details.h

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/***********************************************************************************\
* (c) Copyright 1998-2025 CERN for the benefit of the LHCb and ATLAS collaborations *
*                                                                                   *
* This software is distributed under the terms of the Apache version 2 licence,     *
* copied verbatim in the file "LICENSE".                                            *
*                                                                                   *
* In applying this licence, CERN does not waive the privileges and immunities       *
* granted to it by virtue of its status as an Intergovernmental Organization        *
* or submit itself to any jurisdiction.                                             *
\***********************************************************************************/
#pragma once
 
#include <Gaudi/Algorithm.h>
#include <GaudiKernel/Algorithm.h>
#include <GaudiKernel/DataObjectHandle.h>
#include <GaudiKernel/GaudiException.h>
#include <GaudiKernel/IBinder.h>
#include <GaudiKernel/ThreadLocalContext.h>
#include <cassert>
#include <sstream>
#include <type_traits>
 
#if defined( __cpp_lib_ranges_zip ) && defined( __cpp_lib_ranges_as_const )
#  define GAUDI_FUNCTIONAL_USES_STD_RANGES 1
#  include <ranges>
#else
#  include <range/v3/version.hpp>
#  include <range/v3/view/const.hpp>
#  include <range/v3/view/zip.hpp>
#endif
 
// temporary hack to help in transition to updated constructor
// allows to write code which is forward and backwards compatible
#define GAUDI_FUNCTIONAL_CONSTRUCTOR_USES_TUPLE
 
namespace Gaudi::Functional::details {
 
  // CRJ : Stuff for zipping
  namespace zip {
 
    template <typename OS, typename Arg>
    void printSizes( OS& out, Arg&& arg ) {
      out << "SizeOf'" << System::typeinfoName( typeid( Arg ) ) << "'=" << std::forward<Arg>( arg ).size();
    }
 
    template <typename OS, typename Arg, typename... Args>
    void printSizes( OS& out, Arg&& arg, Args&&... args ) {
      printSizes( out, arg );
      out << ", ";
      printSizes( out, args... );
    }
 
    template <typename A>
    inline bool check_sizes( const A& ) noexcept {
      return true;
    }
 
    template <typename A, typename B>
    inline bool check_sizes( const A& a, const B& b ) noexcept {
      return a.size() == b.size();
    }
 
    template <typename A, typename B, typename... C>
    inline bool check_sizes( const A& a, const B& b, const C&... c ) noexcept {
      return ( check_sizes( a, b ) && check_sizes( b, c... ) );
    }
 
    template <typename... Args>
    inline decltype( auto ) verifySizes( Args&... args ) {
      if ( !check_sizes( args... ) ) {
        std::ostringstream mess;
        mess << "Zipped containers have different sizes : ";
        printSizes( mess, args... );
        throw GaudiException( mess.str(), "Gaudi::Functional::details::zip::verifySizes", StatusCode::FAILURE );
      }
    }
 
    template <typename... Args>
    inline decltype( auto ) range( Args&&... args ) {
#ifndef NDEBUG
      verifySizes( args... );
#endif
#if defined( GAUDI_FUNCTIONAL_USES_STD_RANGES )
      return std::ranges::zip_view( std::forward<Args>( args )... );
#else
      return ranges::views::zip( std::forward<Args>( args )... );
#endif
    }
 
    template <typename... Args>
    inline decltype( auto ) const_range( Args&&... args ) {
#ifndef NDEBUG
      verifySizes( args... );
#endif
#if defined( GAUDI_FUNCTIONAL_USES_STD_RANGES )
      return std::ranges::as_const_view( std::ranges::zip_view( std::forward<Args>( args )... ) );
#else
      return ranges::views::const_( ranges::views::zip( std::forward<Args>( args )... ) );
#endif
    }
  } // namespace zip
 
  inline std::vector<DataObjID> to_DataObjID( const std::vector<std::string>& in ) {
    std::vector<DataObjID> out;
    out.reserve( in.size() );
    std::transform( in.begin(), in.end(), std::back_inserter( out ),
                    []( const std::string& i ) { return DataObjID{ i }; } );
    return out;
  }
 
  template <typename T>
  concept is_optional = requires( T const& t ) {
    t.has_value();
    t.value();
    typename T::value_type;
  };
 
  namespace details2 {
 
    template <typename T>
    struct value_type_of {
      using type = T;
    };
 
    template <is_optional T>
    struct value_type_of<T> {
      using type = T::value_type;
    };
 
