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Introduce stream::select_with_strategy (#2450)
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,230 @@ | ||
| use super::assert_stream; | ||
| use crate::stream::{Fuse, StreamExt}; | ||
| use core::{fmt, pin::Pin}; | ||
| use futures_core::stream::{FusedStream, Stream}; | ||
| use futures_core::task::{Context, Poll}; | ||
| use pin_project_lite::pin_project; | ||
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| /// Type to tell [`SelectWithStrategy`] which stream to poll next. | ||
| #[derive(Debug, PartialEq, Eq, Copy, Clone, Hash)] | ||
| pub enum PollNext { | ||
| /// Poll the first stream. | ||
| Left, | ||
| /// Poll the second stream. | ||
| Right, | ||
| } | ||
|
|
||
| impl PollNext { | ||
| /// Toggle the value and return the old one. | ||
| pub fn toggle(&mut self) -> Self { | ||
| let old = *self; | ||
|
|
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| match self { | ||
| PollNext::Left => *self = PollNext::Right, | ||
| PollNext::Right => *self = PollNext::Left, | ||
| } | ||
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||
| old | ||
| } | ||
| } | ||
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| impl Default for PollNext { | ||
| fn default() -> Self { | ||
| PollNext::Left | ||
| } | ||
| } | ||
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| pin_project! { | ||
| /// Stream for the [`select_with_strategy()`] function. See function docs for details. | ||
| #[must_use = "streams do nothing unless polled"] | ||
| pub struct SelectWithStrategy<St1, St2, Clos, State> { | ||
| #[pin] | ||
| stream1: Fuse<St1>, | ||
| #[pin] | ||
| stream2: Fuse<St2>, | ||
| state: State, | ||
| clos: Clos, | ||
| } | ||
| } | ||
|
|
||
| /// This function will attempt to pull items from both streams. You provide a | ||
| /// closure to tell [`SelectWithStrategy`] which stream to poll. The closure can | ||
| /// store state on `SelectWithStrategy` to which it will receive a `&mut` on every | ||
| /// invocation. This allows basing the strategy on prior choices. | ||
| /// | ||
| /// After one of the two input streams completes, the remaining one will be | ||
| /// polled exclusively. The returned stream completes when both input | ||
| /// streams have completed. | ||
| /// | ||
| /// Note that this function consumes both streams and returns a wrapped | ||
| /// version of them. | ||
| /// | ||
| /// ## Examples | ||
| /// | ||
| /// ### Priority | ||
| /// This example shows how to always prioritize the left stream. | ||
| /// | ||
| /// ```rust | ||
| /// # futures::executor::block_on(async { | ||
| /// use futures::stream::{ repeat, select_with_strategy, PollNext, StreamExt }; | ||
| /// | ||
| /// let left = repeat(1); | ||
| /// let right = repeat(2); | ||
| /// | ||
| /// // We don't need any state, so let's make it an empty tuple. | ||
| /// // We must provide some type here, as there is no way for the compiler | ||
| /// // to infer it. As we don't need to capture variables, we can just | ||
| /// // use a function pointer instead of a closure. | ||
| /// fn prio_left(_: &mut ()) -> PollNext { PollNext::Left } | ||
| /// | ||
| /// let mut out = select_with_strategy(left, right, prio_left); | ||
| /// | ||
| /// for _ in 0..100 { | ||
| /// // Whenever we poll out, we will alwas get `1`. | ||
| /// assert_eq!(1, out.select_next_some().await); | ||
| /// } | ||
| /// # }); | ||
| /// ``` | ||
| /// | ||
| /// ### Round Robin | ||
| /// This example shows how to select from both streams round robin. | ||
| /// Note: this special case is provided by [`futures-util::stream::select`]. | ||
| /// | ||
| /// ```rust | ||
| /// # futures::executor::block_on(async { | ||
| /// use futures::stream::{ repeat, select_with_strategy, PollNext, StreamExt }; | ||
| /// | ||
| /// let left = repeat(1); | ||
| /// let right = repeat(2); | ||
| /// | ||
| /// // We don't need any state, so let's make it an empty tuple. | ||
| /// let rrobin = |last: &mut PollNext| last.toggle(); | ||
| /// | ||
| /// let mut out = select_with_strategy(left, right, rrobin); | ||
| /// | ||
| /// for _ in 0..100 { | ||
| /// // We should be alternating now. | ||
| /// assert_eq!(1, out.select_next_some().await); | ||
| /// assert_eq!(2, out.select_next_some().await); | ||
| /// } | ||
| /// # }); | ||
| /// ``` | ||
| pub fn select_with_strategy<St1, St2, Clos, State>( | ||
