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use super::{AsyncBufRead, AsyncRead};
use core::{
    pin::Pin,
    task::{Context, Poll},
};
use futures::ready;
use pin_project_lite::pin_project;

pin_project! {
    /// The `BufReader` struct adds buffering to any reader.
    ///
    /// It can be excessively inefficient to work directly with a [`AsyncRead`]
    /// instance. A `BufReader` performs large, infrequent reads on the underlying
    /// [`AsyncRead`] and maintains an in-memory buffer of the results.
    ///
    /// `BufReader` can improve the speed of programs that make *small* and
    /// *repeated* read calls to the same file or network socket. It does not
    /// help when reading very large amounts at once, or reading just one or a few
    /// times. It also provides no advantage when reading from a source that is
    /// already in memory, like a `Vec<u8>`.
    ///
    /// When the `BufReader` is dropped, the contents of its buffer will be
    /// discarded. Creating multiple instances of a `BufReader` on the same
    /// stream can cause data loss.
    pub struct BufReader<'buf, R> {
        #[pin]
        inner: R,
        buf: &'buf mut [u8],
        pos: usize,
        cap: usize,
    }
}

impl<'buf, R: AsyncRead> BufReader<'buf, R> {
    /// Creates a new `BufReader` with the specified buffer capacity.
    pub fn new(buf: &'buf mut [u8], inner: R) -> Self {
        Self {
            inner,
            buf,
            pos: 0,
            cap: 0,
        }
    }

    /// Gets a reference to the underlying reader.
    ///
    /// It is inadvisable to directly read from the underlying reader.
    pub fn get_ref(&self) -> &R {
        &self.inner
    }

    /// Gets a mutable reference to the underlying reader.
    ///
    /// It is inadvisable to directly read from the underlying reader.
    pub fn get_mut(&mut self) -> &mut R {
        &mut self.inner
    }

    /// Gets a pinned mutable reference to the underlying reader.
    ///
    /// It is inadvisable to directly read from the underlying reader.
    pub fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut R> {
        self.project().inner
    }

    /// Consumes this `BufReader`, returning the underlying reader.
    ///
    /// Note that any leftover data in the internal buffer is lost.
    pub fn into_inner(self) -> R {
        self.inner
    }

    /// Returns a reference to the internally buffered data.
    ///
    /// Unlike `fill_buf`, this will not attempt to fill the buffer if it is empty.
    pub fn buffer(&self) -> &[u8] {
        &self.buf[self.pos..self.cap]
    }

    /// Invalidates all data in the internal buffer.
    #[inline]
    fn discard_buffer(self: Pin<&mut Self>) {
        let me = self.project();
        *me.pos = 0;
        *me.cap = 0;
    }
}

impl<R: AsyncRead> AsyncRead for BufReader<'_, R> {
    type Error = R::Error;

    fn poll_read(
        mut self: Pin<&mut Self>,
        cx: &mut Context,
        buf: &mut [u8],
    ) -> Poll<Result<usize, R::Error>> {
        // If we don't have any buffered data and we're doing a massive read
        // (larger than our internal buffer), bypass our internal buffer
        // entirely.
        if self.pos == self.cap && buf.len() >= self.buf.len() {
            let res = ready!(self.as_mut().get_pin_mut().poll_read(cx, buf));
            self.discard_buffer();
            return Poll::Ready(res);
        }

        let rem = ready!(self.as_mut().poll_fill_buf(cx))?;
        let used = core::cmp::min(rem.len(), buf.len());
        buf[..used].copy_from_slice(&rem[..used]);
        self.consume(used);
        Poll::Ready(Ok(used))
    }
}

impl<R: AsyncRead> AsyncBufRead for BufReader<'_, R> {
    fn poll_fill_buf(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Result<&[u8], Self::Error>> {
        let me = self.project();

        // If we've reached the end of our internal buffer then we need to fetch
        // some more data from the underlying reader.
        // Branch using `>=` instead of the more correct `==`
        // to tell the compiler that the pos..cap slice is always valid.
        if *me.pos >= *me.cap {
            debug_assert!(*me.pos == *me.cap);

            ready!(me.inner.poll_read(cx, me.buf))?;
            *me.cap = me.buf.len();
            *me.pos = 0;
        }
        Poll::Ready(Ok(&me.buf[*me.pos..*me.cap]))
    }

    fn consume(self: Pin<&mut Self>, amt: usize) {
        let me = self.project();
        *me.pos = core::cmp::min(*me.pos + amt, *me.cap);
    }
}