ordered_stream/join.rs
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use crate::*;
use core::mem;
use core::pin::Pin;
use core::task::{Context, Poll};
pin_project_lite::pin_project! {
/// A stream for the [`join`](fn.join.html) function.
#[derive(Debug)]
pub struct Join<A, B>
where
A: OrderedStream,
B: OrderedStream<Data = A::Data, Ordering=A::Ordering>,
{
#[pin]
stream_a: A,
#[pin]
stream_b: B,
state: JoinState<A::Data, B::Data, A::Ordering>,
}
}
/// Join two streams while preserving the overall ordering of elements.
///
/// You can think of this as implementing the "merge" step of a merge sort on the two streams,
/// producing a single stream that is sorted given two sorted streams. If the streams return
/// [`PollResult::NoneBefore`] as intended, then the joined stream will be able to produce items
/// when only one of the sources has unblocked.
pub fn join<A, B>(stream_a: A, stream_b: B) -> Join<A, B>
where
A: OrderedStream,
B: OrderedStream<Data = A::Data, Ordering = A::Ordering>,
{
Join {
stream_a,
stream_b,
state: JoinState::None,
}
}
#[derive(Debug)]
enum JoinState<A, B, T> {
None,
A(A, T),
B(B, T),
OnlyPollA,
OnlyPollB,
Terminated,
}
impl<A, B, T> JoinState<A, B, T> {
fn take_split(&mut self) -> (PollState<A, T>, PollState<B, T>) {
match mem::replace(self, JoinState::None) {
JoinState::None => (PollState::Pending, PollState::Pending),
JoinState::A(a, t) => (PollState::Item(a, t), PollState::Pending),
JoinState::B(b, t) => (PollState::Pending, PollState::Item(b, t)),
JoinState::OnlyPollA => (PollState::Pending, PollState::Terminated),
JoinState::OnlyPollB => (PollState::Terminated, PollState::Pending),
JoinState::Terminated => (PollState::Terminated, PollState::Terminated),
}
}
}
/// A helper equivalent to Poll<PollResult<T, I>> but easier to match
pub(crate) enum PollState<I, T> {
Item(I, T),
Pending,
NoneBefore,
Terminated,
}
impl<I, T: Ord> PollState<I, T> {
fn ordering(&self) -> Option<&T> {
match self {
Self::Item(_, t) => Some(t),
_ => None,
}
}
fn update(
&mut self,
before: Option<&T>,
other_token: Option<&T>,
retry: bool,
run: impl FnOnce(Option<&T>) -> Poll<PollResult<T, I>>,
) -> bool {
match self {
// Do not re-poll if we have an item already or if we are terminated
Self::Item { .. } | Self::Terminated => return false,
// No need to re-poll if we already declared no items <= before
Self::NoneBefore if retry => return false,
_ => {}
}
// Run the poll with the earlier of the two tokens to avoid transitioning to Pending (which
// will stall the Join) when we could have transitioned to NoneBefore.
let ordering = match (before, other_token) {
(Some(u), Some(o)) => {
if *u > *o {
// The other ordering is earlier - so a retry might let us upgrade a Pending to a
// NoneBefore
Some(o)
} else if retry {
// A retry will not improve matters, so don't bother
return false;
} else {
Some(u)
}
}
(Some(t), None) | (None, Some(t)) => Some(t),
(None, None) => None,
};
*self = run(ordering).into();
matches!(self, Self::Item { .. })
}
}
impl<I, T> From<PollState<I, T>> for Poll<PollResult<T, I>> {
fn from(poll: PollState<I, T>) -> Self {
match poll {
PollState::Item(data, ordering) => Poll::Ready(PollResult::Item { data, ordering }),
PollState::Pending => Poll::Pending,
PollState::NoneBefore => Poll::Ready(PollResult::NoneBefore),
PollState::Terminated => Poll::Ready(PollResult::Terminated),
}
}
}
impl<I, T> From<Poll<PollResult<T, I>>> for PollState<I, T> {
fn from(poll: Poll<PollResult<T, I>>) -> Self {
match poll {
Poll::Ready(PollResult::Item { data, ordering }) => Self::Item(data, ordering),
Poll::Ready(PollResult::NoneBefore) => Self::NoneBefore,
Poll::Ready(PollResult::Terminated) => Self::Terminated,
Poll::Pending => Self::Pending,
}
}
}
impl<A, B> Join<A, B>
where
A: OrderedStream,
B: OrderedStream<Data = A::Data, Ordering = A::Ordering>,
{
/// Split into the source streams.
