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 // This file is part of the ACTS project.
 //
 // Copyright (C) 2016 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 https://mozilla.org/MPL/2.0/.
 
 #pragma once
 
 #include <concepts>
 #include <optional>
 #include <sstream>
 #include <system_error>
 #include <type_traits>
 #include <utility>
 #include <variant>
 
 namespace Acts {
 
 /// Class which encapsulates either a valid result, or an error
 /// @tparam T The valid result value
 /// @tparam E The error, defaults to `std::error_code`
 ///
 template <typename T, typename E = std::error_code>
 class Result {
   /// Private constructor which accepts an external variant.
   /// This is used by the factory static methods to set up
   /// the variant unambiguously in all cases.
   Result(std::variant<T, E>&& var) : m_var(std::move(var)) {}
 
  public:
   using ValueType = T;
   using ErrorType = E;
 
   /// Default construction is disallowed.
   Result() = delete;
 
   /// Copy construction is disallowed
   Result(const Result<T, E>& other) = delete;
 
   /// Assignment is disallowed
   Result<T, E>& operator=(const Result<T, E>& other) = delete;
 
   /// Move construction is allowed
   Result(Result<T, E>&& other) noexcept : m_var(std::move(other.m_var)) {}
 
   /// Move assignment is allowed
   /// @param other The other result instance, rvalue reference
   /// @return The assigned instance
   Result<T, E>& operator=(Result<T, E>&& other) noexcept {
     m_var = std::move(other.m_var);
     return *this;
   }
 
   /// @brief Constructor from arbitrary value
   /// This constructor allows construction from any value. This constructor is
   /// only enabled if T and E are unambiguous, meaning they cannot be implicitly
   /// converted and there is T cannot be constructed from E and vice-versa.
   /// This means that when this is invoked, the value can be propagated to the
   /// underlying variant, and the assignment will be correct, and error will be
   /// an error, and a value will be a value.
   /// @note If T and E are ambiguous, use the `success` and `failure` static
   /// factory methods.
   /// @tparam T2 Type of the potential assignment
   /// @param value The potential value, could be an actual valid value or an
   /// error.
   template <typename T2>
   Result(T2 value) noexcept
     requires(!std::same_as<T, E> && !std::constructible_from<T, E> &&
              !std::convertible_to<T, E> && !std::constructible_from<E, T> &&
              !std::convertible_to<E, T> &&
              !(std::convertible_to<T2, T> && std::convertible_to<T2, E>))
       : m_var(std::conditional_t<std::is_convertible_v<T2, T>, T, E>{
             std::move(value)}) {}
 
   /// @brief Assignment operator from arbitrary value
   /// This operator allows construction from any value. The same rules as for
   /// the `Result(T2 value)` constructor apply.
   /// * @tparam T2 Type of the potential assignment
   /// @param value The potential value, could be an actual valid value or an
   /// error.
   /// @return The assigned instance
   template <typename T2>
   Result<T, E>& operator=(T2 value) noexcept
     requires(!std::same_as<T, E> && !std::constructible_from<T, E> &&
              !std::convertible_to<T, E> && !std::constructible_from<E, T> &&
              !std::convertible_to<E, T> &&
              !(std::convertible_to<T2, T> && std::convertible_to<T2, E>))
   {
     m_var = std::move(std::conditional_t<std::is_convertible_v<T2, T>, T, E>{
         std::move(value)});
     return *this;
   }
 
   /// Static helper factory which forces assignment as valid value.
   /// @param value The valid value to assign. Will not be converted to E.
   /// @return Initialized result object
   static Result<T, E> success(T value) {
     return Result<T, E>(
         std::variant<T, E>{std::in_place_index<0>, std::move(value)});
   }
 
   /// Static helper factory which forces assignment as an error.
   /// @param error The error to assign. Will not be converted to T.
   /// @return Initialized result object
   static Result<T, E> failure(E error) {
     return Result<T, E>(
         std::variant<T, E>{std::in_place_index<1>, std::move(error)});
   }
 
   /// Checks whether this result contains a valid value, and no error.
   /// @return bool Whether result contains an error or not.
   bool ok() const noexcept { return m_var.index() == 0; }
 
   /// Returns a reference into the variant to the valid value.
   /// @note If `!res.ok()`, this method will abort (noexcept)
   /// @return Reference to value stored in the variant.
   T& operator*() noexcept { return std::get<T>(m_var); }
 
   /// Returns a reference into the variant to the valid value.
   /// @note If `!res.ok()`, this method will abort (noexcept)
   /// @return Reference to value stored in the variant.
   const T& operator*() const noexcept { return std::get<T>(m_var); }
 
   /// Allows to access members of the stored object with `res->foo`
   /// similar to `std::optional`.
   /// @note If `!res.ok()`, this method will abort (noexcept)
   /// @return Pointer to value stored in the variant.
   T* operator->() noexcept { return &std::get<T>(m_var); }
 
