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use std::io::{IoSliceMut, Read as _};
use std::pin::Pin;
use std::{cmp, fmt};
use pin_project_lite::pin_project;
use crate::io::{self, BufRead, Read, Seek, SeekFrom, DEFAULT_BUF_SIZE};
use crate::task::{Context, Poll};
pin_project! {
/// Adds buffering to any reader.
///
/// It can be excessively inefficient to work directly with a [`Read`] instance. A `BufReader`
/// performs large, infrequent reads on the underlying [`Read`] and maintains an in-memory buffer
/// of the incoming byte stream.
///
/// `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.
///
/// This type is an async version of [`std::io::BufReader`].
///
/// [`Read`]: trait.Read.html
/// [`std::io::BufReader`]: https://doc.rust-lang.org/std/io/struct.BufReader.html
///
/// # Examples
///
/// ```no_run
/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
/// #
/// use async_std::fs::File;
/// use async_std::io::BufReader;
/// use async_std::prelude::*;
///
/// let mut file = BufReader::new(File::open("a.txt").await?);
///
/// let mut line = String::new();
/// file.read_line(&mut line).await?;
/// #
/// # Ok(()) }) }
/// ```
pub struct BufReader<R> {
#[pin]
inner: R,
buf: Box<[u8]>,
pos: usize,
cap: usize,
}
}
impl<R: io::Read> BufReader<R> {
/// Creates a buffered reader with default buffer capacity.
///
/// The default capacity is currently 8 KB, but may change in the future.
///
/// # Examples
///
/// ```no_run
/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
/// #
/// use async_std::fs::File;
/// use async_std::io::BufReader;
///
/// let f = BufReader::new(File::open("a.txt").await?);
/// #
/// # Ok(()) }) }
/// ```
pub fn new(inner: R) -> BufReader<R> {
BufReader::with_capacity(DEFAULT_BUF_SIZE, inner)
}
/// Creates a new buffered reader with the specified capacity.
///
/// # Examples
///
/// ```no_run
/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
/// #
/// use async_std::fs::File;
/// use async_std::io::BufReader;
///
/// let f = BufReader::with_capacity(1024, File::open("a.txt").await?);
/// #
/// # Ok(()) }) }
/// ```
pub fn with_capacity(capacity: usize, inner: R) -> BufReader<R> {
BufReader {
inner,
buf: vec![0; capacity].into_boxed_slice(),
pos: 0,
cap: 0,
}
}
}
impl<R> BufReader<R> {
/// Gets a reference to the underlying reader.
///
/// It is inadvisable to directly read from the underlying reader.
///
/// # Examples
///
/// ```no_run
/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
/// #
/// use async_std::fs::File;
/// use async_std::io::BufReader;
///
/// let f = BufReader::new(File::open("a.txt").await?);
/// let inner = f.get_ref();
/// #
/// # Ok(()) }) }
/// ```
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.
///
/// # Examples
///
/// ```no_run
/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
/// #
/// use async_std::fs::File;
/// use async_std::io::BufReader;
///
/// let mut file = BufReader::new(File::open("a.txt").await?);
/// let inner = file.get_mut();
/// #
/// # Ok(()) }) }
/// ```
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.
fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut R> {
self.project().inner
}
/// Returns a reference to the internal buffer.
///
/// This function will not attempt to fill the buffer if it is empty.
///
/// # Examples
///
/// ```no_run
/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
/// #
/// use async_std::fs::File;
/// use async_std::io::BufReader;
///
/// let f = BufReader::new(File::open("a.txt").await?);
/// let buffer = f.buffer();
/// #
/// # Ok(()) }) }
/// ```
pub fn buffer(&self) -> &[u8] {
&self.buf[self.pos..self.cap]
}
/// Unwraps the buffered reader, returning the underlying reader.
///
/// Note that any leftover data in the internal buffer is lost.
///
/// # Examples
///
/// ```no_run
/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
/// #
/// use async_std::fs::File;
/// use async_std::io::BufReader;
///
/// let f = BufReader::new(File::open("a.txt").await?);
/// let inner = f.into_inner();
/// #
/// # Ok(()) }) }
/// ```
pub fn into_inner(self) -> R {
self.inner
}
/// Invalidates all data in the internal buffer.
