async_std/stream/stream/
min_by_key.rs

1use core::cmp::Ordering;
2use core::future::Future;
3use core::pin::Pin;
4
5use pin_project_lite::pin_project;
6
7use crate::stream::Stream;
8use crate::task::{Context, Poll};
9
10pin_project! {
11    #[doc(hidden)]
12    #[allow(missing_debug_implementations)]
13    pub struct MinByKeyFuture<S, T, K> {
14        #[pin]
15        stream: S,
16        min: Option<(T, T)>,
17        key_by: K,
18    }
19}
20
21impl<S, T, K> MinByKeyFuture<S, T, K> {
22    pub(super) fn new(stream: S, key_by: K) -> Self {
23        Self {
24            stream,
25            min: None,
26            key_by,
27        }
28    }
29}
30
31impl<S, K> Future for MinByKeyFuture<S, S::Item, K>
32where
33    S: Stream,
34    K: FnMut(&S::Item) -> S::Item,
35    S::Item: Ord,
36{
37    type Output = Option<S::Item>;
38
39    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
40        fn key<B, T>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
41            move |x| (f(&x), x)
42        }
43
44        let this = self.project();
45        let next = futures_core::ready!(this.stream.poll_next(cx));
46
47        match next {
48            Some(new) => {
49                let (key, value) = key(this.key_by)(new);
50                cx.waker().wake_by_ref();
51
52                match this.min.take() {
53                    None => *this.min = Some((key, value)),
54
55                    Some(old) => match key.cmp(&old.0) {
56                        Ordering::Less => *this.min = Some((key, value)),
57                        _ => *this.min = Some(old),
58                    },
59                }
60                Poll::Pending
61            }
62            None => Poll::Ready(this.min.take().map(|min| min.1)),
63        }
64    }
65}
async_std/stream/stream/min_by_key.rs

async_std/stream/stream/
min_by_key.rs
