libexopt/include/boxed.h

#pragma once

#include <memory>
#include <utility>

#include "pointer.h"
#include "util.hh"

#include "exopt.h"

namespace exopt::types { namespace boxed {

	template<typename T>
	struct [[gnu::visibility("internal")]]  boxable_value_type { using type = std::conditional_t
			<std::is_reference_v<T>
			,std::reference_wrapper<std::remove_cvref_t<T> >
			,T>; };

	namespace { 

	template<typename T>
	constexpr std::unique_ptr<T> uniq_clone(std::unique_ptr<T> const& c) noexcept(std::is_nothrow_copy_constructible_v<T>) requires(std::is_copy_constructible_v<T>)
	{ if(__builtin_expect(bool(c), true)) return std::make_unique<T>(*c); return nullptr; }

	template<typename T>
	constexpr std::unique_ptr<T> uniq_clone_unsafe(std::unique_ptr<T> const& c)
#ifndef DEBUG
		noexcept(std::is_nothrow_copy_constructible_v<T>) 
#endif
		requires(std::is_copy_constructible_v<T>)
	{ if(__builtin_expect(!bool(c), false)) 
#ifdef DEBUG
		throw ptr::NullException{}; 
#else
		__builtin_unreachable(); 
#endif
	return std::make_unique<T>(*c); }


	template<typename Error = ptr::NullException>
	constexpr decltype(auto) force_ok(auto&& mv) requires(std::is_convertible_v<decltype(mv), bool> && std::is_default_constructible_v<Error>)
	{
		struct err {
			[[gnu::noinline, gnu::cold, noreturn]]
			static constexpr void _throw() { throw Error{}; }
		};
		if( __builtin_expect(!bool(mv), false)) err::_throw();
		return std::forward<decltype(mv)>(mv);
	}

	} // _impl (anon)
	

	template<typename From, typename To>
	concept binary_convertible = std::is_convertible_v<From, To> and std::is_convertible_v<To, From>;

	template<typename From, typename To>
	concept polymorphic_castable = std::derived_from<std::decay_t<To>, std::decay_t<From>> 
					or std::derived_from<std::decay_t<From>, std::decay_t<To>>
					or std::is_same_v<std::decay_t<From>, std::decay_t<To>>
					or requires(std::decay_t<From> && from) {
		{ static_cast<std::decay_t<To> &&>(from) } -> std::convertible_to<To>;
	};

	template<typename To, typename From> requires(polymorphic_castable<From, To>)
	constexpr std::unique_ptr<To> static_uniq_cast(std::unique_ptr<From>&& from) noexcept
	{
		return std::unique_ptr<To> { 
			static_cast<To*>( std::move(from).release() )
		};
	}

	template<typename To, typename From> requires(polymorphic_castable<From, To>)
	constexpr std::unique_ptr<To> dynamic_uniq_cast(std::unique_ptr<From>&& from) noexcept
	{
		return std::unique_ptr<To> { 
			dynamic_cast<To*>( std::move(from).release() )
		};
	}

	template<typename To, typename From, bool Check = false> requires(polymorphic_castable<From, To>)
	constexpr std::unique_ptr<To> uniq_cast(std::unique_ptr<From>&& from) noexcept
	{
		if constexpr(Check) return dynamic_uniq_cast(std::move(from));
		else return static_uniq_cast(std::move(from));
	}

#if 0
//TODO: Cannot be implemented with this method
	constexpr bool is_boxed_value(auto const& value) noexcept {
		return bool(dynamic_cast<ObjectBase const*>(std::addressof(value)));
	}
	template<typename T>
	constexpr bool is_boxed_type() noexcept {
		using type = std::remove_reference_t<T>;
		constexpr type const* VALUE = nullptr;
		return bool(dynamic_cast<ObjectBase const*>(VALUE));
	}

