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

namespace edm {
namespace soa {

  template <typename... Args>
  class ColumnFillers {
    using Layout = std::tuple<Args...>;
    Layout m_fillers;
    
    template<int I, typename ELEMENT, typename RET>
    auto callFiller(ELEMENT&& iEl, RET*, std::true_type) -> decltype(std::get<I>(m_fillers).m_f(iEl)) {
      return std::get<I>(m_fillers).m_f(iEl);
    }
    
    template<int I, typename ELEMENT, typename RET>
    RET callFiller(ELEMENT&& iEl, RET*, std::false_type) {
      return RET{};
    }
    
    template<int I, typename ELEMENT, typename COLUMN>
    typename COLUMN::type tryValue(ELEMENT&& iEl, COLUMN*, std::true_type, std::true_type) {
      using Pair = typename std::tuple_element<I,Layout>::type;
      using COL = typename Pair::Column_type;
      if(std::is_same<COL,COLUMN>::value) {
        return callFiller<I>(iEl,
                             static_cast<typename COLUMN::type*>(nullptr),
                             std::conditional_t<std::is_same<COL,COLUMN>::value,
                             std::true_type, std::false_type>{});
      }
      return tryValue<I+1>(iEl, static_cast<COLUMN*>(nullptr),
                           std::conditional_t<I+1 == sizeof...(Args),
                           std::false_type, std::true_type>{},
                           std::conditional_t<std::is_same<COL,COLUMN>::value,
                           std::false_type, std::true_type>{});
    }
    
    template<int I, typename ELEMENT, typename COLUMN>
    typename COLUMN::type tryValue(ELEMENT&& iEl, COLUMN*, std::false_type, std::true_type) {
      return value_for_column(iEl,static_cast<COLUMN*>(nullptr));
    }
    template<int I, typename ELEMENT, typename COLUMN>
    typename COLUMN::type tryValue(ELEMENT&& iEl, COLUMN*, std::true_type, std::false_type) {
      return typename COLUMN::type{};
    }
    template<int I, typename ELEMENT, typename COLUMN>
    typename COLUMN::type tryValue(ELEMENT&& iEl, COLUMN*, std::false_type, std::false_type) {
      return typename COLUMN::type{};
    }
    
  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>(iEl, static_cast<COLUMN*>(nullptr), std::true_type{},std::true_type{});
    }
  };

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

}
}


#endif