libexopt/include/util.hh

#pragma once

#include <memory>
#include <string_view>
#include <utility>
#include <array>

#include "exopt.h"

#include "optional.h"

#define _EO_CONSTANT_VALUE(X) ([]() { \
	struct { \
		typedef decltype(X) comptime_constant_t; \
		consteval operator comptime_constant_t() const noexcept { return (X); } \
	} inner; \
	return inner; \
})()

namespace exopt { namespace util [[gnu::visibility("internal")]] {
	template<typename T, typename U>
	concept comptime = std::convertible_to<T, U> and requires{
		typename T::comptime_constant_t;
	};

	template<typename T>
	concept is_complete = requires{ { sizeof(T) == sizeof(T) }; };

	constexpr auto comptime_value(auto value) noexcept {
		using U = decltype(value);
		struct inner {
			typedef U comptime_constant_t;
			consteval operator comptime_constant_t() const noexcept { return std::move(value); }
			U value;

			constexpr inner(std::add_rvalue_reference_t<U> m) noexcept : value(std::move(m)) {}
			constexpr ~inner() = default;
		};
		return inner{ std::move(value) };
	}

	static_assert(comptime_value(std::string_view{"hello"}.size()) == 5, "Bad comptime_value()");
	static_assert(_EO_CONSTANT_VALUE(std::string_view{"hello world"}) == std::string_view{"hello world"}, "Bad CONSTANT_VALUE()");


	template<typename T, size_t N, typename Array= std::array<T, N>, typename _I = std::make_index_sequence<N>>
	constexpr auto array_literal(const T (&a)[N]) noexcept 
		-> Array
	{
		constexpr decltype(auto) _array_create = []<size_t... I>(auto const& a, std::index_sequence<I...>) noexcept 
		{
			return Array { a[I]... };
		};
		return _array_create(std::move(a), _I{});
	}
	template<typename T, size_t N, typename Array= std::array<T, N>, typename _I = std::make_index_sequence<N>>
	constexpr auto array_literal(T (&&a)[N]) noexcept 
		-> Array
	{
		constexpr decltype(auto) _array_create = []<size_t... I>(auto&& a, std::index_sequence<I...>) noexcept 
		{
			return Array { std::move(a[I])... };
		};
		return _array_create(std::move(a), _I{});
	}

	struct [[gnu::visibility("internal")]]  CTInternalFatalError  {
		constexpr CTInternalFatalError() noexcept = default;
		constexpr CTInternalFatalError(CTInternalFatalError const&) noexcept = default;
		constexpr CTInternalFatalError(CTInternalFatalError &&) noexcept = default;
		constexpr CTInternalFatalError& operator=(CTInternalFatalError const&) noexcept = default;
		constexpr CTInternalFatalError& operator=(CTInternalFatalError&&) noexcept = default;
		constexpr virtual ~CTInternalFatalError() noexcept = default;

		constexpr operator std::string_view() const noexcept { return message(); }

		consteval virtual std::string_view message() const noexcept =0;
	};

	[[noreturn, gnu::noinline, gnu::cold]]
	void throw_runtime(std::string_view&&);	

	[[noreturn]]//, gnu::noinline, gnu::cold]]
	constexpr void throw_runtime(std::convertible_to<std::string_view> auto&& msg) {
		std::string_view view{msg};
		if consteval {
			using CTE = CTInternalFatalError; 
			struct CTIFRuntimeError : public CTE {
				using CTE::CTE;
				consteval CTIFRuntimeError(std::string_view&& view) noexcept
					: CTE(), m_message(std::move(view)) {}
				consteval std::string_view message() const noexcept override { return m_message; }
				constexpr virtual ~CTIFRuntimeError() noexcept {}
			private:
				std::string_view m_message;
			};
			throw CTIFRuntimeError(std::move(view));
		} else {
			throw_runtime(std::move(view));
		}
		__builtin_unreachable();
	}
	// Returns an anonymous union with inavtive field: `T value` and active field `assume init` (no unique address).
	template<typename T> requires(!std::is_reference_v<T>)
	constexpr auto uninit() noexcept {
		union UI {
			struct assume{};
			T value;
			[[no_unique_address]] assume init{};
		};
		return UI{};
	}
	template<typename T> requires(!std::is_reference_v<T>)
	struct alignas(T) maybe_uninit {
		using uninit = std::array<unsigned char, sizeof(T)>;

