Add enumerate combinator to Stream (#832)

src/lib.rs

@@ -79,7 +79,7 @@ macro_rules! if_runtime {
     )*)
 }
 
-#[cfg_attr(feature = "rt-full", macro_use)]
+#[macro_use]
 extern crate futures;
 
 #[cfg(feature = "io")]

src/util/enumerate.rs

@@ -0,0 +1,80 @@
+use futures::{Async, Poll, Stream, Sink, StartSend};
+
+/// A stream combinator which combines the yields the current item
+/// plus its count starting from 0.
+///
+/// This structure is produced by the `Stream::enumerate` method.
+#[derive(Debug)]
+#[must_use = "Does nothing unless polled"]
+pub struct Enumerate<T> {
+    inner: T,
+    count: usize,
+}
+
+impl<T> Enumerate<T> {
+    pub(crate) fn new(stream: T) -> Self {
+        Self { inner: stream, count: 0 }
+    }
+
+    /// Acquires a reference to the underlying stream that this combinator is
+    /// pulling from.
+    pub fn get_ref(&self) -> &T {
+        &self.inner
+    }
+
+    /// Acquires a mutable reference to the underlying stream that this
+    /// combinator is pulling from.
+    ///
+    /// Note that care must be taken to avoid tampering with the state of the
+    /// stream which may otherwise confuse this combinator.
+    pub fn get_mut(&mut self) -> &mut T {
+        &mut self.inner
+    }
+
+    /// Consumes this combinator, returning the underlying stream.
+    ///
+    /// Note that this may discard intermediate state of this combinator, so
+    /// care should be taken to avoid losing resources when this is called.
+    pub fn into_inner(self) -> T {
+        self.inner
+    }
+}
+
+impl<T> Stream for Enumerate<T>
+where
+    T: Stream,
+{
+    type Item = (usize, T::Item);
+    type Error = T::Error;
+
+    fn poll(&mut self) -> Poll<Option<Self::Item>, T::Error> {
+        match try_ready!(self.inner.poll()) {
+            Some(item) => {
+                let ret = Some((self.count, item));
+                self.count += 1;
+                Ok(Async::Ready(ret))
+            }
+            None => return Ok(Async::Ready(None)),
+        }
+    }
+}
+
+// Forwarding impl of Sink from the underlying stream
+impl<T> Sink for Enumerate<T>
+    where T: Sink
+{
+    type SinkItem = T::SinkItem;
+    type SinkError = T::SinkError;
+
+    fn start_send(&mut self, item: T::SinkItem) -> StartSend<T::SinkItem, T::SinkError> {
+        self.inner.start_send(item)
+    }
+
+    fn poll_complete(&mut self) -> Poll<(), T::SinkError> {
+        self.inner.poll_complete()
+    }
+
+    fn close(&mut self) -> Poll<(), T::SinkError> {
+        self.inner.close()
+    }
+}

src/util/mod.rs

@@ -9,6 +9,7 @@
 
 mod future;
 mod stream;
+mod enumerate;
 
 pub use self::future::FutureExt;
 pub use self::stream::StreamExt;

src/util/stream.rs

@@ -8,7 +8,7 @@ use futures::Stream;
 
 #[cfg(feature = "timer")]
 use std::time::Duration;
-
+pub use util::enumerate::Enumerate;
 
 /// An extension trait for `Stream` that provides a variety of convenient
 /// combinator functions.
@@ -34,6 +34,24 @@ pub trait StreamExt: Stream {
         Throttle::new(self, duration)
     }
 
+    /// Creates a new stream which gives the current iteration count as well
+    /// as the next value.
+    ///
+    /// The stream returned yields pairs `(i, val)`, where `i` is the
+    /// current index of iteration and `val` is the value returned by the
+    /// iterator.
+    ///
+    /// # Overflow Behavior
+    ///
+    /// The method does no guarding against overflows, so counting elements of
+    /// an iterator with more than [`std::usize::MAX`] elements either produces the
+    /// wrong result or panics.
+    fn enumerate(self) -> Enumerate<Self>
+    where Self: Sized,
+    {
+        Enumerate::new(self)
+    }
+
     /// Creates a new stream which allows `self` until `timeout`.
     ///
     /// This combinator creates a new stream which wraps the receiving stream

tokio-timer/tests/enumerate.rs

@@ -0,0 +1,31 @@
+extern crate futures;
+extern crate tokio;
+extern crate tokio_executor;
+extern crate tokio_timer;
+
+#[macro_use]
+mod support;
+
+use futures::prelude::*;
+use futures::sync::mpsc;
+use tokio::util::StreamExt;
+
+#[test]
+fn enumerate() {
+    use futures::*;
+
+    let (mut tx, rx) = mpsc::channel(1);
+
+    std::thread::spawn(|| {
+        for i in 0..5 {
+            tx = tx.send(i * 2).wait().unwrap();
+        }
+    });
+
+    let mut result = rx.enumerate().collect();
+    assert_eq!(
+        result.wait(),
+        Ok(vec![(0, 0), (1, 2), (2, 4), (3, 6), (4, 8)])
+    );
+
+}