use crate::{Iter, Stream};

use core::option;

use core::pin::Pin;

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

/// Stream for the [`once`](fn@once) function.

#[derive(Debug)]

#[must_use = "streams do nothing unless polled"]

pub struct Once<T> {

    iter: Iter<option::IntoIter<T>>,

}

impl<I> Unpin for Once<I> {}

/// Creates a stream that emits an element exactly once.

///

/// The returned stream is immediately ready and emits the provided value once.

///

/// # Examples

///

/// ```

/// use tokio_stream::{self as stream, StreamExt};

///

/// #[tokio::main]

/// async fn main() {

///     // one is the loneliest number

///     let mut one = stream::once(1);

///

///     assert_eq!(Some(1), one.next().await);

///

///     // just one, that's all we get

///     assert_eq!(None, one.next().await);

/// }

/// ```

pub fn once<T>(value: T) -> Once<T> {

    Once {

        iter: crate::iter(Some(value)),

    }

}

impl<T> Stream for Once<T> {

    type Item = T;

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

        Pin::new(&mut self.iter).poll_next(cx)

    }

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

        self.iter.size_hint()

    }

}