uasync 0.1.1

fast, safe, async executor
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use super::{scheduler::Scheduler, task::Runnable};
use std::{
    cell::{Cell, RefCell},
    ops::Deref,
    rc::Rc,
    sync::Arc,
};

pub(super) struct ThreadContext {
    pub(super) scheduler: Arc<Scheduler>,
    pub(super) queue_index: Option<usize>,
    pub(super) be_fair: Cell<bool>,
    pub(super) lifo_slot: Cell<Option<Arc<dyn Runnable>>>,
}

impl ThreadContext {
    fn try_with<F>(f: impl FnOnce(&mut Option<Rc<Self>>) -> F) -> Result<F, ()> {
        thread_local!(static TLS: RefCell<Option<Rc<ThreadContext>>> = RefCell::new(None));
        TLS.try_with(|ref_cell| f(&mut *ref_cell.borrow_mut()))
            .map_err(|_| ())
    }
}

pub(super) struct Thread {
    context: Rc<ThreadContext>,
}

impl Thread {
    pub(super) const CONTEXT_MISSING_ERROR: &'static str =
        "Thread not running in the context of a Runtime";
    pub(super) const CONTEXT_DESTROYED_ERROR: &'static str =
        "ThreadLocal runtime context was destroyed";

    pub(super) fn try_current() -> Result<Option<Self>, ()> {
        ThreadContext::try_with(|tls| {
            tls.as_ref().map(|context| Self {
                context: context.clone(),
            })
        })
    }

    pub(super) fn current() -> Self {
        match Self::try_current() {
            Ok(Some(thread)) => thread,
            Ok(None) => unreachable!(Self::CONTEXT_MISSING_ERROR),
            Err(_) => unreachable!(Self::CONTEXT_DESTROYED_ERROR),
        }
    }

    pub(super) fn enter(scheduler: &Arc<Scheduler>, queue_index: Option<usize>) -> Self {
        ThreadContext::try_with(|tls| {
            if let Some(context) = tls {
                return Self {
                    context: context.clone(),
                };
            }

            let context = Rc::new(ThreadContext {
                scheduler: scheduler.clone(),
                queue_index,
                be_fair: Cell::new(false),
                lifo_slot: Cell::new(None),
            });

            *tls = Some(context.clone());
            Self { context }
        })
        .unwrap()
    }
}

impl Deref for Thread {
    type Target = ThreadContext;

    fn deref(&self) -> &Self::Target {
        &*self.context
    }
}

impl Drop for Thread {
    fn drop(&mut self) {
        if Rc::strong_count(&self.context) == 2 {
            let context = ThreadContext::try_with(|tls| tls.take());
            let context = context.unwrap().unwrap();
            assert!(Rc::ptr_eq(&context, &self.context));
        }
    }
}