futures_util/future/
select_all.rs

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//! Definition of the `SelectAll`, finding the first future in a list that
//! finishes.

use std::mem;
use std::prelude::v1::*;

use futures_core::{Future, IntoFuture, Poll, Async};
use futures_core::task;

/// Future for the `select_all` combinator, waiting for one of any of a list of
/// futures to complete.
///
/// This is created by the `select_all` function.
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
pub struct SelectAll<A> where A: Future {
    inner: Vec<A>,
}

#[doc(hidden)]
pub type SelectAllNext<A> = A;

/// Creates a new future which will select over a list of futures.
///
/// The returned future will wait for any future within `iter` to be ready. Upon
/// completion or failure the item resolved will be returned, along with the
/// index of the future that was ready and the list of all the remaining
/// futures.
///
/// # Panics
///
/// This function will panic if the iterator specified contains no items.
pub fn select_all<I>(iter: I) -> SelectAll<<I::Item as IntoFuture>::Future>
    where I: IntoIterator,
          I::Item: IntoFuture,
{
    let ret = SelectAll {
        inner: iter.into_iter()
                   .map(|a| a.into_future())
                   .collect(),
    };
    assert!(ret.inner.len() > 0);
    ret
}

impl<A> Future for SelectAll<A>
    where A: Future,
{
    type Item = (A::Item, usize, Vec<A>);
    type Error = (A::Error, usize, Vec<A>);

    fn poll(&mut self, cx: &mut task::Context) -> Poll<Self::Item, Self::Error> {
        let item = self.inner.iter_mut().enumerate().filter_map(|(i, f)| {
            match f.poll(cx) {
                Ok(Async::Pending) => None,
                Ok(Async::Ready(e)) => Some((i, Ok(e))),
                Err(e) => Some((i, Err(e))),
            }
        }).next();
        match item {
            Some((idx, res)) => {
                self.inner.swap_remove(idx);
                let rest = mem::replace(&mut self.inner, Vec::new());
                match res {
                    Ok(e) => Ok(Async::Ready((e, idx, rest))),
                    Err(e) => Err((e, idx, rest)),
                }
            }
            None => Ok(Async::Pending),
        }
    }
}