diff --git a/src/stream/stream/max_by.rs b/src/stream/stream/max_by.rs new file mode 100644 index 00000000..a626b284 --- /dev/null +++ b/src/stream/stream/max_by.rs @@ -0,0 +1,57 @@ +use std::cmp::Ordering; +use std::pin::Pin; + +use pin_project_lite::pin_project; + +use crate::future::Future; +use crate::stream::Stream; +use crate::task::{Context, Poll}; + +pin_project! { + #[doc(hidden)] + #[allow(missing_debug_implementations)] + pub struct MaxByFuture { + #[pin] + stream: S, + compare: F, + max: Option, + } +} + +impl MaxByFuture { + pub(super) fn new(stream: S, compare: F) -> Self { + MaxByFuture { + stream, + compare, + max: None, + } + } +} + +impl Future for MaxByFuture +where + S: Stream, + F: FnMut(&S::Item, &S::Item) -> Ordering, +{ + type Output = Option; + + fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll { + 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.max.take() { + None => *this.max = Some(new), + Some(old) => match (this.compare)(&new, &old) { + Ordering::Greater => *this.max = Some(new), + _ => *this.max = Some(old), + }, + } + Poll::Pending + } + None => Poll::Ready(this.max.take()), + } + } +} diff --git a/src/stream/stream/mod.rs b/src/stream/stream/mod.rs index aadd058b..e8190387 100644 --- a/src/stream/stream/mod.rs +++ b/src/stream/stream/mod.rs @@ -40,6 +40,7 @@ mod last; mod le; mod lt; mod map; +mod max_by; mod min_by; mod min_by_key; mod next; @@ -69,6 +70,7 @@ use gt::GtFuture; use last::LastFuture; use le::LeFuture; use lt::LtFuture; +use max_by::MaxByFuture; use min_by::MinByFuture; use min_by_key::MinByKeyFuture; use next::NextFuture; @@ -758,6 +760,45 @@ extension_trait! { MinByFuture::new(self, compare) } + #[doc = r#" + Returns the element that gives the maximum value with respect to the + specified comparison function. If several elements are equally maximum, + the first element is returned. If the stream is empty, `None` is returned. + + # Examples + + ``` + # fn main() { async_std::task::block_on(async { + # + use std::collections::VecDeque; + + use async_std::prelude::*; + + let s: VecDeque = vec![1, 2, 3].into_iter().collect(); + + let max = s.clone().max_by(|x, y| x.cmp(y)).await; + assert_eq!(max, Some(3)); + + let max = s.max_by(|x, y| y.cmp(x)).await; + assert_eq!(max, Some(1)); + + let max = VecDeque::::new().max_by(|x, y| x.cmp(y)).await; + assert_eq!(max, None); + # + # }) } + ``` + "#] + fn max_by( + self, + compare: F, + ) -> impl Future> [MaxByFuture] + where + Self: Sized, + F: FnMut(&Self::Item, &Self::Item) -> Ordering, + { + MaxByFuture::new(self, compare) + } + #[doc = r#" Returns the nth element of the stream.