stream: filtering utilities (#2001)

Adds `StreamExt::filter` and `StreamExt::filter_map`.

1 files changed, 62 insertions, 0 deletions

diff --git a/tokio/src/stream/filter_map.rs b/tokio/src/stream/filter_map.rs
new file mode 100644
index 00000000..3aeb036f
--- /dev/null
+++ b/tokio/src/stream/filter_map.rs
@@ -0,0 +1,62 @@
+use crate::stream::Stream;
+
+use core::fmt;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream returned by the [`filter_map`](super::StreamExt::filter_map) method.
+    #[must_use = "streams do nothing unless polled"]
+    pub struct FilterMap<St, F> {
+        #[pin]
+        stream: St,
+        f: F,
+    }
+}
+
+impl<St, F> fmt::Debug for FilterMap<St, F>
+where
+    St: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("FilterMap")
+            .field("stream", &self.stream)
+            .finish()
+    }
+}
+
+impl<St, F, T> FilterMap<St, F>
+where
+    St: Stream,
+    F: FnMut(St::Item) -> Option<T>,
+{
+    pub(super) fn new(stream: St, f: F) -> Self {
+        Self { stream, f }
+    }
+}
+
+impl<St, F, T> Stream for FilterMap<St, F>
+where
+    St: Stream,
+    F: FnMut(St::Item) -> Option<T>,
+{
+    type Item = T;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        loop {
+            match ready!(self.as_mut().project().stream.poll_next(cx)) {
+                Some(e) => {
+                    if let Some(e) = (self.as_mut().project().f)(e) {
+                        return Poll::Ready(Some(e));
+                    }
+                }
+                None => return Poll::Ready(None),
+            }
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, self.stream.size_hint().1) // can't know a lower bound, due to the predicate
+    }
+}