1use super::assert_stream;
2use crate::stream::{select_with_strategy, PollNext, SelectWithStrategy};
3use core::pin::Pin;
4use futures_core::stream::{FusedStream, Stream};
5use futures_core::task::{Context, Poll};
6use pin_project_lite::pin_project;
7
8pin_project! {
9 /// Stream for the [`select()`] function.
10 #[derive(Debug)]
11 #[must_use = "streams do nothing unless polled"]
12 pub struct Select<St1, St2> {
13 #[pin]
14 inner: SelectWithStrategy<St1, St2, fn(&mut PollNext)-> PollNext, PollNext>,
15 }
16}
17
18/// This function will attempt to pull items from both streams. Each
19/// stream will be polled in a round-robin fashion, and whenever a stream is
20/// ready to yield an item that item is yielded.
21///
22/// After one of the two input streams completes, the remaining one will be
23/// polled exclusively. The returned stream completes when both input
24/// streams have completed.
25///
26/// Note that this function consumes both streams and returns a wrapped
27/// version of them.
28///
29/// ## Examples
30///
31/// ```rust
32/// # futures::executor::block_on(async {
33/// use futures::stream::{ repeat, select, StreamExt };
34///
35/// let left = repeat(1);
36/// let right = repeat(2);
37///
38/// let mut out = select(left, right);
39///
40/// for _ in 0..100 {
41/// // We should be alternating.
42/// assert_eq!(1, out.select_next_some().await);
43/// assert_eq!(2, out.select_next_some().await);
44/// }
45/// # });
46/// ```
47pub fn select<St1, St2>(stream1: St1, stream2: St2) -> Select<St1, St2>
48where
49 St1: Stream,
50 St2: Stream<Item = St1::Item>,
51{
52 fn round_robin(last: &mut PollNext) -> PollNext {
53 last.toggle()
54 }
55
56 assert_stream::<St1::Item, _>(Select {
57 inner: select_with_strategy(stream1, stream2, round_robin),
58 })
59}
60
61impl<St1, St2> Select<St1, St2> {
62 /// Acquires a reference to the underlying streams that this combinator is
63 /// pulling from.
64 pub fn get_ref(&self) -> (&St1, &St2) {
65 self.inner.get_ref()
66 }
67
68 /// Acquires a mutable reference to the underlying streams that this
69 /// combinator is pulling from.
70 ///
71 /// Note that care must be taken to avoid tampering with the state of the
72 /// stream which may otherwise confuse this combinator.
73 pub fn get_mut(&mut self) -> (&mut St1, &mut St2) {
74 self.inner.get_mut()
75 }
76
77 /// Acquires a pinned mutable reference to the underlying streams that this
78 /// combinator is pulling from.
79 ///
80 /// Note that care must be taken to avoid tampering with the state of the
81 /// stream which may otherwise confuse this combinator.
82 pub fn get_pin_mut(self: Pin<&mut Self>) -> (Pin<&mut St1>, Pin<&mut St2>) {
83 let this = self.project();
84 this.inner.get_pin_mut()
85 }
86
87 /// Consumes this combinator, returning the underlying streams.
88 ///
89 /// Note that this may discard intermediate state of this combinator, so
90 /// care should be taken to avoid losing resources when this is called.
91 pub fn into_inner(self) -> (St1, St2) {
92 self.inner.into_inner()
93 }
94}
95
96impl<St1, St2> FusedStream for Select<St1, St2>
97where
98 St1: Stream,
99 St2: Stream<Item = St1::Item>,
100{
101 fn is_terminated(&self) -> bool {
102 self.inner.is_terminated()
103 }
104}
105
106impl<St1, St2> Stream for Select<St1, St2>
107where
108 St1: Stream,
109 St2: Stream<Item = St1::Item>,
110{
111 type Item = St1::Item;
112
113 fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<St1::Item>> {
114 let this = self.project();
115 this.inner.poll_next(cx)
116 }
117}
118