use crate::stream_ext::Fuse;

use crate::Stream;

use core::pin::Pin;

use core::task::{ready, Context, Poll};

use pin_project_lite::pin_project;

pin_project! {

    /// Stream returned by the [`chain`](super::StreamExt::chain) method.

    pub struct Chain<T, U> {

        #[pin]

        a: Fuse<T>,

        #[pin]

        b: U,

    }

}

impl<T, U> Chain<T, U> {

    pub(super) fn new(a: T, b: U) -> Chain<T, U>

    where

        T: Stream,

        U: Stream,

    {

        Chain { a: Fuse::new(a), b }

    }

}

impl<T, U> Stream for Chain<T, U>

where

    T: Stream,

    U: Stream<Item = T::Item>,

{

    type Item = T::Item;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T::Item>> {

        use Poll::Ready;

        let me = self.project();

        if let Some(v) = ready!(me.a.poll_next(cx)) {

            return Ready(Some(v));

        }

        me.b.poll_next(cx)

    }

    fn size_hint(&self) -> (usize, Option<usize>) {

        super::merge_size_hints(self.a.size_hint(), self.b.size_hint())

    }

}