forked from rust-lang/futures-rs
/
scan.rs
125 lines (112 loc) · 3.42 KB
/
scan.rs
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use crate::unfold_state::UnfoldState;
use core::fmt;
use core::pin::Pin;
use futures_core::future::Future;
use futures_core::ready;
use futures_core::stream::{FusedStream, Stream};
use futures_core::task::{Context, Poll};
#[cfg(feature = "sink")]
use futures_sink::Sink;
use pin_project_lite::pin_project;
pin_project! {
/// Stream for the [`scan`](super::StreamExt::scan) method.
#[must_use = "streams do nothing unless polled"]
pub struct Scan<St: Stream, S, Fut, F> {
#[pin]
stream: St,
f: F,
#[pin]
state: UnfoldState<S, Fut>,
}
}
impl<St, S, Fut, F> fmt::Debug for Scan<St, S, Fut, F>
where
St: Stream + fmt::Debug,
St::Item: fmt::Debug,
S: fmt::Debug,
Fut: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Scan")
.field("stream", &self.stream)
.field("state", &self.state)
.field("done_taking", &self.is_done_taking())
.finish()
}
}
impl<St: Stream, S, Fut, F> Scan<St, S, Fut, F> {
/// Checks if internal state is `None`.
fn is_done_taking(&self) -> bool {
self.state.is_empty()
}
}
impl<B, St, S, Fut, F> Scan<St, S, Fut, F>
where
St: Stream,
F: FnMut(S, St::Item) -> Fut,
Fut: Future<Output = Option<(S, B)>>,
{
pub(super) fn new(stream: St, initial_state: S, f: F) -> Self {
Self { stream, f, state: UnfoldState::Value { value: initial_state } }
}
delegate_access_inner!(stream, St, ());
}
impl<B, St, S, Fut, F> Stream for Scan<St, S, Fut, F>
where
St: Stream,
F: FnMut(S, St::Item) -> Fut,
Fut: Future<Output = Option<(S, B)>>,
{
type Item = B;
fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<B>> {
if self.is_done_taking() {
return Poll::Ready(None);
}
let mut this = self.project();
Poll::Ready(loop {
if let Some(fut) = this.state.as_mut().project_future() {
match ready!(fut.poll(cx)) {
None => {
this.state.set(UnfoldState::Empty);
break None;
}
Some((state, item)) => {
this.state.set(UnfoldState::Value { value: state });
break Some(item);
}
}
} else if let Some(item) = ready!(this.stream.as_mut().poll_next(cx)) {
let state = this.state.as_mut().take_value().unwrap();
this.state.set(UnfoldState::Future { future: (this.f)(state, item) })
} else {
break None;
}
})
}
fn size_hint(&self) -> (usize, Option<usize>) {
if self.is_done_taking() {
(0, Some(0))
} else {
self.stream.size_hint() // can't know a lower bound, due to the predicate
}
}
}
impl<B, St, S, Fut, F> FusedStream for Scan<St, S, Fut, F>
where
St: FusedStream,
F: FnMut(S, St::Item) -> Fut,
Fut: Future<Output = Option<(S, B)>>,
{
fn is_terminated(&self) -> bool {
self.is_done_taking() || !self.state.is_future() && self.stream.is_terminated()
}
}
// Forwarding impl of Sink from the underlying stream
#[cfg(feature = "sink")]
impl<St, S, Fut, F, Item> Sink<Item> for Scan<St, S, Fut, F>
where
St: Stream + Sink<Item>,
{
type Error = St::Error;
delegate_sink!(stream, Item);
}