///
/// This method returns the source streams along with any buffered item and its
/// ordering.
pub fn into_inner(self) -> (A, B, Option<(A::Data, A::Ordering)>) {
let item = match self.state {
JoinState::A(a, o) => Some((a, o)),
JoinState::B(b, o) => Some((b, o)),
_ => None,
};
(self.stream_a, self.stream_b, item)
}
/// Provide direct access to the underlying stream.
///
/// This may be useful if the stream provides APIs beyond [OrderedStream]. Note that the join
/// itself may be buffering an item from this stream, so you should consult
/// [Self::peek_buffered] and, if needed, [Self::take_buffered] before polling it directly.
pub fn stream_a(self: Pin<&mut Self>) -> Pin<&mut A> {
self.project().stream_a
}
/// Provide direct access to the underlying stream.
///
/// This may be useful if the stream provides APIs beyond [OrderedStream]. Note that the join
/// itself may be buffering an item from this stream, so you should consult
/// [Self::peek_buffered] and, if needed, [Self::take_buffered] before polling it directly.
pub fn stream_b(self: Pin<&mut Self>) -> Pin<&mut B> {
self.project().stream_b
}
/// Allow access to the buffered item, if any.
///
/// At most one of the two sides will be `Some`. The returned item is a candidate for being
/// the next item returned by the joined stream, but it could not be returned by the most
/// recent [`OrderedStream::poll_next_before`] call.
pub fn peek_buffered(
self: Pin<&mut Self>,
) -> (
Option<(&mut A::Data, &A::Ordering)>,
Option<(&mut B::Data, &B::Ordering)>,
) {
match self.project().state {
JoinState::A(a, o) => (Some((a, o)), None),
JoinState::B(b, o) => (None, Some((b, o))),
_ => (None, None),
}
}
/// Remove the buffered item, if one is present.
///
/// This does not poll either underlying stream. See [Self::peek_buffered] for details on why
/// buffering exists.
pub fn take_buffered(self: Pin<&mut Self>) -> Option<(A::Data, A::Ordering)> {
let state = self.project().state;
match mem::replace(state, JoinState::None) {
JoinState::A(a, o) => Some((a, o)),
JoinState::B(b, o) => Some((b, o)),
other => {
*state = other;
None
}
}
}
}
impl<A, B> OrderedStream for Join<A, B>
where
A: OrderedStream,
B: OrderedStream<Data = A::Data, Ordering = A::Ordering>,
{
type Data = A::Data;
type Ordering = A::Ordering;
fn poll_next_before(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
before: Option<&Self::Ordering>,
) -> Poll<PollResult<Self::Ordering, Self::Data>> {
let mut this = self.project();
let (mut poll_a, mut poll_b) = this.state.take_split();
poll_a.update(before, poll_b.ordering(), false, |ordering| {
this.stream_a.as_mut().poll_next_before(cx, ordering)
});
if poll_b.update(before, poll_a.ordering(), false, |ordering| {
this.stream_b.as_mut().poll_next_before(cx, ordering)
}) {
// If B just got an item, it's possible that A already knows that it won't have any
// items before that item; we couldn't ask that question before. Ask it now.
poll_a.update(before, poll_b.ordering(), true, |ordering| {
this.stream_a.as_mut().poll_next_before(cx, ordering)
});
}
match (poll_a, poll_b) {
// Both are ready - we can judge ordering directly (simplest case). The first one is
// returned while the other one is buffered for the next poll.
(PollState::Item(a, ta), PollState::Item(b, tb)) => {
if ta <= tb {
*this.state = JoinState::B(b, tb);
Poll::Ready(PollResult::Item {
data: a,
ordering: ta,
})
} else {
*this.state = JoinState::A(a, ta);
Poll::Ready(PollResult::Item {
data: b,
ordering: tb,
})
}
}
// If both sides are terminated, so are we.
