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 #ifndef PODIO_DETAIL_LINK_H
 #define PODIO_DETAIL_LINK_H
 
 #include "podio/detail/LinkFwd.h"
 #include "podio/detail/LinkObj.h"
 #include "podio/utilities/MaybeSharedPtr.h"
 #include "podio/utilities/StaticConcatenate.h"
 #include "podio/utilities/TypeHelpers.h"
 #include <string_view>
 
 #ifdef PODIO_JSON_OUTPUT
   #include "nlohmann/json.hpp"
 #endif
 
 #include <functional>
 #include <ostream>
 #include <type_traits>
 #include <utility> // std::swap
 
 namespace podio {
 
 namespace detail {
   /// Check whether a Link<FromT, ToT> can be initialized from values of types
   /// FromU and ToU in exactly this order, i.e. whether its From element can be
   /// set from a FromU and its To element from a ToU. Passing FromT and ToT in
   /// swapped order checks the reversed order.
   template <typename FromT, typename ToT, typename FromU, typename ToU>
   concept LinkInitializableFrom = requires(podio::MutableLink<FromT, ToT> link, FromU from, ToU to) {
     link.setFrom(from);
     link.setTo(to);
   };
 } // namespace detail
 
 /// Generalized Link type for both Mutable and immutable (default)
 /// versions. User facing classes with the expected naming scheme are defined via
 /// template aliases in LinkFwd.h
 template <typename FromT, typename ToT, bool Mutable>
 class LinkT {
   // The typedefs in LinkFwd.h should make sure that at this point
   // Mutable classes are stripped, i.e. the user should never be able to trigger
   // these!
   static_assert(std::is_same_v<detail::GetDefaultHandleType<FromT>, FromT>,
                 "Links need to be instantiated with the default types!");
   static_assert(std::is_same_v<detail::GetDefaultHandleType<ToT>, ToT>,
                 "Links need to be instantiated with the default types!");
 
   using LinkObjT = LinkObj<FromT, ToT>;
   friend LinkCollection<FromT, ToT>;
   friend LinkCollectionIteratorT<FromT, ToT, Mutable>;
   friend LinkT<FromT, ToT, !Mutable>;
   friend std::hash<LinkT>;
 
   /// Helper member variable to check whether FromU and ToU can be used for this
   /// Link. We need this to make SFINAE trigger in some cases below
   template <typename FromU, typename ToU>
   constexpr static bool sameTypes = std::is_same_v<FromU, FromT> && std::is_same_v<ToU, ToT>;
 
   /// Variable template to for determining whether T is either FromT or ToT.
   /// Mainly defined for convenience
   template <typename T>
   inline constexpr static bool isFromOrToT = detail::isInTuple<T, std::tuple<FromT, ToT>>;
 
   /// Variable template to for determining whether T is either FromT or ToT or
   /// any of their mutable versions.
   template <typename T>
   inline constexpr static bool isMutableFromOrToT =
       detail::isInTuple<T,
                         std::tuple<FromT, ToT, detail::GetMutableHandleType<FromT>, detail::GetMutableHandleType<ToT>>>;
 
 public:
   using mutable_type = podio::MutableLink<FromT, ToT>;
   using value_type = podio::Link<FromT, ToT>;
   using collection_type = podio::LinkCollection<FromT, ToT>;
 
   static constexpr std::string_view typeName =
       utils::static_concatenate_v<detail::link_name_prefix, FromT::typeName, detail::link_name_infix, ToT::typeName,
                                   detail::link_name_suffix>;
   /// Constructor
   LinkT() : m_obj(new LinkObjT{}, podio::utils::MarkOwned) {
   }
 
   /// Constructor with weight
   explicit LinkT(float weight) : m_obj(new LinkObjT{}, podio::utils::MarkOwned) {
     m_obj->data.weight = weight;
   }
 
   /// Copy constructor
   LinkT(const LinkT& other) = default;
 
   /// Assignment operators
   LinkT& operator=(LinkT other) & {
     swap(*this, other);
     return *this;
   }
   LinkT& operator=(LinkT other) && = delete; // Prevent rebinding temporary as the changes wouldn't persist
 
   /// Implicit conversion of mutable to immutable links
   template <typename FromU, typename ToU>
     requires Mutable && sameTypes<FromU, ToU>
   operator LinkT<FromU, ToU, false>() const {
     return Link<FromU, ToU>(m_obj);
   }
 
   /// Create a mutable deep-copy
   ///
   /// @param cloneRelations if set to false only the weight will be cloned but
   ///                       not the relations to the objects this link points
   ///                       to. Defaults to true
   ///
   /// @returns A mutable deep-copy of this link which is independent of the
   ///          original one
   template <typename FromU = FromT, typename ToU = ToT>
     requires sameTypes<FromU, ToU>
   MutableLink<FromU, ToU> clone(bool cloneRelations = true) const {
     auto tmp = new LinkObjT(podio::ObjectID{}, m_obj->data);
     if (cloneRelations) {
       if (m_obj->m_from) {
         tmp->m_from = std::make_unique<FromT>(*m_obj->m_from);
       }
       if (m_obj->m_to) {
         tmp->m_to = std::make_unique<ToT>(*m_obj->m_to);
       }
     }
     return MutableLink<FromU, ToU>(podio::utils::MaybeSharedPtr(tmp, podio::utils::MarkOwned));
   }
 
   /// Create an empty Link handle
   template <bool Mut = Mutable>
     requires(!Mut && !Mutable)
   static Link<FromT, ToT> makeEmpty() {
     return {nullptr};
   }
 
   /// Destructor
   ~LinkT() = default;
 
   /// Get the weight of the link
   float getWeight() const {
     return m_obj->data.weight;
   }
 
   /// Set the weight of the link
   template <bool Mut = Mutable>
     requires(Mut && Mutable)
   void setWeight(float value) {
     m_obj->data.weight = value;
   }
 
   /// Access the related-from object
   const FromT getFrom() const {
     if (!m_obj->m_from) {
       return FromT::makeEmpty();
     }
     return FromT(*(m_obj->m_from));
   }
 
   /// Set the related-from object
   ///
   /// @note All setFrom overloads are equivalent and the correct one is selected
   /// at compile time. We need to differentiate between the handles, only to
   /// make the python bindings work
   template <typename FromU>
     requires(Mutable && std::is_same_v<detail::GetDefaultHandleType<FromU>, FromT> &&
              detail::isDefaultHandleType<FromU>)
   void setFrom(FromU value) {
     m_obj->m_from = std::make_unique<detail::GetDefaultHandleType<FromU>>(value);
   }
 
   /// Set the related-from object
   ///
   /// @note All setFrom overloads are equivalent and the correct one is selected
   /// at compile time. We need to differentiate between the handles, only to
   /// make the python bindings work
   template <typename FromU>
     requires(Mutable && std::is_same_v<detail::GetMutableHandleType<FromT>, FromU> &&
              detail::isMutableHandleType<FromU>)
   void setFrom(FromU value) {
     setFrom(detail::GetDefaultHandleType<FromU>(value));
   }
 
   /// Set the related-from object
   ///
   /// @note All setFrom overloads are equivalent and the correct one is selected
   /// at compile time. We need this overload to allow for an implicit conversion
   /// to interface types in case the relation contains an interface type.
   template <typename FromU>
     requires(Mutable && detail::isInterfaceInitializableFrom<FromT, FromU>)
   void setFrom(FromU value) {
     setFrom(FromT(value));
   }
 
   /// Access the related-to object
   const ToT getTo() const {
     if (!m_obj->m_to) {
       return ToT::makeEmpty();
     }
     return ToT(*(m_obj->m_to));
   }
 
   /// Set the related-to object
   ///
   /// @note All setTo overloads are equivalent and the correct one is selected
   /// at compile time. We need to differentiate between the handles, only to
   /// make the python bindings work
   template <typename ToU>
     requires(Mutable && std::is_same_v<detail::GetDefaultHandleType<ToU>, ToT> && detail::isDefaultHandleType<ToU>)
   void setTo(ToU value) {
     m_obj->m_to = std::make_unique<detail::GetDefaultHandleType<ToU>>(value);
   }
 
   /// Set the related-to object
   ///
   /// @note All setTo overloads are equivalent and the correct one is selected
   /// at compile time. We need to differentiate between the handles, only to
   /// make the python bindings work
   template <typename ToU>
     requires(Mutable && std::is_same_v<detail::GetMutableHandleType<ToT>, ToU> && detail::isMutableHandleType<ToU>)
   void setTo(ToU value) {
     setTo(detail::GetDefaultHandleType<ToU>(value));
   }
 
   /// Set the related-to object
   ///
   /// @note All setTo overloads are equivalent and the correct one is selected
   /// at compile time. We need this overload to allow for an implicit conversion
   /// to interface types in case the relation contains an interface type.
   template <typename ToU>
     requires(Mutable && detail::isInterfaceInitializableFrom<ToT, ToU>)
   void setTo(ToU value) {
     setTo(ToT(value));
   }
 
   /// Templated version for getting an element of the link by type. Only
   /// available for Links where FromT and ToT are **not the same type**,
   /// and if the requested type is actually part of the Link. It is only
   /// possible to get the immutable types from this. Will result in a
   /// compilation error if any of these conditions is not met.
   ///
   /// @tparam T the desired type
   /// @returns T the element of the Link
   template <typename T>
     requires(!std::is_same_v<ToT, FromT> && isFromOrToT<T>)
   T get() const {
     if constexpr (std::is_same_v<T, FromT>) {
       return getFrom();
     } else {
       return getTo();
     }
   }
 
   /// Tuple like index based access to the elements of the Link. Returns
   /// only immutable types of the links. This method enables structured
   /// bindings for Links.
   ///
   /// @tparam Index an index (smaller than 3) to access an element of the Link
   /// @returns Depending on the value of Index:
   ///   - 0: The From element of the Link
   ///   - 1: The To element of the Link
   ///   - 2: The weight of the Link
   template <size_t Index>
     requires(Index < 3)
   auto get() const {
     if constexpr (Index == 0) {
       return getFrom();
     } else if constexpr (Index == 1) {
       return getTo();
     } else {
       return getWeight();
     }
   }
 
