use crate::*;
use std::marker::PhantomData;
//use std::sync::{Arc, Mutex, RwLock};

/// Rendering target
pub trait Output<E: Engine> {
    /// Current output area
    fn area (&self) -> E::Area;
    /// Mutable pointer to area
    fn area_mut (&mut self) -> &mut E::Area;
    /// Render widget in area
    fn place (&mut self, area: E::Area, content: &impl Content<E>);

    #[inline] fn x (&self) -> E::Unit { self.area().x() }
    #[inline] fn y (&self) -> E::Unit { self.area().y() }
    #[inline] fn w (&self) -> E::Unit { self.area().w() }
    #[inline] fn h (&self) -> E::Unit { self.area().h() }
    #[inline] fn wh (&self) -> E::Size { self.area().wh().into() }
}

pub trait Content<E: Engine>: Send + Sync {
    fn content (&self) -> impl Content<E> {
        ()
    }
    fn layout (&self, area: E::Area) -> E::Area {
        self.content().layout(area)
    }
    fn render (&self, output: &mut E::Output) {
        output.place(self.layout(output.area()), &self.content())
    }
}

/// The platonic ideal unit of [Content]: total emptiness at dead center.
impl<E: Engine> Content<E> for () {
    fn layout (&self, area: E::Area) -> E::Area {
        let [x, y] = area.center();
        [x, y, 0.into(), 0.into()].into()
    }
    fn render (&self, _: &mut E::Output) {}
}

impl<E: Engine, T: Content<E>> Content<E> for &T {
    fn content (&self) -> impl Content<E> {
        (*self).content()
    }
    fn layout (&self, area: E::Area) -> E::Area {
        (*self).layout(area)
    }
    fn render (&self, output: &mut E::Output) {
        (*self).render(output)
    }
}

impl<E: Engine, T: Content<E>> Content<E> for Option<T> {
    fn content (&self) -> impl Content<E> {
        self.as_ref()
            .map(|content|content.content())
    }
    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::Output) {
        self.as_ref()
            .map(|content|content.render(output));
    }
}

pub struct Thunk<E: Engine, T: Content<E>, F: Fn()->T + Send + Sync>(F, PhantomData<E>);

impl<E: Engine, T: Content<E>, F: Fn()->T + Send + Sync> Thunk<E, T, F> {
    pub fn new (thunk: F) -> Self {
        Self(thunk, Default::default())
    }
}
impl<E: Engine, T: Content<E>, F: Fn()->T + Send + Sync> Content<E> for Thunk<E, T, F> {
    fn content (&self) -> impl Content<E> {
        (self.0)()
    }
}

#[macro_export] macro_rules! render {
    (($self:ident:$Struct:ty) => $content:expr) => {
        impl <E: Engine> Content<E> for $Struct {
            fn content (&$self) -> impl Content<E> { Some($content) }
        }
    };
    (|$self:ident:$Struct:ident $(<
        $($L:lifetime),* $($T:ident $(:$Trait:path)?),*
    >)?, $to:ident | $render:expr) => {
        impl <$($($L),*)? E: Engine, $($T$(:$Trait)?),*> Content<E>
        for $Struct $(<$($L),* $($T),*>>)? {
            fn render (&$self, $to: &mut E::Output) { $render }
        }
    };
    ($Engine:ty:
        ($self:ident:$Struct:ident $(<$(
            $($L:lifetime)? $($T:ident)? $(:$Trait:path)?
        ),+>)?) => $content:expr
    ) => {
        impl $(<$($($L)? $($T)? $(:$Trait)?),+>)? Content<$Engine>
        for $Struct $(<$($($L)? $($T)?),+>)? {
            fn content (&$self) -> impl Content<$Engine> { $content }
        }
    };

    ($Engine:ty:
        |$self:ident : $Struct:ident $(<$(
            $($L:lifetime)? $($T:ident)? $(:$Trait:path)?
        ),+>)?, $to:ident| $render:expr
    ) => {
        impl $(<$($($L)? $($T)? $(:$Trait)?),+>)? Content<$Engine>
        for $Struct $(<$($($L)? $($T)?),+>)? {
            fn render (&$self, $to: &mut <$Engine as Engine>::Output) { $render }
        }
    };
}