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 // This file is part of the Acts project.
 //
 // Copyright (C) 2021 CERN for the benefit of the Acts project
 //
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
 // file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 #pragma once
 
 #include "Acts/Utilities/TypeTraits.hpp"
 
 #include <cassert>
 #include <functional>
 #include <memory>
 #include <type_traits>
 
 namespace Acts {
 
 /// Ownership enum for @c Delegate
 enum class DelegateType { Owning, NonOwning };
 
 template <auto C>
 struct DelegateFuncTag {
   explicit constexpr DelegateFuncTag() = default;
 };
 
 // Specialization needed for defaulting ownership and for R(Args...) syntax
 template <typename, typename H = void, DelegateType = DelegateType::NonOwning>
 class Delegate;
 
 /// Delegate type that allows type erasure of a callable without allocation
 /// and with a single level of indirection. This type can support:
 /// - a free function pointer
 /// - a pointer to a member function alongside an instance pointer
 /// @note @c Delegate by default does not assume ownership of the instance.
 ///          In that case You need to ensure that the lifetime of the callable
 ///          instance is longer than that of the @c Delegate. If you set @c O
 ///          to @c DelegateType::Owning, it will assume ownership.
 /// @note Currently @c Delegate only supports callables that are ``const``
 /// @tparam R Return type of the function signature
 /// @tparam H Holder type that is used to store an instance
 /// @tparam O Ownership type of the delegate: Owning or NonOwning
 /// @tparam Args Types of the arguments of the function signatures
 ///
 template <typename R, typename H, DelegateType O, typename... Args>
 class Delegate<R(Args...), H, O> {
   static constexpr DelegateType kOwnership = O;
 
   /// Alias of the return type
   using return_type = R;
   using holder_type = H;
   /// Alias to the function pointer type this class will store
   using function_type = return_type (*)(const holder_type *, Args...);
 
   using function_ptr_type = return_type (*)(Args...);
 
   using deleter_type = void (*)(const holder_type *);
 
   template <typename T, typename C>
   using isSignatureCompatible =
       decltype(std::declval<T &>() = std::declval<C>());
 
   using OwningDelegate =
       Delegate<R(Args...), holder_type, DelegateType::Owning>;
   using NonOwningDelegate =
       Delegate<R(Args...), holder_type, DelegateType::NonOwning>;
   template <typename T>
   using isNoFunPtr =
       std::enable_if_t<!std::is_convertible_v<std::decay_t<T>, function_type> &&
                        !std::is_same_v<std::decay_t<T>, OwningDelegate> &&
                        !std::is_same_v<std::decay_t<T>, NonOwningDelegate>>;
 
  public:
   Delegate() = default;
 
   Delegate(Delegate &&) = default;
   Delegate &operator=(Delegate &&) = default;
   Delegate(const Delegate &) = default;
   Delegate &operator=(const Delegate &) = default;
 
   /// Constructor with an explicit runtime callable
   /// @param callable The runtime value of the callable
   /// @note The function signature requires the first argument of the callable is `const void*`.
   ///       i.e. if the signature of the delegate is `void(int)`, the
   ///       callable's signature has to be `void(const void*, int)`.
   Delegate(function_type callable) { connect(callable); }
 
   /// Constructor with a possibly stateful function object.
   /// @tparam Callable Type of the callable
   /// @param callable The callable (function object or lambda)
   /// @note @c Delegate does not assume owner ship over @p callable. You need to ensure
   ///       it's lifetime is longer than that of @c Delegate.
   template <typename Callable, typename = isNoFunPtr<Callable>>
   Delegate(Callable &callable) {
     connect(callable);
   }
 
   /// Constructor with a compile-time free function pointer
   /// @tparam Callable The compile-time free function pointer
   /// @note @c DelegateFuncTag is used to communicate the callable type
   template <auto Callable>
   Delegate(DelegateFuncTag<Callable> /*tag*/) {
     connect<Callable>();
   }
 
