Merge pull request rust-lang#310 from troplin/take_while

Stream::take_while Implementation

src/stream/mod.rs

@@ -35,6 +35,7 @@ mod select;
 mod skip;
 mod skip_while;
 mod take;
+mod take_while;
 mod then;
 mod unfold;
 mod zip;
@@ -58,6 +59,7 @@ pub use self::select::Select;
 pub use self::skip::Skip;
 pub use self::skip_while::SkipWhile;
 pub use self::take::Take;
+pub use self::take_while::TakeWhile;
 pub use self::then::Then;
 pub use self::unfold::{Unfold, unfold};
 pub use self::zip::Zip;
@@ -582,6 +584,20 @@ pub trait Stream {
         skip_while::new(self, pred)
     }
 
+    /// Take elements from this stream while the predicate provided resolves to
+    /// `true`.
+    ///
+    /// This function, like `Iterator::take_while`, will take elements from the
+    /// stream until the `predicate` resolves to `false`. Once one element
+    /// returns false it will always return that the stream is done.
+    fn take_while<P, R>(self, pred: P) -> TakeWhile<Self, P, R>
+        where P: FnMut(&Self::Item) -> R,
+              R: IntoFuture<Item=bool, Error=Self::Error>,
+              Self: Sized
+    {
+        take_while::new(self, pred)
+    }
+
     /// Runs this stream to completion, executing the provided closure for each
     /// element on the stream.
     ///

src/stream/take_while.rs

@@ -0,0 +1,83 @@
+use {Async, Poll, IntoFuture, Future};
+use stream::Stream;
+
+/// A stream combinator which takes elements from a stream while a predicate
+/// holds.
+///
+/// This structure is produced by the `Stream::take_while` method.
+#[must_use = "streams do nothing unless polled"]
+pub struct TakeWhile<S, P, R> where S: Stream, R: IntoFuture {
+    stream: S,
+    pred: P,
+    pending: Option<(R::Future, S::Item)>,
+    done_taking: bool,
+}
+
+pub fn new<S, P, R>(s: S, p: P) -> TakeWhile<S, P, R>
+    where S: Stream,
+          P: FnMut(&S::Item) -> R,
+          R: IntoFuture<Item=bool, Error=S::Error>,
+{
+    TakeWhile {
+        stream: s,
+        pred: p,
+        pending: None,
+        done_taking: false,
+    }
+}
+
+// Forwarding impl of Sink from the underlying stream
+impl<S, P, R> ::sink::Sink for TakeWhile<S, P, R>
+    where S: ::sink::Sink + Stream, R: IntoFuture
+{
+    type SinkItem = S::SinkItem;
+    type SinkError = S::SinkError;
+
+    fn start_send(&mut self, item: S::SinkItem) -> ::StartSend<S::SinkItem, S::SinkError> {
+        self.stream.start_send(item)
+    }
+
+    fn poll_complete(&mut self) -> Poll<(), S::SinkError> {
+        self.stream.poll_complete()
+    }
+}
+
+impl<S, P, R> Stream for TakeWhile<S, P, R>
+    where S: Stream,
+          P: FnMut(&S::Item) -> R,
+          R: IntoFuture<Item=bool, Error=S::Error>,
+{
+    type Item = S::Item;
+    type Error = S::Error;
+
+    fn poll(&mut self) -> Poll<Option<S::Item>, S::Error> {
+        if self.done_taking {
+            return Ok(Async::Ready(None));
+        }
+
+        if self.pending.is_none() {
+            let item = match try_ready!(self.stream.poll()) {
+                Some(e) => e,
+                None => return Ok(Async::Ready(None)),
+            };
+            self.pending = Some(((self.pred)(&item).into_future(), item));
+        }
+
+        assert!(self.pending.is_some());
+        match self.pending.as_mut().unwrap().0.poll() {
+            Ok(Async::Ready(true)) => {
+                let (_, item) = self.pending.take().unwrap();
+                Ok(Async::Ready(Some(item)))
+            },
+            Ok(Async::Ready(false)) => {
+                self.done_taking = true;
+                Ok(Async::Ready(None))
+            }
+            Ok(Async::NotReady) => Ok(Async::NotReady),
+            Err(e) => {
+                self.pending = None;
+                Err(e)
+            }
+        }
+    }
+}

tests/stream.rs

@@ -116,6 +116,12 @@ fn take() {
     assert_done(|| list().take(2).collect(), Ok(vec![1, 2]));
 }
 
+#[test]
+fn take_while() {
+    assert_done(|| list().take_while(|e| Ok(*e < 3)).collect(),
+                Ok(vec![1, 2]));
+}
+
 #[test]
 fn take_passes_errors_through() {
     let mut s = iter(vec![Err(1), Err(2), Ok(3), Ok(4), Err(4)])