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

use futures_io::{AsyncBufRead, AsyncRead};

use std::fmt;

use std::io;

use std::pin::Pin;

/// Reader for the [`empty()`] function.

#[must_use = "readers do nothing unless polled"]

pub struct Empty {

    _priv: (),

}

/// Constructs a new handle to an empty reader.

///

/// All reads from the returned reader will return `Poll::Ready(Ok(0))`.

///

/// # Examples

///

/// A slightly sad example of not reading anything into a buffer:

///

/// ```

/// # futures::executor::block_on(async {

/// use futures::io::{self, AsyncReadExt};

///

/// let mut buffer = String::new();

/// let mut reader = io::empty();

/// reader.read_to_string(&mut buffer).await?;

/// assert!(buffer.is_empty());

/// # Ok::<(), Box<dyn std::error::Error>>(()) }).unwrap();

/// ```

pub fn empty() -> Empty {

    Empty { _priv: () }

}

impl AsyncRead for Empty {

    #[inline]

    fn poll_read(

        self: Pin<&mut Self>,

        _: &mut Context<'_>,

        _: &mut [u8],

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

        Poll::Ready(Ok(0))

    }

}

impl AsyncBufRead for Empty {

    #[inline]

    fn poll_fill_buf(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {

        Poll::Ready(Ok(&[]))

    }

    #[inline]

    fn consume(self: Pin<&mut Self>, _: usize) {}

}

impl fmt::Debug for Empty {

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

        f.pad("Empty { .. }")

    }

}