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use pin_project_lite::pin_project;
use futures_core::stream::Stream;
use std::pin::Pin;
use std::task::{Context, Poll};
pin_project! {
/// Yield the last value received, if any, at each interval.
///
/// If no value was emitted during the last interval, no value is emitted
/// and we skip to the next interval.
#[derive(Debug)]
#[must_use = "streams do nothing unless polled or .awaited"]
pub struct Sample<S: Stream, I> {
#[pin]
stream: S,
#[pin]
interval: I,
state: State,
slot: Option<S::Item>,
}
}
impl<S: Stream, I> Sample<S, I> {
pub(crate) fn new(stream: S, interval: I) -> Self {
Self {
state: State::Streaming,
stream,
interval,
slot: None,
}
}
}
#[derive(Debug)]
enum State {
/// The underlying stream is yielding items.
Streaming,
/// All timers have completed and all data has been yielded.
StreamDone,
/// The closing `Ready(None)` has been yielded.
AllDone,
}
impl<S: Stream, I: Stream> Stream for Sample<S, I> {
type Item = S::Item;
fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
let mut this = self.project();
match this.state {
// The underlying stream is yielding items.
State::Streaming => {
// Poll the underlying stream until we get to `Poll::Pending`.
loop {
match this.stream.as_mut().poll_next(cx) {
Poll::Ready(Some(value)) => {
let _ = this.slot.insert(value);
}
Poll::Ready(None) => {
*this.state = State::StreamDone;
break;
}
Poll::Pending => break,
}
}
// After the stream, always poll the interval timer.
match this.interval.as_mut().poll_next(cx) {
Poll::Ready(_) => {
if let State::StreamDone = this.state {
cx.waker().wake_by_ref();
}
match this.slot.take() {
Some(item) => Poll::Ready(Some(item)),
None => Poll::Pending,
}
}
Poll::Pending => Poll::Pending,
}
}
// All streams have completed and all data has been yielded.
State::StreamDone => {
*this.state = State::AllDone;
Poll::Ready(None)
}
// The closing `Ready(None)` has been yielded.
State::AllDone => panic!("stream polled after completion"),
}
}
}
#[cfg(test)]
mod test {
use crate::prelude::*;
use crate::time::Duration;
use futures_lite::prelude::*;
#[test]
fn smoke() {
async_io::block_on(async {
let interval = Duration::from_millis(100);
let throttle = Duration::from_millis(200);
let take = 4;
let expected = 2;
let mut counter = 0;
crate::stream::interval(interval)
.take(take)
.sample(throttle)
.for_each(|_| counter += 1)
.await;
assert_eq!(counter, expected);
})
}
}