1 use core::fmt;
2 use core::pin::Pin;
3 use futures_core::future::Future;
4 use futures_core::ready;
5 use futures_core::stream::{FusedStream, Stream};
6 use futures_core::task::{Context, Poll};
7 #[cfg(feature = "sink")]
8 use futures_sink::Sink;
9 use pin_project_lite::pin_project;
10 
11 struct StateFn<S, F> {
12     state: S,
13     f: F,
14 }
15 
16 pin_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 
28 impl<St, S, Fut, F> fmt::Debug for Scan<St, S, Fut, F>
29 where
30     St: Stream + fmt::Debug,
31     St::Item: fmt::Debug,
32     S: fmt::Debug,
33     Fut: fmt::Debug,
34 {
fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result35     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 
45 impl<St: Stream, S, Fut, F> Scan<St, S, Fut, F> {
46     /// Checks if internal state is `None`.
is_done_taking(&self) -> bool47     fn is_done_taking(&self) -> bool {
48         self.state_f.is_none()
49     }
50 }
51 
52 impl<B, St, S, Fut, F> Scan<St, S, Fut, F>
53 where
54     St: Stream,
55     F: FnMut(&mut S, St::Item) -> Fut,
56     Fut: Future<Output = Option<B>>,
57 {
new(stream: St, initial_state: S, f: F) -> Self58     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 
65 impl<B, St, S, Fut, F> Stream for Scan<St, S, Fut, F>
66 where
67     St: Stream,
68     F: FnMut(&mut S, St::Item) -> Fut,
69     Fut: Future<Output = Option<B>>,
70 {
71     type Item = B;
72 
poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<B>>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 
size_hint(&self) -> (usize, Option<usize>)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 
108 impl<B, St, S, Fut, F> FusedStream for Scan<St, S, Fut, F>
109 where
110     St: FusedStream,
111     F: FnMut(&mut S, St::Item) -> Fut,
112     Fut: Future<Output = Option<B>>,
113 {
is_terminated(&self) -> bool114     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")]
121 impl<St, S, Fut, F, Item> Sink<Item> for Scan<St, S, Fut, F>
122 where
123     St: Stream + Sink<Item>,
124 {
125     type Error = St::Error;
126 
127     delegate_sink!(stream, Item);
128 }
129