# peek.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.14/src/stream/stream/peek.rs`.
Keywords: rust, rustlang, rust-lang

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    use crate::fns::FnOnce1;
    use crate::stream::{Fuse, StreamExt};
    use core::fmt;
    use core::marker::PhantomData;
    use core::pin::Pin;
    use futures_core::future::{FusedFuture, 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! {
        /// A `Stream` that implements a `peek` method.
        ///
        /// The `peek` method can be used to retrieve a reference
        /// to the next `Stream::Item` if available. A subsequent
        /// call to `poll` will return the owned item.
        #[derive(Debug)]
        #[must_use = "streams do nothing unless polled"]
        pub struct Peekable<St: Stream> {
            #[pin]
            stream: Fuse<St>,
            peeked: Option<St::Item>,
        }
    }

    impl<St: Stream> Peekable<St> {
        pub(super) fn new(stream: St) -> Self {
            Self {
                stream: stream.fuse(),
                peeked: None,
            }
        }

        delegate_access_inner!(stream, St, (.));

        /// Produces a `Peek` future which retrieves a reference to the next item
        /// in the stream, or `None` if the underlying stream terminates.
        pub fn peek(self: Pin<&mut Self>) -> Peek<'_, St> {
            Peek { inner: Some(self) }
        }

        /// Peek retrieves a reference to the next item in the stream.
        ///
        /// This method polls the underlying stream and return either a reference
        /// to the next item if the stream is ready or passes through any errors.
        pub fn poll_peek(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<&St::Item>> {
            let mut this = self.project();

            Poll::Ready(loop {
                if this.peeked.is_some() {
                    break this.peeked.as_ref();
                } else if let Some(item) = ready!(this.stream.as_mut().poll_next(cx)) {
                    *this.peeked = Some(item);
                } else {
                    break None;
                }
            })
        }

        /// Creates a future which will consume and return the next value of this
        /// stream if a condition is true.
        ///
        /// If `func` returns `true` for the next value of this stream, consume and
        /// return it. Otherwise, return `None`.
        ///
        /// # Examples
        ///
        /// Consume a number if it's equal to 0.
        ///
        /// ```
        /// # futures::executor::block_on(async {
        /// use futures::stream::{self, StreamExt};
        /// use futures::pin_mut;
        ///
        /// let stream = stream::iter(0..5).peekable();
        /// pin_mut!(stream);
        /// // The first item of the stream is 0; consume it.
        /// assert_eq!(stream.as_mut().next_if(|&x| x == 0).await, Some(0));
        /// // The next item returned is now 1, so `consume` will return `false`.
        /// assert_eq!(stream.as_mut().next_if(|&x| x == 0).await, None);
        /// // `next_if` saves the value of the next item if it was not equal to `expected`.
        /// assert_eq!(stream.next().await, Some(1));
        /// # });
        /// ```
        ///
        /// Consume any number less than 10.
        ///
        /// ```
        /// # futures::executor::block_on(async {
        /// use futures::stream::{self, StreamExt};
        /// use futures::pin_mut;
        ///
        /// let stream = stream::iter(1..20).peekable();
        /// pin_mut!(stream);
        /// // Consume all numbers less than 10
        /// while stream.as_mut().next_if(|&x| x < 10).await.is_some() {}
        /// // The next value returned will be 10
        /// assert_eq!(stream.next().await, Some(10));
        /// # });
        /// ```
        pub fn next_if<F>(self: Pin<&mut Self>, func: F) -> NextIf<'_, St, F>
        where
            F: FnOnce(&St::Item) -> bool,
        {
            NextIf {
                inner: Some((self, func)),
            }
        }

        /// Creates a future which will consume and return the next item if it is
        /// equal to `expected`.
        ///
        /// # Example
        ///
        /// Consume a number if it's equal to 0.
        ///
        /// ```
        /// # futures::executor::block_on(async {
        /// use futures::stream::{self, StreamExt};
        /// use futures::pin_mut;
        ///
        /// let stream = stream::iter(0..5).peekable();
        /// pin_mut!(stream);
        /// // The first item of the stream is 0; consume it.
        /// assert_eq!(stream.as_mut().next_if_eq(&0).await, Some(0));
        /// // The next item returned is now 1, so `consume` will return `false`.
        /// assert_eq!(stream.as_mut().next_if_eq(&0).await, None);
        /// // `next_if_eq` saves the value of the next item if it was not equal to `expected`.
        /// assert_eq!(stream.next().await, Some(1));
        /// # });
        /// ```
        pub fn next_if_eq<'a, T>(self: Pin<&'a mut Self>, expected: &'a T) -> NextIfEq<'a, St, T>
        where
            T: ?Sized,
            St::Item: PartialEq<T>,
        {
            NextIfEq {
                inner: NextIf {
                    inner: Some((
                        self,
                        NextIfEqFn {
                            expected,
                            _next: PhantomData,
                        },
                    )),
                },
            }
        }
    }

    impl<St: Stream> FusedStream for Peekable<St> {
        fn is_terminated(&self) -> bool {
            self.peeked.is_none() && self.stream.is_terminated()
        }
    }

    impl<S: Stream> Stream for Peekable<S> {
        type Item = S::Item;

        fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
            let this = self.project();
            if let Some(item) = this.peeked.take() {
                return Poll::Ready(Some(item));
            }
            this.stream.poll_next(cx)
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            let peek_len = if self.peeked.is_some() { 1 } else { 0 };
            let (lower, upper) = self.stream.size_hint();
            let lower = lower.saturating_add(peek_len);
            let upper = match upper {
                Some(x) => x.checked_add(peek_len),
                None => None,
            };
            (lower, upper)
        }
    }