   /// Allows to access members of the stored object with `res->foo`
   /// similar to `std::optional`.
   /// @note If `!res.ok()`, this method will abort (noexcept)
   /// @return Pointer to value stored in the variant.
   const T* operator->() const noexcept { return &std::get<T>(m_var); }
 
   /// Returns a reference to the error stored in the result.
   /// @note If `res.ok()` this method will abort (noexcept)
   /// @return Reference to the error
   E& error() & noexcept { return std::get<E>(m_var); }
 
   /// Returns a reference to the error stored in the result.
   /// @note If `res.ok()` this method will abort (noexcept)
   /// @return Reference to the error
   const E& error() const& noexcept { return std::get<E>(m_var); }
 
   /// Returns the error by-value.
   /// @note If `res.ok()` this method will abort (noexcept)
   /// @return The error
   E error() && noexcept { return std::move(std::get<E>(m_var)); }
 
   /// Retrieves the valid value from the result object.
   /// @note This is the lvalue version, returns a reference to the value
   /// @return The valid value as a reference
   T& value() & {
     checkValueAccess();
     return std::get<T>(m_var);
   }
 
   /// Retrieves the valid value from the result object.
   /// @note This is the lvalue version, returns a reference to the value
   /// @return The valid value as a reference
   const T& value() const& {
     checkValueAccess();
     return std::get<T>(m_var);
   }
 
   /// Retrieves the valid value from the result object.
   /// @note This is the rvalue version, returns the value
   /// by-value and moves out of the variant.
   /// @return The valid value by value, moved out of the variant.
   T value() && {
     checkValueAccess();
     return std::move(std::get<T>(m_var));
   }
 
   /// Retrieves the valid value from the result object, or returns a default
   /// value if no valid value exists.
   ///
   /// @param[in] v The default value to use if no valid value exists.
   /// @note This is the lvalue version.
   /// @note This function always returns by value.
   /// @return Either the valid value, or the given substitute.
   template <typename U>
   std::conditional_t<std::is_reference_v<U>, const T&, T> value_or(U&& v) const&
     requires(std::same_as<std::decay_t<U>, T>)
   {
     if (ok()) {
       return value();
     } else {
       return std::forward<U>(v);
     }
   }
 
   /// Retrieves the valid value from the result object, or returns a default
   /// value if no valid value exists.
   ///
   /// @param[in] v The default value to use if no valid value exists.
   /// @note This is the rvalue version which moves the value out.
   /// @note This function always returns by value.
   /// @return Either the valid value, or the given substitute.
   template <typename U>
   T value_or(U&& v) &&
     requires(std::same_as<std::decay_t<U>, T>)
   {
     if (ok()) {
       return std::move(*this).value();
     } else {
       return std::forward<U>(v);
     }
   }
 
   /// Transforms the value contained in this result.
   ///
   /// Applying a function `f` to a valid value `x` returns `f(x)`, while
   /// applying `f` to an invalid value returns another invalid value.
   ///
   /// @param[in] callable The transformation function to apply.
   /// @note This is the lvalue version.
   /// @note This functions is `fmap` on the functor in `A` of `Result<A, E>`.
   /// @return The modified valid value if exists, or an error otherwise.
   template <typename C>
   auto transform(C&& callable) const&
     requires std::invocable<C, const T&>
   {
     using CallableReturnType = decltype(std::declval<C>()(std::declval<T>()));
     using R = Result<std::decay_t<CallableReturnType>, E>;
     if (ok()) {
       return R::success(callable(value()));
     } else {
       return R::failure(error());
     }
   }
 
   /// Transforms the value contained in this result.
   ///
   /// Applying a function `f` to a valid value `x` returns `f(x)`, while
   /// applying `f` to an invalid value returns another invalid value.
   ///
   /// @param[in] callable The transformation function to apply.
   /// @note This is the rvalue version.
   /// @note This functions is `fmap` on the functor in `A` of `Result<A, E>`.
   /// @return The modified valid value if exists, or an error otherwise.
   template <typename C>
   auto transform(C&& callable) &&
     requires std::invocable<C, T&&>
   {
     using CallableReturnType = decltype(std::declval<C>()(std::declval<T>()));
     using R = Result<std::decay_t<CallableReturnType>, E>;
     if (ok()) {
       return R::success(callable(std::move(*this).value()));
     } else {
       return R::failure(std::move(*this).error());
     }
   }
 
   /// Bind a function to this result monadically.
   ///
   /// This function takes a function `f` and, if this result contains a valid
   /// value `x`, returns `f(x)`. If the type of `x` is `T`, then `f` is
   /// expected to accept type `T` and return `Result<U>`. In this case,
   /// `transform` would return the unhelpful type `Result<Result<U>>`, so
   /// `and_then` strips away the outer layer to return `Result<U>`. If the
   /// value is invalid, this returns an invalid value in `Result<U>`.
   ///
   /// @param[in] callable The transformation function to apply.
   /// @note This is the lvalue version.
   /// @note This functions is `>>=` on the functor in `A` of `Result<A, E>`.
   /// @return The modified valid value if exists, or an error otherwise.
   template <typename C>
   auto and_then(C&& callable) const&
     requires std::invocable<C, const T&>
   {
     using R = decltype(std::declval<C>()(std::declval<T>()));
 
     static_assert(std::same_as<typename R::ErrorType, ErrorType>,
                   "bind must take a callable with the same error type");
 
     if (ok()) {
       return callable(value());
     } else {
       return R::failure(error());
     }
   }
 
