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use core::fmt;
use core::pin::Pin;
use futures_core::future::{FusedFuture, Future, TryFuture};
use futures_core::stream::TryStream;
use futures_core::task::{Context, Poll};
use pin_project::{pin_project, project};
#[pin_project]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct TryFold<St, Fut, T, F> {
#[pin]
stream: St,
f: F,
accum: Option<T>,
#[pin]
future: Option<Fut>,
}
impl<St, Fut, T, F> fmt::Debug for TryFold<St, Fut, T, F>
where
St: fmt::Debug,
Fut: fmt::Debug,
T: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("TryFold")
.field("stream", &self.stream)
.field("accum", &self.accum)
.field("future", &self.future)
.finish()
}
}
impl<St, Fut, T, F> TryFold<St, Fut, T, F>
where St: TryStream,
F: FnMut(T, St::Ok) -> Fut,
Fut: TryFuture<Ok = T, Error = St::Error>,
{
pub(super) fn new(stream: St, f: F, t: T) -> TryFold<St, Fut, T, F> {
TryFold {
stream,
f,
accum: Some(t),
future: None,
}
}
}
impl<St, Fut, T, F> FusedFuture for TryFold<St, Fut, T, F>
where St: TryStream,
F: FnMut(T, St::Ok) -> Fut,
Fut: TryFuture<Ok = T, Error = St::Error>,
{
fn is_terminated(&self) -> bool {
self.accum.is_none() && self.future.is_none()
}
}
impl<St, Fut, T, F> Future for TryFold<St, Fut, T, F>
where St: TryStream,
F: FnMut(T, St::Ok) -> Fut,
Fut: TryFuture<Ok = T, Error = St::Error>,
{
type Output = Result<T, St::Error>;
#[project]
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
#[project]
let TryFold { mut stream, f, accum, mut future } = self.project();
Poll::Ready(loop {
if let Some(fut) = future.as_mut().as_pin_mut() {
let res = ready!(fut.try_poll(cx));
future.set(None);
match res {
Ok(a) => *accum = Some(a),
Err(e) => break Err(e),
}
} else if accum.is_some() {
let res = ready!(stream.as_mut().try_poll_next(cx));
let a = accum.take().unwrap();
match res {
Some(Ok(item)) => future.set(Some(f(a, item))),
Some(Err(e)) => break Err(e),
None => break Ok(a),
}
} else {
panic!("Fold polled after completion")
}
})
}
}