use crate::*;

/// Standard numeric type.
pub trait Coordinate: Send + Sync + Copy
    + Add<Self, Output=Self>
    + Sub<Self, Output=Self>
    + Mul<Self, Output=Self>
    + Div<Self, Output=Self>
    + Ord + PartialEq + Eq
    + Debug + Display + Default
    + From<u16> + Into<u16>
    + Into<usize>
    + Into<f64>
{
    fn minus (self, other: Self) -> Self {
        if self >= other {
            self - other
        } else {
            0.into()
        }
    }
}

impl<T> Coordinate for T where T: Send + Sync + Copy
    + Add<Self, Output=Self>
    + Sub<Self, Output=Self>
    + Mul<Self, Output=Self>
    + Div<Self, Output=Self>
    + Ord + PartialEq + Eq
    + Debug + Display + Default
    + From<u16> + Into<u16>
    + Into<usize>
    + Into<f64>
{}

// TODO: return impl Point and impl Size instead of [N;x]
// to disambiguate between usage of 2-"tuple"s

pub trait Size<N: Coordinate> {
    fn x (&self) -> N;
    fn y (&self) -> N;
    #[inline] fn w (&self) -> N { self.x() }
    #[inline] fn h (&self) -> N { self.y() }
    #[inline] fn wh (&self) -> [N;2] { [self.x(), self.y()] }
    #[inline] fn clip_w (&self, w: N) -> [N;2] { [self.w().min(w.into()), self.h()] }
    #[inline] fn clip_h (&self, h: N) -> [N;2] { [self.w(), self.h().min(h.into())] }
    #[inline] fn expect_min (&self, w: N, h: N) -> Usually<&Self> {
        if self.w() < w || self.h() < h {
            Err(format!("min {w}x{h}").into())
        } else {
            Ok(self)
        }
    }
}
impl<N: Coordinate> Size<N> for (N, N) {
    fn x (&self) -> N { self.0 }
    fn y (&self) -> N { self.1 }
}
impl<N: Coordinate> Size<N> for [N;2] {
    fn x (&self) -> N { self[0] }
    fn y (&self) -> N { self[1] }
}

pub trait Area<N: Coordinate>: Copy {
    fn x  (&self) -> N;
    fn y  (&self) -> N;
    fn w  (&self) -> N;
    fn h  (&self) -> N;
    fn x2 (&self) -> N { self.x() + self.w() }
    fn y2 (&self) -> N { self.y() + self.h() }
    #[inline] fn wh       (&self) -> [N;2] { [self.w(), self.h()] }
    #[inline] fn xywh     (&self) -> [N;4] { [self.x(), self.y(), self.w(), self.h()] }
    #[inline] fn lrtb     (&self) -> [N;4] { [self.x(), self.x2(), self.y(), self.y2()] }
    #[inline] fn push_x   (&self, x: N) -> [N;4] { [self.x() + x, self.y(), self.w(), self.h()] }
    #[inline] fn push_y   (&self, y: N) -> [N;4] { [self.x(), self.y() + y, self.w(), self.h()] }
    #[inline] fn shrink_x (&self, x: N) -> [N;4] { [self.x(), self.y(), self.w() - x, self.h()] }
    #[inline] fn shrink_y (&self, y: N) -> [N;4] { [self.x(), self.y(), self.w(), self.h() - y] }
    #[inline] fn set_w    (&self, w: N) -> [N;4] { [self.x(), self.y(), w, self.h()] }
    #[inline] fn set_h    (&self, h: N) -> [N;4] { [self.x(), self.y(), self.w(), h] }
    #[inline] fn clip_h   (&self, h: N) -> [N;4] {
        [self.x(), self.y(), self.w(), self.h().min(h.into())]
    }
    #[inline] fn clip_w   (&self, w: N) -> [N;4] {
        [self.x(), self.y(), self.w().min(w.into()), self.h()]
    }
    #[inline] fn clip     (&self, wh: impl Size<N>) -> [N;4] {
        [self.x(), self.y(), wh.w(), wh.h()]
    }
    #[inline] fn expect_min (&self, w: N, h: N) -> Usually<&Self> {
        if self.w() < w || self.h() < h {
            Err(format!("min {w}x{h}").into())
        } else {
            Ok(self)
        }
    }
    #[inline] fn split_fixed (&self, direction: Direction, a: N) -> ([N;4],[N;4]) {
        match direction {
            Direction::Up => (
                [self.x(), (self.y()+self.h()).minus(a), self.w(), a],
                [self.x(), self.y(), self.w(), self.h().minus(a)],
            ),
            Direction::Down => (
                [self.x(), self.y(), self.w(), a],
                [self.x(), self.y() + a, self.w(), self.h().minus(a)],
            ),
            Direction::Right => (
                [self.x(), self.y(), a, self.h()],
                [self.x() + a, self.y(), self.w().minus(a), self.h()],
            ),
            _ => todo!(),
        }
    }
}

impl<N: Coordinate> Area<N> for (N, N, N, N) {
    #[inline] fn x (&self) -> N { self.0 }
    #[inline] fn y (&self) -> N { self.1 }
    #[inline] fn w (&self) -> N { self.2 }
    #[inline] fn h (&self) -> N { self.3 }
}

impl<N: Coordinate> Area<N> for [N;4] {
    #[inline] fn x (&self) -> N { self[0] }
    #[inline] fn y (&self) -> N { self[1] }
    #[inline] fn w (&self) -> N { self[2] }
    #[inline] fn h (&self) -> N { self[3] }
}

pub trait Layout<E: Engine>: Widget<Engine = E> + Sized {
    fn align_center (self) -> Align<Self> { Align::Center(self) }
    fn align_n   (self) -> Align<Self> { Align::N(self) }
    fn align_s   (self) -> Align<Self> { Align::S(self) }
    fn align_e   (self) -> Align<Self> { Align::E(self) }
    fn align_w   (self) -> Align<Self> { Align::W(self) }
    fn align_nw  (self) -> Align<Self> { Align::NW(self) }
    fn align_sw  (self) -> Align<Self> { Align::SW(self) }
    fn align_ne  (self) -> Align<Self> { Align::NE(self) }
    fn align_se  (self) -> Align<Self> { Align::SE(self) }
    fn align_x   (self) -> Align<Self> { Align::X(self) }
    fn align_y   (self) -> Align<Self> { Align::Y(self) }
    fn fixed_x   (self, x: E::Unit) -> Fixed<E::Unit, Self> { Fixed::X(x, self) }
    fn fixed_y   (self, y: E::Unit) -> Fixed<E::Unit, Self> { Fixed::Y(y, self) }
    fn fixed_xy  (self, x: E::Unit, y: E::Unit) -> Fixed<E::Unit, Self> { Fixed::XY(x, y, self) }
    fn min_x     (self, x: E::Unit) -> Min<E::Unit, Self> { Min::X(x, self) }
    fn min_y     (self, y: E::Unit) -> Min<E::Unit, Self> { Min::Y(y, self) }
    fn min_xy    (self, x: E::Unit, y: E::Unit) -> Min<E::Unit, Self> { Min::XY(x, y, self) }
    fn max_x     (self, x: E::Unit) -> Max<E::Unit, Self> { Max::X(x, self) }
    fn max_y     (self, y: E::Unit) -> Max<E::Unit, Self> { Max::Y(y, self) }
    fn max_xy    (self, x: E::Unit, y: E::Unit) -> Max<E::Unit, Self> { Max::XY(x, y, self) }
    fn push_x    (self, x: E::Unit) -> Push<E::Unit, Self> { