/* A partial result based on std::variant and proposed std::error
(C) 2020-2025 Niall Douglas <http://www.nedproductions.biz/> (11 commits)
File Created: Jan 2020


Boost Software License - Version 1.0 - August 17th, 2003

Permission is hereby granted, free of charge, to any person or organization
obtaining a copy of the software and accompanying documentation covered by
this license (the "Software") to use, reproduce, display, distribute,
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Software, and to permit third-parties to whom the Software is furnished to
do so, all subject to the following:

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the above license grant, this restriction and the following disclaimer,
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SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
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DEALINGS IN THE SOFTWARE.
*/

#ifndef BOOST_OUTCOME_SYSTEM_ERROR2_RESULT_HPP
#define BOOST_OUTCOME_SYSTEM_ERROR2_RESULT_HPP

#include "error.hpp"

#if __cplusplus >= 201703L || _HAS_CXX17
#if __has_include(<variant>)

#include <exception>
#include <variant>

BOOST_OUTCOME_SYSTEM_ERROR2_NAMESPACE_BEGIN

template <class T> inline constexpr std::in_place_type_t<T> in_place_type{};

template <class T> class result;

//! \brief A trait for detecting result types
template <class T> struct is_result : public std::false_type
{
};
template <class T> struct is_result<result<T>> : public std::true_type
{
};

/*! \brief Exception type representing the failure to retrieve an error.
 */
class bad_result_access : public std::exception
{
public:
  bad_result_access() = default;
  //! Return an explanatory string
  virtual const char *what() const noexcept override { return "bad result access"; }  // NOLINT
};

namespace detail
{
  struct void_
  {
  };
  template <class T> using devoid = std::conditional_t<std::is_void_v<T>, void_, T>;
}  // namespace detail

/*! \class result
\brief A imperfect `result<T>` type with its error type hardcoded to `error`, only available on C++ 17 or later.

Note that the proper `result<T>` type does not have the possibility of
valueless by exception state. This implementation is therefore imperfect.
*/
template <class T> class result : protected std::variant<BOOST_OUTCOME_SYSTEM_ERROR2_NAMESPACE::error, detail::devoid<T>>
{
  using _base = std::variant<BOOST_OUTCOME_SYSTEM_ERROR2_NAMESPACE::error, detail::devoid<T>>;
  static_assert(!std::is_reference_v<T>, "Type cannot be a reference");
  static_assert(!std::is_array_v<T>, "Type cannot be an array");
  static_assert(!std::is_same_v<T, BOOST_OUTCOME_SYSTEM_ERROR2_NAMESPACE::error>, "Type cannot be a std::error");
  // not success nor failure types

  struct _implicit_converting_constructor_tag
  {
  };
  struct _explicit_converting_constructor_tag
  {
  };
  struct _implicit_constructor_tag
  {
  };
  struct _implicit_in_place_value_constructor_tag
  {
  };
  struct _implicit_in_place_error_constructor_tag
  {
  };

public:
  //! The value type
  using value_type = T;
  //! The error type
  using error_type = BOOST_OUTCOME_SYSTEM_ERROR2_NAMESPACE::error;
  //! The value type, if it is available, else a usefully named unusable internal type
  using value_type_if_enabled = detail::devoid<T>;
  //! Used to rebind result types
  template <class U> using rebind = result<U>;

protected:
  constexpr void _check() const
  {
    if(_base::index() == 0)
    {
      std::get_if<0>(this)->throw_exception();
    }
  }
  constexpr
#ifdef _MSC_VER
  __declspec(noreturn)
#elif defined(__GNUC__) || defined(__clang__)
        __attribute__((noreturn))
#endif
  void _ub()
  {
    assert(false);  // NOLINT
#if defined(__GNUC__) || defined(__clang__)
    __builtin_unreachable();
#elif defined(_MSC_VER)
    __assume(0);
#endif
  }

public:
  constexpr _base &_internal() noexcept { return *this; }
  constexpr const _base &_internal() const noexcept { return *this; }

  //! Default constructor is disabled
  result() = delete;
  //! Copy constructor
  result(const result &) = delete;
  //! Move constructor
  result(result &&) = default;
  //! Copy assignment
  result &operator=(const result &) = delete;
  //! Move assignment
  result &operator=(result &&) = default;
  //! Destructor
  ~result() = default;