  } // namespace details2
 
  template <typename T>
  using remove_optional_t = typename details2::value_type_of<T>::type;
 
  constexpr struct invoke_optionally_t {
    template <typename F, typename Arg>
      requires( !is_optional<Arg> )
    decltype( auto ) operator()( F&& f, Arg&& arg ) const {
      return std::invoke( std::forward<F>( f ), std::forward<Arg>( arg ) );
    }
    template <typename F, is_optional Arg>
    void operator()( F&& f, Arg&& arg ) const {
      if ( arg ) std::invoke( std::forward<F>( f ), *std::forward<Arg>( arg ) );
    }
  } invoke_optionally{};
 
  template <typename Value, std::size_t... I>
  auto get_values_helper( std::index_sequence<I...> ) {
    return std::make_tuple( ( (void)I, Value{} )... );
  }
 
  template <typename Value, auto N>
  using RepeatValues_ = decltype( get_values_helper<Value>( std::make_index_sequence<N>() ) );
 
  template <std::derived_from<DataObject> Out1, std::convertible_to<Out1> Out2>
  auto put( const DataObjectHandle<Out1>& out_handle, Out2&& out ) {
    return out_handle.put( std::make_unique<Out1>( std::forward<Out2>( out ) ) );
  }
 
  template <typename Out1, std::convertible_to<Out1> Out2>
  auto put( const DataObjectHandle<AnyDataWrapper<Out1>>& out_handle, Out2&& out ) {
    return out_handle.put( std::forward<Out2>( out ) );
  }
 
  // optional put
  template <typename OutHandle, typename OptOut>
    requires( is_optional<OptOut> )
  void put( const OutHandle& out_handle, OptOut&& out ) {
    if ( out ) put( out_handle, *std::forward<OptOut>( out ) );
  }
 
  // adapt to differences between eg. std::vector (which has push_back) and KeyedContainer (which has insert)
  // adapt to getting a T, and a container wanting T* by doing new T{ std::move(out) }
  // adapt to getting a optional<T>
 
  constexpr struct insert_t {
    // for Container<T*>, return T
    template <typename Container>
    using c_remove_ptr_t = std::remove_pointer_t<typename Container::value_type>;
 
    template <typename Container, typename Value>
    auto operator()( Container& c, Value&& v ) const -> decltype( c.push_back( v ) ) {
      return c.push_back( std::forward<Value>( v ) );
    }
 
    template <typename Container, typename Value>
    auto operator()( Container& c, Value&& v ) const -> decltype( c.insert( v ) ) {
      return c.insert( std::forward<Value>( v ) );
    }
 
    // Container<T*> with T&& as argument
    template <typename Container, typename Value>
      requires( std::is_pointer_v<typename Container::value_type> &&
                std::is_convertible_v<Value, c_remove_ptr_t<Container>> )
    auto operator()( Container& c, Value&& v ) const {
      return operator()( c, new c_remove_ptr_t<Container>{ std::forward<Value>( v ) } );
    }
 
  } insert{};
 
 
  constexpr struct deref_t {
    template <typename In>
      requires( !std::is_pointer_v<In> )
    const In& operator()( const In& in ) const {
      return in;
    }
 
    template <typename In>
      requires( !std::is_pointer_v<std::decay_t<In>> )
    In operator()( In&& in ) const {
      return std::forward<In>( in );
    }
 
    template <typename In>
    const In& operator()( const In* in ) const {
      assert( in != nullptr );
      return *in;
    }
  } deref{};
 
  // if Container is a pointer, then we're optional items
  namespace details2 {
    template <typename T>
    constexpr static bool is_gaudi_range_v = false;
 
    template <typename T>
    constexpr static bool is_gaudi_range_v<Gaudi::Range_<T>> = true;
 
    template <typename T>
    constexpr static bool is_gaudi_range_v<Gaudi::NamedRange_<T>> = true;
 
    template <typename T>
    constexpr static bool is_gaudi_range_v<std::optional<Gaudi::NamedRange_<T>>> = true;
 
