coroutine.hh

/*

MIT License

Copyright (c) 2022 PCSX-Redux authors

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
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SOFTWARE.

*/

#pragma once

#include <coroutine>
#include <type_traits>

#include "common/syscalls/syscalls.h"

namespace psyqo {

/**
 * @brief A suitable type to hold and return a C++20 coroutine.
 *
 * @details C++20 introduced the concept of coroutines in the language. This
 * type can be used to properly hold a coroutine and yield and resume it
 * within psyqo. An important caveat of using coroutines is that the language
 * insist on calling `new` and `delete` silently within the coroutine object.
 * This may be a problem for users who don't want to use the heap.
 *
 * @tparam T The type the coroutine returns. `void` by default.
 */

template <typename T = void>
struct Coroutine {
    struct Empty {};
    typedef typename std::conditional<std::is_void<T>::value, Empty, T>::type SafeT;

    Coroutine() = default;
    Coroutine(Coroutine &&other) = default;
    Coroutine &operator=(Coroutine &&other) = default;
    Coroutine(Coroutine const &) = delete;
    Coroutine &operator=(Coroutine const &) = delete;

    /**
     * @brief The awaiter type.
     *
     * @details The awaiter type is the type that is used to suspend the coroutine
     * after scheduling an asychronous operation. The keyword `co_await` can be used
     * on an instance of the object to suspend the current coroutine. Creating an
     * instance of this object is done by calling `coroutine.awaiter()`.
     */
    struct Awaiter {
        Awaiter(Awaiter &&other) = default;
        Awaiter &operator=(Awaiter &&other) = default;
        Awaiter(Awaiter const &) = default;
        Awaiter &operator=(Awaiter const &) = default;
        constexpr bool await_ready() const noexcept {
            bool ret = m_coroutine->m_earlyResume;
            m_coroutine->m_earlyResume = false;
            return ret;
        }
        constexpr void await_suspend(std::coroutine_handle<> h) { m_coroutine->m_suspended = true; }
        constexpr void await_resume() const noexcept {}

      private:
        Awaiter(Coroutine *coroutine) : m_coroutine(coroutine) {}
        Coroutine *m_coroutine;
        friend struct Coroutine;
    };

    /**
     * @brief Creates an `Awaiter` object.
     *
     * @details This method is used to create an instance of the `Awaiter` object.
     * It's used to suspend the coroutine after scheduling an asynchronous operation.
     */
    Awaiter awaiter() { return Awaiter(this); }

    /**
     * @brief Resumes the coroutine.
     *
     * @details This method resumes the coroutine. It's used to resume the coroutine
     * after an asynchronous operation has completed. It is safe to call it from
     * within the coroutine itself, meaning it is safe to call it from a callback
     * which may execute in the same callstack as the coroutine. In this case, the
     * next `co_yield` on the `Awaiter` object will be a no-op.
     */
    void resume() {
        if (!m_handle) return;
        if (!m_suspended) {
            m_earlyResume = true;
            return;
        }
        m_suspended = false;
        m_handle.resume();
    }

    /**
     * @brief Returns the status of the coroutine.
     *
     * @details This method returns the status of the coroutine. It will return
     * `true` if the coroutine is done executing, `false` otherwise. The typical
     * usage of this method is to poll it from the scene loop. The first time it
     * returns `true`, the coroutine will be destroyed. The next times, it will
     * return `true` without doing anything, making the polling loop faster.
     */
    bool done() {
        if (!m_handle) return true;
        bool isDone = m_handle.done();
        if (isDone) {
            if constexpr (!std::is_void<T>::value) {
                m_value = eastl::move(m_handle.promise().m_value);
            }
            m_handle.destroy();
            m_handle = nullptr;
        }
        return isDone;
    }

    /**
     * @brief Returns the value returned by the coroutine.
     *
     * @details This method returns the value returned by the coroutine. It is
     * only valid to call it after the coroutine has finished executing. The
     * typical usage of this method is to call it after the `done` method
     * returns `true`. The coroutine sets its return value using the `co_return`
     * keyword. Since it is possible for the return type to be `void`, the
     * return type of this method is `T` if `T` is not `void`, and `Empty` if
     * `T` is `void`.
     */
    const SafeT &value() const { return m_value; }

  private:
    struct PromiseVoid {
        Coroutine<> get_return_object() {
            return Coroutine<>{eastl::move(std::coroutine_handle<Promise>::from_promise(*this))};
        }
        std::suspend_always initial_suspend() { return {}; }
        std::suspend_always final_suspend() noexcept { return {}; }
        void unhandled_exception() {}
        void return_void() {
            auto awaitingCoroutine = m_awaitingCoroutine;
            if (awaitingCoroutine) {
                // This doesn't feel right, but I don't know how to do it otherwise,
                // since the coroutine is a template and I can't forward the type.
                __builtin_coro_resume(awaitingCoroutine);
            }
        }
        [[no_unique_address]] Empty m_value;
        void *m_awaitingCoroutine = nullptr;
    };
    struct PromiseValue {
        Coroutine<T> get_return_object() {
            return Coroutine{eastl::move(std::coroutine_handle<Promise>::from_promise(*this))};
        }
        std::suspend_always initial_suspend() { return {}; }
        std::suspend_always final_suspend() noexcept { return {}; }
        void unhandled_exception() {}
        void return_value(T &&value) {
            m_value = eastl::move(value);
            auto awaitingCoroutine = m_awaitingCoroutine;
            if (awaitingCoroutine) {
                // This doesn't feel right, but I don't know how to do it otherwise,
                // since the coroutine is a template and I can't forward the type.
                __builtin_coro_resume(awaitingCoroutine);
            }
        }
        T m_value;
        void *m_awaitingCoroutine = nullptr;
    };
    typedef typename std::conditional<std::is_void<T>::value, PromiseVoid, PromiseValue>::type Promise;
    Coroutine(std::coroutine_handle<Promise> &&handle) : m_handle(eastl::move(handle)) {}
    std::coroutine_handle<Promise> m_handle;
    [[no_unique_address]] SafeT m_value;
    void *m_awaitingCoroutine = nullptr;
    bool m_suspended = true;
    bool m_earlyResume = false;

  public:
    using promise_type = Promise;

    constexpr bool await_ready() { return m_handle.done(); }
    template <typename U>
    constexpr void await_suspend(std::coroutine_handle<U> h) {
        auto &promise = m_handle.promise();
        promise.m_awaitingCoroutine = h.address();
        resume();
    }
    constexpr SafeT await_resume() {
        SafeT value = eastl::move(m_handle.promise().m_value);
        m_handle.destroy();
        return value;
    }
};

}  // namespace psyqo