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use core::cmp::Ordering; use core::pin::Pin; use core::future::Future; 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 MinByFuture<S, F, T> { #[pin] stream: S, compare: F, min: Option<T>, } } impl<S, F, T> MinByFuture<S, F, T> { pub(super) fn new(stream: S, compare: F) -> Self { Self { stream, compare, min: None, } } } impl<S, F> Future for MinByFuture<S, F, S::Item> where S: Stream + Unpin + Sized, S::Item: Copy, F: FnMut(&S::Item, &S::Item) -> Ordering, { type Output = Option<S::Item>; fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> { let this = self.project(); let next = futures_core::ready!(this.stream.poll_next(cx)); match next { Some(new) => { cx.waker().wake_by_ref(); match this.min.take() { None => *this.min = Some(new), Some(old) => match (this.compare)(&new, &old) { Ordering::Less => *this.min = Some(new), _ => *this.min = Some(old), }, } Poll::Pending } None => Poll::Ready(*this.min), } } }