add stream-partition

src/stream/stream/mod.rs

@@ -54,6 +54,7 @@ mod ne;
 mod next;
 mod nth;
 mod partial_cmp;
+mod partition;
 mod position;
 mod scan;
 mod skip;
@@ -91,6 +92,7 @@ use ne::NeFuture;
 use next::NextFuture;
 use nth::NthFuture;
 use partial_cmp::PartialCmpFuture;
+use partition::PartitionFuture;
 use position::PositionFuture;
 use try_fold::TryFoldFuture;
 use try_for_each::TryForEachFuture;
@@ -1308,6 +1310,42 @@ extension_trait! {
             FoldFuture::new(self, init, f)
         }
 
+        #[doc = r#"
+            A combinator that applies a function to every element in a stream
+            creating two collections from it.
+
+            # Examples
+
+            Basic usage:
+
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use async_std::prelude::*;
+            use async_std::stream;
+
+            let (even, odd): (Vec<i32>, Vec<i32>) = stream::from_iter(vec![1, 2, 3])
+                .partition(|&n| n % 2 == 0).await;
+
+            assert_eq!(even, vec![2]);
+            assert_eq!(odd, vec![1, 3]);
+
+            #
+            # }) }
+            ```
+        "#]
+        fn partition<B, F>(
+            self,
+            f: F,
+        ) -> impl Future<Output = (B, B)> [PartitionFuture<Self, F, B>]
+        where
+            Self: Sized,
+            F: FnMut(&Self::Item) -> bool,
+            B: Default,
+        {
+            PartitionFuture::new(self, f)
+        }
+
         #[doc = r#"
             Call a closure on each element of the stream.
 

src/stream/stream/partition.rs

@@ -0,0 +1,57 @@
+use std::future::Future;
+use std::pin::Pin;
+use std::default::Default;
+use pin_project_lite::pin_project;
+
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+pin_project! {
+    #[derive(Debug)]
+    pub struct PartitionFuture<S, F, B> {
+        #[pin]
+        stream: S,
+        f: F,
+        res: Option<(B, B)>,
+    }
+}
+
+impl<S, F, B: Default> PartitionFuture<S, F, B> {
+    pub(super) fn new(stream: S, f: F) -> Self {
+        Self {
+            stream,
+            f,
+            res: Some((B::default(), B::default())),
+        }
+    }
+}
+
+impl<S, F, B> Future for PartitionFuture<S, F, B>
+where
+    S: Stream + Sized,
+    F: FnMut(&S::Item) -> bool,
+    B: Default + Extend<S::Item>,
+{
+    type Output = (B, B);
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let mut this = self.project();
+
+        loop {
+            let next = futures_core::ready!(this.stream.as_mut().poll_next(cx));
+
+            match next {
+                Some(v) => {
+                    let mut res = this.res.take().unwrap();
+                    match (this.f)(&v) {
+                        true => res.0.extend(Some(v)),
+                        false => res.1.extend(Some(v)),
+                    };
+
+                    *this.res = Some(res);
+                }
+                None => return Poll::Ready(this.res.take().unwrap()),
+            }
+        }
+    }
+}