1use core::fmt;
2use core::pin::Pin;
3use futures_core::future::Future;
4use futures_core::ready;
5use futures_core::stream::{FusedStream, Stream};
6use futures_core::task::{Context, Poll};
7#[cfg(feature = "sink")]
8use futures_sink::Sink;
9use pin_project_lite::pin_project;
10
11pin_project! {
12 /// Stream for the [`take_while`](super::StreamExt::take_while) method.
13 #[must_use = "streams do nothing unless polled"]
14 pub struct TakeWhile<St: Stream, Fut, F> {
15 #[pin]
16 stream: St,
17 f: F,
18 #[pin]
19 pending_fut: Option<Fut>,
20 pending_item: Option<St::Item>,
21 done_taking: bool,
22 }
23}
24
25impl<St, Fut, F> fmt::Debug for TakeWhile<St, Fut, F>
26where
27 St: Stream + fmt::Debug,
28 St::Item: fmt::Debug,
29 Fut: fmt::Debug,
30{
31 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
32 f.debug_struct("TakeWhile")
33 .field("stream", &self.stream)
34 .field("pending_fut", &self.pending_fut)
35 .field("pending_item", &self.pending_item)
36 .field("done_taking", &self.done_taking)
37 .finish()
38 }
39}
40
41impl<St, Fut, F> TakeWhile<St, Fut, F>
42where
43 St: Stream,
44 F: FnMut(&St::Item) -> Fut,
45 Fut: Future<Output = bool>,
46{
47 pub(super) fn new(stream: St, f: F) -> Self {
48 Self { stream, f, pending_fut: None, pending_item: None, done_taking: false }
49 }
50
51 delegate_access_inner!(stream, St, ());
52}
53
54impl<St, Fut, F> Stream for TakeWhile<St, Fut, F>
55where
56 St: Stream,
57 F: FnMut(&St::Item) -> Fut,
58 Fut: Future<Output = bool>,
59{
60 type Item = St::Item;
61
62 fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<St::Item>> {
63 if self.done_taking {
64 return Poll::Ready(None);
65 }
66
67 let mut this = self.project();
68
69 Poll::Ready(loop {
70 if let Some(fut) = this.pending_fut.as_mut().as_pin_mut() {
71 let take = ready!(fut.poll(cx));
72 let item = this.pending_item.take();
73 this.pending_fut.set(None);
74 if take {
75 break item;
76 } else {
77 *this.done_taking = true;
78 break None;
79 }
80 } else if let Some(item) = ready!(this.stream.as_mut().poll_next(cx)) {
81 this.pending_fut.set(Some((this.f)(&item)));
82 *this.pending_item = Some(item);
83 } else {
84 break None;
85 }
86 })
87 }
88
89 fn size_hint(&self) -> (usize, Option<usize>) {
90 if self.done_taking {
91 return (0, Some(0));
92 }
93
94 let pending_len = usize::from(self.pending_item.is_some());
95 let (_, upper) = self.stream.size_hint();
96 let upper = match upper {
97 Some(x) => x.checked_add(pending_len),
98 None => None,
99 };
100 (0, upper) // can't know a lower bound, due to the predicate
101 }
102}
103
104impl<St, Fut, F> FusedStream for TakeWhile<St, Fut, F>
105where
106 St: FusedStream,
107 F: FnMut(&St::Item) -> Fut,
108 Fut: Future<Output = bool>,
109{
110 fn is_terminated(&self) -> bool {
111 self.done_taking || self.pending_item.is_none() && self.stream.is_terminated()
112 }
113}
114
115// Forwarding impl of Sink from the underlying stream
116#[cfg(feature = "sink")]
117impl<S, Fut, F, Item> Sink<Item> for TakeWhile<S, Fut, F>
118where
119 S: Stream + Sink<Item>,
120{
121 type Error = S::Error;
122
123 delegate_sink!(stream, Item);
124}
125