//! Compatibility between the `tokio::io` and `futures-io` versions of the
//! `AsyncRead` and `AsyncWrite` traits.
use pin_project_lite::pin_project;
use std::io;
use std::pin::Pin;
use std::task::{ready, Context, Poll};

pin_project! {
    /// A compatibility layer that allows conversion between the
    /// `tokio::io` and `futures-io` `AsyncRead` and `AsyncWrite` traits.
    #[derive(Copy, Clone, Debug)]
    pub struct Compat<T> {
        #[pin]
        inner: T,
        seek_pos: Option<io::SeekFrom>,
    }
}

/// Extension trait that allows converting a type implementing
/// `futures_io::AsyncRead` to implement `tokio::io::AsyncRead`.
pub trait FuturesAsyncReadCompatExt: futures_io::AsyncRead {
    /// Wraps `self` with a compatibility layer that implements
    /// `tokio_io::AsyncRead`.
    fn compat(self) -> Compat<Self>
    where
        Self: Sized,
    {
        Compat::new(self)
    }
}

impl<T: futures_io::AsyncRead> FuturesAsyncReadCompatExt for T {}

/// Extension trait that allows converting a type implementing
/// `futures_io::AsyncWrite` to implement `tokio::io::AsyncWrite`.
pub trait FuturesAsyncWriteCompatExt: futures_io::AsyncWrite {
    /// Wraps `self` with a compatibility layer that implements
    /// `tokio::io::AsyncWrite`.
    fn compat_write(self) -> Compat<Self>
    where
        Self: Sized,
    {
        Compat::new(self)
    }
}

impl<T: futures_io::AsyncWrite> FuturesAsyncWriteCompatExt for T {}

/// Extension trait that allows converting a type implementing
/// `tokio::io::AsyncRead` to implement `futures_io::AsyncRead`.
pub trait TokioAsyncReadCompatExt: tokio::io::AsyncRead {
    /// Wraps `self` with a compatibility layer that implements
    /// `futures_io::AsyncRead`.
    fn compat(self) -> Compat<Self>
    where
        Self: Sized,
    {
        Compat::new(self)
    }
}

impl<T: tokio::io::AsyncRead> TokioAsyncReadCompatExt for T {}

/// Extension trait that allows converting a type implementing
/// `tokio::io::AsyncWrite` to implement `futures_io::AsyncWrite`.
pub trait TokioAsyncWriteCompatExt: tokio::io::AsyncWrite {
    /// Wraps `self` with a compatibility layer that implements
    /// `futures_io::AsyncWrite`.
    fn compat_write(self) -> Compat<Self>
    where
        Self: Sized,
    {
        Compat::new(self)
    }
}

impl<T: tokio::io::AsyncWrite> TokioAsyncWriteCompatExt for T {}

// === impl Compat ===

impl<T> Compat<T> {
    fn new(inner: T) -> Self {
        Self {
            inner,
            seek_pos: None,
        }
    }

    /// Get a reference to the `Future`, `Stream`, `AsyncRead`, or `AsyncWrite` object
    /// contained within.
    pub fn get_ref(&self) -> &T {
        &self.inner
    }

    /// Get a mutable reference to the `Future`, `Stream`, `AsyncRead`, or `AsyncWrite` object
    /// contained within.
    pub fn get_mut(&mut self) -> &mut T {
        &mut self.inner
    }

    /// Returns the wrapped item.
    pub fn into_inner(self) -> T {
        self.inner
    }
}

impl<T> tokio::io::AsyncRead for Compat<T>
where
    T: futures_io::AsyncRead,
{
    fn poll_read(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &mut tokio::io::ReadBuf<'_>,
    ) -> Poll<io::Result<()>> {
        // We can't trust the inner type to not peak at the bytes,
        // so we must defensively initialize the buffer.
        let slice = buf.initialize_unfilled();
        let n = ready!(futures_io::AsyncRead::poll_read(
            self.project().inner,
            cx,
            slice
        ))?;
        buf.advance(n);
        Poll::Ready(Ok(()))
    }
}

