Add enumerate combinator to Stream (#832)

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()
+    }
+}