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use core::cmp::Ordering; use core::future::Future; use core::pin::Pin; use pin_project_lite::pin_project; use crate::stream::Stream; use crate::task::{Context, Poll}; pin_project! { #[doc(hidden)] #[allow(missing_debug_implementations)] pub struct MinByKeyFuture<S, T, K> { #[pin] stream: S, min: Option<(T, T)>, key_by: K, } } impl<S, T, K> MinByKeyFuture<S, T, K> { pub(super) fn new(stream: S, key_by: K) -> Self { Self { stream, min: None, key_by, } } } impl<S, K> Future for MinByKeyFuture<S, S::Item, K> where S: Stream, K: FnMut(&S::Item) -> S::Item, S::Item: Ord, { type Output = Option<S::Item>; fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> { fn key<B, T>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) { move |x| (f(&x), x) } let this = self.project(); let next = futures_core::ready!(this.stream.poll_next(cx)); match next { Some(new) => { let (key, value) = key(this.key_by)(new); cx.waker().wake_by_ref(); match this.min.take() { None => *this.min = Some((key, value)), Some(old) => match key.cmp(&old.0) { Ordering::Less => *this.min = Some((key, value)), _ => *this.min = Some(old), }, } Poll::Pending } None => Poll::Ready(this.min.take().map(|min| min.1)), } } }