From d80e0dbd20bb8597afe92f770e39d20440557d1f Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Sat, 25 Jun 2022 10:16:12 -0700
Subject: Demo for a coroutine-based step evaluator.

---
 aisa/coroutine.h | 120 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 120 insertions(+)
 create mode 100644 aisa/coroutine.h

(limited to 'aisa/coroutine.h')

diff --git a/aisa/coroutine.h b/aisa/coroutine.h
new file mode 100644
index 0000000..40a6982
--- /dev/null
+++ b/aisa/coroutine.h
@@ -0,0 +1,120 @@
+#pragma once
+
+#include <coroutine>
+
+namespace aisa {
+
+    inline auto suspend() noexcept { return std::suspend_always{}; }
+
+    template<typename result_t> struct promise;
+
+    template<typename result_t> struct task : public std::coroutine_handle<promise<result_t>> {
+        using handle = std::coroutine_handle<struct promise<result_t>>;
+        using promise_type = struct promise<result_t>;
+        bool await_ready() const noexcept { return handle::done(); }
+        result_t await_resume() const noexcept;
+        template<typename other_t> void await_suspend(std::coroutine_handle<struct promise<other_t>> h) const noexcept;
+        std::optional<result_t> operator()() noexcept;
+    };
+
+    template<> struct task<void> : public std::coroutine_handle<promise<void>> {
+        using handle = std::coroutine_handle<struct promise<void>>;
+        using promise_type = struct promise<void>;
+        bool await_ready() const noexcept { return handle::done(); }
+        void await_resume() const noexcept;
+        template<typename other_t> void await_suspend(std::coroutine_handle<struct promise<other_t>> h) const noexcept;
+        bool operator()() noexcept;
+    };
+
+    template<typename result_t> struct promise {
+        std::coroutine_handle<> precursor;
+        std::optional<result_t> result;
+        promise() = default;
+        promise(const promise &) = delete;
+        task<result_t> get_return_object() noexcept { return task<result_t>{std::coroutine_handle<promise<result_t>>::from_promise(*this)}; }
+        std::suspend_never initial_suspend() const noexcept { return {}; }
+        std::suspend_always final_suspend() const noexcept { return {}; }
+        void unhandled_exception() { }
+        void return_value(result_t x) noexcept { result = std::move(x); }
+    };
+
+    template<> struct promise<void> {
+        std::coroutine_handle<> precursor;
+        promise() = default;
+        promise(const promise &) = delete;
+        task<void> get_return_object() noexcept { return task<void>{std::coroutine_handle<promise<void>>::from_promise(*this)}; }
+        std::suspend_never initial_suspend() const noexcept { return {}; }
+        std::suspend_always final_suspend() const noexcept { return {}; }
+        void unhandled_exception() { }
+        void return_void() noexcept { }
+    };
+
+    template<typename result_t> result_t task<result_t>::await_resume() const noexcept
+    {
+        auto x = std::move(handle::promise().result.value());
+        handle::destroy();
+        return std::move(x);
+    }
+
+    template<typename result_t> template<typename other_t> void task<result_t>::await_suspend(std::coroutine_handle<struct promise<other_t>> h) const noexcept
+    {
+        h.promise().precursor = *this;
+    }
+
+    template<typename result_t> std::optional<result_t> task<result_t>::operator()() noexcept
+    {
+        if (!handle::operator bool())
+            return {};
+        if (!handle::done()) {
+            auto &precursor = handle::promise().precursor;
+            if (precursor) {
+                if (!precursor.done())
+                    precursor.resume();
+                if (precursor.done())
+                    precursor = nullptr;
+            }
+            if (!precursor)
+                handle::resume();
+        }
+        if (handle::done()) {
+            auto x = await_resume();
+            handle::operator=(nullptr);
+            return std::move(x);
+        }
+        return {};
+    }
+
+    inline void task<void>::await_resume() const noexcept
+    {
+        handle::destroy();
+    }
+
+    template<typename other_t> void task<void>::await_suspend(std::coroutine_handle<struct promise<other_t>> h) const noexcept
+    {
+        h.promise().precursor = *this;
+    }
+
+    inline bool task<void>::operator()() noexcept
+    {
+        if (!handle::operator bool())
+            return true;
+        if (!handle::done()) {
+            auto &precursor = handle::promise().precursor;
+            if (precursor) {
+                if (!precursor.done())
+                    precursor.resume();
+                if (precursor.done())
+                    precursor = nullptr;
+            }
+            if (!precursor)
+                handle::resume();
+        }
+        if (handle::done()) {
+            await_resume();
+            handle::operator=(nullptr);
+            return true;
+        }
+        return false;
+    }
+
+}
-- 
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