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    //! Definition of the `SelectOk` combinator, finding the first successful future
    //! in a list.

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

    use {Future, IntoFuture, Poll, Async};

    /// Future for the `select_ok` combinator, waiting for one of any of a list of
    /// futures to successfully complete. Unlike `select_all`, this future ignores all
    /// but the last error, if there are any.
    ///
    /// This is created by the `select_ok` function.
    #[derive(Debug)]
    #[must_use = "futures do nothing unless polled"]
    pub struct SelectOk<A> where A: Future {
        inner: Vec<A>,
    }

    /// 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.
    ///
    /// # Panics
    ///
    /// This function will panic if the iterator specified contains no items.
    pub fn select_ok<I>(iter: I) -> SelectOk<<I::Item as IntoFuture>::Future>
        where I: IntoIterator,
              I::Item: IntoFuture,
    {
        let ret = SelectOk {
            inner: iter.into_iter()
                       .map(|a| a.into_future())
                       .collect(),
        };
        assert!(ret.inner.len() > 0);
        ret
    }

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

        fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
            // loop until we've either exhausted all errors, a success was hit, or nothing is ready
            loop {
                let item = self.inner.iter_mut().enumerate().filter_map(|(i, f)| {
                    match f.poll() {
                        Ok(Async::NotReady) => None,
                        Ok(Async::Ready(e)) => Some((i, Ok(e))),
                        Err(e) => Some((i, Err(e))),
                    }
                }).next();

                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::replace(&mut self.inner, Vec::new());
                                return Ok(Async::Ready((e, rest)))
                            },
                            Err(e) => {
                                if self.inner.is_empty() {
                                    return Err(e)
                                }
                            },
                        }
                    }
                    None => {
                        // based on the filter above, nothing is ready, return
                        return Ok(Async::NotReady)
                    },
                }
            }
        }
    }

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