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 #ifndef PODIO_LINKNAVIGATOR_H
 #define PODIO_LINKNAVIGATOR_H
 
 #include <map>
 #include <tuple>
 #include <type_traits>
 #include <utility>
 #include <vector>
 
 namespace podio {
 
 namespace detail::links {
   /// A small struct that simply bundles an object and its weight for a more
   /// convenient return value for the LinkNavigator
   ///
   /// @note In most uses the names of the members should not really matter as it
   /// is possible to us this via structured bindings
   template <typename T>
   struct WeightedObject {
     WeightedObject(T obj, float w) : o(obj), weight(w) {
     }
     T o;          ///< The object
     float weight; ///< The weight in the link
 
     bool operator==(const WeightedObject<T>& other) const {
       return other.o == o && other.weight == weight;
     }
   };
 
   /// Simple struct tag for overload selection in LinkNavigator below
   struct ReturnFromTag {};
   /// Simple struct tag for overload selection in LinkNavigator below
   struct ReturnToTag {};
 } // namespace detail::links
 
 /// Tag variable to select the lookup of *From* objects have links with a *To*
 /// object in podio::LinkNavigator::getLinked
 static constexpr detail::links::ReturnFromTag ReturnFrom;
 /// Tag variable to select the lookup of *To* objects that have links with a
 /// *From* object in podio::LinkNavigator::getLinked
 static constexpr detail::links::ReturnToTag ReturnTo;
 
 /// A helper class to more easily handle one-to-many links.
 ///
 /// Internally simply populates two maps in its constructor and then queries
 /// them to retrieve objects that are linked with another.
 ///
 /// @note There are no guarantees on the order of the objects in these maps.
 /// Hence, there are also no guarantees on the order of the returned objects,
 /// even if there inherintly is an order to them in the underlying links
 /// collection.
 template <typename LinkCollT>
 class LinkNavigator {
   using FromT = typename LinkCollT::from_type;
   using ToT = typename LinkCollT::to_type;
 
   template <typename T>
   using WeightedObject = detail::links::WeightedObject<T>;
 
 public:
   /// Construct a navigator from an link collection
   LinkNavigator(const LinkCollT& links);
 
   /// We do only construct from a collection
   LinkNavigator() = delete;
   LinkNavigator(const LinkNavigator&) = default;
   LinkNavigator& operator=(const LinkNavigator&) = default;
   LinkNavigator(LinkNavigator&&) = default;
   LinkNavigator& operator=(LinkNavigator&&) = default;
   ~LinkNavigator() = default;
 
   /// Get all the *From* objects and weights that have links with the passed
   /// object
   ///
   /// You will get this overload if you pass the podio::ReturnFrom tag as second
   /// argument
   ///
   /// @note This overload works always, even if the LinkCollection that was used
   /// to construct this instance of the LinkNavigator has the same From and To
   /// types.
   ///
   /// @param object The object that is labeled *To* in the link
   /// @param . tag variable for selecting this overload
   ///
   /// @returns A vector of all objects and their weights that have links with
   ///          the passed object
   std::vector<WeightedObject<FromT>> getLinked(const ToT& object, podio::detail::links::ReturnFromTag) const {
     const auto& [begin, end] = m_to2from.equal_range(object);
     std::vector<WeightedObject<FromT>> result;
     result.reserve(std::distance(begin, end));
 
     for (auto it = begin; it != end; ++it) {
       result.emplace_back(it->second);
     }
     return result;
   }
 
   /// Get all the *From* objects and weights that have links with the passed
   /// object
   ///
   /// @note This overload will automatically do the right thing (TM) in case the
   /// LinkCollection that has been passed to construct this LinkNavigator has
   /// different From and To types.
   ///
   /// @param object The object that is labeled *To* in the link
   ///
   /// @returns A vector of all objects and their weights that have links with
   ///          the passed object
   template <typename ToU = ToT>
   std::enable_if_t<!std::is_same_v<FromT, ToU>, std::vector<WeightedObject<FromT>>> getLinked(const ToT& object) const {
     return getLinked(object, podio::ReturnFrom);
   }
 
   /// Get all the *To* objects and weights that have links with the passed
   /// object
   ///
   /// You will get this overload if you pass the podio::ReturnTo tag as second
   /// argument
   ///
   /// @note This overload works always, even if the LinkCollection that was used
   /// to construct this instance of the LinkNavigator has the same From and To
   /// types.
   ///
   /// @param object The object that is labeled *From* in the link
   /// @param . tag variable for selecting this overload
   ///
   /// @returns A vector of all objects and their weights that have links with
   ///          the passed object
   std::vector<WeightedObject<ToT>> getLinked(const FromT& object, podio::detail::links::ReturnToTag) const {
     const auto& [begin, end] = m_from2to.equal_range(object);
     std::vector<WeightedObject<ToT>> result;
     result.reserve(std::distance(begin, end));
 
     for (auto it = begin; it != end; ++it) {
       result.emplace_back(it->second);
     }
     return result;
   }
 
   /// Get all the *To* objects and weights that have links with the passed
   /// object
   ///
   /// @note This overload will automatically do the right thing (TM) in case the
   /// LinkCollection that has been passed to construct this LinkNavigator has
   /// different From and To types.
   ///
   /// @param object The object that is labeled *From* in the link
   ///
   /// @returns A vector of all objects and their weights that have links with
   ///          the passed object
   template <typename FromU = FromT>
   std::enable_if_t<!std::is_same_v<FromU, ToT>, std::vector<WeightedObject<ToT>>> getLinked(const FromT& object) const {
     return getLinked(object, podio::ReturnTo);
   }
 
 private:
   std::multimap<FromT, WeightedObject<ToT>> m_from2to{}; ///< Map the from to the to objects
   std::multimap<ToT, WeightedObject<FromT>> m_to2from{}; ///< Map the to to the from objects
 };
 
 template <typename LinkCollT>
 LinkNavigator<LinkCollT>::LinkNavigator(const LinkCollT& links) {
   for (const auto& [from, to, weight] : links) {
     m_from2to.emplace(std::piecewise_construct, std::forward_as_tuple(from), std::forward_as_tuple(to, weight));
     m_to2from.emplace(std::piecewise_construct, std::forward_as_tuple(to), std::forward_as_tuple(from, weight));
   }
 }
 
 } // namespace podio
 
 #endif // PODIO_LINKNAVIGATOR_H

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