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use {Async, Poll};
use stream::Stream;

/// A stream combinator used to filter the results of a stream and only yield
/// some values.
///
/// This structure is produced by the `Stream::filter` method.
#[must_use = "streams do nothing unless polled"]
pub struct Filter<S, F> {
    stream: S,
    f: F,
}

pub fn new<S, F>(s: S, f: F) -> Filter<S, F>
    where S: Stream,
          F: FnMut(&S::Item) -> bool,
{
    Filter {
        stream: s,
        f: f,
    }
}

// Forwarding impl of Sink from the underlying stream
impl<S, F> ::sink::Sink for Filter<S, F>
    where S: ::sink::Sink
{
    type SinkItem = S::SinkItem;
    type SinkError = S::SinkError;

    fn start_send(&mut self, item: S::SinkItem) -> ::StartSend<S::SinkItem, S::SinkError> {
        self.stream.start_send(item)
    }

    fn poll_complete(&mut self) -> Poll<(), S::SinkError> {
        self.stream.poll_complete()
    }
}

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

    fn poll(&mut self) -> Poll<Option<S::Item>, S::Error> {
        loop {
            match try_ready!(self.stream.poll()) {
                Some(e) => {
                    if (self.f)(&e) {
                        return Ok(Async::Ready(Some(e)))
                    }
                }
                None => return Ok(Async::Ready(None)),
            }
        }
    }
}