// We need either: Tagged pointers (XOR, bit-mangled, manual heap, or GOT (external allocation) kind), \
	or (preferred): layout of Box<T> to be `{ aligned<T>(T, box_type_tag) } unique*` \
		box_type_tag should be: One pointer width, starting at offset *T+1 (aligned to T within internal tuple struct), provides a way to access RTTI for `*T`, a pointer to static (non-allocated) type information or dynamic_cast used lambda would be ideal.
#endif

	template<typename T>
	struct Box final {
		typedef boxable_value_type<T>::type type;

		template<std::derived_from<T> U, bool RuntimeCheck=false>
		constexpr static Box<T> new_dynamic(U&& v) noexcept(!RuntimeCheck) {
			if constexpr(!RuntimeCheck) {
				auto boxed = std::make_unique<U>(std::move(v));
				return Box<T>{std::unique_ptr<T>{static_cast<T*>(boxed.release())}};
			} else
			return Box<T>{std::make_unique<T>(dynamic_cast<T&&>(std::move(v)))};
		}

		template<std::derived_from<T> U, bool Check = false>
		constexpr Box<U> upcast() && noexcept(!Check) 
		{ return Box<U> { force_ok(uniq_cast<U, T, Check>(std::move(m_ptr))) }; }

		template<typename U, bool Check = true> requires(std::derived_from<T, U>)
		constexpr Box<U> downcast() && noexcept(!Check)
		{ return Box<U> { force_ok(uniq_cast<U, T, Check>(std::move(m_ptr))) }; }

		template<polymorphic_castable<T> U, bool Check = !std::derived_from<U, T> > // Default dynamic check set to false only if we can statically determine that T is derived from U and the conversion is therfore infallible
		constexpr Box<U> polycast() && noexcept(!Check)
		{ return Box<U> { force_ok(uniq_cast<U, T, Check>(std::move(m_ptr))) }; }

		constexpr Box() noexcept(std::is_nothrow_default_constructible_v<T>) requires(std::is_default_constructible_v<T>)
			: m_ptr{std::make_unique<T>()} {}

		template<typename... Args> requires(std::is_constructible_v<T, Args...>)
		constexpr Box(Args&&... ctor) noexcept(std::is_nothrow_constructible_v<T, Args...>)
			: m_ptr{std::make_unique<T>(std::forward<Args>(ctor)...)} {}

		constexpr Box(T&& value) noexcept(std::is_nothrow_constructible_v<T>)
			: m_ptr{std::make_unique<T>(std::move(value))} {}

		constexpr Box(Box&&) noexcept = default;
		constexpr Box& operator=(Box&&) noexcept = default;
		constexpr Box(const Box& copy) noexcept(std::is_nothrow_copy_constructible_v<T>) requires(std::is_copy_constructible_v<T>)
			: m_ptr{uniq_clone_unsafe(copy.m_ptr)} {}
		constexpr Box& operator=(Box const& copy) noexcept(std::is_nothrow_copy_assignable_v<T>) requires(std::is_copy_assignable_v<T>)
		{
			if(__builtin_expect(this != std::addressof(copy), true)) {
				m_ptr = uniq_clone_unsafe(copy.m_ptr);
			} return *this;
		}

		[[gnu::nonnull(1)]]
		constexpr static Box<T> from_raw_ptr(T* ptr) noexcept {
			return Box<T>{ std::unique_ptr<T>{ ptr } };
		}
		constexpr static Box<T> from_raw_ptr(std::unique_ptr<T>&& uniq) 
#if DEBUG
			{ if(__builtin_expect(!uniq, false)) throw ptr::NullException{};
#else

			noexcept {
			_EO_ASSUME(uniq.get() != nullptr);
#endif
			return Box<T>{ std::move(uniq) };
		}
		[[gnu::returns_nonnull]]
		constexpr T* get() const noexcept { return m_ptr.get(); }
		[[gnu::returns_nonnull]]
		constexpr T* release() noexcept { return m_ptr.release(); }

		constexpr std::unique_ptr<T>&& into_unique() &&noexcept { return std::move(m_ptr); }

	//TODO: Accessors, `(explicit?) operator std::unique_ptr<T> const&[&]`?, etc.

		constexpr ~Box() = default;
	private:
		constexpr explicit Box(std::unique_ptr<T>&& ptr)
			: m_ptr(std::move(ptr)) {
			_EO_ASSUME(m_ptr.get());
		}

#if 0
// TODO: Identifying if a value `this` pointer is boxed
		struct alignas(type) inner_layout {
			constexpr inner_layout() noexcept = default;
			constexpr inner_layout(const inner_layout&) noexcept = default;
			constexpr ~inner_layout() noexcept = default;

			type value;
			box_type_tag identifier{box_tag_of<T>()};
		};
#endif
		std::unique_ptr<T> m_ptr; // Unique<T> m_ptr;
	};

