futures_util/stream/stream/
scan.rs

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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
11struct StateFn<S, F> {
12    state: S,
13    f: F,
14}
15
16pin_project! {
17    /// Stream for the [`scan`](super::StreamExt::scan) method.
18    #[must_use = "streams do nothing unless polled"]
19    pub struct Scan<St: Stream, S, Fut, F> {
20        #[pin]
21        stream: St,
22        state_f: Option<StateFn<S, F>>,
23        #[pin]
24        future: Option<Fut>,
25    }
26}
27
28impl<St, S, Fut, F> fmt::Debug for Scan<St, S, Fut, F>
29where
30    St: Stream + fmt::Debug,
31    St::Item: fmt::Debug,
32    S: fmt::Debug,
33    Fut: fmt::Debug,
34{
35    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
36        f.debug_struct("Scan")
37            .field("stream", &self.stream)
38            .field("state", &self.state_f.as_ref().map(|s| &s.state))
39            .field("future", &self.future)
40            .field("done_taking", &self.is_done_taking())
41            .finish()
42    }
43}
44
45impl<St: Stream, S, Fut, F> Scan<St, S, Fut, F> {
46    /// Checks if internal state is `None`.
47    fn is_done_taking(&self) -> bool {
48        self.state_f.is_none()
49    }
50}
51
52impl<B, St, S, Fut, F> Scan<St, S, Fut, F>
53where
54    St: Stream,
55    F: FnMut(&mut S, St::Item) -> Fut,
56    Fut: Future<Output = Option<B>>,
57{
58    pub(super) fn new(stream: St, initial_state: S, f: F) -> Self {
59        Self { stream, state_f: Some(StateFn { state: initial_state, f }), future: None }
60    }
61
62    delegate_access_inner!(stream, St, ());
63}
64
65impl<B, St, S, Fut, F> Stream for Scan<St, S, Fut, F>
66where
67    St: Stream,
68    F: FnMut(&mut S, St::Item) -> Fut,
69    Fut: Future<Output = Option<B>>,
70{
71    type Item = B;
72
73    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<B>> {
74        if self.is_done_taking() {
75            return Poll::Ready(None);
76        }
77
78        let mut this = self.project();
79
80        Poll::Ready(loop {
81            if let Some(fut) = this.future.as_mut().as_pin_mut() {
82                let item = ready!(fut.poll(cx));
83                this.future.set(None);
84
85                if item.is_none() {
86                    *this.state_f = None;
87                }
88
89                break item;
90            } else if let Some(item) = ready!(this.stream.as_mut().poll_next(cx)) {
91                let state_f = this.state_f.as_mut().unwrap();
92                this.future.set(Some((state_f.f)(&mut state_f.state, item)))
93            } else {
94                break None;
95            }
96        })
97    }
98
99    fn size_hint(&self) -> (usize, Option<usize>) {
100        if self.is_done_taking() {
101            (0, Some(0))
102        } else {
103            self.stream.size_hint() // can't know a lower bound, due to the predicate
104        }
105    }
106}
107
108impl<B, St, S, Fut, F> FusedStream for Scan<St, S, Fut, F>
109where
110    St: FusedStream,
111    F: FnMut(&mut S, St::Item) -> Fut,
112    Fut: Future<Output = Option<B>>,
113{
114    fn is_terminated(&self) -> bool {
115        self.is_done_taking() || self.future.is_none() && self.stream.is_terminated()
116    }
117}
118
119// Forwarding impl of Sink from the underlying stream
120#[cfg(feature = "sink")]
121impl<St, S, Fut, F, Item> Sink<Item> for Scan<St, S, Fut, F>
122where
123    St: Stream + Sink<Item>,
124{
125    type Error = St::Error;
126
127    delegate_sink!(stream, Item);
128}
futures_util/stream/stream/scan.rs

futures_util/stream/stream/
scan.rs
