use bytes::BufMut;

use crate::io::util::poll_proceed_and_make_progress;
use crate::io::{AsyncRead, ReadBuf};

use std::io;
use std::pin::Pin;
use std::task::{ready, Context, Poll};

cfg_io_util! {
    /// An async reader which yields one byte over and over and over and over and
    /// over and...
    ///
    /// This struct is generally created by calling [`repeat`][repeat]. Please
    /// see the documentation of `repeat()` for more details.
    ///
    /// This is an asynchronous version of [`std::io::Repeat`][std].
    ///
    /// [repeat]: fn@repeat
    /// [std]: std::io::Repeat
    #[derive(Debug)]
    pub struct Repeat {
        byte: u8,
    }

    /// Creates an instance of an async reader that infinitely repeats one byte.
    ///
    /// All reads from this reader will succeed by filling the specified buffer with
    /// the given byte.
    ///
    /// This is an asynchronous version of [`std::io::repeat`][std].
    ///
    /// [std]: std::io::repeat
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::io::{self, AsyncReadExt};
    ///
    /// # #[tokio::main(flavor = "current_thread")]
    /// # async fn main() {
    /// let mut buffer = [0; 3];
    /// io::repeat(0b101).read_exact(&mut buffer).await.unwrap();
    /// assert_eq!(buffer, [0b101, 0b101, 0b101]);
    /// # }
    /// ```
    pub fn repeat(byte: u8) -> Repeat {
        Repeat { byte }
    }
}

impl AsyncRead for Repeat {
    #[inline]
    fn poll_read(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &mut ReadBuf<'_>,
    ) -> Poll<io::Result<()>> {
        ready!(crate::trace::trace_leaf());
        ready!(poll_proceed_and_make_progress(cx));
        buf.put_bytes(self.byte, buf.remaining());
        Poll::Ready(Ok(()))
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn assert_unpin() {
        crate::is_unpin::<Repeat>();
    }
}