#[inline]
fn discard_buffer(self: Pin<&mut Self>) {
let this = self.project();
*this.pos = 0;
*this.cap = 0;
}
}
impl<R: Read> Read for BufReader<R> {
fn poll_read(
mut self: Pin<&mut Self>,
cx: &mut Context<'_>,
buf: &mut [u8],
) -> Poll<io::Result<usize>> {
// 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 = futures_core::ready!(self.as_mut().get_pin_mut().poll_read(cx, buf));
self.discard_buffer();
return Poll::Ready(res);
}
let mut rem = futures_core::ready!(self.as_mut().poll_fill_buf(cx))?;
let nread = rem.read(buf)?;
self.consume(nread);
Poll::Ready(Ok(nread))
}
fn poll_read_vectored(
mut self: Pin<&mut Self>,
cx: &mut Context<'_>,
bufs: &mut [IoSliceMut<'_>],
) -> Poll<io::Result<usize>> {
let total_len = bufs.iter().map(|b| b.len()).sum::<usize>();
if self.pos == self.cap && total_len >= self.buf.len() {
let res =
futures_core::ready!(self.as_mut().get_pin_mut().poll_read_vectored(cx, bufs));
self.discard_buffer();
return Poll::Ready(res);
}
let mut rem = futures_core::ready!(self.as_mut().poll_fill_buf(cx))?;
let nread = rem.read_vectored(bufs)?;
self.consume(nread);
Poll::Ready(Ok(nread))
}
}
impl<R: Read> BufRead for BufReader<R> {
fn poll_fill_buf<'a>(
self: Pin<&'a mut Self>,
cx: &mut Context<'_>,
) -> Poll<io::Result<&'a [u8]>> {
let mut this = 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 *this.pos >= *this.cap {
debug_assert!(*this.pos == *this.cap);
*this.cap = futures_core::ready!(this.inner.as_mut().poll_read(cx, this.buf))?;
*this.pos = 0;
}
Poll::Ready(Ok(&this.buf[*this.pos..*this.cap]))
}
fn consume(self: Pin<&mut Self>, amt: usize) {
let this = self.project();
*this.pos = cmp::min(*this.pos + amt, *this.cap);
}
}
impl<R: io::Read + fmt::Debug> fmt::Debug for BufReader<R> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("BufReader")
.field("reader", &self.inner)
.field(
"buffer",
&format_args!("{}/{}", self.cap - self.pos, self.buf.len()),
)
.finish()
}
}
impl<R: Seek> Seek for BufReader<R> {
/// Seeks to an offset, in bytes, in the underlying reader.
///
/// The position used for seeking with `SeekFrom::Current(_)` is the position the underlying
/// reader would be at if the `BufReader` had no internal buffer.
///
/// Seeking always discards the internal buffer, even if the seek position would otherwise fall
/// within it. This guarantees that calling `.into_inner()` immediately after a seek yields the
/// underlying reader at the same position.
///
/// See [`Seek`] for more details.
///
/// Note: In the edge case where you're seeking with `SeekFrom::Current(n)` where `n` minus the
/// internal buffer length overflows an `i64`, two seeks will be performed instead of one. If
/// the second seek returns `Err`, the underlying reader will be left at the same position it
/// would have if you called `seek` with `SeekFrom::Current(0)`.
///
/// [`Seek`]: trait.Seek.html
fn poll_seek(
mut self: Pin<&mut Self>,
cx: &mut Context<'_>,
pos: SeekFrom,
) -> Poll<io::Result<u64>> {
let result: u64;
if let SeekFrom::Current(n) = pos {
let remainder = (self.cap - self.pos) as i64;
// it should be safe to assume that remainder fits within an i64 as the alternative
// means we managed to allocate 8 exbibytes and that's absurd.
// But it's not out of the realm of possibility for some weird underlying reader to
// support seeking by i64::min_value() so we need to handle underflow when subtracting
// remainder.
if let Some(offset) = n.checked_sub(remainder) {
result = futures_core::ready!(
self.as_mut()
.get_pin_mut()
.poll_seek(cx, SeekFrom::Current(offset))
)?;
} else {
// seek backwards by our remainder, and then by the offset
futures_core::ready!(
self.as_mut()
.get_pin_mut()
.poll_seek(cx, SeekFrom::Current(-remainder))
)?;
self.as_mut().discard_buffer();
result = futures_core::ready!(
self.as_mut()
.get_pin_mut()
.poll_seek(cx, SeekFrom::Current(n))
)?;
}
} else {
// Seeking with Start/End doesn't care about our buffer length.
result = futures_core::ready!(self.as_mut().get_pin_mut().poll_seek(cx, pos))?;
}
self.discard_buffer();
Poll::Ready(Ok(result))
}
}