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use super::assert_sink;
use crate::unfold_state::UnfoldState;
use core::{future::Future, pin::Pin};
use futures_core::ready;
use futures_core::task::{Context, Poll};
use futures_sink::Sink;
use pin_project_lite::pin_project;
pin_project! {
#[derive(Debug)]
#[must_use = "sinks do nothing unless polled"]
pub struct Unfold<T, F, R> {
function: F,
#[pin]
state: UnfoldState<T, R>,
}
}
pub fn unfold<T, F, R, Item, E>(init: T, function: F) -> Unfold<T, F, R>
where
F: FnMut(T, Item) -> R,
R: Future<Output = Result<T, E>>,
{
assert_sink::<Item, E, _>(Unfold {
function,
state: UnfoldState::Value { value: init },
})
}
impl<T, F, R, Item, E> Sink<Item> for Unfold<T, F, R>
where
F: FnMut(T, Item) -> R,
R: Future<Output = Result<T, E>>,
{
type Error = E;
fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
self.poll_flush(cx)
}
fn start_send(self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error> {
let mut this = self.project();
let future = match this.state.as_mut().take_value() {
Some(value) => (this.function)(value, item),
None => panic!("start_send called without poll_ready being called first"),
};
this.state.set(UnfoldState::Future { future });
Ok(())
}
fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
let mut this = self.project();
Poll::Ready(if let Some(future) = this.state.as_mut().project_future() {
match ready!(future.poll(cx)) {
Ok(state) => {
this.state.set(UnfoldState::Value { value: state });
Ok(())
}
Err(err) => Err(err),
}
} else {
Ok(())
})
}
fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
self.poll_flush(cx)
}
}