Merge pull request #409 from yjhmelody/stream-min

Add Stream min

Author: yoshuawuyts

src/stream/stream/min.rs

@@ -0,0 +1,60 @@
+use std::marker::PhantomData;
+use std::cmp::{Ordering, Ord};
+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 MinFuture<S, F, T> {
+        #[pin]
+        stream: S,
+        _compare: PhantomData<F>,
+        min: Option<T>,
+    }
+}
+
+impl<S, F, T> MinFuture<S, F, T> {
+    pub(super) fn new(stream: S) -> Self {
+        Self {
+            stream,
+            _compare: PhantomData,
+            min: None,
+        }
+    }
+}
+
+impl<S, F> Future for MinFuture<S, F, S::Item>
+where
+    S: Stream,
+    S::Item: Ord,
+    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 new.cmp(&old) {
+                        Ordering::Less => *this.min = Some(new),
+                        _ => *this.min = Some(old),
+                    },
+                }
+                Poll::Pending
+            }
+            None => Poll::Ready(this.min.take()),
+        }
+    }
+}

src/stream/stream/mod.rs

@@ -42,6 +42,7 @@ mod le;
 mod lt;
 mod map;
 mod max_by;
+mod min;
 mod min_by;
 mod min_by_key;
 mod ne;
@@ -75,6 +76,7 @@ use last::LastFuture;
 use le::LeFuture;
 use lt::LtFuture;
 use max_by::MaxByFuture;
+use min::MinFuture;
 use min_by::MinByFuture;
 use min_by_key::MinByKeyFuture;
 use ne::NeFuture;
@@ -766,6 +768,40 @@ extension_trait! {
             MinByFuture::new(self, compare)
         }
 
+        #[doc = r#"
+            Returns the element that gives the minimum value. If several elements are equally minimum,
+            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<usize> = vec![1, 2, 3].into_iter().collect();
+
+            let min = s.clone().min().await;
+            assert_eq!(min, Some(1));
+
+            let min = VecDeque::<usize>::new().min().await;
+            assert_eq!(min, None);
+            #
+            # }) }
+            ```
+        "#]
+        fn min<F>(
+            self,
+        ) -> impl Future<Output = Option<Self::Item>> [MinFuture<Self, F, Self::Item>]
+        where
+            Self: Sized,
+            F: FnMut(&Self::Item, &Self::Item) -> Ordering,
+        {
+            MinFuture::new(self)
+        }
+
          #[doc = r#"
             Returns the element that gives the maximum value with respect to the
             specified comparison function. If several elements are equally maximum,