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use super::assert_future;
use crate::future::TryFutureExt;
use alloc::vec::Vec;
use core::iter::FromIterator;
use core::mem;
use core::pin::Pin;
use futures_core::future::{Future, TryFuture};
use futures_core::task::{Context, Poll};
/// Future for the [`select_ok`] function.
#[derive(Debug)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct SelectOk<Fut> {
inner: Vec<Fut>,
}
impl<Fut: Unpin> Unpin for SelectOk<Fut> {}
/// Creates a new future which will select the first successful future over a list of futures.
///
/// The returned future will wait for any future within `iter` to be ready and Ok. Unlike
/// `select_all`, this will only return the first successful completion, or the last
/// failure. This is useful in contexts where any success is desired and failures
/// are ignored, unless all the futures fail.
///
/// This function is only available when the `std` or `alloc` feature of this
/// library is activated, and it is activated by default.
///
/// # Panics
///
/// This function will panic if the iterator specified contains no items.
pub fn select_ok<I>(iter: I) -> SelectOk<I::Item>
where
I: IntoIterator,
I::Item: TryFuture + Unpin,
{
let ret = SelectOk { inner: iter.into_iter().collect() };
assert!(!ret.inner.is_empty(), "iterator provided to select_ok was empty");
assert_future::<
Result<(<I::Item as TryFuture>::Ok, Vec<I::Item>), <I::Item as TryFuture>::Error>,
_,
>(ret)
}
impl<Fut: TryFuture + Unpin> Future for SelectOk<Fut> {
type Output = Result<(Fut::Ok, Vec<Fut>), Fut::Error>;
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
// loop until we've either exhausted all errors, a success was hit, or nothing is ready
loop {
let item =
self.inner.iter_mut().enumerate().find_map(|(i, f)| match f.try_poll_unpin(cx) {
Poll::Pending => None,
Poll::Ready(e) => Some((i, e)),
});
match item {
Some((idx, res)) => {
// always remove Ok or Err, if it's not the last Err continue looping
drop(self.inner.remove(idx));
match res {
Ok(e) => {
let rest = mem::take(&mut self.inner);
return Poll::Ready(Ok((e, rest)));
}
Err(e) => {
if self.inner.is_empty() {
return Poll::Ready(Err(e));
}
}
}
}
None => {
// based on the filter above, nothing is ready, return
return Poll::Pending;
}
}
}
}
}
impl<Fut: TryFuture + Unpin> FromIterator<Fut> for SelectOk<Fut> {
fn from_iter<T: IntoIterator<Item = Fut>>(iter: T) -> Self {
select_ok(iter)
}
}
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