(PollState::Terminated, PollState::Terminated) => {
*this.state = JoinState::Terminated;
Poll::Ready(PollResult::Terminated)
}
// If one side is terminated, we can produce items directly from the other side.
(a, PollState::Terminated) => {
*this.state = JoinState::OnlyPollA;
a.into()
}
(PollState::Terminated, b) => {
*this.state = JoinState::OnlyPollB;
b.into()
}
// If one side is pending, we can't return Ready until that gets resolved. Because we
// have already requested that our child streams wake us when it is possible to make
// any kind of progress, we meet the requirements to return Poll::Pending.
(PollState::Item(a, t), PollState::Pending) => {
*this.state = JoinState::A(a, t);
Poll::Pending
}
(PollState::Pending, PollState::Item(b, t)) => {
*this.state = JoinState::B(b, t);
Poll::Pending
}
(PollState::Pending, PollState::Pending) => Poll::Pending,
(PollState::Pending, PollState::NoneBefore) => Poll::Pending,
(PollState::NoneBefore, PollState::Pending) => Poll::Pending,
// If both sides report NoneBefore, so can we.
(PollState::NoneBefore, PollState::NoneBefore) => Poll::Ready(PollResult::NoneBefore),
(PollState::Item(data, ordering), PollState::NoneBefore) => {
// B was polled using either the Some value of (before) or using A's ordering.
//
// If before is set and is earlier than A's ordering, then B might later produce a
// value with (bt >= before && bt < at), so we can't return A's item yet and must
// buffer it. However, we can return None because neither stream will produce
// items before the ordering passed in before.
//
// If before is either None or after A's ordering, B's NoneBefore return represents a
// promise to not produce an item before A's, so we can return A's item now.
match before {
Some(before) if ordering > *before => {
*this.state = JoinState::A(data, ordering);
Poll::Ready(PollResult::NoneBefore)
}
_ => Poll::Ready(PollResult::Item { data, ordering }),
}
}
(PollState::NoneBefore, PollState::Item(data, ordering)) => {
// A was polled using either the Some value of (before) or using B's ordering.
//
// By a mirror of the above argument, this NoneBefore result gives us permission to
// produce either B's item or NoneBefore.
match before {
Some(before) if ordering > *before => {
*this.state = JoinState::B(data, ordering);
Poll::Ready(PollResult::NoneBefore)
}
_ => Poll::Ready(PollResult::Item { data, ordering }),
}
}
}
}
fn position_hint(&self) -> Option<MaybeBorrowed<'_, Self::Ordering>> {
let (a, b) = match &self.state {
JoinState::None => (self.stream_a.position_hint(), self.stream_b.position_hint()),
JoinState::A(_, t) => (
Some(MaybeBorrowed::Borrowed(t)),
self.stream_b.position_hint(),
),
JoinState::B(_, t) => (
self.stream_b.position_hint(),
Some(MaybeBorrowed::Borrowed(t)),
),
JoinState::OnlyPollA => return self.stream_a.position_hint(),
JoinState::OnlyPollB => return self.stream_b.position_hint(),
JoinState::Terminated => return None,
};
// We can only provide a hint if we have a valid hint for both sides
match (a, b) {
(Some(a), Some(b)) if *a <= *b => Some(a),
(Some(_), Some(b)) => Some(b),
_ => None,
}
}
fn size_hint(&self) -> (usize, Option<usize>) {
let extra = match &self.state {
JoinState::None => 0,
JoinState::A { .. } => 1,
JoinState::B { .. } => 1,
JoinState::OnlyPollA => return self.stream_a.size_hint(),
JoinState::OnlyPollB => return self.stream_b.size_hint(),
JoinState::Terminated => return (0, Some(0)),
};
