Add stream::Peekable::{next_if, next_if_eq}

futures-util/src/stream/mod.rs

@@ -19,7 +19,7 @@ pub use futures_core::stream::{FusedStream, Stream, TryStream};
 mod stream;
 pub use self::stream::{
     Chain, Collect, Concat, Cycle, Enumerate, Filter, FilterMap, FlatMap, Flatten, Fold, ForEach,
-    Fuse, Inspect, Map, Next, Peek, Peekable, Scan, SelectNextSome, Skip, SkipWhile, StreamExt,
+    Fuse, Inspect, Map, Next, NextIf, NextIfEq, Peek, Peekable, Scan, SelectNextSome, Skip, SkipWhile, StreamExt,
     StreamFuture, Take, TakeUntil, TakeWhile, Then, Unzip, Zip,
 };
 

futures-util/src/stream/stream/mod.rs

@@ -123,7 +123,7 @@ pub use self::select_next_some::SelectNextSome;
 
 mod peek;
 #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
-pub use self::peek::{Peek, Peekable};
+pub use self::peek::{Peek, Peekable, NextIf, NextIfEq};
 
 mod skip;
 #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411

futures-util/src/stream/stream/peek.rs

@@ -1,5 +1,7 @@
+use crate::fns::FnOnce1;
 use crate::stream::{Fuse, StreamExt};
 use core::fmt;
+use core::marker::PhantomData;
 use core::pin::Pin;
 use futures_core::future::{FusedFuture, Future};
 use futures_core::ready;
@@ -44,10 +46,7 @@ impl<St: Stream> Peekable<St> {
     ///
     /// This method polls the underlying stream and return either a reference
     /// to the next item if the stream is ready or passes through any errors.
-    pub fn poll_peek(
-        self: Pin<&mut Self>,
-        cx: &mut Context<'_>,
-    ) -> Poll<Option<&St::Item>> {
+    pub fn poll_peek(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<&St::Item>> {
         let mut this = self.project();
 
         Poll::Ready(loop {
@@ -60,6 +59,96 @@ impl<St: Stream> Peekable<St> {
             }
         })
     }
+
+    /// Creates a future which will consume and return the next value of this
+    /// stream if a condition is true.
+    ///
+    /// If `func` returns `true` for the next value of this stream, consume and
+    /// return it. Otherwise, return `None`.
+    ///
+    /// # Examples
+    ///
+    /// Consume a number if it's equal to 0.
+    ///
+    /// ```
+    /// # futures::executor::block_on(async {
+    /// use futures::stream::{self, StreamExt};
+    /// use futures::pin_mut;
+    ///
+    /// let stream = stream::iter(0..5).peekable();
+    /// pin_mut!(stream);
+    /// // The first item of the stream is 0; consume it.
+    /// assert_eq!(stream.as_mut().next_if(|&x| x == 0).await, Some(0));
+    /// // The next item returned is now 1, so `consume` will return `false`.
+    /// assert_eq!(stream.as_mut().next_if(|&x| x == 0).await, None);
+    /// // `next_if` saves the value of the next item if it was not equal to `expected`.
+    /// assert_eq!(stream.next().await, Some(1));
+    /// # });
+    /// ```
+    ///
+    /// Consume any number less than 10.
+    ///
+    /// ```
+    /// # futures::executor::block_on(async {
+    /// use futures::stream::{self, StreamExt};
+    /// use futures::pin_mut;
+    ///
+    /// let stream = stream::iter(1..20).peekable();
+    /// pin_mut!(stream);
+    /// // Consume all numbers less than 10
+    /// while stream.as_mut().next_if(|&x| x < 10).await.is_some() {}
+    /// // The next value returned will be 10
+    /// assert_eq!(stream.next().await, Some(10));
+    /// # });
+    /// ```
+    pub fn next_if<F>(self: Pin<&mut Self>, func: F) -> NextIf<'_, St, F>
+    where
+        F: FnOnce(&St::Item) -> bool,
+    {
+        NextIf {
+            inner: Some((self, func)),
+        }
+    }
+
+    /// Creates a future which will consume and return the next item if it is
+    /// equal to `expected`.
+    ///
+    /// # Example
+    ///
+    /// Consume a number if it's equal to 0.
+    ///
+    /// ```
+    /// # futures::executor::block_on(async {
+    /// use futures::stream::{self, StreamExt};
+    /// use futures::pin_mut;
+    ///
+    /// let stream = stream::iter(0..5).peekable();
+    /// pin_mut!(stream);
+    /// // The first item of the stream is 0; consume it.
+    /// assert_eq!(stream.as_mut().next_if_eq(&0).await, Some(0));
+    /// // The next item returned is now 1, so `consume` will return `false`.
+    /// assert_eq!(stream.as_mut().next_if_eq(&0).await, None);
+    /// // `next_if_eq` saves the value of the next item if it was not equal to `expected`.
+    /// assert_eq!(stream.next().await, Some(1));
+    /// # });
+    /// ```
+    pub fn next_if_eq<'a, T>(self: Pin<&'a mut Self>, expected: &'a T) -> NextIfEq<'a, St, T>
+    where
+        T: ?Sized,
+        St::Item: PartialEq<T>,
+    {
+        NextIfEq {
+            inner: NextIf {
+                inner: Some((
+                    self,
+                    NextIfEqFn {
+                        expected,
+                        _next: PhantomData,
+                    },
+                )),
+            },
+        }
+    }
 }
 