    template <typename In>
    struct get_from_handle {
      template <template <typename> class Handle, std::convertible_to<In> I>
      auto operator()( const Handle<I>& h ) -> const In& {
        return *h.get();
      }
      template <template <typename> class Handle, typename I>
      auto operator()( const Handle<Gaudi::Range_<I>>& h ) -> const In {
        return h.get();
      }
      template <template <typename> class Handle, typename I>
      auto operator()( const Handle<Gaudi::NamedRange_<I>>& h ) -> const In {
        return h.get();
      }
      template <template <typename> class Handle, typename I>
      auto operator()( const Handle<std::optional<Gaudi::NamedRange_<I>>>& h ) -> const In {
        return h.get();
      }
      template <template <typename> class Handle, typename I>
        requires( std::is_convertible_v<I*, In> )
      auto operator()( const Handle<I>& h ) -> const In {
        return h.getIfExists();
      } // In is-a pointer
    };
 
    template <typename Iterator>
    class indirect_iterator {
      using traits = std::iterator_traits<Iterator>;
      Iterator m_iter;
 
    public:
      using iterator_category [[maybe_unused]] = typename traits::iterator_category;
      using difference_type [[maybe_unused]]   = typename traits::difference_type;
      using reference                          = decltype( **m_iter );
      using value_type [[maybe_unused]]        = std::remove_reference_t<reference>;
      using pointer [[maybe_unused]]           = std::add_pointer_t<value_type>;
 
      indirect_iterator() = default;
      constexpr explicit indirect_iterator( Iterator i ) : m_iter( std::move( i ) ) {}
 
      constexpr reference operator*() const { return **m_iter; }
 
      constexpr bool operator==( const indirect_iterator& rhs ) const { return m_iter == rhs.m_iter; }
 
      constexpr indirect_iterator& operator++() {
        ++m_iter;
        return *this;
      }
      constexpr indirect_iterator operator++( int ) {
        auto i = *this;
        ++*this;
        return i;
      }
 
      // bidirectional iterator operations (if possible)
      constexpr indirect_iterator& operator--()
        requires std::bidirectional_iterator<Iterator>
      {
        --m_iter;
        return *this;
      }
      constexpr indirect_iterator operator--( int )
        requires std::bidirectional_iterator<Iterator>
      {
        auto i = *this;
        --*this;
        return i;
      }
 
      // random access iterator operations (if possible)
      constexpr indirect_iterator& operator+=( difference_type n )
        requires std::random_access_iterator<Iterator>
      {
        m_iter += n;
        return *this;
      }
      constexpr indirect_iterator operator+( difference_type n ) const
        requires std::random_access_iterator<Iterator>
      {
        auto i = *this;
        return i += n;
      }
      constexpr indirect_iterator& operator-=( difference_type n )
        requires std::random_access_iterator<Iterator>
      {
        m_iter -= n;
        return *this;
      }
      constexpr indirect_iterator operator-( difference_type n ) const
        requires std::random_access_iterator<Iterator>
      {
        auto i = *this;
        return i -= n;
      }
      constexpr difference_type operator-( const indirect_iterator& other ) const
        requires std::random_access_iterator<Iterator>
      {
        return m_iter - other.m_iter;
      }
      constexpr reference operator[]( difference_type n ) const
        requires std::random_access_iterator<Iterator>
      {
        return **( m_iter + n );
      }
    };
 
  } // namespace details2
 
  template <typename Container>
  class vector_of_const_ {
    static constexpr bool is_pointer = std::is_pointer_v<Container>;
    static constexpr bool is_range   = details2::is_gaudi_range_v<Container>;
    // TODO: refuse pointer to a range... range must always be by value
    using val_t           = std::add_const_t<std::remove_pointer_t<Container>>;
    using ptr_t           = std::add_pointer_t<val_t>;
    using ContainerVector = std::vector<std::conditional_t<is_range, std::remove_const_t<val_t>, ptr_t>>;
    ContainerVector m_containers;
 
    constexpr static decltype( auto ) wrap( ContainerVector::const_reference t ) {
      if constexpr ( is_pointer || is_range ) {
        return t;
      } else {
        return *t;
      }
    }
    constexpr static auto wrap( ContainerVector::const_iterator i ) {
      if constexpr ( is_pointer || is_range ) {
        return i;
      } else {
        return details2::indirect_iterator{ i };
      }
    }
 
