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Introduce futures_util::stream::select_bias.
The current select function only supports round robin. To change that would be a breaking change, so for now I propose a `select_bias` which takes a closure that defines the selection strategy. - When a breaking change is planned, we could re-consider this, but it does have 4 generic parameters of which one is a closure, compared to the 2 streams in `select`. - note the name select_bias. It's not consistent with `select_biased` used elsewhere in the lib. This can obviously be changed, but I feel the "ed" at the end does not add any semantic value but makes function names longer. - There isn't really integration tests included right now, but the examples in the docs get tested by doc test.
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use super::assert_stream; | ||
use crate::stream::{Fuse, StreamExt}; | ||
use core::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 [`SelectBias`] 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, | ||
} | ||
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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_bias()`] function. See function docs for details. | ||
#[derive(Debug)] | ||
#[must_use = "streams do nothing unless polled"] | ||
pub struct SelectBias<St1, St2, Clos, State> { | ||
#[pin] | ||
stream1: Fuse<St1>, | ||
#[pin] | ||
stream2: Fuse<St2>, | ||
state: State, | ||
clos: Clos, | ||
} | ||
} | ||
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/// This function will attempt to pull items from both streams. You provide a | ||
/// closure to tell the [`SelectBias`] which stream to poll. The closure can | ||
/// store state on `SelectBias` to which it will receive a `&mut` on every | ||
/// invocation. This allows basing the strategy on prior choices. | ||
/// | ||
/// After one of the two input stream 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_util::stream::{ select_bias, PollNext, StreamExt }; | ||
/// use futures::stream::repeat; | ||
/// | ||
/// let left = repeat(1); | ||
/// let right = repeat(2); | ||
/// | ||
/// // We don't need any state, so lets make it an empty tuple. | ||
/// // We must provide some type here, as there is no way for the compiler | ||
/// // to infer it. | ||
/// let prio_left = |_: &mut ()| PollNext::Left; | ||
/// | ||
/// let mut out = select_bias(left, right, prio_left); | ||
/// | ||
/// for _ in 0..100 { | ||
/// // Whenever we poll out, we will alwasy get `1`. | ||
/// assert_eq!(1, out.select_next_some().await); | ||
/// } | ||
/// | ||
/// }); | ||
/// ``` | ||
/// | ||
/// ### Round Robin | ||
/// This example shows how to select from both streams round robin. | ||
/// | ||
/// ```rust | ||
/// | ||
/// # futures::executor::block_on(async { | ||
/// use futures_util::stream::{ select_bias, PollNext, StreamExt }; | ||
/// use futures::stream::repeat; | ||
/// | ||
/// let left = repeat(1); | ||
/// let right = repeat(2); | ||
/// | ||
/// // We don't need any state, so lets make it an empty tuple. | ||
/// let rrobin = |last: &mut PollNext| last.toggle(); | ||
/// | ||
/// let mut out = select_bias(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_bias<St1, St2, Clos, State>( | ||
stream1: St1, | ||
stream2: St2, | ||
which: Clos, | ||
) -> SelectBias<St1, St2, Clos, State> | ||
where | ||
St1: Stream, | ||
St2: Stream<Item = St1::Item>, | ||
Clos: FnMut(&mut State) -> PollNext, | ||
State: Default, | ||
{ | ||
assert_stream::<St1::Item, _>(SelectBias { | ||
stream1: stream1.fuse(), | ||
stream2: stream2.fuse(), | ||
state: Default::default(), | ||
clos: which, | ||
}) | ||
} | ||
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impl<St1, St2, Clos, State> SelectBias<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()) | ||
} | ||
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/// 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()) | ||
} | ||
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/// 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 SelectBias<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 SelectBias<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, | ||
} | ||
} |