// This source code is released into the public domain.
export module nihil.core:error;

// error: a type representing an error.
//
// An error consists of an immediate cause, which may be a string or
// std:error_code, and an optional proximate cause, which is another error
// object.  Any number of error objects may be stacked.
//
// For example, a failure to open a file might be a stack of two errors:
//
// - string, "failed to open /etc/somefile",
// - std::error_code, "No such file or directory".
//
// Calling .str() will format the entire stack starting at that error,
// for example: "failed to open /etc/somefile: No such file or directory".
//
// Errors may be moved and (relatively) cheaply copied, since the cause
// chain is refcounted.
//
// error derives from std::exception, so it may be thrown and caught and
// provides a useful what().  When throwing errors, creating a derived
// error will make it easier to distinguish errors when catching them.

import nihil.std;

import :match;

namespace nihil {

// Things which can be errors.
using error_t = std::variant<std::monostate, std::string, std::error_code, std::error_condition>;

// A proxy class used when constructing errors.  This has implicit constructors from various types,
// which means we don't have to handle every possible combination of types in error itself.
export struct error_proxy
{
	// Construct from...

	// ... a string_view
	error_proxy(std::string_view const what) // NOLINT
		: m_error(std::string(what))
	{
	}

	// ... an std::string; so we can move the string into place if it's an rvalue.
	error_proxy(auto &&what) // NOLINT
	requires(std::same_as<std::remove_cvref_t<decltype(what)>, std::string>)
		: m_error(std::forward<decltype(what)>(what))
	{
	}

	// ... a C string
	error_proxy(char const *what) // NOLINT
		: m_error(std::string(what))
	{
	}

	// ... an std::error_code
	error_proxy(std::error_code const what) // NOLINT
		: m_error(what)
	{
	}

	// ... an std::error_condition
	error_proxy(std::error_condition const what) // NOLINT
		: m_error(what)
	{
	}

	// ... an error code enum
	template <typename T>
	requires(std::is_error_code_enum<T>::value)
	error_proxy(T what) // NOLINT
		: m_error(make_error_code(what))
	{
	}

	// ... an error condition enum
	template <typename T>
	requires(std::is_error_condition_enum<T>::value)
	error_proxy(T what) // NOLINT
		: m_error(make_error_condition(what))
	{
	}

	// Not copyable.
	error_proxy(error_proxy const &) = delete;
	auto operator=(error_proxy const &) -> error_proxy & = delete;

	// Not movable.
	error_proxy(error_proxy &&) = delete;
	auto operator=(error_proxy &&) -> error_proxy & = delete;

	~error_proxy() = default;

	// Let error extract the error_t.
	[[nodiscard]] auto error() && -> error_t &&
	{
		return std::move(m_error);
	}

private:
	// The error.
	error_t m_error;
};

// The error class.
export struct error : std::exception
{
	// Create an empty error, representing success.
	error() = default;

	// Destroy an error.
	~error() override = default;

	// Create an error from an error proxy.
	explicit error(error_proxy &&proxy)
		: m_error(std::move(std::move(proxy).error()))
	{
	}

	// Create an error from an error proxy and an error cause.
	error(error_proxy &&proxy, error cause)
		: m_error(std::move(std::move(proxy).error()))
		, m_cause(std::make_shared<error>(std::move(cause)))
	{
	}

	// Create an error from an error proxy and an error_proxy cause.
	// For example, error("cannot open file", std::errc::permission_denied).
	error(error_proxy &&proxy, error_proxy &&cause)
		: m_error(std::move(std::move(proxy).error()))
		, m_cause(std::make_shared<error>(std::move(cause)))
	{
	}

	// Copyable.
	error(error const &) = default;
	auto operator=(error const &) -> error & = default;

	// Movable.
	error(error &&) noexcept = default;
	auto operator=(error &&) noexcept -> error & = default;

	// Return the cause of this error.
	[[nodiscard]] auto cause(this error const &self) -> std::shared_ptr<error> const &
	{
		return self.m_cause;
	}

	// Return the root cause of this error, which may be this object.
	// For errors caused by an OS error, this will typically be the
	// error_code error.
	[[nodiscard]] auto root_cause(this error const &self) -> error const &
	{
		auto const *cause = &self;
		while (cause->m_cause)
			cause = cause->m_cause.get();
		return *cause;
	}