  //! Implicit result converting move constructor
  BOOST_OUTCOME_SYSTEM_ERROR2_TEMPLATE(class U)
  BOOST_OUTCOME_SYSTEM_ERROR2_TREQUIRES(BOOST_OUTCOME_SYSTEM_ERROR2_TPRED(std::is_convertible_v<U, T>)) constexpr result(result<U> &&o, _implicit_converting_constructor_tag = {}) noexcept(std::is_nothrow_constructible_v<T, U>)
      : _base(std::move(o))
  {
  }
  //! Implicit result converting copy constructor
  BOOST_OUTCOME_SYSTEM_ERROR2_TEMPLATE(class U)
  BOOST_OUTCOME_SYSTEM_ERROR2_TREQUIRES(BOOST_OUTCOME_SYSTEM_ERROR2_TPRED(std::is_convertible_v<U, T>)) constexpr result(const result<U> &o, _implicit_converting_constructor_tag = {}) noexcept(std::is_nothrow_constructible_v<T, U>)
      : _base(o)
  {
  }
  //! Explicit result converting move constructor
  BOOST_OUTCOME_SYSTEM_ERROR2_TEMPLATE(class U)
  BOOST_OUTCOME_SYSTEM_ERROR2_TREQUIRES(BOOST_OUTCOME_SYSTEM_ERROR2_TPRED(std::is_constructible_v<T, U>)) constexpr explicit result(result<U> &&o, _explicit_converting_constructor_tag = {}) noexcept(std::is_nothrow_constructible_v<T, U>)
      : _base(std::move(o))
  {
  }
  //! Explicit result converting copy constructor
  BOOST_OUTCOME_SYSTEM_ERROR2_TEMPLATE(class U)
  BOOST_OUTCOME_SYSTEM_ERROR2_TREQUIRES(BOOST_OUTCOME_SYSTEM_ERROR2_TPRED(std::is_constructible_v<T, U>)) constexpr explicit result(const result<U> &o, _explicit_converting_constructor_tag = {}) noexcept(std::is_nothrow_constructible_v<T, U>)
      : _base(o)
  {
  }

  //! Anything which `std::variant<error, T>` will construct from, we shall implicitly construct from
  using _base::_base;

  //! Special case `in_place_type_t<void>`
  constexpr explicit result(std::in_place_type_t<void> /*unused*/) noexcept
      : _base(in_place_type<detail::void_>)
  {
  }

  //! Implicit in-place converting error constructor
  BOOST_OUTCOME_SYSTEM_ERROR2_TEMPLATE(class Arg1, class Arg2, class... Args, long = 5)                                                                                              //
  BOOST_OUTCOME_SYSTEM_ERROR2_TREQUIRES(BOOST_OUTCOME_SYSTEM_ERROR2_TPRED(!(std::is_constructible_v<value_type, Arg1, Arg2, Args...> && std::is_constructible_v<error_type, Arg1, Arg2, Args...>)  //
                                              &&std::is_constructible_v<error_type, Arg1, Arg2, Args...>))
  constexpr result(Arg1 &&arg1, Arg2 &&arg2, Args &&...args) noexcept(std::is_nothrow_constructible_v<error_type, Arg1, Arg2, Args...>)
      : _base(std::in_place_index<0>, std::forward<Arg1>(arg1), std::forward<Arg2>(arg2), std::forward<Args>(args)...)
  {
  }

  //! Implicit in-place converting value constructor
  BOOST_OUTCOME_SYSTEM_ERROR2_TEMPLATE(class Arg1, class Arg2, class... Args, int = 5)                                                                                               //
  BOOST_OUTCOME_SYSTEM_ERROR2_TREQUIRES(BOOST_OUTCOME_SYSTEM_ERROR2_TPRED(!(std::is_constructible_v<value_type, Arg1, Arg2, Args...> && std::is_constructible_v<error_type, Arg1, Arg2, Args...>)  //
                                              &&std::is_constructible_v<value_type, Arg1, Arg2, Args...>))
  constexpr result(Arg1 &&arg1, Arg2 &&arg2, Args &&...args) noexcept(std::is_nothrow_constructible_v<value_type, Arg1, Arg2, Args...>)
      : _base(std::in_place_index<1>, std::forward<Arg1>(arg1), std::forward<Arg2>(arg2), std::forward<Args>(args)...)
  {
  }

  //! Implicit construction from any type where an ADL discovered `make_status_code(T, Args ...)` returns a `status_code`.
  BOOST_OUTCOME_SYSTEM_ERROR2_TEMPLATE(class U, class... Args,                                                                            //
                         class MakeStatusCodeResult = typename detail::safe_get_make_status_code_result<U, Args...>::type)  // Safe ADL lookup of make_status_code(), returns void if not found
  BOOST_OUTCOME_SYSTEM_ERROR2_TREQUIRES(BOOST_OUTCOME_SYSTEM_ERROR2_TPRED(!std::is_same<typename std::decay<U>::type, result>::value                    // not copy/move of self
                                              && !std::is_same<typename std::decay<U>::type, value_type>::value             // not copy/move of value type
                                              && is_status_code<MakeStatusCodeResult>::value                                // ADL makes a status code
                                              && std::is_constructible<error_type, MakeStatusCodeResult>::value))           // ADLed status code is compatible
  constexpr result(U &&v, Args &&...args) noexcept(noexcept(make_status_code(std::declval<U>(), std::declval<Args>()...)))  // NOLINT
      : _base(std::in_place_index<0>, make_status_code(static_cast<U &&>(v), static_cast<Args &&>(args)...))
  {
  }

