1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
use core::future::Future; use core::pin::Pin; use core::task::{Context, Poll}; use crate::stream::DoubleEndedStream; #[doc(hidden)] #[allow(missing_debug_implementations)] pub struct NthBackFuture<'a, S> { stream: &'a mut S, n: usize, } impl<'a, S> NthBackFuture<'a, S> { pub(crate) fn new(stream: &'a mut S, n: usize) -> Self { NthBackFuture { stream, n } } } impl<'a, S> Future for NthBackFuture<'a, S> where S: DoubleEndedStream + Sized + Unpin, { type Output = Option<S::Item>; fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> { let next = futures_core::ready!(Pin::new(&mut *self.stream).poll_next_back(cx)); match next { Some(v) => match self.n { 0 => Poll::Ready(Some(v)), _ => { self.n -= 1; cx.waker().wake_by_ref(); Poll::Pending } }, None => Poll::Ready(None), } } }