DoubleEndedStream extension

src/stream/double_ended_stream/mod.rs

@@ -0,0 +1,241 @@
+use crate::stream::Stream;
+
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+mod next_back;
+mod nth_back;
+mod rfind;
+mod rfold;
+mod try_rfold;
+
+use next_back::NextBackFuture;
+use nth_back::NthBackFuture;
+use rfind::RFindFuture;
+use rfold::RFoldFuture;
+use try_rfold::TryRFoldFuture;
+
+/// A stream able to yield elements from both ends.
+///
+/// Something that implements `DoubleEndedStream` has one extra capability
+/// over something that implements [`Stream`]: the ability to also take
+/// `Item`s from the back, as well as the front.
+///
+/// [`Stream`]: trait.Stream.html
+#[cfg(feature = "unstable")]
+#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
+pub trait DoubleEndedStream: Stream {
+    #[doc = r#"
+        Attempts to receive the next item from the back of the stream.
+
+        There are several possible return values:
+
+        * `Poll::Pending` means this stream's next_back value is not ready yet.
+        * `Poll::Ready(None)` means this stream has been exhausted.
+        * `Poll::Ready(Some(item))` means `item` was received out of the stream.
+
+        # Examples
+
+        ```
+        # fn main() { async_std::task::block_on(async {
+        #
+        use std::pin::Pin;
+
+        use async_std::prelude::*;
+        use async_std::stream;
+        use async_std::task::{Context, Poll};
+
+        fn increment(
+            s: impl DoubleEndedStream<Item = i32> + Unpin,
+        ) -> impl DoubleEndedStream<Item = i32> + Unpin {
+            struct Increment<S>(S);
+
+            impl<S: DoubleEndedStream<Item = i32> + Unpin> Stream for Increment<S> {
+                type Item = S::Item;
+
+                fn poll_next(
+                    mut self: Pin<&mut Self>,
+                    cx: &mut Context<'_>,
+                ) -> Poll<Option<Self::Item>> {
+                    match Pin::new(&mut self.0).poll_next(cx) {
+                        Poll::Pending => Poll::Pending,
+                        Poll::Ready(None) => Poll::Ready(None),
+                        Poll::Ready(Some(item)) => Poll::Ready(Some(item + 1)),
+                    }
+                }
+            }
+
+            impl<S: DoubleEndedStream<Item = i32> + Unpin> DoubleEndedStream for Increment<S> {
+                fn poll_next_back(
+                    mut self: Pin<&mut Self>,
+                    cx: &mut Context<'_>,
+                ) -> Poll<Option<Self::Item>> {
+                    match Pin::new(&mut self.0).poll_next_back(cx) {
+                        Poll::Pending => Poll::Pending,
+                        Poll::Ready(None) => Poll::Ready(None),
+                        Poll::Ready(Some(item)) => Poll::Ready(Some(item + 1)),
+                    }
+                }
+            }
+
+            Increment(s)
+        }
+
+        let mut s = increment(stream::once(7));
+
+        assert_eq!(s.next_back().await, Some(8));
+        assert_eq!(s.next_back().await, None);
+        #
+        # }) }
+        ```
+    "#]
+    fn poll_next_back(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>>;
+
+    #[doc = r#"
+            Advances the stream and returns the next value.
+
+            Returns [`None`] when iteration is finished. Individual stream implementations may
+            choose to resume iteration, and so calling `next()` again may or may not eventually
+            start returning more values.
+
+            [`None`]: https://doc.rust-lang.org/std/option/enum.Option.html#variant.None
+
+            # Examples
+
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use async_std::stream::double_ended_stream::{self, DoubleEndedStream};
+
+            let mut s = double_ended_stream::from_iter(vec![7u8]);
+
+            assert_eq!(s.next_back().await, Some(7));
+            assert_eq!(s.next_back().await, None);
+            #
+            # }) }
+            ```
+        "#]
+    fn next_back(&mut self) -> NextBackFuture<'_, Self>
+    where
+        Self: Unpin,
+    {
+        NextBackFuture { stream: self }
+    }
+
+    #[doc = r#"
+            Returns the nth element from the back of the stream.
+
+            # Examples
+
+            Basic usage:
+
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use async_std::stream::double_ended_stream::{self, DoubleEndedStream};
+
+            let mut s = double_ended_stream::from_iter(vec![1u8, 2, 3, 4, 5]);
+
+            let second = s.nth_back(1).await;
+            assert_eq!(second, Some(4));
+            #
+            # }) }
+            ```
+        "#]
+    fn nth_back(&mut self, n: usize) -> NthBackFuture<'_, Self>
+    where
+        Self: Unpin + Sized,
+    {
+        NthBackFuture::new(self, n)
+    }
+
+    #[doc = r#"
+            Returns the the frist element from the right that matches the predicate.
