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

use bytes::{Buf, BufMut};
use std::io::{self, IoSlice};
use std::mem::MaybeUninit;
use std::pin::Pin;
use std::task::{ready, Context, Poll};

/// Try to read data from an `AsyncRead` into an implementer of the [`BufMut`] trait.
///
/// [`BufMut`]: bytes::Buf
///
/// # Example
///
/// ```
/// use bytes::{Bytes, BytesMut};
/// use tokio_stream as stream;
/// use tokio::io::Result;
/// use tokio_util::io::{StreamReader, poll_read_buf};
/// use std::future::poll_fn;
/// use std::pin::Pin;
/// # #[tokio::main]
/// # async fn main() -> std::io::Result<()> {
///
/// // Create a reader from an iterator. This particular reader will always be
/// // ready.
/// let mut read = StreamReader::new(stream::iter(vec![Result::Ok(Bytes::from_static(&[0, 1, 2, 3]))]));
///
/// let mut buf = BytesMut::new();
/// let mut reads = 0;
///
/// loop {
///     reads += 1;
///     let n = poll_fn(|cx| poll_read_buf(Pin::new(&mut read), cx, &mut buf)).await?;
///
///     if n == 0 {
///         break;
///     }
/// }
///
/// // one or more reads might be necessary.
/// assert!(reads >= 1);
/// assert_eq!(&buf[..], &[0, 1, 2, 3]);
/// # Ok(())
/// # }
/// ```
#[cfg_attr(not(feature = "io"), allow(unreachable_pub))]
pub fn poll_read_buf<T: AsyncRead + ?Sized, B: BufMut>(
    io: Pin<&mut T>,
    cx: &mut Context<'_>,
    buf: &mut B,
) -> Poll<io::Result<usize>> {
    if !buf.has_remaining_mut() {
        return Poll::Ready(Ok(0));
    }

    let n = {
        let dst = buf.chunk_mut();

        // Safety: `chunk_mut()` returns a `&mut UninitSlice`, and `UninitSlice` is a
        // transparent wrapper around `[MaybeUninit<u8>]`.
        let dst = unsafe { &mut *(dst as *mut _ as *mut [MaybeUninit<u8>]) };
        let mut buf = ReadBuf::uninit(dst);
        let ptr = buf.filled().as_ptr();
        ready!(io.poll_read(cx, &mut buf)?);

        // Ensure the pointer does not change from under us
        assert_eq!(ptr, buf.filled().as_ptr());
        buf.filled().len()
    };

    // Safety: This is guaranteed to be the number of initialized (and read)
    // bytes due to the invariants provided by `ReadBuf::filled`.
    unsafe {
        buf.advance_mut(n);
    }

    Poll::Ready(Ok(n))
}

/// Try to write data from an implementer of the [`Buf`] trait to an
/// [`AsyncWrite`], advancing the buffer's internal cursor.
///
/// This function will use [vectored writes] when the [`AsyncWrite`] supports
/// vectored writes.
///
/// # Examples
///
/// [`File`] implements [`AsyncWrite`] and [`Cursor<&[u8]>`] implements
/// [`Buf`]:
///
/// ```no_run
/// use tokio_util::io::poll_write_buf;
/// use tokio::io;
/// use tokio::fs::File;
///
/// use bytes::Buf;
/// use std::future::poll_fn;
/// use std::io::Cursor;
/// use std::pin::Pin;
///
/// #[tokio::main]
/// async fn main() -> io::Result<()> {
///     let mut file = File::create("foo.txt").await?;
///     let mut buf = Cursor::new(b"data to write");
///
///     // Loop until the entire contents of the buffer are written to
///     // the file.
///     while buf.has_remaining() {
///         poll_fn(|cx| poll_write_buf(Pin::new(&mut file), cx, &mut buf)).await?;
///     }
///
///     Ok(())
/// }
/// ```
///
/// [`Buf`]: bytes::Buf
/// [`AsyncWrite`]: tokio::io::AsyncWrite
/// [`File`]: tokio::fs::File
/// [vectored writes]: tokio::io::AsyncWrite::poll_write_vectored
#[cfg_attr(not(feature = "io"), allow(unreachable_pub))]
pub fn poll_write_buf<T: AsyncWrite + ?Sized, B: Buf>(
    io: Pin<&mut T>,
    cx: &mut Context<'_>,
    buf: &mut B,
) -> Poll<io::Result<usize>> {
    const MAX_BUFS: usize = 64;

    if !buf.has_remaining() {
        return Poll::Ready(Ok(0));
    }

    let n = if io.is_write_vectored() {
        let mut slices = [IoSlice::new(&[]); MAX_BUFS];
        let cnt = buf.chunks_vectored(&mut slices);
        ready!(io.poll_write_vectored(cx, &slices[..cnt]))?
    } else {
        ready!(io.poll_write(cx, buf.chunk()))?
    };

    buf.advance(n);

    Poll::Ready(Ok(n))
}