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use core::pin::Pin;
use futures_core::stream::{FusedStream, Stream};
use futures_core::task::{Context, Poll};
use pin_project::{pin_project, project};
#[pin_project]
#[derive(Debug)]
#[must_use = "streams do nothing unless polled"]
pub struct Chain<St1, St2> {
#[pin]
first: Option<St1>,
#[pin]
second: St2,
}
impl<St1, St2> Chain<St1, St2>
where St1: Stream,
St2: Stream<Item = St1::Item>,
{
pub(super) fn new(stream1: St1, stream2: St2) -> Chain<St1, St2> {
Chain {
first: Some(stream1),
second: stream2,
}
}
}
impl<St1, St2> FusedStream for Chain<St1, St2>
where St1: Stream,
St2: FusedStream<Item=St1::Item>,
{
fn is_terminated(&self) -> bool {
self.first.is_none() && self.second.is_terminated()
}
}
impl<St1, St2> Stream for Chain<St1, St2>
where St1: Stream,
St2: Stream<Item=St1::Item>,
{
type Item = St1::Item;
#[project]
fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
) -> Poll<Option<Self::Item>> {
#[project]
let Chain { mut first, second } = self.project();
if let Some(first) = first.as_mut().as_pin_mut() {
if let Some(item) = ready!(first.poll_next(cx)) {
return Poll::Ready(Some(item))
}
}
first.set(None);
second.poll_next(cx)
}
fn size_hint(&self) -> (usize, Option<usize>) {
if let Some(first) = &self.first {
let (first_lower, first_upper) = first.size_hint();
let (second_lower, second_upper) = self.second.size_hint();
let lower = first_lower.saturating_add(second_lower);
let upper = match (first_upper, second_upper) {
(Some(x), Some(y)) => x.checked_add(y),
_ => None
};
(lower, upper)
} else {
self.second.size_hint()
}
}
}