From 6a8ae2de4261fecd0281685b9eea66bbd82bd7fb Mon Sep 17 00:00:00 2001
From: Bent Bisballe Nyeng <deva@aasimon.org>
Date: Tue, 1 Aug 2023 08:27:01 +0200
Subject: A4: WIP

---
 a4/generator.h | 86 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 86 insertions(+)
 create mode 100644 a4/generator.h

(limited to 'a4/generator.h')

diff --git a/a4/generator.h b/a4/generator.h
new file mode 100644
index 0000000..c857a40
--- /dev/null
+++ b/a4/generator.h
@@ -0,0 +1,86 @@
+// -*- c++ -*-
+#pragma once
+
+// The code in this file has been taken directly from:
+// https://en.cppreference.com/w/cpp/language/coroutines
+
+#include <coroutine>
+#include <cstdint>
+#include <exception>
+#include <iostream>
+
+template <typename T>
+struct Generator
+{
+	// The class name 'Generator' is our choice and it is not required for coroutine
+	// magic. Compiler recognizes coroutine by the presence of 'co_yield' keyword.
+	// You can use name 'MyGenerator' (or any other name) instead as long as you include
+	// nested struct promise_type with 'MyGenerator get_return_object()' method.
+
+	struct promise_type;
+	using handle_type = std::coroutine_handle<promise_type>;
+
+	struct promise_type // required
+	{
+		T value_;
+		std::exception_ptr exception_;
+
+		Generator get_return_object()
+		{
+			return Generator(handle_type::from_promise(*this));
+		}
+		std::suspend_always initial_suspend() { return {}; }
+		std::suspend_always final_suspend() noexcept { return {}; }
+		void unhandled_exception() { exception_ = std::current_exception(); } // saving
+		// exception
+
+		template <std::convertible_to<T> From> // C++20 concept
+		std::suspend_always yield_value(From&& from)
+		{
+			value_ = std::forward<From>(from); // caching the result in promise
+			return {};
+		}
+		void return_void() { }
+	};
+
+	handle_type h_;
+
+	Generator(handle_type h)
+		: h_(h)
+	{
+	}
+	~Generator() { h_.destroy(); }
+	explicit operator bool()
+	{
+		fill(); // The only way to reliably find out whether or not we finished coroutine,
+		        // whether or not there is going to be a next value generated (co_yield)
+		        // in coroutine via C++ getter (operator () below) is to execute/resume
+		        // coroutine until the next co_yield point (or let it fall off end).
+		        // Then we store/cache result in promise to allow getter (operator() below
+		        // to grab it without executing coroutine).
+		return !h_.done();
+	}
+	T operator()()
+	{
+		fill();
+		full_ = false; // we are going to move out previously cached
+		// result to make promise empty again
+		return std::move(h_.promise().value_);
+	}
+
+private:
+	bool full_ = false;
+
+	void fill()
+	{
+		if (!full_)
+		{
+			h_();
+			if (h_.promise().exception_)
+				std::rethrow_exception(h_.promise().exception_);
+			// propagate coroutine exception in called context
+
+			full_ = true;
+		}
+	}
+};
-- 
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