From 121d1334a945a7c3f9400afc09fffbb542181229 Mon Sep 17 00:00:00 2001
From: Vegard Storheil Eriksen <zyp@jvnv.net>
Date: Wed, 15 Sep 2021 23:17:33 +0200
Subject: async: Add generic coroutine type.

---
 async/async.h | 107 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 107 insertions(+)
 create mode 100644 async/async.h

diff --git a/async/async.h b/async/async.h
new file mode 100644
index 0000000..80a66f4
--- /dev/null
+++ b/async/async.h
@@ -0,0 +1,107 @@
+#pragma once
+
+#include <coroutine>
+
+template <typename T>
+class promise_return {
+    private:
+        T val;
+
+    public:
+        void return_value(T v) {
+            val = v;
+        }
+
+        T value() {
+            return val;
+        }
+};
+
+template <>
+class promise_return<void> {
+    public:
+        void return_void() {}
+        void value() {}
+};
+
+/// Coroutine future for asynchronous function. Implements \term{awaitable}, \term{co_await}, \term{co_return}.
+///
+/// \tparam T Return type.
+///
+/// Manages coroutine state.
+/// Awaiting an empty object or destroying the coroutine state while it's being awaited results in undefined behavior.
+///
+template <typename T = void, typename initial_awaitable = std::suspend_always>
+class async {
+    public:
+        class promise_type : public promise_return<T> {
+            public:
+                using handle_type = std::coroutine_handle<promise_type>;
+
+                async get_return_object() {
+                    return {handle_type::from_promise(*this)};
+                }
+
+                std::coroutine_handle<> parent;
+
+                class final_awaitable {
+                    public:
+                        bool await_ready() { return false; }
+
+                        std::coroutine_handle<> await_suspend(handle_type h) {
+                            return h.promise().parent;
+                        }
+
+                        void await_resume() {}
+                };
+
+                initial_awaitable initial_suspend() { return {}; }
+                final_awaitable final_suspend() { return {}; }
+        };
+
+        bool await_ready() {
+            return handle.done();
+        }
+
+        auto await_suspend(std::coroutine_handle<> h) {
+            handle.promise().parent = h;
+
+            return handle;
+        }
+
+        T await_resume() {
+            return handle.promise().value();
+        }
+
+        /// Construct empty object.
+        async() : handle(nullptr) {}
+
+        /// Transfer ownership of coroutine state, leaving *other* empty.
+        async(async&& other) {
+            handle = other.handle;
+            other.handle = nullptr;
+        }
+
+        /// Transfer ownership of coroutine state, leaving *other* empty.
+        async& operator=(async&& other) {
+            handle = other.handle;
+            other.handle = nullptr;
+
+            return *this;
+        }
+
+        /// Destroy coroutine state if not empty.
+        ~async() {
+            if(handle) {
+                handle.destroy();
+            }
+        }
+
+    private:
+        promise_type::handle_type handle;
+
+        async(promise_type::handle_type h) : handle(h) {}
+
+        async(const async&) = delete;
+        async& operator=(const async&) = delete;
+};
-- 
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