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use pin_project::pin_project;

use super::{task, Future, Pin, Poll};

pub(crate) trait Started: Future {
    fn started(&self) -> bool;
}

pub(crate) fn lazy<F, R>(func: F) -> Lazy<F, R>
where
    F: FnOnce() -> R,
    R: Future + Unpin,
{
    Lazy {
        inner: Inner::Init(func),
    }
}

// FIXME: allow() required due to `impl Trait` leaking types to this lint
#[allow(missing_debug_implementations)]
#[pin_project]
pub(crate) struct Lazy<F, R> {
    #[pin]
    inner: Inner<F, R>,
}

#[pin_project(project = InnerProj, project_replace = InnerProjReplace)]
enum Inner<F, R> {
    Init(F),
    Fut(#[pin] R),
    Empty,
}

impl<F, R> Started for Lazy<F, R>
where
    F: FnOnce() -> R,
    R: Future,
{
    fn started(&self) -> bool {
        match self.inner {
            Inner::Init(_) => false,
            Inner::Fut(_) | Inner::Empty => true,
        }
    }
}

impl<F, R> Future for Lazy<F, R>
where
    F: FnOnce() -> R,
    R: Future,
{
    type Output = R::Output;

    fn poll(self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
        let mut this = self.project();

        if let InnerProj::Fut(f) = this.inner.as_mut().project() {
            return f.poll(cx);
        }

        match this.inner.as_mut().project_replace(Inner::Empty) {
            InnerProjReplace::Init(func) => {
                this.inner.set(Inner::Fut(func()));
                if let InnerProj::Fut(f) = this.inner.project() {
                    return f.poll(cx);
                }
                unreachable!()
            }
            _ => unreachable!("lazy state wrong"),
        }
    }
}