 impl<St: Stream> FusedStream for Peekable<St> {
@@ -103,7 +192,7 @@ where
 }
 
 pin_project! {
-    /// Future for the [`Peekable::peek()`](self::Peekable::peek) function from [`Peekable`]
+    /// Future for the [`Peekable::peek`](self::Peekable::peek) method.
     #[must_use = "futures do nothing unless polled"]
     pub struct Peek<'a, St: Stream> {
         inner: Option<Pin<&'a mut Peekable<St>>>,
@@ -116,9 +205,7 @@ where
     St::Item: fmt::Debug,
 {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_struct("Peek")
-            .field("inner", &self.inner)
-            .finish()
+        f.debug_struct("Peek").field("inner", &self.inner).finish()
     }
 }
 
@@ -133,6 +220,7 @@ where
     St: Stream,
 {
     type Output = Option<&'a St::Item>;
+
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
         let inner = self.project().inner;
         if let Some(peekable) = inner {
@@ -144,3 +232,127 @@ where
         }
     }
 }
+
+pin_project! {
+    /// Future for the [`Peekable::next_if`](self::Peekable::next_if) method.
+    #[must_use = "futures do nothing unless polled"]
+    pub struct NextIf<'a, St: Stream, F> {
+        inner: Option<(Pin<&'a mut Peekable<St>>, F)>,
+    }
+}
+
+impl<St, F> fmt::Debug for NextIf<'_, St, F>
+where
+    St: Stream + fmt::Debug,
+    St::Item: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("NextIf")
+            .field("inner", &self.inner.as_ref().map(|(s, _f)| s))
+            .finish()
+    }
+}
+
+#[allow(single_use_lifetimes)] // https://github.com/rust-lang/rust/issues/55058
+impl<St, F> FusedFuture for NextIf<'_, St, F>
+where
+    St: Stream,
+    F: for<'a> FnOnce1<&'a St::Item, Output = bool>,
+{
+    fn is_terminated(&self) -> bool {
+        self.inner.is_none()
+    }
+}
+
+#[allow(single_use_lifetimes)] // https://github.com/rust-lang/rust/issues/55058
+impl<St, F> Future for NextIf<'_, St, F>
+where
+    St: Stream,
+    F: for<'a> FnOnce1<&'a St::Item, Output = bool>,
+{
+    type Output = Option<St::Item>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let inner = self.project().inner;
+        if let Some((peekable, _)) = inner {
+            let res = ready!(peekable.as_mut().poll_next(cx));
+
+            let (peekable, func) = inner.take().unwrap();
+            match res {
+                Some(ref matched) if func.call_once(matched) => Poll::Ready(res),
+                other => {
+                    let peekable = peekable.project();
+                    // Since we called `self.next()`, we consumed `self.peeked`.
+                    assert!(peekable.peeked.is_none());
+                    *peekable.peeked = other;
+                    Poll::Ready(None)
+                }
+            }
+        } else {