   /// Templated version for setting an element of the link by type. Only
   /// available for Links where FromT and ToT are **not the same type**,
   /// and if the requested type is actually part of the Link. Will result
   /// in a compilation error if any of these conditions is not met.
   ///
   /// @tparam T type of value (**inferred!**)
   /// @param value the element to set for this link.
   template <typename T>
     requires(Mutable && !std::is_same_v<ToT, FromT> &&
              (isMutableFromOrToT<T> || detail::isInterfaceInitializableFrom<ToT, T> ||
               detail::isInterfaceInitializableFrom<FromT, T>))
   void set(T value) {
     if constexpr (std::is_same_v<T, FromT>) {
       setFrom(std::move(value));
     } else if constexpr (std::is_same_v<T, ToT>) {
       setTo(std::move(value));
     } else if constexpr (detail::isInterfaceInitializableFrom<FromT, T> &&
                          !detail::isInterfaceInitializableFrom<ToT, T>) {
       setFrom(std::move(value));
     } else if constexpr (detail::isInterfaceInitializableFrom<ToT, T> &&
                          !detail::isInterfaceInitializableFrom<FromT, T>) {
       setTo(std::move(value));
     } else {
       static_assert(detail::always_false<T>, "Argument type is ambiguous, can't determine link direction");
     }
   }
 
   /// check whether the object is actually available
   bool isAvailable() const {
     return m_obj;
   }
 
   /// disconnect from Link instance
   void unlink() {
     m_obj = podio::utils::MaybeSharedPtr<LinkObjT>(nullptr);
   }
 
   /// Get the ObjectID
   podio::ObjectID getObjectID() const {
     if (m_obj) {
       return m_obj->id;
     }
     return podio::ObjectID{};
   }
 
   podio::ObjectID id() const {
     return getObjectID();
   }
 
   constexpr bool operator==(const LinkT& other) const {
     return m_obj == other.m_obj;
   }
 
   constexpr bool operator!=(const LinkT& other) const {
     return !(*this == other);
   }
 
   template <typename FromU, typename ToU>
     requires sameTypes<FromU, ToU>
   constexpr bool operator==(const LinkT<FromU, ToU, !Mutable>& other) const {
     return m_obj == other.m_obj;
   }
 
   template <typename FromU, typename ToU>
     requires sameTypes<FromU, ToU>
   constexpr bool operator!=(const LinkT<FromU, ToU, !Mutable>& other) const {
     return !(*this == other);
   }
 
   constexpr bool operator<(const LinkT& other) const {
     return m_obj < other.m_obj;
   }
 
   friend void swap(LinkT& a, LinkT& b) {
     using std::swap;
     swap(a.m_obj, b.m_obj); // swap out the internal pointers
   }
 
 private:
   /// Constructor from existing LinkObj
   explicit LinkT(podio::utils::MaybeSharedPtr<LinkObjT> obj) : m_obj(std::move(obj)) {
   }
 
   template <bool Mut = Mutable>
     requires(!Mut && !Mutable)
   LinkT(LinkObjT* obj) : m_obj(podio::utils::MaybeSharedPtr<LinkObjT>(obj)) {
   }
 
   podio::utils::MaybeSharedPtr<LinkObjT> m_obj{nullptr};
 };
 
 template <typename FromT, typename ToT>
 std::ostream& operator<<(std::ostream& os, const Link<FromT, ToT>& link) {
   if (!link.isAvailable()) {
     return os << "[not available]";
   }
 
   return os << " id: " << link.id() << '\n'
             << " weight: " << link.getWeight() << '\n'
             << " from: " << link.getFrom().id() << '\n'
             << " to: " << link.getTo().id() << '\n';
 }
 
 #if defined(PODIO_JSON_OUTPUT) && !defined(__CLING__)
 template <typename FromT, typename ToT>
 void to_json(nlohmann::json& j, const podio::LinkT<FromT, ToT, false>& link) {
   j = nlohmann::json{{"weight", link.getWeight()}};
 
   j["from"] = nlohmann::json{{"collectionID", link.getFrom().getObjectID().collectionID},
                              {"index", link.getFrom().getObjectID().index}};
 
   j["to"] = nlohmann::json{{"collectionID", link.getTo().getObjectID().collectionID},
                            {"index", link.getTo().getObjectID().index}};
 }
 #endif
 
 } // namespace podio
 
 template <typename FromT, typename ToT, bool Mutable>
 struct std::hash<podio::LinkT<FromT, ToT, Mutable>> {
   std::size_t operator()(const podio::LinkT<FromT, ToT, Mutable>& obj) const {
     return std::hash<typename podio::LinkT<FromT, ToT, Mutable>::LinkObjT*>{}(obj.m_obj.get());
   }
 };
 
 #endif // PODIO_DETAIL_LINK_H

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