add max_by_key (#408)

* add max_by_key

* fix conflict

* fmt code

src/stream/stream/max_by_key.rs

@@ -0,0 +1,60 @@
+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 MaxByKeyFuture<S, T, K> {
+        #[pin]
+        stream: S,
+        max: Option<T>,
+        key_by: K,
+    }
+}
+
+impl<S, T, K> MaxByKeyFuture<S, T, K> {
+    pub(super) fn new(stream: S, key_by: K) -> Self {
+        Self {
+            stream,
+            max: None,
+            key_by,
+        }
+    }
+}
+
+impl<S, K> Future for MaxByKeyFuture<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> {
+        let this = self.project();
+        let next = futures_core::ready!(this.stream.poll_next(cx));
+
+        match next {
+            Some(new) => {
+                let new = (this.key_by)(&new);
+                cx.waker().wake_by_ref();
+                match this.max.take() {
+                    None => *this.max = Some(new),
+
+                    Some(old) => match new.cmp(&old) {
+                        Ordering::Greater => *this.max = Some(new),
+                        _ => *this.max = Some(old),
+                    },
+                }
+                Poll::Pending
+            }
+            None => Poll::Ready(this.max.take()),
+        }
+    }
+}

src/stream/stream/mod.rs

@@ -43,6 +43,7 @@ mod le;
 mod lt;
 mod map;
 mod max_by;
+mod max_by_key;
 mod min;
 mod min_by;
 mod min_by_key;
@@ -77,6 +78,7 @@ use last::LastFuture;
 use le::LeFuture;
 use lt::LtFuture;
 use max_by::MaxByFuture;
+use max_by_key::MaxByKeyFuture;
 use min::MinFuture;
 use min_by::MinByFuture;
 use min_by_key::MinByKeyFuture;
@@ -767,6 +769,43 @@ extension_trait! {
             MinByKeyFuture::new(self, key_by)
         }
 
+         #[doc = r#"
+            Returns the element that gives the maximum value with respect to the
+            specified key 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<i32> = vec![-1, -2, -3].into_iter().collect();
+
+            let max = s.clone().max_by_key(|x| x.abs()).await;
+            assert_eq!(max, Some(3));
+
+            let max = VecDeque::<isize>::new().max_by_key(|x| x.abs()).await;
+            assert_eq!(max, None);
+            #
+            # }) }
+            ```
+        "#]
+        fn max_by_key<K>(
+            self,
+            key_by: K,
+        ) -> impl Future<Output = Option<Self::Item>> [MaxByKeyFuture<Self, Self::Item, K>]
+        where
+            Self: Sized,
+            Self::Item: Ord,
+            K: FnMut(&Self::Item) -> Self::Item,
+        {
+            MaxByKeyFuture::new(self, key_by)
+        }
+
         #[doc = r#"
             Returns the element that gives the minimum value with respect to the
             specified comparison function. If several elements are equally minimum,