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use crate::stream::{self, 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, 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: stream::iter(Some(value).into_iter()),
    }
}

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()
    }
}