//! Split a single value implementing `AsyncRead + AsyncWrite` into separate

//! `AsyncRead` and `AsyncWrite` handles.

//!

//! To restore this read/write object from its `split::ReadHalf` and

//! `split::WriteHalf` use `unsplit`.

use crate::io::{AsyncRead, AsyncWrite, ReadBuf};

use std::fmt;

use std::io;

use std::pin::Pin;

use std::sync::Arc;

use std::sync::Mutex;

use std::task::{Context, Poll};

cfg_io_util! {

    /// The readable half of a value returned from [`split`](split()).

    pub struct ReadHalf<T> {

        inner: Arc<Inner<T>>,

    }

    /// The writable half of a value returned from [`split`](split()).

    pub struct WriteHalf<T> {

        inner: Arc<Inner<T>>,

    }

    /// Splits a single value implementing `AsyncRead + AsyncWrite` into separate

    /// `AsyncRead` and `AsyncWrite` handles.

    ///

    /// To restore this read/write object from its `ReadHalf` and

    /// `WriteHalf` use [`unsplit`](ReadHalf::unsplit()).

    pub fn split<T>(stream: T) -> (ReadHalf<T>, WriteHalf<T>)

    where

        T: AsyncRead + AsyncWrite,

    {

        let is_write_vectored = stream.is_write_vectored();

        let inner = Arc::new(Inner {

            stream: Mutex::new(stream),

            is_write_vectored,

        });

        let rd = ReadHalf {

            inner: inner.clone(),

        };

        let wr = WriteHalf { inner };

        (rd, wr)

    }

}

struct Inner<T> {

    stream: Mutex<T>,

    is_write_vectored: bool,

}

impl<T> Inner<T> {

    fn with_lock<R>(&self, f: impl FnOnce(Pin<&mut T>) -> R) -> R {

        let mut guard = self.stream.lock().unwrap();

        // safety: we do not move the stream.

        let stream = unsafe { Pin::new_unchecked(&mut *guard) };

        f(stream)

    }

}

impl<T> ReadHalf<T> {

    /// Checks if this `ReadHalf` and some `WriteHalf` were split from the same

    /// stream.

    pub fn is_pair_of(&self, other: &WriteHalf<T>) -> bool {

        other.is_pair_of(self)

    }

    /// Reunites with a previously split `WriteHalf`.

    ///

    /// # Panics

    ///

    /// If this `ReadHalf` and the given `WriteHalf` do not originate from the

    /// same `split` operation this method will panic.

    /// This can be checked ahead of time by calling [`is_pair_of()`](Self::is_pair_of).

    #[track_caller]

    pub fn unsplit(self, wr: WriteHalf<T>) -> T

    where

        T: Unpin,

    {

        if self.is_pair_of(&wr) {

            drop(wr);

            let inner = Arc::try_unwrap(self.inner)

                .ok()

                .expect("`Arc::try_unwrap` failed");

            inner.stream.into_inner().unwrap()

        } else {

            panic!("Unrelated `split::Write` passed to `split::Read::unsplit`.")

        }

    }

}

impl<T> WriteHalf<T> {

    /// Checks if this `WriteHalf` and some `ReadHalf` were split from the same

    /// stream.

    pub fn is_pair_of(&self, other: &ReadHalf<T>) -> bool {

        Arc::ptr_eq(&self.inner, &other.inner)

    }

}

impl<T: AsyncRead> AsyncRead for ReadHalf<T> {

    fn poll_read(

        self: Pin<&mut Self>,

        cx: &mut Context<'_>,

        buf: &mut ReadBuf<'_>,

    ) -> Poll<io::Result<()>> {

        self.inner.with_lock(|stream| stream.poll_read(cx, buf))

    }

}

impl<T: AsyncWrite> AsyncWrite for WriteHalf<T> {

    fn poll_write(

        self: Pin<&mut Self>,

        cx: &mut Context<'_>,

        buf: &[u8],

    ) -> Poll<Result<usize, io::Error>> {

        self.inner.with_lock(|stream| stream.poll_write(cx, buf))

    }

    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {

        self.inner.with_lock(|stream| stream.poll_flush(cx))

    }

    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {

        self.inner.with_lock(|stream| stream.poll_shutdown(cx))

    }

    fn poll_write_vectored(

        self: Pin<&mut Self>,

        cx: &mut Context<'_>,

        bufs: &[io::IoSlice<'_>],

    ) -> Poll<Result<usize, io::Error>> {

        self.inner

            .with_lock(|stream| stream.poll_write_vectored(cx, bufs))

    }

    fn is_write_vectored(&self) -> bool {

        self.inner.is_write_vectored

    }

}

unsafe impl<T: Send> Send for ReadHalf<T> {}

unsafe impl<T: Send> Send for WriteHalf<T> {}

unsafe impl<T: Sync> Sync for ReadHalf<T> {}

unsafe impl<T: Sync> Sync for WriteHalf<T> {}

impl<T: fmt::Debug> fmt::Debug for ReadHalf<T> {

    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {

        fmt.debug_struct("split::ReadHalf").finish()

    }

}

impl<T: fmt::Debug> fmt::Debug for WriteHalf<T> {

    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {

        fmt.debug_struct("split::WriteHalf").finish()

    }

}