use std::cmp;
use std::pin::Pin;
use pin_project_lite::pin_project;
use crate::io::{self, BufRead, Read};
use crate::task::{Context, Poll};
pin_project! {
/// Reader adaptor which limits the bytes read from an underlying reader.
///
/// This struct is generally created by calling [`take`] on a reader.
/// Please see the documentation of [`take`] for more details.
///
/// [`take`]: trait.Read.html#method.take
#[derive(Debug)]
pub struct Take<T> {
#[pin]
pub(crate) inner: T,
pub(crate) limit: u64,
}
}
impl<T> Take<T> {
/// Returns the number of bytes that can be read before this instance will
/// return EOF.
///
/// # Note
///
/// This instance may reach `EOF` after reading fewer bytes than indicated by
/// this method if the underlying [`Read`] instance reaches EOF.
///
/// [`Read`]: trait.Read.html
///
/// # Examples
///
/// ```no_run
/// # fn main() -> async_std::io::Result<()> { async_std::task::block_on(async {
/// #
/// use async_std::prelude::*;
/// use async_std::fs::File;
///
/// let f = File::open("foo.txt").await?;
///
/// // read at most five bytes
/// let handle = f.take(5);
///
/// println!("limit: {}", handle.limit());
/// #
/// # Ok(()) }) }
/// ```
pub fn limit(&self) -> u64 {
self.limit
}
/// Sets the number of bytes that can be read before this instance will
/// return EOF. This is the same as constructing a new `Take` instance, so
/// the amount of bytes read and the previous limit value don't matter when
/// calling this method.
///
/// # Examples
///
/// ```no_run
/// # fn main() -> async_std::io::Result<()> { async_std::task::block_on(async {
/// #
/// use async_std::prelude::*;
/// use async_std::fs::File;
///
/// let f = File::open("foo.txt").await?;
///
/// // read at most five bytes
/// let mut handle = f.take(5);
/// handle.set_limit(10);
///
/// assert_eq!(handle.limit(), 10);
/// #
/// # Ok(()) }) }
/// ```
pub fn set_limit(&mut self, limit: u64) {
self.limit = limit;
}
/// Consumes the `Take`, returning the wrapped reader.
///
/// # Examples
///
/// ```no_run
/// # fn main() -> async_std::io::Result<()> { async_std::task::block_on(async {
/// #
/// use async_std::prelude::*;
/// use async_std::fs::File;
///
/// let file = File::open("foo.txt").await?;
///
/// let mut buffer = [0; 5];
/// let mut handle = file.take(5);
/// handle.read(&mut buffer).await?;
///
/// let file = handle.into_inner();
/// #
/// # Ok(()) }) }
/// ```
pub fn into_inner(self) -> T {
self.inner
}
/// Gets a reference to the underlying reader.
///
/// # Examples
///
/// ```no_run
/// # fn main() -> async_std::io::Result<()> { async_std::task::block_on(async {
/// #
/// use async_std::prelude::*;
/// use async_std::fs::File;
///
/// let file = File::open("foo.txt").await?;
///
/// let mut buffer = [0; 5];
/// let mut handle = file.take(5);
/// handle.read(&mut buffer).await?;
///
/// let file = handle.get_ref();
/// #
/// # Ok(()) }) }
/// ```
pub fn get_ref(&self) -> &T {
&self.inner
}
/// Gets a mutable reference to the underlying reader.
///
/// Care should be taken to avoid modifying the internal I/O state of the
/// underlying reader as doing so may corrupt the internal limit of this
/// `Take`.
///
/// # Examples
///
/// ```no_run
/// # fn main() -> async_std::io::Result<()> { async_std::task::block_on(async {
/// #
/// use async_std::prelude::*;
/// use async_std::fs::File;
///
/// let file = File::open("foo.txt").await?;
///
/// let mut buffer = [0; 5];
/// let mut handle = file.take(5);
/// handle.read(&mut buffer).await?;
///
/// let file = handle.get_mut();
/// #
/// # Ok(()) }) }
/// ```
pub fn get_mut(&mut self) -> &mut T {
&mut self.inner
}
}
impl<T: Read> Read for Take<T> {
/// Attempt to read from the `AsyncRead` into `buf`.
fn poll_read(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
buf: &mut [u8],
) -> Poll<io::Result<usize>> {
let this = self.project();
take_read_internal(this.inner, cx, buf, this.limit)
}
}
pub fn take_read_internal<R: Read + ?Sized>(
mut rd: Pin<&mut R>,
cx: &mut Context<'_>,
buf: &mut [u8],
limit: &mut u64,
) -> Poll<io::Result<usize>> {
// Don't call into inner reader at all at EOF because it may still block
if *limit == 0 {
return Poll::Ready(Ok(0));
}
let max = cmp::min(buf.len() as u64, *limit) as usize;
match futures_core::ready!(rd.as_mut().poll_read(cx, &mut buf[..max])) {
Ok(n) => {
*limit -= n as u64;
Poll::Ready(Ok(n))
}
Err(e) => Poll::Ready(Err(e)),
}
}
impl<T: BufRead> BufRead for Take<T> {
fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
let this = self.project();
if *this.limit == 0 {
return Poll::Ready(Ok(&[]));
}
match futures_core::ready!(this.inner.poll_fill_buf(cx)) {
Ok(buf) => {
let cap = cmp::min(buf.len() as u64, *this.limit) as usize;
Poll::Ready(Ok(&buf[..cap]))
}
Err(e) => Poll::Ready(Err(e)),
}
}
fn consume(self: Pin<&mut Self>, amt: usize) {
let this = self.project();
// Don't let callers reset the limit by passing an overlarge value
let amt = cmp::min(amt as u64, *this.limit) as usize;
*this.limit -= amt as u64;
this.inner.consume(amt);
}
}
#[cfg(test)]
mod tests {
use crate::io;
use crate::prelude::*;
#[test]
fn test_take_basics() -> std::io::Result<()> {
crate::task::block_on( async move {
let source: io::Cursor<Vec<u8>> = io::Cursor::new(vec![0, 1, 2, 3, 4, 5, 6, 7, 8]);
let mut buffer = [0u8; 5];
// read at most five bytes
let mut handle = source.take(5);
handle.read(&mut buffer).await?;
assert_eq!(buffer, [0, 1, 2, 3, 4]);
// check that the we are actually at the end
assert_eq!(handle.read(&mut buffer).await.unwrap(), 0);
Ok(())
})
}
}