   /// Constructor with a compile-time member function pointer and instance
   /// @tparam Callable The compile-time member function pointer
   /// @tparam Type The type of the instance the member function should be called on
   /// @param instance The instance on which the member function pointer should be called on
   /// @note @c Delegate does not assume owner ship over @p instance.
   /// @note @c DelegateFuncTag is used to communicate the callable type
   template <auto Callable, typename Type, DelegateType T = kOwnership,
             typename = std::enable_if_t<T == DelegateType::NonOwning>>
   Delegate(DelegateFuncTag<Callable> /*tag*/, const Type *instance) {
     connect<Callable>(instance);
   }
 
   /// Constructor from rvalue reference is deleted, should catch construction
   /// with temporary objects and thus invalid pointers
   template <typename Callable, typename = isNoFunPtr<Callable>>
   Delegate(Callable &&) = delete;
 
   /// Assignment operator with an explicit runtime callable
   /// @param callable The runtime value of the callable
   /// @note The function signature requires the first argument of the callable is `const void*`.
   ///       i.e. if the signature of the delegate is `void(int)`, the
   ///       callable's signature has to be `void(const void*, int)`.
   void operator=(function_type callable) { connect(callable); }
 
   /// Assignment operator with possibly stateful function object.
   /// @tparam Callable Type of the callable
   /// @param callable The callable (function object or lambda)
   /// @note @c Delegate does not assume owner ship over @p callable. You need to ensure
   ///       it's lifetime is longer than that of @c Delegate.
   template <typename Callable, typename = isNoFunPtr<Callable>>
   void operator=(Callable &callable) {
     connect(callable);
   }
 
   /// Assignment operator from rvalue reference is deleted, should catch
   /// assignment from temporary objects and thus invalid pointers
   template <typename Callable, typename = isNoFunPtr<Callable>>
   void operator=(Callable &&) = delete;
 
   /// Connect a free function pointer.
   /// @note The function pointer must be ``constexpr`` for @c Delegate to accept it
   /// @tparam Callable The compile-time free function pointer
   template <auto Callable>
   void connect() {
     m_payload.payload = nullptr;
 
     static_assert(
         Concepts::is_detected<isSignatureCompatible, function_ptr_type,
                               decltype(Callable)>::value,
         "Callable given does not correspond exactly to required call "
         "signature");
 
     m_function = [](const holder_type * /*payload*/,
                     Args... args) -> return_type {
       return std::invoke(Callable, std::forward<Args>(args)...);
     };
   }
 
   /// Assignment operator with possibly stateful function object.
   /// @tparam Callable Type of the callable
   /// @param callable The callable (function object or lambda)
   /// @note @c Delegate does not assume owner ship over @p callable. You need to ensure
   ///       it's lifetime is longer than that of @c Delegate.
   template <typename Callable, typename = isNoFunPtr<Callable>>
   void connect(Callable &callable) {
     connect<&Callable::operator(), Callable>(&callable);
   }
 
   /// Connection with rvalue reference is deleted, should catch assignment
   /// from temporary objects and thus invalid pointers
   template <typename Callable, typename = isNoFunPtr<Callable>>
   void connect(Callable &&) = delete;
 
   /// Connect anything that is assignable to the function pointer
   /// @param callable The runtime value of the callable
   /// @note The function signature requires the first argument of the callable is `const void*`.
   ///       i.e. if the signature of the delegate is `void(int)`, the
   ///       callable's signature has to be `void(const void*, int)`.
   void connect(function_type callable) {
     if constexpr (kOwnership == DelegateType::NonOwning) {
       m_payload.payload = nullptr;
     }
     m_function = callable;
   }
 
   /// Connect a member function to be called on an instance
   /// @tparam Callable The compile-time member function pointer
   /// @tparam Type The type of the instance the member function should be called on
   /// @param instance The instance on which the member function pointer should be called on
   /// @note @c Delegate does not assume owner ship over @p instance. You need to ensure
   ///       it's lifetime is longer than that of @c Delegate.
   template <auto Callable, typename Type, DelegateType T = kOwnership,
             typename = std::enable_if_t<T == DelegateType::NonOwning>>
   void connect(const Type *instance) {
     using member_ptr_type = return_type (Type::*)(Args...) const;
 
     static_assert(Concepts::is_detected<isSignatureCompatible, member_ptr_type,
                                         decltype(Callable)>::value,
                   "Callable given does not correspond exactly to required call "
                   "signature");
 
     m_payload.payload = instance;
 
     m_function = [](const holder_type *payload, Args... args) -> return_type {
       assert(payload != nullptr && "Payload is required, but not set");
       const auto *concretePayload = static_cast<const Type *>(payload);
       return std::invoke(Callable, concretePayload,
                          std::forward<Args>(args)...);
     };
   }
 
