use super::assert_stream;
use crate::unfold_state::UnfoldState;
use core::fmt;
use core::pin::Pin;
use futures_core::future::Future;
use futures_core::ready;
use futures_core::stream::{FusedStream, Stream};
use futures_core::task::{Context, Poll};
use pin_project_lite::pin_project;
pub fn unfold<T, F, Fut, Item>(init: T, f: F) -> Unfold<T, F, Fut>
where
F: FnMut(T) -> Fut,
Fut: Future<Output = Option<(Item, T)>>,
{
assert_stream::<Item, _>(Unfold { f, state: UnfoldState::Value { value: init } })
}
pin_project! {
#[must_use = "streams do nothing unless polled"]
pub struct Unfold<T, F, Fut> {
f: F,
#[pin]
state: UnfoldState<T, Fut>,
}
}
impl<T, F, Fut> fmt::Debug for Unfold<T, F, Fut>
where
T: fmt::Debug,
Fut: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Unfold").field("state", &self.state).finish()
}
}
impl<T, F, Fut, Item> FusedStream for Unfold<T, F, Fut>
where
F: FnMut(T) -> Fut,
Fut: Future<Output = Option<(Item, T)>>,
{
fn is_terminated(&self) -> bool {
if let UnfoldState::Empty = self.state {
true
} else {
false
}
}
}
impl<T, F, Fut, Item> Stream for Unfold<T, F, Fut>
where
F: FnMut(T) -> Fut,
Fut: Future<Output = Option<(Item, T)>>,
{
type Item = Item;
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.as_mut().take_value() {
this.state.set(UnfoldState::Future { future: (this.f)(state) });
}
let step = match this.state.as_mut().project_future() {
Some(fut) => ready!(fut.poll(cx)),
None => panic!("Unfold must not be polled after it returned `Poll::Ready(None)`"),
};
if let Some((item, next_state)) = step {
this.state.set(UnfoldState::Value { value: next_state });
Poll::Ready(Some(item))
} else {
this.state.set(UnfoldState::Empty);
Poll::Ready(None)
}
}
}