  public:
    using value_type = std::conditional_t<is_pointer, ptr_t, val_t>;
    using size_type  = typename ContainerVector::size_type;
 
    vector_of_const_() = default;
    void reserve( size_type size ) { m_containers.reserve( size ); }
    template <typename T>
    void push_back( T&& container ) {
      if constexpr ( is_pointer || is_range ) {
        m_containers.push_back( container );
      } else {
        // note: does not copy its argument, so we're not really a container...
        m_containers.push_back( &container );
      }
    }
    auto             begin() const { return wrap( m_containers.begin() ); }
    auto             end() const { return wrap( m_containers.end() ); }
    decltype( auto ) front() const { return wrap( m_containers.front() ); }
    decltype( auto ) back() const { return wrap( m_containers.back() ); }
    decltype( auto ) operator[]( size_type i ) const { return wrap( m_containers[i] ); }
    decltype( auto ) at( size_type i ) const { return wrap( m_containers.at( i ) ); }
    size_type        size() const { return m_containers.size(); }
  };
 
  namespace detail2 { // utilities for detected_or_t{,_} usage
 
    // keep only for backwards compatibility... for now.
    template <typename Tr>
    using BaseClass_t = typename Tr::BaseClass;
 
    template <typename Tr, typename Default>
    struct BaseClass {
      using type = Default;
    };
    template <typename Tr, typename Default>
      requires requires { typename Tr::BaseClass; }
    struct BaseClass<Tr, Default> {
      using type = Tr::BaseClass;
    };
 
    template <typename T, typename Tr, template <typename...> typename Default>
    struct OutputHandle {
      using type = Default<T>;
    };
    template <typename T, typename Tr, template <typename...> typename Default>
      requires requires { typename Tr::template OutputHandle<T>; }
    struct OutputHandle<T, Tr, Default> {
      using type = Tr::template OutputHandle<T>;
    };
 
    template <typename T, typename Tr, template <typename...> typename Default>
    struct InputHandle {
      using type = Default<T>;
    };
    template <typename T, typename Tr, template <typename...> typename Default>
      requires requires { typename Tr::template InputHandle<T>; }
    struct InputHandle<T, Tr, Default> {
      using type = Tr::template InputHandle<T>;
    };
 
    template <typename T>
    using DefaultInputHandle =
        std::conditional_t<std::derived_from<std::decay_t<T>, IAlgTool>,
                           ToolHandle<Gaudi::Interface::Bind::IBinder<std::decay_t<T>>>, DataObjectReadHandle<T>>;
  } // namespace detail2
 
  // check whether Tr::BaseClass is a valid type,
  // if so, define BaseClass_t<Tr> as being Tr::BaseClass
  // else   define                 as being Gaudi::Algorithm
  template <typename Tr, typename Default = Gaudi::Algorithm>
  using BaseClass_t = detail2::BaseClass<Tr, Default>::type;
 
  // check whether Traits::{Input,Output}Handle<T> is a valid type,
  // if so, define {Input,Output}Handle_t<Traits,T> as being Traits::{Input,Output}Handle<T>
  // else   define                                  as being DataObject{Read,,Write}Handle<T>
  template <typename Tr, typename T>
  using OutputHandle_t = typename detail2::OutputHandle<T, Tr, DataObjectWriteHandle>::type;
 
  template <typename Tr, typename T>
  using InputHandle_t = typename detail2::InputHandle<T, Tr, detail2::DefaultInputHandle>::type;
 
  template <typename Traits>
  inline constexpr bool isLegacy =
      std::is_base_of_v<Gaudi::details::LegacyAlgorithmAdapter, details::BaseClass_t<Traits>>;
 
#define GAUDI_FUNCTIONAL_MAKE_VECTOR_OF_HANDLES_USES_DATAOBJID
 
  template <typename Handles>
  Handles make_vector_of_handles( IDataHandleHolder* owner, const std::vector<DataObjID>& init ) {
    Handles handles;
    handles.reserve( init.size() );
    std::transform( init.begin(), init.end(), std::back_inserter( handles ),
                    [&]( const auto& loc ) -> typename Handles::value_type {
                      return { loc, owner };
                    } );
    return handles;
  }
 
  template <typename Handle, typename Algo>
  auto get( const Handle& handle, const Algo&,
            const EventContext& ) -> decltype( details::deref( handle.get() ) ) // make it SFINAE friendly...
  {
    return details::deref( handle.get() );
  }
 
  template <typename IFace, typename Algo>
  auto get( const ToolHandle<Gaudi::Interface::Bind::IBinder<IFace>>& handle, const Algo&, const EventContext& ctx ) {
    return handle.bind( ctx );
  }
 
  template <typename Handle>
  auto getKey( const Handle& h ) -> decltype( h.objKey() ) {
    return h.objKey();
  }
 
