use crate::*;

/// Composable renderable with static dispatch.
pub trait Content<E: Output> {
    /// Return a [Render]able of a specific type.
    fn content (&self) -> impl Render<E> + '_ {
        ()
    }
    /// Perform layout. By default, delegates to [Self::content].
    fn layout (&self, area: E::Area) -> E::Area {
        self.content().layout(area)
    }
    /// Draw to output. By default, delegates to [Self::content].
    fn render (&self, output: &mut E) {
        self.content().render(output) 
    }
}

/// Every pointer to [Content] is a [Content].
impl<E: Output, C: Content<E>> Content<E> for &C {
    fn content (&self) -> impl Render<E> + '_ { (*self).content() }
    fn layout (&self, area: E::Area) -> E::Area { (*self).layout(area) }
    fn render (&self, output: &mut E) { (*self).render(output) }
}

/// The platonic ideal unit of [Content]: total emptiness at dead center (e=1vg^sqrt(-1))
impl<E: Output> Content<E> for () {
    fn layout (&self, area: E::Area) -> E::Area { area.center().to_area_pos().into() }
    fn render (&self, _: &mut E) {}
}

impl<E: Output, T: Content<E>> Content<E> for Option<T> {
    fn content (&self) -> impl Render<E> + '_ {
        self.as_ref()
    }
    fn layout (&self, area: E::Area) -> E::Area {
        self.as_ref()
            .map(|content|content.layout(area))
            .unwrap_or([0.into(), 0.into(), 0.into(), 0.into(),].into())
    }
    fn render (&self, output: &mut E) {
        self.as_ref()
            .map(|content|content.render(output));
    }
}

/// You can render from a box.
impl<E: Output> Content<E> for RenderBox<E> {
    fn content (&self) -> impl Render<E> + '_ { self.deref() }
    //fn boxed <'b> (self) -> RenderBox<'b, E> where Self: Sized + 'b { self }
}

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

/// Implement [Content] with custom rendering for a struct.
#[macro_export] macro_rules! render {
    (|$self:ident:$Struct:ident $(<
        $($L:lifetime),* $($T:ident $(:$Trait:path)?),*
    >)?, $to:ident | $render:expr) => {
        impl <$($($L),*)? E: Output, $($T$(:$Trait)?),*> Content<E>
        for $Struct $(<$($L),* $($T),*>>)? {
            fn render (&$self, $to: &mut E) { $render }
        }
    };
    ($Output:ty:|
        $self:ident:
        $Struct:ident $(<$($($L:lifetime)? $($T:ident)? $(:$Trait:path)?),+>)?, $to:ident
    |$render:expr) => {
        impl $(<$($($L)? $($T)? $(:$Trait)?),+>)? Content<$Output>
        for $Struct $(<$($($L)? $($T)?),+>)? {
            fn render (&$self, $to: &mut $Output) { $render }
        }
    };
}