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use super::assert_stream;
use core::fmt;
use core::pin::Pin;
use futures_core::future::TryFuture;
use futures_core::ready;
use futures_core::stream::Stream;
use futures_core::task::{Context, Poll};
use pin_project_lite::pin_project;
pub fn try_unfold<T, F, Fut, Item>(init: T, f: F) -> TryUnfold<T, F, Fut>
where
F: FnMut(T) -> Fut,
Fut: TryFuture<Ok = Option<(Item, T)>>,
{
assert_stream::<Result<Item, Fut::Error>, _>(TryUnfold { f, state: Some(init), fut: None })
}
pin_project! {
#[must_use = "streams do nothing unless polled"]
pub struct TryUnfold<T, F, Fut> {
f: F,
state: Option<T>,
#[pin]
fut: Option<Fut>,
}
}
impl<T, F, Fut> fmt::Debug for TryUnfold<T, F, Fut>
where
T: fmt::Debug,
Fut: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("TryUnfold").field("state", &self.state).field("fut", &self.fut).finish()
}
}
impl<T, F, Fut, Item> Stream for TryUnfold<T, F, Fut>
where
F: FnMut(T) -> Fut,
Fut: TryFuture<Ok = Option<(Item, T)>>,
{
type Item = Result<Item, Fut::Error>;
fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
let mut this = self.project();
if let Some(state) = this.state.take() {
this.fut.set(Some((this.f)(state)));
}
match this.fut.as_mut().as_pin_mut() {
None => {
Poll::Ready(None)
}
Some(future) => {
let step = ready!(future.try_poll(cx));
this.fut.set(None);
match step {
Ok(Some((item, next_state))) => {
*this.state = Some(next_state);
Poll::Ready(Some(Ok(item)))
}
Ok(None) => Poll::Ready(None),
Err(e) => Poll::Ready(Some(Err(e))),
}
}
}
}
}