//! Provides an `IntervalBuffer` that can be used to buffer a stream and emit the values at a regular interval.
//!
//! This is useful for when you receive streaming data but want to parse it in bulk.
//!
//! ```rust,no_run
//! use futures::prelude::*;
//! use irc::client::prelude::*;
//! use tokio_interval_buffer::IntervalBuffer;
//!
//! #[tokio::main]
//! async fn main() {
//!     let mut client = Client::from_config(Config {
//!         nickname: Some(String::from("...")),
//!         server: Some(String::from("...")),
//!         channels: vec![String::from("...")],
//!         ..Default::default()
//!     })
//!     .await
//!     .expect("Could not create an irc client");
//!
//!     // Take the IRC stream and process all the messages every 10 seconds
//!     let mut buffered_receiver = IntervalBuffer::<_, _, failure::Error>::new(
//!         client.stream().unwrap().map_err(|e| e.into()),
//!         std::time::Duration::from_secs(10),
//!     );
//!
//!     while let Some(item) = buffered_receiver.next().await {
//!         println!("Buffer: {:?}", item);
//!     }
//! }
//! ```

use std::{
    pin::Pin,
    task::{Context, Poll},
    time::Duration,
};
use tokio::time::Interval;

/// This buffer takes a stream and an interval, and will emit a Vec<Stream::Item> every interval.
///
/// If no messages are send in that interval, the stream will not send anything.
/// If the initial stream ends, this stream ends as well.
///
/// If either the stream or the interval timer emits an error, this stream will emit the same error.
/// For the timer, `From<tokio::timer::Error>` has to be emitted for the error.
/// In the future I want to be able to configure a `.map_err` function for this.
pub struct IntervalBuffer<Stream, Item, Error, Container: Insertable<Item> = Vec<Item>>
where
    Stream: futures::Stream<Item = Result<Item, Error>>,
    Error: From<tokio::time::error::Error>,
{
    stream: Stream,
    timer: Interval,
    buffer: Container,
}

/// A generic component used for the container for the IntervalBuffer.
/// This can be implemented for any type, and is only implemented for Vec<T> by default.
///
/// For performance-specific implementations this should be replaced for whatever works for your situation.
pub trait Insertable<T>: Sized {
    /// Insert an item in the buffer
    fn insert(&mut self, t: T);

    /// Return the current content of the buffer, and clear itself.
    ///
    /// If the container is empty, you can return None.
    ///
    /// For Vec<T> this is implemented as `std::mem::replace(self, Vec::new())`
    fn return_content_and_clear(&mut self) -> Option<Self>;
}

impl<T> Insertable<T> for Vec<T> {
    fn insert(&mut self, t: T) {
        self.push(t);
    }

    fn return_content_and_clear(&mut self) -> Option<Vec<T>> {
        if self.is_empty() {
            None
        } else {
            // Make sure to preserve the capacity so we have a decent estimate of how big the buffer should be.
            // TODO: Maybe keep a history of the last 5 capacities and take the average of this? Currently this will stay the largest size the application has seen.
            let new_vec = Vec::with_capacity(self.capacity());
            Some(std::mem::replace(self, new_vec))
        }
    }
}

impl<Stream, Item, Error, Container> IntervalBuffer<Stream, Item, Error, Container>
where
    Stream: futures::Stream<Item = Result<Item, Error>>,
    Error: From<tokio::time::error::Error>,
    Container: Insertable<Item>,
{
    /// Create a new IntervalBuffer with a default container. This will simply call `new_with_container(.., .., Container::default())`. See that function for more informaiton.
    pub fn new(stream: Stream, interval: Duration) -> Self
    where
        Container: Default,
    {
        Self::new_with_container(stream, interval, Container::default())
    }

    /// Create a new IntervalBuffer with a given stream, interval and container.
    ///
    /// The first time this stream will be able to send data is after `interval: Duration`. This will not emit immediately.
    ///
    /// If either the stream or the internal timer emits an error, this stream will emit an error.
    ///
    /// If the stream ends (by returning `Ok(Poll::Ready(None))`), this stream will immediately return. The internal timer will not be polled.
    pub fn new_with_container(stream: Stream, interval: Duration, container: Container) -> Self {
        let timer = tokio::time::interval(interval);
        IntervalBuffer {
            stream,
            timer,
            buffer: container,
        }
    }

    /// Get a reference to the internal buffer
    pub fn buffer(&self) -> &Container {
        &self.buffer
    }

    /// Get a mutable reference to the internal buffer.
    ///
    /// This can be used to e.g. set the capacity for a Vec.
    pub fn buffer_mut(&mut self) -> &mut Container {
        &mut self.buffer
    }
}

impl<Stream, Item, Error, Container> futures::Stream
    for IntervalBuffer<Stream, Item, Error, Container>
where
    Stream: futures::Stream<Item = Result<Item, Error>>,
    Error: From<tokio::time::error::Error>,
    Container: Insertable<Item>,
{
    type Item = Result<Container, Error>;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        let inner = unsafe { Pin::get_unchecked_mut(self) };
        let mut stream = unsafe { Pin::new_unchecked(&mut inner.stream) };

        loop {
            match stream.as_mut().poll_next(cx) {
                Poll::Pending => break,
                Poll::Ready(Some(Ok(v))) => inner.buffer.insert(v),
                Poll::Ready(Some(Err(e))) => return Poll::Ready(Some(Err(e))),
                Poll::Ready(None) => return Poll::Ready(None),
            }
        }

        let mut timer = unsafe { Pin::new_unchecked(&mut inner.timer) };
        match timer.poll_tick(cx) {
            Poll::Pending => Poll::Pending,
            Poll::Ready(_) => {
                let result = inner.buffer.return_content_and_clear();
                if let Some(container) = result {
                    Poll::Ready(Some(Ok(container)))
                } else {
                    Poll::Pending
                }
            }
        }
    }
}