  // given a pack, return a corresponding tuple
  template <typename... In>
  struct filter_evtcontext_t {
    using type = std::tuple<In...>;
 
    static_assert( !std::disjunction_v<std::is_same<EventContext, In>...>,
                   "EventContext can only appear as first argument" );
 
    template <typename Algorithm, typename Handles>
    static auto apply( const Algorithm& algo, Handles& handles ) {
      return std::apply(
          [&]( const auto&... handle ) { return algo( get( handle, algo, Gaudi::Hive::currentContext() )... ); },
          handles );
    }
    template <typename Algorithm, typename Handles>
    static auto apply( const Algorithm& algo, const EventContext& ctx, Handles& handles ) {
      return std::apply( [&]( const auto&... handle ) { return algo( get( handle, algo, ctx )... ); }, handles );
    }
  };
 
  // except when it starts with EventContext, then drop it
  template <typename... In>
  struct filter_evtcontext_t<EventContext, In...> {
    using type = std::tuple<In...>;
 
    static_assert( !std::disjunction_v<std::is_same<EventContext, In>...>,
                   "EventContext can only appear as first argument" );
 
    template <typename Algorithm, typename Handles>
    static auto apply( const Algorithm& algo, const EventContext& ctx, Handles& handles ) {
      return std::apply( [&]( const auto&... handle ) { return algo( ctx, get( handle, algo, ctx )... ); }, handles );
    }
 
    template <typename Algorithm, typename Handles>
    static auto apply( const Algorithm& algo, Handles& handles ) {
      return apply( algo, Gaudi::Hive::currentContext(), handles );
    }
  };
 
  template <typename... In>
  using filter_evtcontext = typename filter_evtcontext_t<In...>::type;
 
  template <typename OutputSpec, typename InputSpec, typename Traits_>
  class DataHandleMixin;
 
  template <typename Out, typename In, typename Tr>
  void updateHandleLocation( DataHandleMixin<Out, In, Tr>& parent, const std::string& prop,
                             const std::string& newLoc ) {
    auto sc = parent.setProperty( prop, newLoc );
    if ( sc.isFailure() ) throw GaudiException( "Could not set Property", prop + " -> " + newLoc, sc );
  }
 
  template <typename Out, typename In, typename Tr>
  void updateHandleLocations( DataHandleMixin<Out, In, Tr>& parent, const std::string& prop,
                              const std::vector<std::string>& newLocs ) {
    std::ostringstream ss;
    GaudiUtils::details::ostream_joiner( ss << '[', newLocs, ", ", []( std::ostream& os, const auto& i ) -> auto& {
      return os << "'" << i << "'";
    } ) << ']';
    auto sc = parent.setProperty( prop, ss.str() );
    if ( sc.isFailure() ) throw GaudiException( "Could not set Property", prop + " -> " + ss.str(), sc );
  }
 
  template <typename... Out, typename... In, typename Traits_>
  class DataHandleMixin<std::tuple<Out...>, std::tuple<In...>, Traits_> : public BaseClass_t<Traits_> {
    static_assert( std::is_base_of_v<Algorithm, BaseClass_t<Traits_>>, "BaseClass must inherit from Algorithm" );
 
    template <typename IArgs, typename OArgs, std::size_t... I, std::size_t... J>
    DataHandleMixin( std::string name, ISvcLocator* pSvcLocator, const IArgs& inputs, std::index_sequence<I...>,
                     const OArgs& outputs, std::index_sequence<J...> )
        : BaseClass_t<Traits_>( std::move( name ), pSvcLocator )
        , m_inputs( std::tuple_cat( std::forward_as_tuple( this ), std::get<I>( inputs ) )... )
        , m_outputs( std::tuple_cat( std::forward_as_tuple( this ), std::get<J>( outputs ) )... ) {
      // make sure this algorithm is seen as reentrant by Gaudi
      this->setProperty( "Cardinality", 0 ).ignore();
    }
 
  public:
    constexpr static std::size_t N_in  = sizeof...( In );
    constexpr static std::size_t N_out = sizeof...( Out );
 
    using KeyValue  = std::pair<std::string, std::string>;
    using KeyValues = std::pair<std::string, std::vector<std::string>>;
 
