libexopt/include/either.hh

#pragma once

namespace exopt::types {
	namespace either [[gnu::visibility("internal")]] {
		//TODO: A version with std::shared_ptr<T> instead
		template<typename T>//, typename P = T*>
		class Cow {
			using var_t =	std::variant<pointer_type  // Borrowed
					, value_type>;  // Owned
		public:
			//TODO: Use pointer traits of P to deduce `pointer_type`, so we can have P be a shared_ptr too. Or perhaps, a specialisation for `Cow<std::shared_ptr<T>>` would be better
			using value_type = std::remove_cvref_t<T>;

			constexpr static inline bool Writeable = std::is_copy_constructible_v<value_type>;
			constexpr static inline bool NTWriteable = std::is_nothrow_copy_constructible_v<value_type>;

			using reference_type = value_type &;
			using const_reference_type = value_type const&;
			using move_reference_type = value_type &&;
			using pointer_type = value_type *;
			using const_pointer_type = value_type const*;

			constexpr Cow(std::add_rvalue_reference_t<T> value) noexcept(std::is_nothrow_constructible_v<value_type, decltype(value)>)
				: m_value{std::move(value)} {}

			constexpr explicit(std::is_convertible_v<std::add_rvalue_reference<T>, pointer_type>) 
				Cow(pointer_type p) noexcept
				: m_value{p} {}
			constexpr explicit Cow(std::nullptr_t) noexcept
				: Cow(std::move(static_cast<pointer_type>(nullptr))) {}

			constexpr Cow(Cow const& copy) noexcept
				: m_value(copy.get()) {}
			constexpr Cow(Cow&& move) noexcept(std::is_nothrow_move_constructible_v<value_type>)
				: m_value(std::move(move.m_value)) { move.clear(); /* XXX: Is this needed? I think it is, to ensure that Cow<T> knows if it's been moved, even though std::variant already knows */ }

			/// Return a newly copy-constructed owned object `T` from the referred (or held) value.
			constexpr Cow clone() const noexcept(std::is_nothrow_copy_constructible_v<value_type>) requires(std::is_copy_constructible_v<T>)
			{
				if(const auto* pref = this->get())
					return { *pref };
				else return { nullptr };
			}
			//TODO: Operator overloads for access to `value_type`: operator*, operator->, etc.

			constexpr Cow(Cow&& move) noexcept(std::is_nothrow_move_constructible_v<value_type>) {
				if(this != std::addressof(move)) {
					if(__builtin_expect(move.is_empty(), false)) clear();
					else {
						m_value.emplace<value_type>(std::move(move).as_ref())
						move.clear();
					}
				} return *this;
			}
			constexpr Cow(Cow const& copy) noexcept(std::is_nothrow_destructible_v<value_type>) {
				if(this != std::addressof(copy)) {
					m_value.emplace<pointer_type>(copy.get());
				} return *this;
			}

			constexpr ~Cow() noexcept(std::is_nothrow_destructible_v<value_type>) = default;

			constexpr move_reference_type as_ref() && {
				return make_owned();
			}
			constexpr reference_type as_ref() & {
				return make_owned();
			}
			constexpr const_reference_type as_ref() const {
				return std::visit([](const auto& arg) -> const_reference_type {
					using U = std::remove_cvref_t<decltype(arg)>>;
					if constexpr(std::is_same_v<U, value_type)
						return std::forward<decltype(arg)>(arg);
					else 	return *arg;
				}, m_value);
			}
			//TODO: ^ UNLIKELY(is_empty()) null-checks for `*arg`.

			constexpr const_pointer_type get() const noexcept { 
				return std::visit([](const auto& arg) -> const_pointer_type {
					using U = std::remove_cvref_t<decltype(arg)>;
					if constexpr(std::is_same_v<U, value_type>)
						return std::addressof(arg);
					else 	return arg;
				}, m_value);
			}
			constexpr pointer_type get() noexcept { 
				return std::addressof(make_owned());
				/*
				return std::visit([](auto& arg) -> pointer_type {
					using U = std::remove_cvref_t<decltype(arg)>;
					if constexpr(std::is_same_v<U, value_type>)
						return std::addressof(arg);
					else 	return arg;
				}, m_value);*/
			}
			constexpr reference_type make_owned() & noexcept(NTWriteable) requires(Writeable) {
				if(const auto* ct_from = std::visit([&m_value](auto& arg) -> auto* {
					using U = std::remove_cvref_t<decltype(arg)>;

					if constexpr(std::is_same_v<U, pointer_type>) return arg;
					else return nullptr;
					//if constexpr(std::is_same_v<U, value_type>) return false;
					//else if(auto * refp = std::get_if<pointer_type>(m_value))

