blob: 64e2d6f9e5f57bd4909e2bcca0123d0aa1927221 [file] [log] [blame]
use std::cmp::{Eq, PartialEq, PartialOrd, Ord, Ordering};
use std::collections::BinaryHeap;
use std::fmt::{self, Debug};
use std::iter::FromIterator;
use {Async, Future, IntoFuture, Poll, Stream};
use stream::FuturesUnordered;
struct OrderWrapper<T> {
item: T,
index: usize,
impl<T> PartialEq for OrderWrapper<T> {
fn eq(&self, other: &Self) -> bool {
self.index == other.index
impl<T> Eq for OrderWrapper<T> {}
impl<T> PartialOrd for OrderWrapper<T> {
fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
impl<T> Ord for OrderWrapper<T> {
fn cmp(&self, other: &Self) -> Ordering {
// BinaryHeap is a max heap, so compare backwards here.
impl<T> Future for OrderWrapper<T>
where T: Future
type Item = OrderWrapper<T::Item>;
type Error = T::Error;
fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
let result = try_ready!(self.item.poll());
Ok(Async::Ready(OrderWrapper {
item: result,
index: self.index
/// An unbounded queue of futures.
/// This "combinator" is similar to `FuturesUnordered`, but it imposes an order
/// on top of the set of futures. While futures in the set will race to
/// completion in parallel, results will only be returned in the order their
/// originating futures were added to the queue.
/// Futures are pushed into this queue and their realized values are yielded in
/// order. This structure is optimized to manage a large number of futures.
/// Futures managed by `FuturesOrdered` will only be polled when they generate
/// notifications. This reduces the required amount of work needed to coordinate
/// large numbers of futures.
/// When a `FuturesOrdered` is first created, it does not contain any futures.
/// Calling `poll` in this state will result in `Ok(Async::Ready(None))` to be
/// returned. Futures are submitted to the queue using `push`; however, the
/// future will **not** be polled at this point. `FuturesOrdered` will only
/// poll managed futures when `FuturesOrdered::poll` is called. As such, it
/// is important to call `poll` after pushing new futures.
/// If `FuturesOrdered::poll` returns `Ok(Async::Ready(None))` this means that
/// the queue is currently not managing any futures. A future may be submitted
/// to the queue at a later time. At that point, a call to
/// `FuturesOrdered::poll` will either return the future's resolved value
/// **or** `Ok(Async::NotReady)` if the future has not yet completed. When
/// multiple futures are submitted to the queue, `FuturesOrdered::poll` will
/// return `Ok(Async::NotReady)` until the first future completes, even if
/// some of the later futures have already completed.
/// Note that you can create a ready-made `FuturesOrdered` via the
/// `futures_ordered` function in the `stream` module, or you can start with an
/// empty queue with the `FuturesOrdered::new` constructor.
#[must_use = "streams do nothing unless polled"]
pub struct FuturesOrdered<T>
where T: Future
in_progress: FuturesUnordered<OrderWrapper<T>>,
queued_results: BinaryHeap<OrderWrapper<T::Item>>,
next_incoming_index: usize,
next_outgoing_index: usize,
/// Converts a list of futures into a `Stream` of results from the futures.
/// This function will take an list of futures (e.g. a vector, an iterator,
/// etc), and return a stream. The stream will yield items as they become
/// available on the futures internally, in the order that their originating
/// futures were submitted to the queue. If the futures complete out of order,
/// items will be stored internally within `FuturesOrdered` until all preceding
/// items have been yielded.
/// Note that the returned queue can also be used to dynamically push more
/// futures into the queue as they become available.
pub fn futures_ordered<I>(futures: I) -> FuturesOrdered<<I::Item as IntoFuture>::Future>
where I: IntoIterator,
I::Item: IntoFuture
let mut queue = FuturesOrdered::new();
for future in futures {
return queue
impl<T> FuturesOrdered<T>
where T: Future
/// Constructs a new, empty `FuturesOrdered`
/// The returned `FuturesOrdered` does not contain any futures and, in this
/// state, `FuturesOrdered::poll` will return `Ok(Async::Ready(None))`.
pub fn new() -> FuturesOrdered<T> {
FuturesOrdered {
in_progress: FuturesUnordered::new(),
queued_results: BinaryHeap::new(),
next_incoming_index: 0,
next_outgoing_index: 0,
/// Returns the number of futures contained in the queue.
/// This represents the total number of in-flight futures, both
/// those currently processing and those that have completed but
/// which are waiting for earlier futures to complete.
pub fn len(&self) -> usize {
self.in_progress.len() + self.queued_results.len()
/// Returns `true` if the queue contains no futures
pub fn is_empty(&self) -> bool {
self.in_progress.is_empty() && self.queued_results.is_empty()
/// Push a future into the queue.
/// This function submits the given future to the internal set for managing.
/// This function will not call `poll` on the submitted future. The caller
/// must ensure that `FuturesOrdered::poll` is called in order to receive
/// task notifications.
pub fn push(&mut self, future: T) {
let wrapped = OrderWrapper {
item: future,
index: self.next_incoming_index,
self.next_incoming_index += 1;
impl<T> Stream for FuturesOrdered<T>
where T: Future
type Item = T::Item;
type Error = T::Error;
fn poll(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
// Get any completed futures from the unordered set.
loop {
match self.in_progress.poll()? {
Async::Ready(Some(result)) => self.queued_results.push(result),
Async::Ready(None) | Async::NotReady => break,
if let Some(next_result) = self.queued_results.peek() {
// PeekMut::pop is not stable yet QQ
if next_result.index != self.next_outgoing_index {
return Ok(Async::NotReady);
} else if !self.in_progress.is_empty() {
return Ok(Async::NotReady);
} else {
return Ok(Async::Ready(None));
let next_result = self.queued_results.pop().unwrap();
self.next_outgoing_index += 1;
impl<T: Debug> Debug for FuturesOrdered<T>
where T: Future
fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
write!(fmt, "FuturesOrdered {{ ... }}")
impl<F: Future> FromIterator<F> for FuturesOrdered<F> {
fn from_iter<T>(iter: T) -> Self
where T: IntoIterator<Item = F>
let mut new = FuturesOrdered::new();
for future in iter.into_iter() {