let (al, ah) = self.stream_a.size_hint();
let (bl, bh) = self.stream_b.size_hint();
let min = al.saturating_add(bl).saturating_add(extra);
let max = ah
.and_then(|a| bh.and_then(|b| a.checked_add(b)))
.and_then(|h| h.checked_add(extra));
(min, max)
}
}
impl<A, B> FusedOrderedStream for Join<A, B>
where
A: OrderedStream,
B: OrderedStream<Data = A::Data, Ordering = A::Ordering>,
{
fn is_terminated(&self) -> bool {
matches!(self.state, JoinState::Terminated)
}
}
#[cfg(test)]
mod test {
extern crate alloc;
use crate::join;
use crate::FromStream;
use crate::OrderedStream;
use crate::OrderedStreamExt;
use crate::PollResult;
use alloc::rc::Rc;
use core::cell::Cell;
use core::pin::Pin;
use core::task::{Context, Poll};
use futures_executor::block_on;
use futures_util::pin_mut;
use futures_util::stream::iter;
#[derive(Debug, PartialEq)]
pub struct Message {
serial: u32,
}
#[test]
fn join_two() {
block_on(async {
let stream1 = iter([
Message { serial: 1 },
Message { serial: 4 },
Message { serial: 5 },
]);
let stream2 = iter([
Message { serial: 2 },
Message { serial: 3 },
Message { serial: 6 },
]);
let mut joined = join(
FromStream::with_ordering(stream1, |m| m.serial),
FromStream::with_ordering(stream2, |m| m.serial),
);
for i in 0..6 {
let msg = joined.next().await.unwrap();
assert_eq!(msg.serial, i as u32 + 1);
}
});
}
#[test]
fn join_one_slow() {
futures_executor::block_on(async {
pub struct DelayStream(Rc<Cell<u8>>);
impl OrderedStream for DelayStream {
type Ordering = u32;
type Data = Message;
fn poll_next_before(
self: Pin<&mut Self>,
_: &mut Context<'_>,
before: Option<&Self::Ordering>,
) -> Poll<PollResult<Self::Ordering, Self::Data>> {
match self.0.get() {
0 => Poll::Pending,
1 if matches!(before, Some(&1)) => Poll::Ready(PollResult::NoneBefore),
1 => Poll::Pending,
2 => {
self.0.set(3);
Poll::Ready(PollResult::Item {
data: Message { serial: 4 },
ordering: 4,
})
}
_ => Poll::Ready(PollResult::Terminated),
}
}
}
let stream1 = iter([
Message { serial: 1 },
Message { serial: 3 },
Message { serial: 5 },
]);
let stream1 = FromStream::with_ordering(stream1, |m| m.serial);
let go = Rc::new(Cell::new(0));
let stream2 = DelayStream(go.clone());
let join = join(stream1, stream2);
let waker = futures_util::task::noop_waker();
let mut ctx = core::task::Context::from_waker(&waker);
pin_mut!(join);
// When the DelayStream has no information about what it contains, join returns Pending
// (since there could be a serial-0 message output of DelayStream)
assert_eq!(
join.as_mut().poll_next_before(&mut ctx, None),
Poll::Pending
);
go.set(1);
// Now the DelayStream will return NoneBefore on serial 1
assert_eq!(
join.as_mut().poll_next_before(&mut ctx, None),
Poll::Ready(PollResult::Item {
data: Message { serial: 1 },
ordering: 1,
})
);
// however, it does not (yet) do so for serial 3
assert_eq!(
join.as_mut().poll_next_before(&mut ctx, None),
Poll::Pending
);
go.set(2);
assert_eq!(
join.as_mut().poll_next_before(&mut ctx, None),
Poll::Ready(PollResult::Item {
data: Message { serial: 3 },
ordering: 3,
})
);
assert_eq!(
join.as_mut().poll_next_before(&mut ctx, None),
Poll::Ready(PollResult::Item {
data: Message { serial: 4 },
ordering: 4,
})
);
assert_eq!(
join.as_mut().poll_next_before(&mut ctx, None),
Poll::Ready(PollResult::Item {
data: Message { serial: 5 },
ordering: 5,
})
);
assert_eq!(
join.as_mut().poll_next_before(&mut ctx, None),
Poll::Ready(PollResult::Terminated)
);
});
}
}