+            panic!("NextIf polled after completion")
+        }
+    }
+}
+
+pin_project! {
+    /// Future for the [`Peekable::next_if_eq`](self::Peekable::next_if_eq) method.
+    #[must_use = "futures do nothing unless polled"]
+    pub struct NextIfEq<'a, St: Stream, T: ?Sized> {
+        #[pin]
+        inner: NextIf<'a, St, NextIfEqFn<'a, T, St::Item>>,
+    }
+}
+
+impl<St, T> fmt::Debug for NextIfEq<'_, St, T>
+where
+    St: Stream + fmt::Debug,
+    St::Item: fmt::Debug,
+    T: ?Sized,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("NextIfEq")
+            .field("inner", &self.inner.inner.as_ref().map(|(s, _f)| s))
+            .finish()
+    }
+}
+
+impl<St, T> FusedFuture for NextIfEq<'_, St, T>
+where
+    St: Stream,
+    T: ?Sized,
+    St::Item: PartialEq<T>,
+{
+    fn is_terminated(&self) -> bool {
+        self.inner.is_terminated()
+    }
+}
+
+impl<St, T> Future for NextIfEq<'_, St, T>
+where
+    St: Stream,
+    T: ?Sized,
+    St::Item: PartialEq<T>,
+{
+    type Output = Option<St::Item>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        self.project().inner.poll(cx)
+    }
+}
+
+struct NextIfEqFn<'a, T: ?Sized, Item> {
+    expected: &'a T,
+    _next: PhantomData<Item>,
+}
+
+impl<T, Item> FnOnce1<&Item> for NextIfEqFn<'_, T, Item>
+where
+    T: ?Sized,
+    Item: PartialEq<T>,
+{
+    type Output = bool;
+
+    fn call_once(self, next: &Item) -> Self::Output {
+        next == self.expected
+    }
+}

futures/tests/stream_peekable.rs

@@ -11,3 +11,29 @@ fn peekable() {
         assert_eq!(peekable.collect::<Vec<u8>>().await, vec![1, 2, 3]);
     });
 }
+
+#[test]
+fn peekable_next_if_eq() {
+    block_on(async {
+        // first, try on references
+        let s = stream::iter(vec!["Heart", "of", "Gold"]).peekable();
+        pin_mut!(s);
+        // try before `peek()`
+        assert_eq!(s.as_mut().next_if_eq(&"trillian").await, None);
+        assert_eq!(s.as_mut().next_if_eq(&"Heart").await, Some("Heart"));
+        // try after peek()
+        assert_eq!(s.as_mut().peek().await, Some(&"of"));
+        assert_eq!(s.as_mut().next_if_eq(&"of").await, Some("of"));
+        assert_eq!(s.as_mut().next_if_eq(&"zaphod").await, None);
+        // make sure `next()` still behaves
+        assert_eq!(s.next().await, Some("Gold"));
+
+        // make sure comparison works for owned values
+        let s = stream::iter(vec![String::from("Ludicrous"), "speed".into()]).peekable();
+        pin_mut!(s);
+        // make sure basic functionality works
+        assert_eq!(s.as_mut().next_if_eq("Ludicrous").await, Some("Ludicrous".into()));
+        assert_eq!(s.as_mut().next_if_eq("speed").await, Some("speed".into()));
+        assert_eq!(s.as_mut().next_if_eq("").await, None);
+    });
+}