From 034cd404a129103a8dd7747e6bd00ffd5550da93 Mon Sep 17 00:00:00 2001
From: Lexi Winter <lexi@le-fay.org>
Date: Mon, 30 Jun 2025 07:51:23 +0100
Subject: refactoring

---
 nihil.generator/generator_promise_base.ccm | 205 +++++++++++++++++++++++++++++
 1 file changed, 205 insertions(+)
 create mode 100644 nihil.generator/generator_promise_base.ccm

(limited to 'nihil.generator/generator_promise_base.ccm')

diff --git a/nihil.generator/generator_promise_base.ccm b/nihil.generator/generator_promise_base.ccm
new file mode 100644
index 0000000..fec9b1b
--- /dev/null
+++ b/nihil.generator/generator_promise_base.ccm
@@ -0,0 +1,205 @@
+///////////////////////////////////////////////////////////////////////////////
+// Reference implementation of std::generator proposal P2168.
+//
+// See https://wg21.link/P2168 for details.
+//
+///////////////////////////////////////////////////////////////////////////////
+// Copyright Lewis Baker, Corentin Jabot
+//
+// Use, modification and distribution is subject to the Boost Software License,
+// Version 1.0.
+// (See accompanying file LICENSE or http://www.boost.org/LICENSE_1_0.txt)
+///////////////////////////////////////////////////////////////////////////////
+
+module;
+
+#include <coroutine>
+#include <exception>
+#include <memory>
+
+export module nihil.generator:generator_promise_base;
+
+import :elements_of;
+import :forward;
+import :manual_lifetime;
+
+namespace nihil {
+
+template <typename Ref>
+struct generator_promise_base
+{
+	template <typename Ref2, typename Value, typename Alloc>
+	friend class generator;
+
+	generator_promise_base *m_root;
+	std::coroutine_handle<> m_parent_or_leaf;
+
+	// Note: Using manual_lifetime here to avoid extra calls to exception_ptr
+	// constructor/destructor in cases where it is not needed (i.e. where this
+	// generator coroutine is not used as a nested coroutine).
+	// This member is lazily constructed by the __yield_sequence_awaiter::await_suspend()
+	// method if this generator is used as a nested generator.
+	manual_lifetime<std::exception_ptr> m_exception;
+	manual_lifetime<Ref> m_value;
+
+	explicit generator_promise_base(std::coroutine_handle<> this_coro) noexcept
+		: m_root(this)
+		, m_parent_or_leaf(this_coro)
+	{
+	}
+
+	~generator_promise_base()
+	{
+		if (m_root != this) {
+			// This coroutine was used as a nested generator and so will
+			// have constructed its __exception_ member which needs to be
+			// destroyed here.
+			m_exception.destruct();
+		}
+	}
+
+	auto initial_suspend() noexcept -> std::suspend_always
+	{
+		return {};
+	}
+
+	auto return_void() noexcept -> void
+	{
+	}
+
+	auto unhandled_exception() -> void
+	{
+		if (m_root != this)
+			m_exception.get() = std::current_exception();
+		else
+			throw;
+	}
+
+	// Transfers control back to the parent of a nested coroutine
+	struct final_awaiter
+	{
+		auto await_ready() noexcept -> bool
+		{
+			return false;
+		}
+
+		template <typename Promise>
+		auto
+		await_suspend(std::coroutine_handle<Promise> h) noexcept -> std::coroutine_handle<>
+		{
+			Promise &promise = h.promise();
+			generator_promise_base &root = *promise.m_root;
+			if (&root != &promise) {
+				auto parent = promise.m_parent_or_leaf;
+				root.m_parent_or_leaf = parent;
+				return parent;
+			}
+			return std::noop_coroutine();
+		}
+
+		void await_resume() noexcept
+		{
+		}
+	};
+
+	auto final_suspend() noexcept -> final_awaiter
+	{
+		return {};
+	}
+
+	auto yield_value(Ref &&x) noexcept(std::is_nothrow_move_constructible_v<Ref>)
+		-> std::suspend_always
+	{
+		m_root->m_value.construct((Ref &&)x);
+		return {};
+	}
+
+	template <typename T>
+	requires(!std::is_reference_v<Ref>) && std::is_convertible_v<T, Ref>
+	auto
+	yield_value(T &&x) noexcept(std::is_nothrow_constructible_v<Ref, T>) -> std::suspend_always
+	{
+		m_root->m_value.construct((T &&)x);
+		return {};
+	}
+
+	template <typename Gen>
+	struct yield_sequence_awaiter
+	{
+		Gen m_gen;
+
+		yield_sequence_awaiter(Gen &&g) noexcept
+			// Taking ownership of the generator ensures frame are destroyed
+		        // in the reverse order of their execution.
+			: m_gen((Gen &&)g)
+		{
+		}
+
+		auto await_ready() noexcept -> bool
+		{
+			return false;
+		}
+
+		// set the parent, root and exceptions pointer and
+		// resume the nested
+		template <typename Promise>
+		auto
+		await_suspend(std::coroutine_handle<Promise> h) noexcept -> std::coroutine_handle<>
+		{
+			generator_promise_base &current = h.promise();
+			generator_promise_base &nested = *m_gen.get_promise();
+			generator_promise_base &root = *current.m_root;
+
+			nested.m_root = current.m_root;
+			nested.m_parent_or_leaf = h;
+
+			// Lazily construct the __exception_ member here now that we
+			// know it will be used as a nested generator. This will be
+			// destroyed by the promise destructor.
+			nested.m_exception.construct();
+			root.m_parent_or_leaf = m_gen.get_coro();
+
+			// Immediately resume the nested coroutine (nested generator)
+			return m_gen.get_coro();
+		}
+
+		void await_resume()
+		{
+			generator_promise_base &nested_promise = *m_gen.get_promise();
+
+			if (nested_promise.m_exception.get()) {
+				std::rethrow_exception(nested_promise.m_exception.get());
+			}
+		}
+	};
+
+	template <typename OValue, typename OAlloc>
+	auto yield_value(nihil::elements_of<generator<Ref, OValue, OAlloc>> g) noexcept
+		-> yield_sequence_awaiter<generator<Ref, OValue, OAlloc>>
+
+	{
+		return std::move(g).get();
+	}
+
+	template <std::ranges::range Rng, typename Allocator>
+	auto yield_value(nihil::elements_of<Rng, Allocator> &&x)
+		-> yield_sequence_awaiter<generator<Ref, std::remove_cvref_t<Ref>, Allocator>>
+
+	{
+		return [](std::allocator_arg_t, Allocator,
+		          auto &&rng) -> generator<Ref, std::remove_cvref_t<Ref>, Allocator> {
+			for (auto &&e : rng)
+				co_yield static_cast<decltype(e)>(e);
+		}(std::allocator_arg, x.get_allocator(), std::forward<Rng>(x.get()));
+	}
+
+	void resume()
+	{
+		m_parent_or_leaf.resume();
+	}
+
+	// Disable use of co_await within this coroutine.
+	void await_transform() = delete;
+};
+
+} // namespace nihil
-- 
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