   /// Connect a member function to be called on an instance
   /// @tparam Callable The compile-time member function pointer
   /// @tparam Type The type of the instance the member function should be called on
   /// @param instance The instance on which the member function pointer should be called on
   /// @note @c Delegate assumes owner ship over @p instance.
   template <auto Callable, typename Type, DelegateType T = kOwnership,
             typename = std::enable_if_t<T == DelegateType::Owning>>
   void connect(std::unique_ptr<const Type> instance) {
     using member_ptr_type = return_type (Type::*)(Args...) const;
     static_assert(Concepts::is_detected<isSignatureCompatible, member_ptr_type,
                                         decltype(Callable)>::value,
                   "Callable given does not correspond exactly to required call "
                   "signature");
 
     m_payload.payload = std::unique_ptr<const holder_type, deleter_type>(
         instance.release(), [](const holder_type *payload) {
           const auto *concretePayload = static_cast<const Type *>(payload);
           delete concretePayload;
         });
 
     m_function = [](const holder_type *payload, Args... args) -> return_type {
       assert(payload != nullptr && "Payload is required, but not set");
       const auto *concretePayload = static_cast<const Type *>(payload);
       return std::invoke(Callable, concretePayload,
                          std::forward<Args>(args)...);
     };
   }
 
   /// The call operator that exposes the functionality of the @c Delegate type.
   /// @param args The arguments to call the contained function with
   /// @return Return value of the contained function
   return_type operator()(Args... args) const {
     assert(connected() && "Delegate is not connected");
     return std::invoke(m_function, m_payload.ptr(),
                        std::forward<Args>(args)...);
   }
 
   /// Return whether this delegate is currently connected
   /// @return True if this delegate is connected
   bool connected() const { return m_function != nullptr; }
 
   /// Return whether this delegate is currently connected
   /// @return True if this delegate is connected
   operator bool() const { return connected(); }
 
   /// Disconnect this delegate, meaning it cannot be called anymore
   void disconnect() {
     m_payload.clear();
     m_function = nullptr;
   }
 
   template <typename holder_t = holder_type,
             typename = std::enable_if_t<!std::is_same_v<holder_t, void>>>
   const holder_type *instance() const {
     return m_payload.ptr();
   }
 
  private:
   // Deleter that does not do anything
   static void noopDeleter(const holder_type * /*unused*/) {}
 
   /// @cond
 
   // Payload object without a deleter
   struct NonOwningPayload {
     void clear() { payload = nullptr; }
 
     const holder_type *ptr() const { return payload; }
 
     const holder_type *payload{nullptr};
   };
 
   // Payload object with a deleter
   struct OwningPayload {
     void clear() { payload.reset(); }
 
     const holder_type *ptr() const { return payload.get(); }
 
     std::unique_ptr<const holder_type, deleter_type> payload{nullptr,
                                                              &noopDeleter};
   };
 
   /// Stores the instance pointer and maybe a deleter
   std::conditional_t<kOwnership == DelegateType::NonOwning, NonOwningPayload,
                      OwningPayload>
       m_payload;
 
   /// @endcond
 
   /// Stores the function pointer wrapping the compile time function pointer given in @c connect().
   function_type m_function{nullptr};
 };
 
 template <typename, typename H = void>
 class OwningDelegate;
 
 /// Alias for an owning delegate
 template <typename R, typename H, typename... Args>
 class OwningDelegate<R(Args...), H>
     : public Delegate<R(Args...), H, DelegateType::Owning> {};
 
 }  // namespace Acts

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