futures_util/stream/stream/
fold.rs

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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}
futures_util/stream/stream/fold.rs

futures_util/stream/stream/
fold.rs
