1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128

use core::fmt;
use core::pin::Pin;
use futures_core::future::Future;
use futures_core::ready;
use futures_core::stream::{FusedStream, Stream};
use futures_core::task::{Context, Poll};
#[cfg(feature = "sink")]
use futures_sink::Sink;
use pin_project_lite::pin_project;

struct StateFn<S, F> {
    state: S,
    f: F,
}

pin_project! {
    /// Stream for the [`scan`](super::StreamExt::scan) method.
    #[must_use = "streams do nothing unless polled"]
    pub struct Scan<St: Stream, S, Fut, F> {
        #[pin]
        stream: St,
        state_f: Option<StateFn<S, F>>,
        #[pin]
        future: Option<Fut>,
    }
}

impl<St, S, Fut, F> fmt::Debug for Scan<St, S, Fut, F>
where
    St: Stream + fmt::Debug,
    St::Item: fmt::Debug,
    S: fmt::Debug,
    Fut: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Scan")
            .field("stream", &self.stream)
            .field("state", &self.state_f.as_ref().map(|s| &s.state))
            .field("future", &self.future)
            .field("done_taking", &self.is_done_taking())
            .finish()
    }
}

impl<St: Stream, S, Fut, F> Scan<St, S, Fut, F> {
    /// Checks if internal state is `None`.
    fn is_done_taking(&self) -> bool {
        self.state_f.is_none()
    }
}

impl<B, St, S, Fut, F> Scan<St, S, Fut, F>
where
    St: Stream,
    F: FnMut(&mut S, St::Item) -> Fut,
    Fut: Future<Output = Option<B>>,
{
    pub(super) fn new(stream: St, initial_state: S, f: F) -> Self {
        Self { stream, state_f: Some(StateFn { state: initial_state, f }), future: None }
    }

    delegate_access_inner!(stream, St, ());
}

impl<B, St, S, Fut, F> Stream for Scan<St, S, Fut, F>
where
    St: Stream,
    F: FnMut(&mut S, St::Item) -> Fut,
    Fut: Future<Output = Option<B>>,
{
    type Item = B;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<B>> {
        if self.is_done_taking() {
            return Poll::Ready(None);
        }

        let mut this = self.project();

        Poll::Ready(loop {
            if let Some(fut) = this.future.as_mut().as_pin_mut() {
                let item = ready!(fut.poll(cx));
                this.future.set(None);

                if item.is_none() {
                    *this.state_f = None;
                }

                break item;
            } else if let Some(item) = ready!(this.stream.as_mut().poll_next(cx)) {
                let state_f = this.state_f.as_mut().unwrap();
                this.future.set(Some((state_f.f)(&mut state_f.state, item)))
            } else {
                break None;
            }
        })
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.is_done_taking() {
            (0, Some(0))
        } else {
            self.stream.size_hint() // can't know a lower bound, due to the predicate
        }
    }
}

impl<B, St, S, Fut, F> FusedStream for Scan<St, S, Fut, F>
where
    St: FusedStream,
    F: FnMut(&mut S, St::Item) -> Fut,
    Fut: Future<Output = Option<B>>,
{
    fn is_terminated(&self) -> bool {
        self.is_done_taking() || self.future.is_none() && self.stream.is_terminated()
    }
}

// Forwarding impl of Sink from the underlying stream
#[cfg(feature = "sink")]
impl<St, S, Fut, F, Item> Sink<Item> for Scan<St, S, Fut, F>
where
    St: Stream + Sink<Item>,
{
    type Error = St::Error;

    delegate_sink!(stream, Item);
}