#define _EO_ADD_RV std::add_rvalue_reference_v

	template<typename Base, typename T = Base>
	constexpr inline bool is_boxable_v = binary_convertible<_EO_ADD_RV<T>, _EO_ADD_RV<Base> > and requires(T&& value) { 
		typename Box<Base>;
		typename Box<Base>::type;
		{ std::make_unique<T>(std::move(value)) } -> std::same_as<std::unique_ptr<T>>;
	};
	template<typename T> concept is_boxable = is_boxable_v<T>;

	template<typename T>
	constexpr inline bool is_nothrow_boxable_v = is_boxable_v<T> && std::is_nothrow_constructible_v<Box<T>, T>;

#undef _EO_ADD_RV

	template<typename T, typename U> requires(requires(T&& o) { static_cast<U&&>(o); })
	constexpr Box<U> static_box_cast(Box<T>&& b) noexcept { return Box<U>::from_raw_ptr(static_cast<U*>(b.release())); }

	template<typename T, typename U> requires(requires(T&& o) { static_cast<U&&>(o); })
	constexpr Box<U> dynamic_box_cast(Box<T>&& b) {	
		auto* rel = b.release();
		try {
			return Box<U>::from_raw_ptr(std::addressof(dynamic_cast<U&&>(std::move(*rel))));
		} catch(...) {
			delete rel;
			throw;
		}
	} //TODO: Overload for `const&` that does the type check *before* the copy allocation.

#if 0
	template<typename T, typename R /*XXX: Maybe remove this, and put the requires after the definition, using `auto&& value` instead of `R&& value`*/> requires(std::derived_from<R, T>)
	constexpr Box<T> box(R&& value, util::comptime<bool> auto Check = _EO_CONSTANT_VALUE(false)) noexcept(std::is_nothrow_invocable_v< Box<T>::new_dynamic<R, util::comptime_eval_v<bool, Check>>) //XXX: This seems illegal...
	{ return Box<T>::template new_dynamic<R, Check>(std::move(value)); }

	template<typename T>
	constexpr Box<T> box(T&& value) noexcept(std::is_nothrow_constructible_v<Box<T>, T>)
	{ return { std::move(value) }; }
#endif
	template<typename T>
	constexpr Box<T> box(T&& value) noexcept(std::is_nothrow_constructible_v<Box<T>, decltype(value)>)
	{
		/*if constexpr(requires(decltype(value) v) { static_cast<T&&>(std::move(v)); }) {
			return Box<T>::template new_dynamic<T>(std::move(value));
		} else */return Box<T> { std::move(value) };
	}

	template<typename T>
	constexpr Box<T> box(std::unique_ptr<T>&& value) noexcept { return Box<T> { std::move(value) }; }

	template<typename Base, typename T> /*requires(polymorphic_castable<Base, T>)
	constexpr Box<Base> polybox(T&& value) noexcept
	{
		return Box<T>{ std::move(value) }.polycast<Base>()
	}*/ requires(std::derived_from<T, Base>)
	constexpr Box<Base> polybox(T&& value) noexcept {
		return Box<T>{ std::move(value) }.polycast<Base, false>(); // Convertion from derived to base infallible, no need for check.
	}

	template<typename Base, typename T> /*requires(polymorphic_castable<Base, T>)
	constexpr Box<Base> polybox(T&& value) noexcept
	{
		return Box<T>{ std::move(value) }.polycast<Base>()
	}*/ requires(std::derived_from<T, Base>)
	constexpr Box<Base> polybox(std::unique_ptr<T>&& value) noexcept {
		return Box<T>{ std::move(value) }.polycast<Base, false>(); // Convertion from derived to base infallible, no need for check.
	}
}
using boxed::Box;
}

namespace exopt::types { namespace optional [[gnu::visibility("internal")]] {
	template<typename T>
	struct null_optimise<boxed::Box<T> > { constexpr static inline bool value = true;
	//XXX: Eh.. Idk if Option<T>'s lifetime design can make this work...
		using held_type = boxed::Box<T>;
		using type = boxed::Box<T>&;
		constexpr static decltype(auto) convert_to_held(std::add_rvalue_reference_t<type> t) noexcept { return std::move(t); }
		constexpr static decltype(auto) convert_from_held(std::add_rvalue_reference_t<held_type> t) noexcept { return std::forward<type>(t); }

		constexpr static inline T* sentinel = nullptr;
	};


}