use std::{
    fmt::{Debug, Formatter},
    future::Future,
    mem::replace,
    pin::Pin,
};
#[cfg(feature = "hanging_detection")]
use std::{
    sync::Arc,
    task::{Poll, ready},
    time::Duration,
};

#[cfg(feature = "hanging_detection")]
use tokio::time::{Timeout, timeout};

pub struct Event {
    #[cfg(feature = "hanging_detection")]
    description: Arc<dyn Fn() -> String + Sync + Send>,
    event: event_listener::Event,
}

impl Event {
    /// See [`event_listener::Event::new`]. May attach a description that may optionally be read
    /// later.
    ///
    /// This confusingly takes a closure ([`FnOnce`]) that returns a nested closure ([`Fn`]).
    ///
    /// When `hanging_detection` is disabled, `description` is never called.
    ///
    /// When `hanging_detection` is enabled, the outer closure is called immediately. The outer
    /// closure can have an ephemeral lifetime. The inner closure must be `'static`, but is called
    /// only when the `description` is actually read.
    ///
    /// The outer closure allows avoiding extra lookups (e.g. task type info) that may be needed to
    /// capture information needed for constructing (moving into) the inner closure.
    #[inline(always)]
    pub fn new<InnerFn>(_description: impl FnOnce() -> InnerFn) -> Self
    where
        InnerFn: Fn() -> String + Sync + Send + 'static,
    {
        #[cfg(not(feature = "hanging_detection"))]
        return Self {
            event: event_listener::Event::new(),
        };
        #[cfg(feature = "hanging_detection")]
        return Self {
            description: Arc::new((_description)()),
            event: event_listener::Event::new(),
        };
    }

    /// See [`event_listener::Event::listen`].
    pub fn listen(&self) -> EventListener {
        #[cfg(not(feature = "hanging_detection"))]
        return EventListener {
            listener: self.event.listen(),
        };
        #[cfg(feature = "hanging_detection")]
        return EventListener {
            description: self.description.clone(),
            note: Arc::new(String::new),
            future: Some(Box::pin(timeout(
                Duration::from_secs(30),
                self.event.listen(),
            ))),
            duration: Duration::from_secs(30),
        };
    }

    /// See [`event_listener::Event::listen`]. May attach a note that may optionally be read later.
    ///
    /// This confusingly takes a closure ([`FnOnce`]) that returns a nested closure ([`Fn`]).
    ///
    /// When `hanging_detection` is disabled, `note` is never called.
    ///
    /// When `hanging_detection` is enabled, the outer closure is called immediately. The outer
    /// closure can have an ephemeral lifetime. The inner closer must be `'static`, but is called
    /// only when the `note` is actually read.
    ///
    /// The outer closure allow avoiding extra lookups (e.g. task type info) that may be needed to
    /// capture information needed for constructing (moving into) the inner closure.
    pub fn listen_with_note<InnerFn>(&self, _note: impl FnOnce() -> InnerFn) -> EventListener
    where
        InnerFn: Fn() -> String + Sync + Send + 'static,
    {
        #[cfg(not(feature = "hanging_detection"))]
        return EventListener {
            listener: self.event.listen(),
        };
        #[cfg(feature = "hanging_detection")]
        return EventListener {
            description: self.description.clone(),
            note: Arc::new((_note)()),
            future: Some(Box::pin(timeout(
                Duration::from_secs(30),
                self.event.listen(),
            ))),
            duration: Duration::from_secs(30),
        };
    }

    /// pulls out the event listener, leaving a new, empty event in its place.
    pub fn take(&mut self) -> Event {
        #[cfg(not(feature = "hanging_detection"))]
        return Self {
            event: replace(&mut self.event, event_listener::Event::new()),
        };
        #[cfg(feature = "hanging_detection")]
        return Self {
            description: self.description.clone(),
            event: replace(&mut self.event, event_listener::Event::new()),
        };
    }
}

impl Event {
    /// see [`event_listener::Event::notify`]
    pub fn notify(&self, n: usize) {
        self.event.notify(n);
    }
}

impl Debug for Event {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        let mut t = f.debug_tuple("Event");
        #[cfg(feature = "hanging_detection")]
        t.field(&(self.description)());
        t.finish()
    }
}