		constexpr maybe_uninit() noexcept {}


		constexpr maybe_uninit(const maybe_uninit& c) noexcept
			: m_uninit(c.m_uninit) {}
		constexpr maybe_uninit(maybe_uninit&& m) noexcept
			: m_uninit(std::move(m.m_uninit)) {}

		constexpr maybe_uninit& operator=(maybe_uninit const& c) noexcept
		{ m_uninit = c.m_uninit; return *this; }

		constexpr maybe_uninit& operator=(maybe_uninit&& c) noexcept
		{ m_uninit = std::move(c.m_uninit); return *this; }

		constexpr T& operator=(T&& value) noexcept {
			m_init = std::move(value); return *this;
		}
		constexpr T& operator=(T const& copy) noexcept {
			m_init = maybe_uninit{copy}; return *this; //XXX: These assignment operators may not be a good idea, we can't guarantee which active destriminant it is.
		}

		constexpr T& assume_init() & noexcept = delete;
		constexpr T&& assume_init() && noexcept { return std::move(m_init); }
		constexpr T const& assume_init() const& noexcept { return m_init; }
		constexpr T const&& assume_init() const&& noexcept { return m_init; }

		constexpr uninit& data() &noexcept { return m_uninit; }
		constexpr uninit&& data() &&noexcept { return std::move(m_uninit); }
		constexpr uninit const& data() const&noexcept { return m_uninit; }
		constexpr uninit const&& data() const&& noexcept { return std::move(m_uninit); }

		constexpr T* get() noexcept { return std::addressof(m_init); }
		constexpr const T* get() const noexcept { return std::addressof(m_init); }

		//TODO: accessors, `assume_init_*()`, etc.

		constexpr ~maybe_uninit() noexcept {}

		constexpr void assume_init_drop() noexcept(std::is_nothrow_destructible_v<T>)
		{ m_init.~T(); m_uninit = {}; }

		[[nodiscard]]
		constexpr T assume_init_take() noexcept(std::is_nothrow_move_constructible_v<T>)
		{
			auto v = std::move(m_init);
			assume_init_drop();
			return v;
		}

		constexpr maybe_uninit<T> init(T&& value) noexcept(std::is_nothrow_move_constructible_v<T>)
		{ return maybe_uninit<T>{std::move(value), EXPLICIT_INIT_TAG<EXPLICIT_INIT_MOVE>{}}; }

		template<typename... Args> requires(std::is_constructible_v<T, Args...>)
		constexpr maybe_uninit<T> init_in_place(Args&&... args) noexcept(std::is_nothrow_constructible_v<T, Args...>)
		{ return maybe_uninit<T>{EXPLICIT_INIT_TAG<EXPLICIT_INIT_IN_PLACE>{}, std::forward<Args>(args)...}; }

	private:
		enum EXPLICIT_INIT_TYPE {
			EXPLICIT_INIT_IN_PLACE,
			EXPLICIT_INIT_MOVE,
		};
		template<EXPLICIT_INIT_TYPE>
		struct EXPLICIT_INIT_TAG {};
		constexpr maybe_uninit(T&& value, EXPLICIT_INIT_TAG<EXPLICIT_INIT_MOVE>) noexcept(std::is_nothrow_move_constructible_v<T>)
			: m_init{std::move(value)} {}
		template<typename... Args> requires(std::is_constructible_v<T, Args...>)
		constexpr maybe_uninit(EXPLICIT_INIT_TAG<EXPLICIT_INIT_IN_PLACE> _a, Args&&... args) noexcept(std::is_nothrow_constructible_v<T, Args...>)
			: m_init{std::forward<Args>(args)...} { (void) _a; } 

		union {
			T m_init;
			[[no_unique_address]] alignas(T) uninit m_uninit{};
		};
	};

	namespace {
		constexpr std::string_view ui_test() noexcept {
			auto ui = uninit<std::string_view>();

			maybe_uninit<std::string_view> mi;

			ui.init.~assume();
			ui.value = std::string_view{"Hello"};
			return std::move(ui.value);
		}
		static_assert(ui_test()[0] == 'H', "Bad uninit<T>()");
	}
} }