| stream1: St1, | ||
| stream2: St2, | ||
| which: Clos, | ||
| ) -> SelectWithStrategy<St1, St2, Clos, State> | ||
| where | ||
| St1: Stream, | ||
| St2: Stream<Item = St1::Item>, | ||
| Clos: FnMut(&mut State) -> PollNext, | ||
| State: Default, | ||
| { | ||
| assert_stream::<St1::Item, _>(SelectWithStrategy { | ||
| stream1: stream1.fuse(), | ||
| stream2: stream2.fuse(), | ||
| state: Default::default(), | ||
| clos: which, | ||
| }) | ||
| } | ||
|
|
||
| impl<St1, St2, Clos, State> SelectWithStrategy<St1, St2, Clos, State> { | ||
| /// Acquires a reference to the underlying streams that this combinator is | ||
| /// pulling from. | ||
| pub fn get_ref(&self) -> (&St1, &St2) { | ||
| (self.stream1.get_ref(), self.stream2.get_ref()) | ||
| } | ||
|
|
||
| /// Acquires a mutable reference to the underlying streams that this | ||
| /// combinator is pulling from. | ||
| /// | ||
| /// Note that care must be taken to avoid tampering with the state of the | ||
| /// stream which may otherwise confuse this combinator. | ||
| pub fn get_mut(&mut self) -> (&mut St1, &mut St2) { | ||
| (self.stream1.get_mut(), self.stream2.get_mut()) | ||
| } | ||
|
|
||
| /// Acquires a pinned mutable reference to the underlying streams that this | ||
| /// combinator is pulling from. | ||
| /// | ||
| /// Note that care must be taken to avoid tampering with the state of the | ||
| /// stream which may otherwise confuse this combinator. | ||
| pub fn get_pin_mut(self: Pin<&mut Self>) -> (Pin<&mut St1>, Pin<&mut St2>) { | ||
| let this = self.project(); | ||
| (this.stream1.get_pin_mut(), this.stream2.get_pin_mut()) | ||
| } | ||
|
|
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| /// Consumes this combinator, returning the underlying streams. | ||
| /// | ||
| /// Note that this may discard intermediate state of this combinator, so | ||
| /// care should be taken to avoid losing resources when this is called. | ||
| pub fn into_inner(self) -> (St1, St2) { | ||
| (self.stream1.into_inner(), self.stream2.into_inner()) | ||
| } | ||
| } | ||
|
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| impl<St1, St2, Clos, State> FusedStream for SelectWithStrategy<St1, St2, Clos, State> | ||
| where | ||
| St1: Stream, | ||
| St2: Stream<Item = St1::Item>, | ||
| Clos: FnMut(&mut State) -> PollNext, | ||
| { | ||
| fn is_terminated(&self) -> bool { | ||
| self.stream1.is_terminated() && self.stream2.is_terminated() | ||
| } | ||
| } | ||
|
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| impl<St1, St2, Clos, State> Stream for SelectWithStrategy<St1, St2, Clos, State> | ||
| where | ||
| St1: Stream, | ||
| St2: Stream<Item = St1::Item>, | ||
| Clos: FnMut(&mut State) -> PollNext, | ||
| { | ||
| type Item = St1::Item; | ||
|
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| fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<St1::Item>> { | ||
| let this = self.project(); | ||
|
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| match (this.clos)(this.state) { | ||
| PollNext::Left => poll_inner(this.stream1, this.stream2, cx), | ||
| PollNext::Right => poll_inner(this.stream2, this.stream1, cx), | ||
| } | ||
| } | ||
| } | ||
|
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| fn poll_inner<St1, St2>( | ||
| a: Pin<&mut St1>, | ||
| b: Pin<&mut St2>, | ||
| cx: &mut Context<'_>, | ||
| ) -> Poll<Option<St1::Item>> | ||
| where | ||
| St1: Stream, | ||
| St2: Stream<Item = St1::Item>, | ||
| { | ||
| let a_done = match a.poll_next(cx) { | ||
| Poll::Ready(Some(item)) => return Poll::Ready(Some(item)), | ||
| Poll::Ready(None) => true, | ||
| Poll::Pending => false, | ||
| }; | ||
|
|
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| match b.poll_next(cx) { | ||
| Poll::Ready(Some(item)) => Poll::Ready(Some(item)), | ||
| Poll::Ready(None) if a_done => Poll::Ready(None), | ||
| Poll::Ready(None) | Poll::Pending => Poll::Pending, | ||
| } | ||
| } | ||
|
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| impl<St1, St2, Clos, State> fmt::Debug for SelectWithStrategy<St1, St2, Clos, State> | ||
| where | ||
| St1: fmt::Debug, | ||
| St2: fmt::Debug, | ||
| State: fmt::Debug, | ||
| { | ||
| fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { | ||
| f.debug_struct("SelectWithStrategy") | ||
| .field("stream1", &self.stream1) | ||
| .field("stream2", &self.stream2) | ||
| .field("state", &self.state) | ||
| .finish() | ||
| } | ||
| } |