<Args...>;
      Layout m_fillers;

      template <int I, typename ELEMENT>
      decltype(auto) callFiller(ELEMENT&& iEl) {
        return std::get<I>(m_fillers).m_f(iEl);
      }

      template <int I, typename COLUMN, typename ELEMENT>
      typename COLUMN::type tryValue(ELEMENT&& iEl) {
        if constexpr (I < sizeof...(Args)) {
          using Pair = typename std::tuple_element<I, Layout>::type;
          using COL = typename Pair::Column_type;
          if constexpr (std::is_same<COL, COLUMN>::value) {
            return callFiller<I>(iEl);
          } else {
            //try another filler to see if it matches
            return tryValue<I + 1, COLUMN>(iEl);
          }
        } else {
          //no matches so call overload function
          return value_for_column(iEl, static_cast<COLUMN*>(nullptr));
        }
      }

    public:
      ColumnFillers(Args... iArgs) : m_fillers(std::forward<Args>(iArgs)...) {}

      template <typename ELEMENT, typename COLUMN>
      typename COLUMN::type value(ELEMENT&& iEl, COLUMN*) {
        return tryValue<0, COLUMN>(iEl);
      }
    };

    template <typename... Args>
    ColumnFillers<Args...> column_fillers(Args... iArgs) {
      return ColumnFillers<Args...>(std::forward<Args>(iArgs)...);
    }

  }  // namespace soa
}  // namespace edm

#endif