use core::mem;

use futures_core::{Stream, Async, Poll};
use futures_core::task;

/// State of chain stream.
#[derive(Debug)]
enum State<S1, S2> {
    /// Emitting elements of first stream
    First(S1, S2),
    /// Emitting elements of second stream
    Second(S2),
    /// Temporary value to replace first with second
    Temp,
}

/// An adapter for chaining the output of two streams.
///
/// The resulting stream produces items from first stream and then
/// from second stream.
#[derive(Debug)]
#[must_use = "streams do nothing unless polled"]
pub struct Chain<S1, S2> {
    state: State<S1, S2>
}

pub fn new<S1, S2>(s1: S1, s2: S2) -> Chain<S1, S2>
    where S1: Stream, S2: Stream<Item=S1::Item, Error=S1::Error>,
{
    Chain { state: State::First(s1, s2) }
}

impl<S1, S2> Stream for Chain<S1, S2>
    where S1: Stream, S2: Stream<Item=S1::Item, Error=S1::Error>,
{
    type Item = S1::Item;
    type Error = S1::Error;

    fn poll_next(&mut self, cx: &mut task::Context) -> Poll<Option<Self::Item>, Self::Error> {
        loop {
            match self.state {
                State::First(ref mut s1, ref _s2) => match s1.poll_next(cx) {
                    Ok(Async::Ready(None)) => (), // roll
                    x => return x,
                },
                State::Second(ref mut s2) => return s2.poll_next(cx),
                State::Temp => unreachable!(),
            }

            self.state = match mem::replace(&mut self.state, State::Temp) {
                State::First(_s1, s2) => State::Second(s2),
                _ => unreachable!(),
            };
        }
    }
}