stream: add StreamExt::partition() method

This allows filtering items that match or don't match into separate
stream. It is analogous to std::iter::Iterator::partition()

```rust
let stream = stream::iter(0..4);
let (mut even, mut odd) = stream.partition(|v| v % 2 == 0);
assert_eq!(Some(0), even.next().await);
assert_eq!(Some(1), odd.next().await);
```

tokio-stream/src/lib.rs

@@ -81,8 +81,8 @@ pub use stream_ext::{collect::FromStream, StreamExt};
 /// Adapters for [`Stream`]s created by methods in [`StreamExt`].
 pub mod adapters {
     pub use crate::stream_ext::{
-        Chain, Filter, FilterMap, Fuse, Map, MapWhile, Merge, Peekable, Skip, SkipWhile, Take,
-        TakeWhile, Then,
+        Chain, FalsePartition, Filter, FilterMap, Fuse, Map, MapWhile, Merge, Peekable, Skip,
+        SkipWhile, Take, TakeWhile, Then, TruePartition,
     };
     cfg_time! {
         pub use crate::stream_ext::{ChunksTimeout, Timeout, TimeoutRepeating};

tokio-stream/src/stream_ext.rs

@@ -37,6 +37,10 @@ pub use merge::Merge;
 mod next;
 use next::Next;
 
+mod partition;
+use partition::Partition;
+pub use partition::{FalsePartition, TruePartition};
+
 mod skip;
 pub use skip::Skip;
 
@@ -841,6 +845,35 @@ pub trait StreamExt: Stream {
         FoldFuture::new(self, init, f)
     }
 
+    /// Partitions the stream into two streams based on the provided predicate.
+    ///
+    /// The first stream contains all elements for which `f` returns `true`, and
+    /// the second contains all elements for which `f` returns `false`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio_stream::{self as stream, StreamExt};
+    ///
+    /// let s = stream::iter(vec![1, 2, 3, 4, 5]);
+    /// let (even, odd) = s.partition(|x| x % 2 == 0);
+    ///
+    /// let even: Vec<_> = even.collect().await;
+    /// let odd: Vec<_> = odd.collect().await;
+    ///
+    /// assert_eq!(even, vec![2, 4]);
+    /// assert_eq!(odd, vec![1, 3, 5]);
+    /// # }
+    fn partition<F>(self, f: F) -> (TruePartition<Self, F>, FalsePartition<Self, F>)
+    where
+        F: FnMut(&Self::Item) -> bool,
+        Self: Sized,
+    {
+        Partition::new(self, f).split()
+    }
+
     /// Drain stream pushing all emitted values into a collection.
     ///
     /// Equivalent to:

tokio-stream/src/stream_ext/partition.rs

@@ -0,0 +1,153 @@
+#![allow(dead_code)]
+
+use crate::Stream;
+
+use core::fmt;
+use std::{
+    collections::VecDeque,
+    pin::Pin,
+    sync::{Arc, Mutex},
+    task::{Context, Poll, Waker},
+};
+
+pub(super) struct Partition<St, F>
+where
+    St: Stream,
+{
+    stream: St,
+    f: F,
+    true_buffer: VecDeque<St::Item>,
+    false_buffer: VecDeque<St::Item>,
+    true_waker: Option<Waker>,
+    false_waker: Option<Waker>,
+}
+
+impl<St, F> Partition<St, F>
+where
+    St: Stream,
+{
+    pub(super) fn new(stream: St, f: F) -> Self {
+        Self {
+            stream,
+            f,
+            true_buffer: VecDeque::new(),
+            false_buffer: VecDeque::new(),
+            true_waker: None,
+            false_waker: None,
+        }
+    }
+
+    pub(super) fn split(self) -> (TruePartition<St, F>, FalsePartition<St, F>) {
+        let partition = Arc::new(Mutex::new(self));
+        let true_partition = TruePartition::new(partition.clone());
+        let false_partition = FalsePartition::new(partition.clone());
+        (true_partition, false_partition)
+    }
+}
+
+/// A stream that only yields elements that satisfy a predicate.
+///
+/// This stream is produced by the [`StreamExt::partition`] method.
+pub struct TruePartition<St: Stream, F> {
+    partition: Arc<Mutex<Partition<St, F>>>,
+}
+
+impl<St, F> fmt::Debug for TruePartition<St, F>
+where
+    St: Stream,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("TruePartition").finish_non_exhaustive()
+    }
+}
+
+impl<St, F> TruePartition<St, F>
+where
+    St: Stream,
+{
+    fn new(partition: Arc<Mutex<Partition<St, F>>>) -> Self {
+        Self { partition }
+    }
+}
+
+impl<St, F> Stream for TruePartition<St, F>
+where
+    St: Stream + Unpin,
+    F: FnMut(&St::Item) -> bool,
+{
+    type Item = St::Item;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let mut partition = self.partition.lock().unwrap();
+        if let Some(item) = partition.true_buffer.pop_front() {
+            return Poll::Ready(Some(item));
+        }
+
+        match Pin::new(&mut partition.stream).poll_next(cx) {
+            Poll::Ready(None) => Poll::Ready(None),
+            Poll::Ready(Some(item)) if (partition.f)(&item) => Poll::Ready(Some(item)),
+            Poll::Ready(Some(item)) => {
+                partition.false_buffer.push_back(item);
+                partition.false_waker = Some(cx.waker().clone());
+                cx.waker().wake_by_ref();
+                Poll::Pending
+            }
+            Poll::Pending => {
+                partition.true_waker = Some(cx.waker().clone());
+                Poll::Pending
+            }
+        }
+    }
+}
+
+/// A stream that only yields elements that do not satisfy a predicate.
+///
+/// This stream is produced by the [`StreamExt::partition`] method.
+pub struct FalsePartition<St: Stream, F> {
+    partition: Arc<Mutex<Partition<St, F>>>,
+}
+
+impl<St, F> fmt::Debug for FalsePartition<St, F>
+where
+    St: Stream,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("FalsePartition").finish_non_exhaustive()
+    }
+}
+
+impl<St: Stream, F> FalsePartition<St, F> {
+    fn new(partition: Arc<Mutex<Partition<St, F>>>) -> Self {
+        Self { partition }
+    }
+}
+
+impl<St, F> Stream for FalsePartition<St, F>
+where
+    St: Stream + Unpin,
+    F: FnMut(&St::Item) -> bool,
+{
+    type Item = St::Item;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let mut partition = self.partition.lock().unwrap();
+        if let Some(item) = partition.false_buffer.pop_front() {
+            return Poll::Ready(Some(item));
+        }
+
+        match Pin::new(&mut partition.stream).poll_next(cx) {
+            Poll::Ready(None) => Poll::Ready(None),
+            Poll::Ready(Some(item)) if !(partition.f)(&item) => Poll::Ready(Some(item)),
+            Poll::Ready(Some(item)) => {
+                partition.true_buffer.push_back(item);
+                partition.true_waker = Some(cx.waker().clone());
+                cx.waker().wake_by_ref();
+                Poll::Pending
+            }
+            Poll::Pending => {
+                partition.false_waker = Some(cx.waker().clone());
+                Poll::Pending
+            }
+        }
+    }
+}

tokio-stream/tests/stream_partition.rs

@@ -0,0 +1,29 @@
+use tokio_stream::{self as stream, StreamExt};
+
+mod support {
+    pub(crate) mod mpsc;
+}
+
+#[tokio::test]
+async fn partition() {
+    let stream = stream::iter(0..4);
+    let (mut even, mut odd) = stream.partition(|v| v % 2 == 0);
+    assert_eq!(Some(0), even.next().await);
+    assert_eq!(Some(1), odd.next().await);
+    assert_eq!(Some(2), even.next().await);
+    assert_eq!(Some(3), odd.next().await);
+    assert_eq!(None, even.next().await);
+    assert_eq!(None, odd.next().await);
+}
+
+#[tokio::test]
+async fn partition_buffers() {
+    let stream = stream::iter(0..4);
+    let (mut even, mut odd) = stream.partition(|v| v % 2 == 0);
+    assert_eq!(Some(1), odd.next().await);
+    assert_eq!(Some(3), odd.next().await);
+    assert_eq!(None, odd.next().await);
+    assert_eq!(Some(0), even.next().await);
+    assert_eq!(Some(2), even.next().await);
+    assert_eq!(None, odd.next().await);
+}