blob: 5e429e36b9d93d078a8d9e8c924a42447f11b2cf [file] [log] [blame]
use crate::stream::{StreamExt, Fuse};
use core::marker::Unpin;
use core::pin::Pin;
use futures_core::stream::{FusedStream, Stream};
use futures_core::task::{LocalWaker, Poll};
/// An adapter for merging the output of two streams.
///
/// The merged stream will attempt to pull items from both input streams. Each
/// stream will be polled in a round-robin fashion, and whenever a stream is
/// ready to yield an item that item is yielded.
///
/// After one of the two input stream completes, the remaining one will be
/// polled exclusively. The returned stream completes when both input
/// streams have completed.
#[derive(Debug)]
#[must_use = "streams do nothing unless polled"]
pub struct Select<St1, St2> {
stream1: Fuse<St1>,
stream2: Fuse<St2>,
flag: bool,
}
impl<St1: Unpin, St2: Unpin> Unpin for Select<St1, St2> {}
impl<St1, St2> Select<St1, St2>
where St1: Stream,
St2: Stream<Item = St1::Item>
{
pub(super) fn new(stream1: St1, stream2: St2) -> Select<St1, St2> {
Select {
stream1: stream1.fuse(),
stream2: stream2.fuse(),
flag: false,
}
}
}
impl<St1, St2> FusedStream for Select<St1, St2> {
fn is_terminated(&self) -> bool {
self.stream1.is_terminated() && self.stream2.is_terminated()
}
}
impl<St1, St2> Stream for Select<St1, St2>
where St1: Stream,
St2: Stream<Item = St1::Item>
{
type Item = St1::Item;
fn poll_next(
self: Pin<&mut Self>,
lw: &LocalWaker
) -> Poll<Option<St1::Item>> {
let Select { flag, stream1, stream2 } =
unsafe { Pin::get_mut_unchecked(self) };
let stream1 = unsafe { Pin::new_unchecked(stream1) };
let stream2 = unsafe { Pin::new_unchecked(stream2) };
if *flag {
poll_inner(flag, stream1, stream2, lw)
} else {
poll_inner(flag, stream2, stream1, lw)
}
}
}
fn poll_inner<St1, St2>(
flag: &mut bool,
a: Pin<&mut St1>,
b: Pin<&mut St2>,
lw: &LocalWaker
) -> Poll<Option<St1::Item>>
where St1: Stream, St2: Stream<Item = St1::Item>
{
let a_done = match a.poll_next(lw) {
Poll::Ready(Some(item)) => return Poll::Ready(Some(item)),
Poll::Ready(None) => true,
Poll::Pending => false,
};
match b.poll_next(lw) {
Poll::Ready(Some(item)) => {
// If the other stream isn't finished yet, give them a chance to
// go first next time as we pulled something off `b`.
if !a_done {
*flag = !*flag;
}
Poll::Ready(Some(item))
}
Poll::Ready(None) if a_done => Poll::Ready(None),
Poll::Ready(None) | Poll::Pending => Poll::Pending,
}
}