+
+            # Examples
+
+            Basic usage:
+
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use async_std::stream::double_ended_stream::{self, DoubleEndedStream};
+
+            let mut s = double_ended_stream::from_iter(vec![1u8, 2, 3, 4, 5]);
+
+            let second = s.rfind(|v| v % 2 == 0).await;
+            assert_eq!(second, Some(4));
+            #
+            # }) }
+            ```
+        "#]
+    fn rfind<P>(&mut self, p: P) -> RFindFuture<'_, Self, P>
+    where
+        Self: Unpin + Sized,
+        P: FnMut(&Self::Item) -> bool,
+    {
+        RFindFuture::new(self, p)
+    }
+
+    #[doc = r#"
+            # Examples
+
+            Basic usage:
+
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use async_std::stream::double_ended_stream::{self, DoubleEndedStream};
+
+            let s = double_ended_stream::from_iter(vec![1u8, 2, 3, 4, 5]);
+
+            let second = s.rfold(0, |acc, v| v + acc).await;
+
+            assert_eq!(second, 15);
+            #
+            # }) }
+            ```
+        "#]
+    fn rfold<B, F>(self, accum: B, f: F) -> RFoldFuture<Self, F, B>
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        RFoldFuture::new(self, accum, f)
+    }
+
+    #[doc = r#"
+            A combinator that applies a function as long as it returns successfully, producing a single, final value.
+            Immediately returns the error when the function returns unsuccessfully.
+
+            # Examples
+
+            Basic usage:
+
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use async_std::stream::double_ended_stream::{self, DoubleEndedStream};
+
+            let s = double_ended_stream::from_iter(vec![1u8, 2, 3, 4, 5]);
+            let sum = s.try_rfold(0, |acc, v| {
+                if (acc+v) % 2 == 1 {
+                    Ok(v+3)
+                } else {
+                    Err("fail")
+                }
+            }).await;
+
+            assert_eq!(sum, Err("fail"));
+            #
+            # }) }
+            ```
+        "#]
+    fn try_rfold<B, F, E>(self, accum: B, f: F) -> TryRFoldFuture<Self, F, B>
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> Result<B, E>,
+    {
+        TryRFoldFuture::new(self, accum, f)
+    }
+}

src/stream/double_ended_stream/next_back.rs

@@ -0,0 +1,19 @@
+use std::pin::Pin;
+use std::future::Future;
+
+use crate::stream::DoubleEndedStream;
+use crate::task::{Context, Poll};
+
+#[doc(hidden)]
+#[allow(missing_debug_implementations)]
+pub struct NextBackFuture<'a, T: Unpin + ?Sized> {
+    pub(crate) stream: &'a mut T,
+}
+
+impl<T: DoubleEndedStream + Unpin + ?Sized> Future for NextBackFuture<'_, T> {
+    type Output = Option<T::Item>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        Pin::new(&mut *self.stream).poll_next_back(cx)
+    }
+}

src/stream/double_ended_stream/nth_back.rs

@@ -0,0 +1,41 @@
+use std::future::Future;
+use std::pin::Pin;
+use std::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),
+        }
+    }
+}
+

src/stream/double_ended_stream/rfind.rs

@@ -0,0 +1,41 @@
+use std::task::{Context, Poll};
+use std::future::Future;
+use std::pin::Pin;
+
+use crate::stream::DoubleEndedStream;
+
+#[doc(hidden)]
+#[allow(missing_debug_implementations)]
+pub struct RFindFuture<'a, S, P> {
+    stream: &'a mut S,
+    p: P,
+}
+
+impl<'a, S, P> RFindFuture<'a, S, P> {
+    pub(super) fn new(stream: &'a mut S, p: P) -> Self {
+        RFindFuture { stream, p }
+    }
+}
+
+impl<S: Unpin, P> Unpin for RFindFuture<'_, S, P> {}
+
+impl<'a, S, P> Future for RFindFuture<'a, S, P>
+where
+    S: DoubleEndedStream + Unpin + Sized,
+    P: FnMut(&S::Item) -> bool,
+{
+    type Output = Option<S::Item>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let item = futures_core::ready!(Pin::new(&mut *self.stream).poll_next_back(cx));
+
+        match item {
+            Some(v) if (&mut self.p)(&v) => Poll::Ready(Some(v)),
+            Some(_) => {
+                cx.waker().wake_by_ref();
+                Poll::Pending
+            }
+            None => Poll::Ready(None),
+        }
+    }
+}