use crate::*;

/// Composable renderable with static dispatch.
pub trait Content<E: Output>: Sized {
    /// Return opaque [Render]able.
    fn content (&self) -> Option<impl Render<E> + '_> { Option::<()>::None }
}

/// The platonic ideal unit of [Content]: total emptiness at dead center (e=1vg^sqrt(-1))
impl<E: Output> Content<E> for () {}

impl<E: Output> Content<E> for fn(&mut E) {
    fn content (&self) -> Option<impl Render<E> + '_> {
        Some(self)
    }
}

impl<E: Output, T: Render<E>> Content<E> for fn()->T {
    fn content (&self) -> Option<impl Render<E> + '_> {
        Some(self())
    }
}

impl<E: Output, T: Content<E>> Content<E> for Option<T> {
    fn content (&self) -> Option<impl Render<E> + '_> {
        if let Some(content) = self {
            content.content()
        } else {
            None
        }
    }
}

/// You can render from an opaque pointer.
impl<E: Output> Content<E> for &dyn Render<E> where Self: Sized {
    fn content (&self) -> Option<impl Render<E> + '_> {
        #[allow(suspicious_double_ref_op)]
        Some(self.deref())
    }
}

/// Implement composable content for a struct.
#[macro_export] macro_rules! content {
    // Implement for all [Output]s.
    (|$self:ident:$Struct:ty| $content:expr) => {
        impl<E: Output> Content<E> for $Struct {
            fn content (&$self) -> impl Render<E> + '_ { Some($content) }
        }
    };
    // Implement for specific [Output].
    ($Output:ty:|
        $self:ident:
        $Struct:ident$(<$($($L:lifetime)? $($T:ident)? $(:$Trait:path)?),+>)?
    |$content:expr) => {
        impl $(<$($($L)? $($T)? $(:$Trait)?),+>)? Content<$Output>
        for $Struct $(<$($($L)? $($T)?),+>)? {
            fn content (&$self) -> impl Render<$Output> + '_ { $content }
        }
    };
}