use super::assert_future;

use core::pin::Pin;

use futures_core::future::{FusedFuture, Future};

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

/// Future for the [`lazy`] function.

#[derive(Debug)]

#[must_use = "futures do nothing unless you `.await` or poll them"]

pub struct Lazy<F> {

    f: Option<F>,

}

// safe because we never generate `Pin<&mut F>`

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

/// Creates a new future that allows delayed execution of a closure.

///

/// The provided closure is only run once the future is polled.

///

/// # Examples

///

/// ```

/// # futures::executor::block_on(async {

/// use futures::future;

///

/// let a = future::lazy(|_| 1);

/// assert_eq!(a.await, 1);

///

/// let b = future::lazy(|_| -> i32 {

///     panic!("oh no!")

/// });

/// drop(b); // closure is never run

/// # });

/// ```

pub fn lazy<F, R>(f: F) -> Lazy<F>

where

    F: FnOnce(&mut Context<'_>) -> R,

{

    assert_future::<R, _>(Lazy { f: Some(f) })

}

impl<F, R> FusedFuture for Lazy<F>

where

    F: FnOnce(&mut Context<'_>) -> R,

{

    fn is_terminated(&self) -> bool {

        self.f.is_none()

    }

}

impl<F, R> Future for Lazy<F>

where

    F: FnOnce(&mut Context<'_>) -> R,

{

    type Output = R;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<R> {

        Poll::Ready((self.f.take().expect("Lazy polled after completion"))(cx))

    }

}