src/stream_ext/take_while.rs

use crate::Stream;

use core::fmt;
use core::pin::Pin;
use core::task::{Context, Poll};
use pin_project_lite::pin_project;

pin_project! {
    /// Stream for the [`take_while`](super::StreamExt::take_while) method.
    #[must_use = "streams do nothing unless polled"]
    pub struct TakeWhile<St, F> {
        #[pin]
        stream: St,
        predicate: F,
        done: bool,
    }
}

impl<St, F> fmt::Debug for TakeWhile<St, F>
where
    St: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("TakeWhile")
            .field("stream", &self.stream)
            .field("done", &self.done)
            .finish()
    }
}

impl<St, F> TakeWhile<St, F> {
    pub(super) fn new(stream: St, predicate: F) -> Self {
        Self {
            stream,
            predicate,
            done: false,
        }
    }
}

impl<St, F> Stream for TakeWhile<St, F>
where
    St: Stream,
    F: FnMut(&St::Item) -> bool,
{
    type Item = St::Item;

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        if !*self.as_mut().project().done {
            self.as_mut().project().stream.poll_next(cx).map(|ready| {
                let ready = ready.and_then(|item| {
                    if !(self.as_mut().project().predicate)(&item) {
                        None
                    } else {
                        Some(item)
                    }
                });

                if ready.is_none() {
                    *self.as_mut().project().done = true;
                }

                ready
            })
        } else {
            Poll::Ready(None)
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.done {
            return (0, Some(0));
        }

        let (_, upper) = self.stream.size_hint();

        (0, upper)
    }
}