#![allow(unused)]

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

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

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)
}

#[derive(Default)]
pub struct Executor {
    tasks:    Slab<Task>,
    queue:    TaskQueue,
    to_spawn: SpawnQueue,
    wakers:   BTreeMap<TaskId, Waker>,
}

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

    pub fn run(&mut self) {
        {
            let mut global_spawner = SPAWN_QUEUE.write();
            if global_spawner.is_some() {
                panic!("Task executor is already running");
            }

            *global_spawner = Some(Arc::clone(&self.to_spawn));
        }

        loop {
            when! {
                let Some(id) = self
                    .to_spawn
                    .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 task from empty slot: {}", id.0);
                    };

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

                    match task.poll(&mut cx) {
                        Poll::Ready(()) => {
                            self.tasks.remove(id.0);
                            self.wakers.remove(&id);
                        }
                        Poll::Pending => (),
                    }
                },
                self.tasks.is_empty() => break,
                _ => (),
            }
        }

        *SPAWN_QUEUE.write() = None;
    }
}

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

impl Task {
    pub fn new(future: impl Future<Output = ()> + Send + 'static) -> Self {
        log::trace!("New task scheduled");
        Self {
            future: Box::pin(future),
        }
    }

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

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

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

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

impl Wake for TaskWaker {
    fn wake(self: Arc<Self>) {
        log::trace!("Woke Task-{:?}", self.id);
        self.wake_by_ref();
    }

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