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
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146

use crate::stream::{Fuse, StreamExt};
use core::fmt;
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;
            }
        })
    }
}

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) function from [`Peekable`]
    #[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")
        }
    }
}