  //! Swap with another result
  constexpr void swap(result &o) noexcept(std::is_nothrow_swappable_v<_base>) { _base::swap(o); }

  //! Clone the result
  constexpr result clone() const { return has_value() ? result(value()) : result(error().clone()); }

  //! True if result has a value
  constexpr bool has_value() const noexcept { return _base::index() == 1; }
  //! True if result has a value
  explicit operator bool() const noexcept { return has_value(); }
  //! True if result has an error
  constexpr bool has_error() const noexcept { return _base::index() == 0; }

  //! Accesses the value if one exists, else calls `.error().throw_exception()`.
  constexpr value_type_if_enabled &value() &
  {
    _check();
    return std::get<1>(*this);
  }
  //! Accesses the value if one exists, else calls `.error().throw_exception()`.
  constexpr const value_type_if_enabled &value() const &
  {
    _check();
    return std::get<1>(*this);
  }
  //! Accesses the value if one exists, else calls `.error().throw_exception()`.
  constexpr value_type_if_enabled &&value() &&
  {
    _check();
    return std::get<1>(std::move(*this));
  }
  //! Accesses the value if one exists, else calls `.error().throw_exception()`.
  constexpr const value_type_if_enabled &&value() const &&
  {
    _check();
    return std::get<1>(std::move(*this));
  }

  //! Accesses the error if one exists, else throws `bad_result_access`.
  constexpr error_type &error() &
  {
    if(!has_error())
    {
#ifndef BOOST_NO_EXCEPTIONS
      throw bad_result_access();
#else
      abort();
#endif
    }
    return *std::get_if<0>(this);
  }
  //! Accesses the error if one exists, else throws `bad_result_access`.
  constexpr const error_type &error() const &
  {
    if(!has_error())
    {
#ifndef BOOST_NO_EXCEPTIONS
      throw bad_result_access();
#else
      abort();
#endif
    }
    return *std::get_if<0>(this);
  }
  //! Accesses the error if one exists, else throws `bad_result_access`.
  constexpr error_type &&error() &&
  {
    if(!has_error())
    {
#ifndef BOOST_NO_EXCEPTIONS
      throw bad_result_access();
#else
      abort();
#endif
    }
    return std::move(*std::get_if<0>(this));
  }
  //! Accesses the error if one exists, else throws `bad_result_access`.
  constexpr const error_type &&error() const &&
  {
    if(!has_error())
    {
#ifndef BOOST_NO_EXCEPTIONS
      throw bad_result_access();
#else
      abort();
#endif
    }
    return std::move(*std::get_if<0>(this));
  }

  //! Accesses the value, being UB if none exists
  constexpr value_type_if_enabled &assume_value() &noexcept
  {
    if(!has_value())
    {
      _ub();
    }
    return *std::get_if<1>(this);
  }
  //! Accesses the error, being UB if none exists
  constexpr const value_type_if_enabled &assume_value() const &noexcept
  {
    if(!has_value())
    {
      _ub();
    }
    return *std::get_if<1>(this);
  }
  //! Accesses the error, being UB if none exists
  constexpr value_type_if_enabled &&assume_value() &&noexcept
  {
    if(!has_value())
    {
      _ub();
    }
    return std::move(*std::get_if<1>(this));
  }
  //! Accesses the error, being UB if none exists
  constexpr const value_type_if_enabled &&assume_value() const &&noexcept
  {
    if(!has_value())
    {
      _ub();
    }
    return std::move(*std::get_if<1>(this));
  }

  //! Accesses the error, being UB if none exists
  constexpr error_type &assume_error() &noexcept
  {
    if(!has_error())
    {
      _ub();
    }
    return *std::get_if<0>(this);
  }
  //! Accesses the error, being UB if none exists
  constexpr const error_type &assume_error() const &noexcept
  {
    if(!has_error())
    {
      _ub();
    }
    return *std::get_if<0>(this);
  }
  //! Accesses the error, being UB if none exists
  constexpr error_type &&assume_error() &&noexcept
  {
    if(!has_error())
    {
      _ub();
    }
    return std::move(*std::get_if<0>(this));
  }
  //! Accesses the error, being UB if none exists
  constexpr const error_type &&assume_error() const &&noexcept
  {
    if(!has_error())
    {
      _ub();
    }
    return std::move(*std::get_if<0>(this));
  }
};

//! True if the two results compare equal.
template <class T, class U, typename = decltype(std::declval<T>() == std::declval<U>())> constexpr inline bool operator==(const result<T> &a, const result<U> &b) noexcept
{
  const auto &x = a._internal();
  return x == b;
}
//! True if the two results compare unequal.
template <class T, class U, typename = decltype(std::declval<T>() != std::declval<U>())> constexpr inline bool operator!=(const result<T> &a, const result<U> &b) noexcept
{
  const auto &x = a._internal();
  return x != b;
}

BOOST_OUTCOME_SYSTEM_ERROR2_NAMESPACE_END

#endif
#endif
#endif
