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use crate::stream::Stream;
use core::future::Future;
use core::marker::PhantomPinned;
use core::pin::Pin;
use core::task::{Context, Poll};
use pin_project_lite::pin_project;
pin_project! {
/// Future returned by the [`fold`](super::StreamExt::fold) method.
#[derive(Debug)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct FoldFuture<St, B, F> {
#[pin]
stream: St,
acc: Option<B>,
f: F,
// Make this future `!Unpin` for compatibility with async trait methods.
#[pin]
_pin: PhantomPinned,
}
}
impl<St, B, F> FoldFuture<St, B, F> {
pub(super) fn new(stream: St, init: B, f: F) -> Self {
Self {
stream,
acc: Some(init),
f,
_pin: PhantomPinned,
}
}
}
impl<St, B, F> Future for FoldFuture<St, B, F>
where
St: Stream,
F: FnMut(B, St::Item) -> B,
{
type Output = B;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let mut me = self.project();
loop {
let next = ready!(me.stream.as_mut().poll_next(cx));
match next {
Some(v) => {
let old = me.acc.take().unwrap();
let new = (me.f)(old, v);
*me.acc = Some(new);
}
None => return Poll::Ready(me.acc.take().unwrap()),
}
}
}
}
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