				}, m_value)) return m_value.emplace<value_type>(*ct_from); //XXX: Do we need to create a temporary here for creating invariants by reading the old invariant?
			}
			constexpr move_reference_type make_owned() && noexcept(NTWriteable) requires(Writeable) {
				if(const auto* ct_from = std::visit([&m_value](auto&& arg) -> auto* {
					using U = std::remove_cvref_t<decltype(arg)>;

					//TODO: I don't think this works the way I want it to... We'll need to emplace inside this function I think, since the moved temporary has been moved. It might not matter though.
					if constexpr(std::is_same_v<U, pointer_type>) return arg;
					else return nullptr;
					//if constexpr(std::is_same_v<U, value_type>) return false;
					//else if(auto * refp = std::get_if<pointer_type>(m_value))

				}, std::move(m_value))) return std::move(m_value.emplace<value_type>(*ct_from)); //XXX: Do we need to create a temporary here for creating invariants by reading the old invariant?
			}


			//These commented out visits modify the pointee of the borrowed invariant. THIS IS NOT WHAT WE WANT. Instead, we emplace a copy and then copy it again (possibly elided: &) or return it as an rvalue reference (elided: &&)
			constexpr value_type into_owned() & noexcept(NTWriteable) requires(Writeable) {
				return make_owned();
				/*return std::visit([](auto&& arg) -> move_reference_type {
					using U = std::remove_cvref_t<decltype(arg)>;
					if constexpr(std::is_same_v<U, value_type>)
						return std::move(arg);
					else 	return std::move(*arg);
				}, std::move(m_value));*/
			}
			constexpr move_reference_type into_owned() && noexcept(NTWriteable) requires(Writeable) {
				return make_owned();
				/*return std::move(std::visit([](auto&& arg) -> move_reference_type {
					using U = std::remove_cvref_t<decltype(arg)>;
					if constexpr(std::is_same_v<U, value_type>)
						return std::move(arg);
					else 	return std::move(*arg);
				}, std::move(m_value)));*/
			}
		protected:
			constexpr bool is_empty() const noexcept {
				if(const const_pointer_type* p_ref = std::get_if<pointer_type>(&m_value))
					if(!p_ref) return true;
				return false;
			}
			constexpr void clear() noexcept(std::is_nothrow_destructible_v<value_type>) {
				m_value.emplace<pointer_type>(nullptr);	
			} 	
		private:
			var_t m_value;
		};

		template<typename T>
		class Cow<std::shared_ptr<T>> {
			using var_t = std::variant<
					std::weak_ptr<value_type> // Borrowed
					, std::shared_ptr<value_type>>; // Owned
		public:
			using value_type = std::remove_cvref_t<T>;

			using reference_type = value_type &;
			using const_reference_type = value_type const&;
			using move_reference_type = value_type &&;
			using pointer_type = value_type;
			using const_pointer_type = value_type const*;
			//TODO: AutoCow: See above comment about RC'd lifetimes with shallow coppies.
			// How should we implement this... I think weak for borrowed and shared for owned is good, since we can still use shared_ptr's aliasing ctor to apply clone() (copy-construction) to the `value_type` directly while keeping the ref-count. 
			// XXX: Therfore, there should never be *any* strictly-aliasing variants both exposing the same `value_type`, despite *all* the variants managing the lifetime of the `value_type`. I think... Hnm... Can we get shared_ptr<T>'s aliasing constructor to *expose* a newly constructed `value_type` while managing the new one and the old one? XXX: Should we even manage the old one at all after a `clone()`? I actually don't think we should, since `clone()` should detach lifetime from the instance that it was called since it's a newly constructed (owned) object.
	
		private:
			var_t m_value;
		};
	}

	/// Manually lifetime managed Cow<T>
	using either::Cow;

	/// RC-lifetime managed Cow<T>
	template<typename T>
	using AutoCow = Cow<std::shared_ptr<T>>;
}