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use core::pin::Pin;
use futures_core::future::Future;
use futures_core::stream::{FusedStream, Stream};
use futures_core::task::{Context, Poll};
use pin_utils::{unsafe_pinned, unsafe_unpinned};
pub fn unfold<T, F, Fut, It>(init: T, f: F) -> Unfold<T, F, Fut>
where F: FnMut(T) -> Fut,
Fut: Future<Output = Option<(It, T)>>,
{
Unfold {
f,
state: Some(init),
fut: None,
}
}
#[derive(Debug)]
#[must_use = "streams do nothing unless polled"]
pub struct Unfold<T, F, Fut> {
f: F,
state: Option<T>,
fut: Option<Fut>,
}
impl<T, F, Fut: Unpin> Unpin for Unfold<T, F, Fut> {}
impl<T, F, Fut> Unfold<T, F, Fut> {
unsafe_unpinned!(f: F);
unsafe_unpinned!(state: Option<T>);
unsafe_pinned!(fut: Option<Fut>);
}
impl<T, F, Fut> FusedStream for Unfold<T, F, Fut> {
fn is_terminated(&self) -> bool {
self.state.is_none() && self.fut.is_none()
}
}
impl<T, F, Fut, It> Stream for Unfold<T, F, Fut>
where F: FnMut(T) -> Fut,
Fut: Future<Output = Option<(It, T)>>,
{
type Item = It;
fn poll_next(
mut self: Pin<&mut Self>,
cx: &mut Context<'_>,
) -> Poll<Option<It>> {
if let Some(state) = self.as_mut().state().take() {
let fut = (self.as_mut().f())(state);
self.as_mut().fut().set(Some(fut));
}
let step = ready!(self.as_mut().fut().as_pin_mut().unwrap().poll(cx));
self.as_mut().fut().set(None);
if let Some((item, next_state)) = step {
*self.as_mut().state() = Some(next_state);
return Poll::Ready(Some(item))
} else {
return Poll::Ready(None)
}
}
}