    // generic constructor:  N -> M
    DataHandleMixin( std::string name, ISvcLocator* pSvcLocator, RepeatValues_<KeyValue, N_in> const& inputs,
                     RepeatValues_<KeyValue, N_out> const& outputs )
        : DataHandleMixin( std::move( name ), pSvcLocator, inputs, std::index_sequence_for<In...>{}, outputs,
                           std::index_sequence_for<Out...>{} ) {}
 
    // special cases: forward to the generic case...
    // 1 -> 1
    DataHandleMixin( std::string name, ISvcLocator* locator, const KeyValue& input, const KeyValue& output )
        : DataHandleMixin( std::move( name ), locator, std::forward_as_tuple( input ),
                           std::forward_as_tuple( output ) ) {}
    // 1 -> N
    DataHandleMixin( std::string name, ISvcLocator* locator, const KeyValue& input,
                     RepeatValues_<KeyValue, N_out> const& outputs )
        : DataHandleMixin( std::move( name ), locator, std::forward_as_tuple( input ), outputs ) {}
    // N -> 1
    DataHandleMixin( std::string name, ISvcLocator* locator, RepeatValues_<KeyValue, N_in> const& inputs,
                     const KeyValue& output )
        : DataHandleMixin( std::move( name ), locator, inputs, std::forward_as_tuple( output ) ) {}
 
    template <std::size_t N = 0>
    decltype( auto ) inputLocation() const {
      return getKey( std::get<N>( m_inputs ) );
    }
    template <typename T>
    decltype( auto ) inputLocation() const {
      return getKey( std::get<details::InputHandle_t<Traits_, std::decay_t<T>>>( m_inputs ) );
    }
    constexpr unsigned int inputLocationSize() const { return N_in; }
 
    template <std::size_t N = 0>
    decltype( auto ) outputLocation() const {
      return getKey( std::get<N>( m_outputs ) );
    }
    template <typename T>
    decltype( auto ) outputLocation() const {
      return getKey( std::get<details::OutputHandle_t<Traits_, std::decay_t<T>>>( m_outputs ) );
    }
    constexpr unsigned int outputLocationSize() const { return N_out; }
 
  protected:
    bool isReEntrant() const override { return true; }
 
    std::tuple<details::InputHandle_t<Traits_, In>...>   m_inputs;
    std::tuple<details::OutputHandle_t<Traits_, Out>...> m_outputs;
  };
 
  template <typename Traits_>
  class DataHandleMixin<std::tuple<>, std::tuple<>, Traits_> : public BaseClass_t<Traits_> {
    static_assert( std::is_base_of_v<Algorithm, BaseClass_t<Traits_>>, "BaseClass must inherit from Algorithm" );
 
  public:
    using KeyValue  = std::pair<std::string, std::string>;
    using KeyValues = std::pair<std::string, std::vector<std::string>>;
    DataHandleMixin( std::string name, ISvcLocator* pSvcLocator, std::tuple<> = {}, std::tuple<> = {} )
        : BaseClass_t<Traits_>( std::move( name ), pSvcLocator ) {
      // make sure this algorithm is seen as reentrant by Gaudi
      this->setProperty( "Cardinality", 0 ).ignore();
    }
 
  protected:
    bool isReEntrant() const override { return true; }
 
    std::tuple<> m_inputs;
  };
 
  template <typename... In, typename Traits_>
  class DataHandleMixin<std::tuple<>, std::tuple<In...>, Traits_> : public BaseClass_t<Traits_> {
    static_assert( std::is_base_of_v<Algorithm, BaseClass_t<Traits_>>, "BaseClass must inherit from Algorithm" );
 
    template <typename IArgs, std::size_t... I>
    DataHandleMixin( std::string name, ISvcLocator* pSvcLocator, const IArgs& inputs, std::index_sequence<I...> )
        : BaseClass_t<Traits_>( std::move( name ), pSvcLocator )
        , m_inputs( std::tuple_cat( std::forward_as_tuple( this ), std::get<I>( inputs ) )... ) {
      // make sure this algorithm is seen as reentrant by Gaudi
      this->setProperty( "Cardinality", 0 ).ignore();
    }
 
  public:
    using KeyValue                    = std::pair<std::string, std::string>;
    using KeyValues                   = std::pair<std::string, std::vector<std::string>>;
    constexpr static std::size_t N_in = sizeof...( In );
 
