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use super::assert_future;
use crate::future::{Either, FutureExt};
use core::pin::Pin;
use futures_core::future::{FusedFuture, Future};
use futures_core::task::{Context, Poll};
/// Future for the [`select()`] function.
#[must_use = "futures do nothing unless you `.await` or poll them"]
#[derive(Debug)]
pub struct Select<A, B> {
inner: Option<(A, B)>,
}
impl<A: Unpin, B: Unpin> Unpin for Select<A, B> {}
/// Waits for either one of two differently-typed futures to complete.
///
/// This function will return a new future which awaits for either one of both
/// futures to complete. The returned future will finish with both the value
/// resolved and a future representing the completion of the other work.
///
/// Note that this function consumes the receiving futures and returns a
/// wrapped version of them.
///
/// Also note that if both this and the second future have the same
/// output type you can use the `Either::factor_first` method to
/// conveniently extract out the value at the end.
///
/// # Examples
///
/// A simple example
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::{
/// pin_mut,
/// future::Either,
/// future::self,
/// };
///
/// // These two futures have different types even though their outputs have the same type.
/// let future1 = async {
/// future::pending::<()>().await; // will never finish
/// 1
/// };
/// let future2 = async {
/// future::ready(2).await
/// };
///
/// // 'select' requires Future + Unpin bounds
/// pin_mut!(future1);
/// pin_mut!(future2);
///
/// let value = match future::select(future1, future2).await {
/// Either::Left((value1, _)) => value1, // `value1` is resolved from `future1`
/// // `_` represents `future2`
/// Either::Right((value2, _)) => value2, // `value2` is resolved from `future2`
/// // `_` represents `future1`
/// };
///
/// assert!(value == 2);
/// # });
/// ```
///
/// A more complex example
///
/// ```
/// use futures::future::{self, Either, Future, FutureExt};
///
/// // A poor-man's join implemented on top of select
///
/// fn join<A, B>(a: A, b: B) -> impl Future<Output=(A::Output, B::Output)>
/// where A: Future + Unpin,
/// B: Future + Unpin,
/// {
/// future::select(a, b).then(|either| {
/// match either {
/// Either::Left((x, b)) => b.map(move |y| (x, y)).left_future(),
/// Either::Right((y, a)) => a.map(move |x| (x, y)).right_future(),
/// }
/// })
/// }
/// ```
pub fn select<A, B>(future1: A, future2: B) -> Select<A, B>
where
A: Future + Unpin,
B: Future + Unpin,
{
assert_future::<Either<(A::Output, B), (B::Output, A)>, _>(Select {
inner: Some((future1, future2)),
})
}
impl<A, B> Future for Select<A, B>
where
A: Future + Unpin,
B: Future + Unpin,
{
type Output = Either<(A::Output, B), (B::Output, A)>;
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
/// When compiled with `-C opt-level=z`, this function will help the compiler eliminate the `None` branch, where
/// `Option::unwrap` does not.
#[inline(always)]
fn unwrap_option<T>(value: Option<T>) -> T {
match value {
None => unreachable!(),
Some(value) => value,
}
}
let (a, b) = self.inner.as_mut().expect("cannot poll Select twice");
if let Poll::Ready(val) = a.poll_unpin(cx) {
return Poll::Ready(Either::Left((val, unwrap_option(self.inner.take()).1)));
}
if let Poll::Ready(val) = b.poll_unpin(cx) {
return Poll::Ready(Either::Right((val, unwrap_option(self.inner.take()).0)));
}
Poll::Pending
}
}
impl<A, B> FusedFuture for Select<A, B>
where
A: Future + Unpin,
B: Future + Unpin,
{
fn is_terminated(&self) -> bool {
self.inner.is_none()
}
}