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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);
}
}