1use core::fmt;
2use core::pin::Pin;
3use futures_core::future::{FusedFuture, Future};
4use futures_core::ready;
5use futures_core::stream::Stream;
6use futures_core::task::{Context, Poll};
7use pin_project_lite::pin_project;
8
9pin_project! {
10 /// Future for the [`fold`](super::StreamExt::fold) method.
11 #[must_use = "futures do nothing unless you `.await` or poll them"]
12 pub struct Fold<St, Fut, T, F> {
13 #[pin]
14 stream: St,
15 f: F,
16 accum: Option<T>,
17 #[pin]
18 future: Option<Fut>,
19 }
20}
21
22impl<St, Fut, T, F> fmt::Debug for Fold<St, Fut, T, F>
23where
24 St: fmt::Debug,
25 Fut: fmt::Debug,
26 T: fmt::Debug,
27{
28 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
29 f.debug_struct("Fold")
30 .field("stream", &self.stream)
31 .field("accum", &self.accum)
32 .field("future", &self.future)
33 .finish()
34 }
35}
36
37impl<St, Fut, T, F> Fold<St, Fut, T, F>
38where
39 St: Stream,
40 F: FnMut(T, St::Item) -> Fut,
41 Fut: Future<Output = T>,
42{
43 pub(super) fn new(stream: St, f: F, t: T) -> Self {
44 Self { stream, f, accum: Some(t), future: None }
45 }
46}
47
48impl<St, Fut, T, F> FusedFuture for Fold<St, Fut, T, F>
49where
50 St: Stream,
51 F: FnMut(T, St::Item) -> Fut,
52 Fut: Future<Output = T>,
53{
54 fn is_terminated(&self) -> bool {
55 self.accum.is_none() && self.future.is_none()
56 }
57}
58
59impl<St, Fut, T, F> Future for Fold<St, Fut, T, F>
60where
61 St: Stream,
62 F: FnMut(T, St::Item) -> Fut,
63 Fut: Future<Output = T>,
64{
65 type Output = T;
66
67 fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
68 let mut this = self.project();
69 Poll::Ready(loop {
70 if let Some(fut) = this.future.as_mut().as_pin_mut() {
71 // we're currently processing a future to produce a new accum value
72 *this.accum = Some(ready!(fut.poll(cx)));
73 this.future.set(None);
74 } else if this.accum.is_some() {
75 // we're waiting on a new item from the stream
76 let res = ready!(this.stream.as_mut().poll_next(cx));
77 let a = this.accum.take().unwrap();
78 if let Some(item) = res {
79 this.future.set(Some((this.f)(a, item)));
80 } else {
81 break a;
82 }
83 } else {
84 panic!("Fold polled after completion")
85 }
86 })
87 }
88}
89