    // generic constructor:  N -> 0
    DataHandleMixin( std::string name, ISvcLocator* pSvcLocator, RepeatValues_<KeyValue, N_in> const& inputs )
        : DataHandleMixin( std::move( name ), pSvcLocator, inputs, std::index_sequence_for<In...>{} ) {}
 
    // special cases: forward to the generic case...
    // 1 -> 0
    DataHandleMixin( std::string name, ISvcLocator* locator, const KeyValue& input )
        : DataHandleMixin( std::move( name ), locator, std::forward_as_tuple( input ) ) {}
 
    template <std::size_t N = 0>
    decltype( auto ) inputLocation() const {
      return getKey( std::get<N>( m_inputs ) );
    }
    template <typename T>
    decltype( auto ) inputLocation() const {
      return getKey( std::get<details::InputHandle_t<Traits_, std::decay_t<T>>>( m_inputs ) );
    }
    constexpr unsigned int inputLocationSize() const { return N_in; }
 
  protected:
    bool isReEntrant() const override { return true; }
 
    std::tuple<details::InputHandle_t<Traits_, In>...> m_inputs;
  };
 
  template <typename Traits_>
  class DataHandleMixin<std::tuple<void>, std::tuple<>, Traits_>
      : public DataHandleMixin<std::tuple<>, std::tuple<>, Traits_> {
  public:
    using DataHandleMixin<std::tuple<>, std::tuple<>, Traits_>::DataHandleMixin;
  };
 
  template <typename... Out, typename Traits_>
  class DataHandleMixin<std::tuple<Out...>, std::tuple<>, Traits_> : public BaseClass_t<Traits_> {
    static_assert( std::is_base_of_v<Algorithm, BaseClass_t<Traits_>>, "BaseClass must inherit from Algorithm" );
 
    template <typename OArgs, std::size_t... J>
    DataHandleMixin( std::string name, ISvcLocator* pSvcLocator, const OArgs& outputs, std::index_sequence<J...> )
        : BaseClass_t<Traits_>( std::move( name ), pSvcLocator )
        , m_outputs( std::tuple_cat( std::forward_as_tuple( this ), std::get<J>( outputs ) )... ) {
      // make sure this algorithm is seen as reentrant by Gaudi
      this->setProperty( "Cardinality", 0 ).ignore();
    }
 
  public:
    constexpr static std::size_t N_out = sizeof...( Out );
    using KeyValue                     = std::pair<std::string, std::string>;
    using KeyValues                    = std::pair<std::string, std::vector<std::string>>;
 
    // generic constructor:  0 -> N
    DataHandleMixin( std::string name, ISvcLocator* pSvcLocator, RepeatValues_<KeyValue, N_out> const& outputs )
        : DataHandleMixin( std::move( name ), pSvcLocator, outputs, std::index_sequence_for<Out...>{} ) {}
 
    // 0 -> 1
    DataHandleMixin( std::string name, ISvcLocator* locator, const KeyValue& output )
        : DataHandleMixin( std::move( name ), locator, std::forward_as_tuple( output ) ) {}
 
    template <std::size_t N = 0>
    decltype( auto ) outputLocation() const {
      return getKey( std::get<N>( m_outputs ) );
    }
    constexpr unsigned int outputLocationSize() const { return N_out; }
 
  protected:
    bool isReEntrant() const override { return true; }
 
    std::tuple<details::OutputHandle_t<Traits_, Out>...> m_outputs;
  };
 
  template <typename Fun, typename Container, typename... Args>
  constexpr void applyPostProcessing( const Fun&, Container&, Args... ) {
    static_assert( sizeof...( Args ) == 0, "Args should not be used!" );
  }
 
  template <typename Fun, typename Container>
  auto applyPostProcessing( const Fun& fun, Container& c ) -> decltype( fun.postprocess( c ), void() ) {
    fun.postprocess( c );
  }
 
} // namespace Gaudi::Functional::details