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use super::AsyncRead;
use core::{
    pin::Pin,
    task::{Context, Poll},
};

pub trait AsyncBufRead: AsyncRead {
    /// Attempts to return the contents of the internal buffer, filling it with more data
    /// from the inner reader if it is empty.
    ///
    /// On success, returns `Poll::Ready(Ok(buf))`.
    ///
    /// If no data is available for reading, the method returns
    /// `Poll::Pending` and arranges for the current task (via
    /// `cx.waker().wake_by_ref()`) to receive a notification when the object becomes
    /// readable or is closed.
    ///
    /// This function is a lower-level call. It needs to be paired with the
    /// [`consume`] method to function properly. When calling this
    /// method, none of the contents will be "read" in the sense that later
    /// calling [`poll_read`] may return the same contents. As such, [`consume`] must
    /// be called with the number of bytes that are consumed from this buffer to
    /// ensure that the bytes are never returned twice.
    ///
    /// An empty buffer returned indicates that the stream has reached EOF.
    ///
    /// [`poll_read`]: AsyncRead::poll_read
    /// [`consume`]: AsyncBufRead::consume
    fn poll_fill_buf(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Result<&[u8], Self::Error>>;

    /// Tells this buffer that `amt` bytes have been consumed from the buffer,
    /// so they should no longer be returned in calls to [`poll_read`].
    ///
    /// This function is a lower-level call. It needs to be paired with the
    /// [`poll_fill_buf`] method to function properly. This function does
    /// not perform any I/O, it simply informs this object that some amount of
    /// its buffer, returned from [`poll_fill_buf`], has been consumed and should
    /// no longer be returned. As such, this function may do odd things if
    /// [`poll_fill_buf`] isn't called before calling it.
    ///
    /// The `amt` must be `<=` the number of bytes in the buffer returned by
    /// [`poll_fill_buf`].
    ///
    /// [`poll_read`]: AsyncRead::poll_read
    /// [`poll_fill_buf`]: AsyncBufRead::poll_fill_buf
    fn consume(self: Pin<&mut Self>, amt: usize);
}

impl AsyncBufRead for &[u8] {
    fn poll_fill_buf(
        self: Pin<&mut Self>,
        _cx: &mut Context<'_>,
    ) -> Poll<Result<&[u8], Self::Error>> {
        Poll::Ready(Ok(*self))
    }

    fn consume(mut self: Pin<&mut Self>, amt: usize) {
        *self = &self[amt..];
    }
}