impl<T> futures_io::AsyncRead for Compat<T>
where
    T: tokio::io::AsyncRead,
{
    fn poll_read(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        slice: &mut [u8],
    ) -> Poll<io::Result<usize>> {
        let mut buf = tokio::io::ReadBuf::new(slice);
        ready!(tokio::io::AsyncRead::poll_read(
            self.project().inner,
            cx,
            &mut buf
        ))?;
        Poll::Ready(Ok(buf.filled().len()))
    }
}

impl<T> tokio::io::AsyncBufRead for Compat<T>
where
    T: futures_io::AsyncBufRead,
{
    fn poll_fill_buf<'a>(
        self: Pin<&'a mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<io::Result<&'a [u8]>> {
        futures_io::AsyncBufRead::poll_fill_buf(self.project().inner, cx)
    }

    fn consume(self: Pin<&mut Self>, amt: usize) {
        futures_io::AsyncBufRead::consume(self.project().inner, amt)
    }
}

impl<T> futures_io::AsyncBufRead for Compat<T>
where
    T: tokio::io::AsyncBufRead,
{
    fn poll_fill_buf<'a>(
        self: Pin<&'a mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<io::Result<&'a [u8]>> {
        tokio::io::AsyncBufRead::poll_fill_buf(self.project().inner, cx)
    }

    fn consume(self: Pin<&mut Self>, amt: usize) {
        tokio::io::AsyncBufRead::consume(self.project().inner, amt)
    }
}

impl<T> tokio::io::AsyncWrite for Compat<T>
where
    T: futures_io::AsyncWrite,
{
    fn poll_write(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<io::Result<usize>> {
        futures_io::AsyncWrite::poll_write(self.project().inner, cx, buf)
    }

    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        futures_io::AsyncWrite::poll_flush(self.project().inner, cx)
    }

    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        futures_io::AsyncWrite::poll_close(self.project().inner, cx)
    }
}

impl<T> futures_io::AsyncWrite for Compat<T>
where
    T: tokio::io::AsyncWrite,
{
    fn poll_write(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<io::Result<usize>> {
        tokio::io::AsyncWrite::poll_write(self.project().inner, cx, buf)
    }

    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        tokio::io::AsyncWrite::poll_flush(self.project().inner, cx)
    }

    fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        tokio::io::AsyncWrite::poll_shutdown(self.project().inner, cx)
    }
}

impl<T: tokio::io::AsyncSeek> futures_io::AsyncSeek for Compat<T> {
    fn poll_seek(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        pos: io::SeekFrom,
    ) -> Poll<io::Result<u64>> {
        if self.seek_pos != Some(pos) {
            // Ensure previous seeks have finished before starting a new one
            ready!(self.as_mut().project().inner.poll_complete(cx))?;
            self.as_mut().project().inner.start_seek(pos)?;
            *self.as_mut().project().seek_pos = Some(pos);
        }
        let res = ready!(self.as_mut().project().inner.poll_complete(cx));
        *self.as_mut().project().seek_pos = None;
        Poll::Ready(res)
    }
}

impl<T: futures_io::AsyncSeek> tokio::io::AsyncSeek for Compat<T> {
    fn start_seek(mut self: Pin<&mut Self>, pos: io::SeekFrom) -> io::Result<()> {
        *self.as_mut().project().seek_pos = Some(pos);
        Ok(())
    }

    fn poll_complete(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
        let pos = match self.seek_pos {
            None => {
                // tokio 1.x AsyncSeek recommends calling poll_complete before start_seek.
                // We don't have to guarantee that the value returned by
                // poll_complete called without start_seek is correct,
                // so we'll return 0.
                return Poll::Ready(Ok(0));
            }
            Some(pos) => pos,
        };
        let res = ready!(self.as_mut().project().inner.poll_seek(cx, pos));
        *self.as_mut().project().seek_pos = None;
        Poll::Ready(res)
    }
}

#[cfg(unix)]
impl<T: std::os::unix::io::AsRawFd> std::os::unix::io::AsRawFd for Compat<T> {
    fn as_raw_fd(&self) -> std::os::unix::io::RawFd {
        self.inner.as_raw_fd()
    }
}

#[cfg(windows)]
impl<T: std::os::windows::io::AsRawHandle> std::os::windows::io::AsRawHandle for Compat<T> {
    fn as_raw_handle(&self) -> std::os::windows::io::RawHandle {
        self.inner.as_raw_handle()
    }
}
