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use std::{fmt, marker::PhantomData, ops::Deref};

use crate::Resource;

/// Reference to a resource.
#[derive(Clone)]
pub struct Ref<'a, R: 'a> {
    inner: rt_map::Ref<'a, Box<dyn Resource>>,
    phantom: PhantomData<&'a R>,
}

impl<'a, R> Ref<'a, R> {
    pub fn new(inner: rt_map::Ref<'a, Box<dyn Resource>>) -> Self {
        Self {
            inner,
            phantom: PhantomData,
        }
    }
}

impl<'a, R> Deref for Ref<'a, R>
where
    R: Resource,
{
    type Target = R;

    fn deref(&self) -> &R {
        (*self.inner)
            .downcast_ref::<R>()
            .unwrap_or_else(|| panic!("Failed to downcast to {}", std::any::type_name::<R>()))
    }
}

impl<'a, R> fmt::Debug for Ref<'a, R>
where
    R: Resource + fmt::Debug + 'a,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let inner: &R = &*self;
        f.debug_struct("Ref").field("inner", inner).finish()
    }
}

impl<'a, R> PartialEq for Ref<'a, R>
where
    R: Resource + PartialEq + 'a,
{
    fn eq(&self, other: &Self) -> bool {
        let r_self: &R = &*self;
        let r_other: &R = &*other;
        r_self == r_other
    }
}

#[cfg(test)]
mod tests {
    use std::{
        fmt::{self, Write},
        sync::atomic::AtomicUsize,
    };

    use rt_map::CellRef;

    use crate::Resource;

    use super::Ref;

    #[test]
    fn debug_includes_inner_field() -> fmt::Result {
        let flag = AtomicUsize::new(0);
        let value: Box<dyn Resource> = Box::new(A(1));
        let r#ref = Ref::<A>::new(rt_map::Ref::new(CellRef {
            flag: &flag,
            value: &value,
        }));

        let mut debug_string = String::with_capacity(64);
        write!(&mut debug_string, "{:?}", r#ref)?;
        assert_eq!("Ref { inner: A(1) }", debug_string.as_str());

        Ok(())
    }

    #[test]
    fn partial_eq_compares_value() -> fmt::Result {
        let flag = AtomicUsize::new(0);
        let value: Box<dyn Resource> = Box::new(A(1));
        let r#ref = Ref::<A>::new(rt_map::Ref::new(CellRef {
            flag: &flag,
            value: &value,
        }));

        assert_eq!(
            Ref::<A>::new(rt_map::Ref::new(CellRef {
                flag: &flag,
                value: &value,
            })),
            r#ref
        );
        assert_ne!(
            Ref::<A>::new(rt_map::Ref::new(CellRef {
                flag: &flag,
                value: &(Box::new(A(2)) as Box<dyn Resource>),
            })),
            r#ref
        );

        Ok(())
    }

    #[derive(Debug, Clone, PartialEq)]
    struct A(usize);
}