From 799c535a95094297a38d922a0e9903b2e8a1a4bd Mon Sep 17 00:00:00 2001
From: Vegard Storheil Eriksen <zyp@jvnv.net>
Date: Wed, 15 Sep 2021 23:18:10 +0200
Subject: async: Add coroutine tasks and scheduler.

---
 async/scheduler.h | 163 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 163 insertions(+)
 create mode 100644 async/scheduler.h

(limited to 'async/scheduler.h')

diff --git a/async/scheduler.h b/async/scheduler.h
new file mode 100644
index 0000000..0196aa5
--- /dev/null
+++ b/async/scheduler.h
@@ -0,0 +1,163 @@
+#pragma once
+
+#include <coroutine>
+
+struct schedulable {
+    schedulable* next = nullptr;
+
+    std::coroutine_handle<> awaiter;
+
+    schedulable() {}
+    schedulable(std::coroutine_handle<> h) : awaiter(h) {}
+};
+
+struct scheduler_t {
+    schedulable* first = nullptr;
+    schedulable** last_next_p = &first;
+
+    void run() {
+        while(first) {
+            auto sch = pop_next();
+            sch.awaiter.resume();
+        }
+    }
+
+    schedulable& pop_next() {
+        critical_section lock;
+
+        auto sch = first;
+        first = first->next;
+
+        if(!first) {
+            last_next_p = &first;
+        }
+
+        return *sch;
+    }
+
+    void schedule(schedulable& sch) {
+        critical_section lock;
+
+        sch.next = nullptr;
+        *last_next_p = &sch;
+        last_next_p = &sch.next;
+    }
+};
+
+scheduler_t scheduler;
+
+struct yield : public schedulable {
+    bool await_ready() {
+        return false;
+    }
+
+    bool await_suspend(std::coroutine_handle<> h) {
+        awaiter = h;
+        scheduler.schedule(*this);
+        return true;
+    }
+
+    void await_resume() {}
+};
+
+template <typename T = void>
+struct future : public schedulable {
+    std::optional<T> value;
+
+    bool await_ready() {
+        return bool(value);
+    }
+
+    bool await_suspend(std::coroutine_handle<> h) {
+        if(value) {
+            return false;
+        } else {
+            awaiter = h;
+            return true;
+        }
+    }
+
+    T await_resume() {
+        return *value; // TODO: move?
+    }
+
+    bool done() {
+        return bool(value);
+    }
+
+    void set(T v) {
+        if(value) {
+            return;
+        }
+
+        value = v;
+
+        if(awaiter) {
+            scheduler.schedule(*this);
+        }
+    }
+
+    void reset() {
+        value.reset();
+        awaiter = nullptr;
+    }
+};
+
+template <>
+struct future<void> : public schedulable {
+    bool ready;
+
+    future() : ready(false) {}
+
+    bool await_ready() {
+        return ready;
+    }
+
+    bool await_suspend(std::coroutine_handle<> h) {
+        if(ready) {
+            return false;
+        } else {
+            awaiter = h;
+            return true;
+        }
+    }
+
+    void await_resume() {
+        
+    }
+
+    bool done() {
+        return ready;
+    }
+
+    void set() {
+        if(ready) {
+            return;
+        }
+
+        ready = true;
+
+        if(awaiter) {
+            scheduler.schedule(*this);
+        }
+    }
+
+    void reset() {
+        awaiter = nullptr;
+        ready = false;
+    }
+};
+
+struct task : public schedulable {
+    struct promise_type {
+        task get_return_object() {
+            auto handle = std::coroutine_handle<promise_type>::from_promise(*this);
+            return {handle};
+        }
+        std::suspend_always initial_suspend() { return {}; }
+        std::suspend_never final_suspend() { return {}; }
+        void return_void() {}
+    };
+
+    task(std::coroutine_handle<promise_type> h) : schedulable(h) {}
+};
-- 
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