blob: 36f1bc42fcdc59713b72cb3a5fea4b8e6882bbd8 [file] [log] [blame]
use crate::stream::{StreamExt, Fuse};
use core::marker::Unpin;
use core::mem::PinMut;
use futures_core::stream::Stream;
use futures_core::task::{self, 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> Stream for Select<St1, St2>
where St1: Stream,
St2: Stream<Item = St1::Item>
{
type Item = St1::Item;
fn poll_next(
self: PinMut<Self>,
cx: &mut task::Context
) -> Poll<Option<St1::Item>> {
let Select { flag, stream1, stream2 } =
unsafe { PinMut::get_mut_unchecked(self) };
let stream1 = unsafe { PinMut::new_unchecked(stream1) };
let stream2 = unsafe { PinMut::new_unchecked(stream2) };
if *flag {
poll_inner(flag, stream1, stream2, cx)
} else {
poll_inner(flag, stream2, stream1, cx)
}
}
}
fn poll_inner<St1, St2>(
flag: &mut bool,
a: PinMut<St1>,
b: PinMut<St2>,
cx: &mut task::Context
) -> Poll<Option<St1::Item>>
where St1: Stream, St2: Stream<Item = St1::Item>
{
let a_done = match a.poll_next(cx) {
Poll::Ready(Some(item)) => return Poll::Ready(Some(item)),
Poll::Ready(None) => true,
Poll::Pending => false,
};
match b.poll_next(cx) {
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,
}
}