   /// Bind a function to this result monadically.
   ///
   /// This function takes a function `f` and, if this result contains a valid
   /// value `x`, returns `f(x)`. If the type of `x` is `T`, then `f` is
   /// expected to accept type `T` and return `Result<U>`. In this case,
   /// `transform` would return the unhelpful type `Result<Result<U>>`, so
   /// `and_then` strips away the outer layer to return `Result<U>`. If the
   /// value is invalid, this returns an invalid value in `Result<U>`.
   ///
   /// @param[in] callable The transformation function to apply.
   /// @note This is the rvalue version.
   /// @note This functions is `>>=` on the functor in `A` of `Result<A, E>`.
   /// @return The modified valid value if exists, or an error otherwise.
   template <typename C>
   auto and_then(C&& callable) &&
     requires std::invocable<C, T&&>
   {
     using R = decltype(std::declval<C>()(std::declval<T>()));
 
     static_assert(std::same_as<typename R::ErrorType, ErrorType>,
                   "bind must take a callable with the same error type");
 
     if (ok()) {
       return callable(std::move(*this).value());
     } else {
       return R::failure(std::move(*this).error());
     }
   }
 
  private:
   std::variant<T, E> m_var;
 
   void checkValueAccess() const {
     if (m_var.index() != 0) {
       if constexpr (std::is_same_v<E, std::error_code>) {
         std::stringstream ss;
         const auto& e = std::get<E>(m_var);
         ss << "Value called on error value: " << e.category().name() << ": "
            << e.message() << " [" << e.value() << "]";
         throw std::runtime_error(ss.str());
       } else {
         throw std::runtime_error("Value called on error value");
       }
     }
   }
 };
 
 /// Template specialization for the void case.
 /// This specialization handles the case where there is no actual return value,
 /// but
 /// an error might be returned. Returning the error directly would make handling
 /// different from other functions using the `Result<T, E>` mechanism.
 /// `Result<void, E>` does not have the dereference operator, and value methods.
 /// The static `success` factory does not accept a value.
 /// @note To ease usage, this `Result<void, E>` is default constructible in the
 /// *ok*
 /// state, whereas `Result<T, E>` is not.
 /// @tparam E The type of the error
 ///
 template <typename E>
 class Result<void, E> {
  public:
   /// Default constructor which initializes the result in the ok state.
   Result() = default;
 
   /// The copy constructor is deleted.
   Result(const Result<void, E>& other) = default;
 
   /// The (self) assignment operator is deleted.
   Result<void, E>& operator=(const Result<void, E>& other) = default;
 
   /// Move constructor
   /// @param other The other result object, rvalue ref
   Result(Result<void, E>&& other) noexcept : m_opt(std::move(other.m_opt)) {}
 
   /// Move assignment operator
   /// @param other The other result object, rvalue ref
   Result<void, E>& operator=(Result<void, E>&& other) noexcept {
     m_opt = std::move(other.m_opt);
     return *this;
   }
 
   /// Constructor from error. This implicitly requires E2 to be convertible to
   /// E.
   /// @tparam E2 The type of the actual error
   /// @param error The instance of the actual error
   template <typename E2>
   Result(E2 error) noexcept : m_opt(std::move(error)) {}
 
   /// Assignment operator from an error.
   /// @tparam E2 The type of the actual error
   /// @param error The instance of the actual error
   /// @return The assigned instance
   template <typename E2>
   Result<void, E>& operator=(E2 error) {
     m_opt = std::move(error);
     return *this;
   }
 
   /// Static factory function to initialize the result in the ok state.
   /// @return Result object, in ok state
   static Result<void, E> success() { return Result<void, E>(); }
 
   /// Static factory function to initialize the result in the error state.
   /// @param error The error to initialize with.
   /// @return Result object, in error state.
   static Result<void, E> failure(E error) {
     return Result<void, E>(std::move(error));
   }
 
   /// Checks whether this result is in the ok state, and no error.
   /// @return bool Whether result contains an error or not.
   bool ok() const noexcept { return !m_opt; }
 
   /// Returns a reference to the error stored in the result.
   /// @note If `res.ok()` this method will abort (noexcept)
   /// @return Reference to the error
   E& error() & noexcept { return m_opt.value(); }
 
   /// Returns a reference to the error stored in the result.
   /// @note If `res.ok()` this method will abort (noexcept)
   /// @return Reference to the error
   const E& error() const& noexcept { return m_opt.value(); }
 
   /// Returns the error by-value.
   /// @note If `res.ok()` this method will abort (noexcept)
   /// @return Reference to the error
   E error() && noexcept { return std::move(m_opt.value()); }
 
  private:
   std::optional<E> m_opt;
 };
 
 }  // namespace Acts

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