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use {Future, Poll, Async};
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
pub struct Select<A, B> where A: Future, B: Future<Item=A::Item, Error=A::Error> {
inner: Option<(A, B)>,
}
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
pub struct SelectNext<A, B> where A: Future, B: Future<Item=A::Item, Error=A::Error> {
inner: OneOf<A, B>,
}
#[derive(Debug)]
enum OneOf<A, B> where A: Future, B: Future {
A(A),
B(B),
}
pub fn new<A, B>(a: A, b: B) -> Select<A, B>
where A: Future,
B: Future<Item=A::Item, Error=A::Error>
{
Select {
inner: Some((a, b)),
}
}
impl<A, B> Future for Select<A, B>
where A: Future,
B: Future<Item=A::Item, Error=A::Error>,
{
type Item = (A::Item, SelectNext<A, B>);
type Error = (A::Error, SelectNext<A, B>);
fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
let (ret, is_a) = match self.inner {
Some((ref mut a, ref mut b)) => {
match a.poll() {
Err(a) => (Err(a), true),
Ok(Async::Ready(a)) => (Ok(a), true),
Ok(Async::NotReady) => {
match b.poll() {
Err(a) => (Err(a), false),
Ok(Async::Ready(a)) => (Ok(a), false),
Ok(Async::NotReady) => return Ok(Async::NotReady),
}
}
}
}
None => panic!("cannot poll select twice"),
};
let (a, b) = self.inner.take().unwrap();
let next = if is_a {OneOf::B(b)} else {OneOf::A(a)};
let next = SelectNext { inner: next };
match ret {
Ok(a) => Ok(Async::Ready((a, next))),
Err(e) => Err((e, next)),
}
}
}
impl<A, B> Future for SelectNext<A, B>
where A: Future,
B: Future<Item=A::Item, Error=A::Error>,
{
type Item = A::Item;
type Error = A::Error;
fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
match self.inner {
OneOf::A(ref mut a) => a.poll(),
OneOf::B(ref mut b) => b.poll(),
}
}
}