    // Forwarding impl of Sink from the underlying stream
    #[cfg(feature = "sink")]
    impl<S, Item> Sink<Item> for Peekable<S>
    where
        S: Sink<Item> + Stream,
    {
        type Error = S::Error;

        delegate_sink!(stream, Item);
    }

    pin_project! {
        /// Future for the [`Peekable::peek`](self::Peekable::peek) method.
        #[must_use = "futures do nothing unless polled"]
        pub struct Peek<'a, St: Stream> {
            inner: Option<Pin<&'a mut Peekable<St>>>,
        }
    }

    impl<St> fmt::Debug for Peek<'_, St>
    where
        St: Stream + fmt::Debug,
        St::Item: fmt::Debug,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Peek").field("inner", &self.inner).finish()
        }
    }

    impl<St: Stream> FusedFuture for Peek<'_, St> {
        fn is_terminated(&self) -> bool {
            self.inner.is_none()
        }
    }

    impl<'a, St> Future for Peek<'a, St>
    where
        St: Stream,
    {
        type Output = Option<&'a St::Item>;

        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            let inner = self.project().inner;
            if let Some(peekable) = inner {
                ready!(peekable.as_mut().poll_peek(cx));

                inner.take().unwrap().poll_peek(cx)
            } else {
                panic!("Peek polled after completion")
            }
        }
    }

    pin_project! {
        /// Future for the [`Peekable::next_if`](self::Peekable::next_if) method.
        #[must_use = "futures do nothing unless polled"]
        pub struct NextIf<'a, St: Stream, F> {
            inner: Option<(Pin<&'a mut Peekable<St>>, F)>,
        }
    }

    impl<St, F> fmt::Debug for NextIf<'_, St, F>
    where
        St: Stream + fmt::Debug,
        St::Item: fmt::Debug,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("NextIf")
                .field("inner", &self.inner.as_ref().map(|(s, _f)| s))
                .finish()
        }
    }

    #[allow(single_use_lifetimes)] // https://github.com/rust-lang/rust/issues/55058
    impl<St, F> FusedFuture for NextIf<'_, St, F>
    where
        St: Stream,
        F: for<'a> FnOnce1<&'a St::Item, Output = bool>,
    {
        fn is_terminated(&self) -> bool {
            self.inner.is_none()
        }
    }

    #[allow(single_use_lifetimes)] // https://github.com/rust-lang/rust/issues/55058
    impl<St, F> Future for NextIf<'_, St, F>
    where
        St: Stream,
        F: for<'a> FnOnce1<&'a St::Item, Output = bool>,
    {
        type Output = Option<St::Item>;

        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            let inner = self.project().inner;
            if let Some((peekable, _)) = inner {
                let res = ready!(peekable.as_mut().poll_next(cx));

                let (peekable, func) = inner.take().unwrap();
                match res {
                    Some(ref matched) if func.call_once(matched) => Poll::Ready(res),
                    other => {
                        let peekable = peekable.project();
                        // Since we called `self.next()`, we consumed `self.peeked`.
                        assert!(peekable.peeked.is_none());
                        *peekable.peeked = other;
                        Poll::Ready(None)
                    }
                }
            } else {
                panic!("NextIf polled after completion")
            }
        }
    }

    pin_project! {
        /// Future for the [`Peekable::next_if_eq`](self::Peekable::next_if_eq) method.
        #[must_use = "futures do nothing unless polled"]
        pub struct NextIfEq<'a, St: Stream, T: ?Sized> {
            #[pin]
            inner: NextIf<'a, St, NextIfEqFn<'a, T, St::Item>>,
        }
    }

    impl<St, T> fmt::Debug for NextIfEq<'_, St, T>
    where
        St: Stream + fmt::Debug,
        St::Item: fmt::Debug,
        T: ?Sized,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("NextIfEq")
                .field("inner", &self.inner.inner.as_ref().map(|(s, _f)| s))
                .finish()
        }
    }

    impl<St, T> FusedFuture for NextIfEq<'_, St, T>
    where
        St: Stream,
        T: ?Sized,
        St::Item: PartialEq<T>,
    {
        fn is_terminated(&self) -> bool {
            self.inner.is_terminated()
        }
    }

    impl<St, T> Future for NextIfEq<'_, St, T>
    where
        St: Stream,
        T: ?Sized,
        St::Item: PartialEq<T>,
    {
        type Output = Option<St::Item>;

        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            self.project().inner.poll(cx)
        }
    }

    struct NextIfEqFn<'a, T: ?Sized, Item> {
        expected: &'a T,
        _next: PhantomData<Item>,
    }

    impl<T, Item> FnOnce1<&Item> for NextIfEqFn<'_, T, Item>
    where
        T: ?Sized,
        Item: PartialEq<T>,
    {
        type Output = bool;

        fn call_once(self, next: &Item) -> Self::Output {
            next == self.expected
        }
    }