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use core::mem;
use {Future, Poll, IntoFuture, Async};
use stream::Stream;
#[derive(Debug)]
#[must_use = "streams do nothing unless polled"]
pub struct Fold<S, F, Fut, T> where Fut: IntoFuture {
stream: S,
f: F,
state: State<T, Fut::Future>,
}
#[derive(Debug)]
enum State<T, F> where F: Future {
Empty,
Ready(T),
Processing(F),
}
pub fn new<S, F, Fut, T>(s: S, f: F, t: T) -> Fold<S, F, Fut, T>
where S: Stream,
F: FnMut(T, S::Item) -> Fut,
Fut: IntoFuture<Item = T>,
S::Error: From<Fut::Error>,
{
Fold {
stream: s,
f: f,
state: State::Ready(t),
}
}
impl<S, F, Fut, T> Future for Fold<S, F, Fut, T>
where S: Stream,
F: FnMut(T, S::Item) -> Fut,
Fut: IntoFuture<Item = T>,
S::Error: From<Fut::Error>,
{
type Item = T;
type Error = S::Error;
fn poll(&mut self) -> Poll<T, S::Error> {
loop {
match mem::replace(&mut self.state, State::Empty) {
State::Empty => panic!("cannot poll Fold twice"),
State::Ready(state) => {
match self.stream.poll()? {
Async::Ready(Some(e)) => {
let future = (self.f)(state, e);
let future = future.into_future();
self.state = State::Processing(future);
}
Async::Ready(None) => return Ok(Async::Ready(state)),
Async::NotReady => {
self.state = State::Ready(state);
return Ok(Async::NotReady)
}
}
}
State::Processing(mut fut) => {
match fut.poll()? {
Async::Ready(state) => self.state = State::Ready(state),
Async::NotReady => {
self.state = State::Processing(fut);
return Ok(Async::NotReady)
}
}
}
}
}
}
}