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

/// A stream combinator which skips a number of elements before continuing.
///
/// This structure is produced by the `Stream::skip` method.
#[derive(Debug)]
#[must_use = "streams do nothing unless polled"]
pub struct Skip<S> {
    stream: S,
    remaining: u64,
}

pub fn new<S>(s: S, amt: u64) -> Skip<S>
    where S: Stream,
{
    Skip {
        stream: s,
        remaining: amt,
    }
}

impl<S> Skip<S> {
    /// Acquires a reference to the underlying stream that this combinator is
    /// pulling from.
    pub fn get_ref(&self) -> &S {
        &self.stream
    }

    /// Acquires a mutable reference to the underlying stream that this
    /// combinator is pulling from.
    ///
    /// Note that care must be taken to avoid tampering with the state of the
    /// stream which may otherwise confuse this combinator.
    pub fn get_mut(&mut self) -> &mut S {
        &mut self.stream
    }

    /// Consumes this combinator, returning the underlying stream.
    ///
    /// Note that this may discard intermediate state of this combinator, so
    /// care should be taken to avoid losing resources when this is called.
    pub fn into_inner(self) -> S {
        self.stream
    }
}

// Forwarding impl of Sink from the underlying stream
impl<S> ::sink::Sink for Skip<S>
    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()
    }

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

impl<S> Stream for Skip<S>
    where S: Stream,
{
    type Item = S::Item;
    type Error = S::Error;

    fn poll(&mut self) -> Poll<Option<S::Item>, S::Error> {
        while self.remaining > 0 {
            match try_ready!(self.stream.poll()) {
                Some(_) => self.remaining -= 1,
                None => return Ok(Async::Ready(None)),
            }
        }

        self.stream.poll()
    }
}