diff options
Diffstat (limited to 'src/stream_ext/then.rs')
-rw-r--r-- | src/stream_ext/then.rs | 83 |
1 files changed, 83 insertions, 0 deletions
diff --git a/src/stream_ext/then.rs b/src/stream_ext/then.rs new file mode 100644 index 0000000..7f6b5a2 --- /dev/null +++ b/src/stream_ext/then.rs @@ -0,0 +1,83 @@ +use crate::Stream; + +use core::fmt; +use core::future::Future; +use core::pin::Pin; +use core::task::{Context, Poll}; +use pin_project_lite::pin_project; + +pin_project! { + /// Stream for the [`then`](super::StreamExt::then) method. + #[must_use = "streams do nothing unless polled"] + pub struct Then<St, Fut, F> { + #[pin] + stream: St, + #[pin] + future: Option<Fut>, + f: F, + } +} + +impl<St, Fut, F> fmt::Debug for Then<St, Fut, F> +where + St: fmt::Debug, +{ + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + f.debug_struct("Then") + .field("stream", &self.stream) + .finish() + } +} + +impl<St, Fut, F> Then<St, Fut, F> { + pub(super) fn new(stream: St, f: F) -> Self { + Then { + stream, + future: None, + f, + } + } +} + +impl<St, F, Fut> Stream for Then<St, Fut, F> +where + St: Stream, + Fut: Future, + F: FnMut(St::Item) -> Fut, +{ + type Item = Fut::Output; + + fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Fut::Output>> { + let mut me = self.project(); + + loop { + if let Some(future) = me.future.as_mut().as_pin_mut() { + match future.poll(cx) { + Poll::Ready(item) => { + me.future.set(None); + return Poll::Ready(Some(item)); + } + Poll::Pending => return Poll::Pending, + } + } + + match me.stream.as_mut().poll_next(cx) { + Poll::Ready(Some(item)) => { + me.future.set(Some((me.f)(item))); + } + Poll::Ready(None) => return Poll::Ready(None), + Poll::Pending => return Poll::Pending, + } + } + } + + fn size_hint(&self) -> (usize, Option<usize>) { + let future_len = if self.future.is_some() { 1 } else { 0 }; + let (lower, upper) = self.stream.size_hint(); + + let lower = lower.saturating_add(future_len); + let upper = upper.and_then(|upper| upper.checked_add(future_len)); + + (lower, upper) + } +} |