#[cfg(not(feature = "hanging_detection"))]
pub struct EventListener {
    listener: event_listener::EventListener,
}

#[cfg(not(feature = "hanging_detection"))]
impl Debug for EventListener {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        f.debug_tuple("EventListener").finish()
    }
}

#[cfg(not(feature = "hanging_detection"))]
impl Future for EventListener {
    type Output = ();

    fn poll(
        self: Pin<&mut Self>,
        cx: &mut std::task::Context<'_>,
    ) -> std::task::Poll<Self::Output> {
        let listener = unsafe { self.map_unchecked_mut(|s| &mut s.listener) };
        listener.poll(cx)
    }
}

#[cfg(feature = "hanging_detection")]
pub struct EventListener {
    description: Arc<dyn Fn() -> String + Sync + Send>,
    note: Arc<dyn Fn() -> String + Sync + Send>,
    // Timeout need to stay pinned while polling and also while it's dropped.
    // So it's important to put it into a pinned Box to be able to take it out of the Option.
    future: Option<std::pin::Pin<Box<Timeout<event_listener::EventListener>>>>,
    duration: Duration,
}

#[cfg(feature = "hanging_detection")]
impl Debug for EventListener {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        let mut t = f.debug_tuple("EventListener");
        t.field(&(self.description)());
        let note = (self.note)();
        if !note.is_empty() {
            t.field(&note);
        }
        t.finish()
    }
}

#[cfg(feature = "hanging_detection")]
impl Future for EventListener {
    type Output = ();

    fn poll(
        mut self: Pin<&mut Self>,
        cx: &mut std::task::Context<'_>,
    ) -> std::task::Poll<Self::Output> {
        while let Some(future) = self.future.as_mut() {
            match ready!(future.as_mut().poll(cx)) {
                Ok(_) => {
                    self.future = None;
                    return Poll::Ready(());
                }
                Err(_) => {
                    let note = (self.note)();
                    let description = (self.description)();
                    if note.is_empty() {
                        eprintln!(
                            "EventListener({}) is potentially hanging, waiting for {}s",
                            description,
                            self.duration.as_secs(),
                        );
                    } else {
                        eprintln!(
                            "EventListener({}) is potentially hanging, waiting for {}s from {}",
                            description,
                            self.duration.as_secs(),
                            note
                        );
                    }
                    self.duration *= 2;
                    // SAFETY: Taking from Option is safe because the value is inside of a pinned
                    // Box. Pinning must continue until dropped.
                    let future = self.future.take().unwrap();
                    self.future = Some(Box::pin(timeout(
                        self.duration,
                        // SAFETY: We can move the inner future since it's an EventListener and
                        // that is Unpin.
                        unsafe { std::pin::Pin::into_inner_unchecked(future) }.into_inner(),
                    )));
                }
            }
        }
        // EventListener was awaited again after completion
        Poll::Ready(())
    }
}

#[cfg(all(test, not(feature = "hanging_detection")))]
mod tests {
    use std::hint::black_box;

    use tokio::time::{Duration, timeout};

    use super::*;

    // The closures used for descriptions/notes should be eliminated. This may only happen at higher
    // optimization levels (that would be okay), but in practice it seems to work even for
    // opt-level=0.
    #[tokio::test]
    async fn ensure_dead_code_elimination() {
        fn dead_fn() {
            // This code triggers a build error when it's not removed.
            unsafe {
                unsafe extern "C" {
                    fn trigger_link_error() -> !;
                }
                trigger_link_error();
            }
        }

        let event = black_box(Event::new(|| {
            dead_fn();
            || {
                dead_fn();
                String::new()
            }
        }));
        let listener = black_box(event.listen_with_note(|| {
            dead_fn();
            || {
                dead_fn();
                String::new()
            }
        }));

        let _ = black_box(timeout(Duration::from_millis(10), listener)).await;
    }
}