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use core::pin::Pin;

use crate::stream::Stream;
use crate::task::{Context, Poll};

/// A stream that repeats elements of type `T` endlessly by applying a provided closure.

///

/// This stream is created by the [`repeat_with`] function. See its

/// documentation for more.

///

/// [`repeat_with`]: fn.repeat_with.html

#[derive(Clone, Debug)]
pub struct RepeatWith<F> {
    f: F,
}

impl<F> Unpin for RepeatWith<F> {}

/// Creates a new stream that repeats elements of type `A` endlessly by applying the provided closure.

///

/// # Examples

///

/// Basic usage:

///

/// ```

/// # async_std::task::block_on(async {

/// #

/// use async_std::prelude::*;

/// use async_std::stream;

///

/// let s = stream::repeat_with(|| 1);

///

/// pin_utils::pin_mut!(s);

///

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

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

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

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

/// # })

/// ```

///

/// Going finite:

///

/// ```

/// # async_std::task::block_on(async {

/// #

/// use async_std::prelude::*;

/// use async_std::stream;

///

/// let mut n = 1;

/// let s = stream::repeat_with(|| {

///     let item = n;

///     n *= 2;

///     item

/// })

/// .take(4);

///

/// pin_utils::pin_mut!(s);

///

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

/// assert_eq!(s.next().await, Some(2));

/// assert_eq!(s.next().await, Some(4));

/// assert_eq!(s.next().await, Some(8));

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

/// # })

/// ```

pub fn repeat_with<T, F>(repeater: F) -> RepeatWith<F>
where
    F: FnMut() -> T,
{
    RepeatWith { f: repeater }
}

impl<T, F> Stream for RepeatWith<F>
where
    F: FnMut() -> T,
{
    type Item = T;

    fn poll_next(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        let item = (&mut self.f)();
        Poll::Ready(Some(item))
    }
}