	// Format this error without its cause as a string.
	[[nodiscard]] auto this_str(this error const &self) -> std::string
	{
		return self.m_error | match {
			[] (std::monostate) -> std::string {
				return "No error";
			},
			[] (std::error_code const &m) {
				return m.message();
			},
			[] (std::error_condition const &m) {
				return m.message();
			},
			[] (std::string const &m) {
				return m;
			}
		};
	}

	// Format this error and its cause as a string.
	[[nodiscard]] auto full_str(this error const &self) -> std::string
	{
		auto str = self.this_str();

		auto cause = self.cause();
		while (cause) {
			str += ": " + cause->this_str();
			cause = cause->cause();
		}

		return str;
	}

	// Return this error's error_code, if any.
	[[nodiscard]] auto code(this error const &self) -> std::optional<std::error_code>
	{
		auto const *code = std::get_if<std::error_code>(&self.m_error);
		if (code != nullptr)
			return {*code};
		return {};
	}

	// Return this error's error_condition, if any.
	[[nodiscard]] auto condition(this error const &self) -> std::optional<std::error_condition>
	{
		auto const *condition = std::get_if<std::error_condition>(&self.m_error);
		if (condition != nullptr)
			return {*condition};
		return {};
	}

	// Format this error and its cause as a C string and return it.  This is for
	// compatibility with std::exception.  The lifetime of the returned string
	// is the same as the error object.
	[[nodiscard]] auto what() const noexcept -> char const * final
	{
		if (!m_what)
			m_what = this->full_str();
		return m_what->c_str();
	}

	// Allow error to be implicitly converted to std::expected and std::unexpected, to make
	// using it with std::expected easier.

	template <typename T>
	operator std::expected<T, error> () && // NOLINT
	{
		return std::unexpected{std::move(*this)};
	}

	template<typename T>
	operator std::expected<T, error> () const // NOLINT
	{
		return std::unexpected{*this};
	}

	operator std::unexpected<error> () && // NOLINT
	{
		return std::unexpected{std::move(*this)};
	}

	operator std::unexpected<error> () const // NOLINT
	{
		return std::unexpected{*this};
	}

private:
	// This error.
	error_t m_error = make_error_code(std::errc());

	// The cause of this error, if any.
	std::shared_ptr<error> m_cause;

	// For std::exception::what(), we need to keep the string valid
	// until we're destroyed.
	mutable std::optional<std::string> m_what;

	// Equality comparison.
	[[nodiscard]] friend auto operator==(error const &lhs, error const &rhs) -> bool
	{
		return lhs.m_error == rhs.m_error;
	}

	[[nodiscard]] friend auto operator<=>(error const &lhs, error const &rhs) -> std::strong_ordering
	{
		return lhs.m_error <=> rhs.m_error;
	}

	// Compare an error with an std::error_code.
	[[nodiscard]] friend auto operator==(error const &lhs, std::error_code const &rhs) -> bool
	{
		return lhs.code() == rhs;
	}

	// Compare an error to an std::error_condition.
	[[nodiscard]] friend auto operator==(error const &lhs, std::error_condition const &rhs) -> bool
	{
		return lhs.condition() == rhs;
	}

	// Compare an error to an std::error_code enum.
	[[nodiscard]] friend auto operator==(error const &lhs, auto rhs) -> bool
	requires(std::is_error_code_enum<decltype(rhs)>::value)
	{
		return lhs.code() == rhs;
	}

	// Compare an error to an std::error_condition enum.
	[[nodiscard]] friend auto operator==(error const &lhs, auto rhs) -> bool
	requires(std::is_error_condition_enum<decltype(rhs)>::value)
	{
		return lhs.condition() == rhs;
	}

	// Print an error to a stream.
	friend auto operator<<(std::ostream &strm, error const &e) -> std::ostream &
	{
		return strm << e.full_str();
	}
};

} // namespace nihil

// Make error formattable.
export template <>
struct std::formatter<nihil::error, char>
{
	template <typename ParseContext>
	constexpr auto parse(ParseContext &ctx) -> ParseContext::iterator
	{
		return ctx.begin();
	}

	template <typename FormatContext>
	auto format(nihil::error const &e, FormatContext &ctx) const -> FormatContext::iterator
	{
		return std::ranges::copy(e.full_str(), ctx.out()).out;
	}
};