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use crate::ring_buffer::RingBuffer;
use async_stream::stream;
use futures::{
    task::{Context, Poll},
    Sink, Stream,
};
use std::pin::Pin;
use std::sync::Arc;

/// An error thrown by the [`AsyncPublisher`] as part of the `Sink` trait implementation.
///
#[derive(Debug, Clone)]
pub struct PublishError;

impl std::fmt::Display for PublishError {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(
            f,
            "async-publisher could not get an overwriteable envelope from the ring"
        )
    }
}

/// A handle to asynchronously publish to the event-bus.
///
/// Implements the [`Sink`] trait to asynchronously publish a stream of events to the event-bus.
///
/// # Example
///
/// Basic usage:
///
/// ```ignore
/// let eventbus = Eventador::new(4)?;
/// let mut publisher: AsyncPublisher<usize> = eventbus.async_publisher(10);
///
/// let mut i: usize = 1234;
/// publisher.send(i).await?;
/// ```
///
pub struct AsyncPublisher<T> {
    ring: Arc<RingBuffer>,
    buffer_size: usize,
    events: Vec<T>,
    sequence_stream: Pin<Box<dyn Stream<Item = u64> + Send + Sync>>,
}

impl<T: 'static + Send + Sync + Unpin> AsyncPublisher<T> {
    pub(crate) fn new(ring: Arc<RingBuffer>, buffer: usize) -> Self {
        let buffer = if buffer == 0 { buffer + 1 } else { buffer };

        let stream_ring = ring.clone();
        let stream = Box::pin(stream! {
            loop {
                yield stream_ring.async_next().await;
            }
        });

        Self {
            ring,
            buffer_size: buffer,
            events: Vec::with_capacity(buffer),
            sequence_stream: stream,
        }
    }
}

impl<T: 'static + Send + Sync + Unpin> Sink<T> for AsyncPublisher<T> {
    type Error = PublishError;

    fn poll_ready(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        if self.events.len() >= self.buffer_size {
            Poll::Pending
        } else {
            Poll::Ready(Ok(()))
        }
    }

    fn start_send(mut self: Pin<&mut Self>, event: T) -> Result<(), Self::Error> {
        self.events.push(event);

        Ok(())
    }

    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        while !self.events.is_empty() {
            match self.sequence_stream.as_mut().poll_next(cx) {
                Poll::Ready(Some(sequence)) => {
                    if let Some(event) = self.events.pop() {
                        let envelope = self
                            .ring
                            .get_envelope(sequence)
                            .expect("ring buffer was not pre-populated with empty event envelopes");

                        envelope.overwrite(sequence, event);
                    }
                }

                Poll::Ready(None) => return Poll::Ready(Err(PublishError)),

                Poll::Pending => return Poll::Pending,
            }
        }

        Poll::Ready(Ok(()))
    }

    fn poll_close(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        self.buffer_size = 0;
        self.poll_flush(cx)
    }
}

#[cfg(test)]
mod tests {
    // use crate::async_publisher::*;
}