//! Async task and executor

use alloc::{boxed::Box, collections::BTreeMap, sync::Arc, task::Wake};
use core::{
    future::Future,
    pin::Pin,
    task::{Context, Poll, Waker},
};
use crossbeam_queue::SegQueue;
use slab::Slab;
use spin::RwLock;

type TaskQueue = Arc<SegQueue<TaskId>>;
type SpawnQueue = Arc<SegQueue<Task>>;

static SPAWN_QUEUE: RwLock<Option<SpawnQueue>> = RwLock::new(None);

/// Spawn a new task
pub fn spawn(future: impl Future<Output = ()> + Send + 'static) {
    match &*SPAWN_QUEUE.read() {
        Some(s) => s.push(Task::new(future)),
        None => panic!("no task executor is running"),
    }
}

/// Forcibly yield a task
pub fn yield_now() -> impl Future<Output = ()> {
    struct YieldNow(bool);
    impl Future for YieldNow {
        type Output = ();

        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            if self.0 {
                Poll::Ready(())
            } else {
                self.0 = true;
                cx.waker().wake_by_ref();
                Poll::Pending
            }
        }
    }

    YieldNow(false)
}

/// Tasks executor
#[derive(Default)]
pub struct Executor {
    /// All spawned tasks' stash
    tasks: Slab<Task>,

    /// Awake tasks' queue
    queue: TaskQueue,

    /// Incoming tasks to enqueue
    incoming: SpawnQueue,

    /// Wakers
    wakers: BTreeMap<TaskId, Waker>,
}

impl Executor {
    /// Spawn a task
    pub fn spawn(&mut self, future: impl Future<Output = ()> + Send + 'static) {
        self.queue
            .push(TaskId(self.tasks.insert(Task::new(future))));
    }

    /// Spin poll loop until it runs out of tasks
    pub fn run(&mut self) {
        // Assign `self.incoming` to global spawn queue to spawn tasks
        // from within
        {
            let mut spawner = SPAWN_QUEUE.write();
            if spawner.is_some() {
                panic!("task executor is already running");
            }

            *spawner = Some(Arc::clone(&self.incoming));
        }

        // Try to get incoming task, if none available, poll
        // enqueued one
        while let Some(id) = self
            .incoming
            .pop()
            .map(|t| TaskId(self.tasks.insert(t)))
            .or_else(|| self.queue.pop())
        {
            let Some(task) = self.tasks.get_mut(id.0) else {
                panic!("attempted to get non-extant task with id {}", id.0)
            };

            let mut cx = Context::from_waker(
                self.wakers
                    .entry(id)
                    .or_insert_with(|| TaskWaker::new(id, Arc::clone(&self.queue))),
            );

            match task.poll(&mut cx) {
                Poll::Ready(()) => {
                    // Task done, unregister
                    self.tasks.remove(id.0);
                    self.wakers.remove(&id);
                }
                Poll::Pending => (),
            }
        }

        *SPAWN_QUEUE.write() = None;
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct TaskId(usize);

/// Async task
struct Task {
    future: Pin<Box<dyn Future<Output = ()> + Send>>,
}

impl Task {
    /// Create a new task from a future
    fn new(future: impl Future<Output = ()> + Send + 'static) -> Self {
        Self {
            future: Box::pin(future),
        }
    }

    fn poll(&mut self, cx: &mut Context) -> Poll<()> {
        self.future.as_mut().poll(cx)
    }
}

struct TaskWaker {
    id: TaskId,
    queue: TaskQueue,
}

impl TaskWaker {
    fn new(id: TaskId, queue: TaskQueue) -> Waker {
        Waker::from(Arc::new(Self { id, queue }))
    }
}

impl Wake for TaskWaker {
    fn wake(self: Arc<Self>) {
        self.wake_by_ref();
    }

    fn wake_by_ref(self: &Arc<Self>) {
        self.queue.push(self.id);
    }
}