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use crate::stream_ext::Fuse;
use crate::Stream;
use core::pin::Pin;
use core::task::{Context, Poll};
use pin_project_lite::pin_project;
pin_project! {
/// Stream returned by the [`merge`](super::StreamExt::merge) method.
pub struct Merge<T, U> {
#[pin]
a: Fuse<T>,
#[pin]
b: Fuse<U>,
// When `true`, poll `a` first, otherwise, `poll` b`.
a_first: bool,
}
}
impl<T, U> Merge<T, U> {
pub(super) fn new(a: T, b: U) -> Merge<T, U>
where
T: Stream,
U: Stream,
{
Merge {
a: Fuse::new(a),
b: Fuse::new(b),
a_first: true,
}
}
}
impl<T, U> Stream for Merge<T, U>
where
T: Stream,
U: Stream<Item = T::Item>,
{
type Item = T::Item;
fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T::Item>> {
let me = self.project();
let a_first = *me.a_first;
// Toggle the flag
*me.a_first = !a_first;
if a_first {
poll_next(me.a, me.b, cx)
} else {
poll_next(me.b, me.a, cx)
}
}
fn size_hint(&self) -> (usize, Option<usize>) {
super::merge_size_hints(self.a.size_hint(), self.b.size_hint())
}
}
fn poll_next<T, U>(
first: Pin<&mut T>,
second: Pin<&mut U>,
cx: &mut Context<'_>,
) -> Poll<Option<T::Item>>
where
T: Stream,
U: Stream<Item = T::Item>,
{
use Poll::*;
let mut done = true;
match first.poll_next(cx) {
Ready(Some(val)) => return Ready(Some(val)),
Ready(None) => {}
Pending => done = false,
}
match second.poll_next(cx) {
Ready(Some(val)) => return Ready(Some(val)),
Ready(None) => {}
Pending => done = false,
}
if done {
Ready(None)
} else {
Pending
}
}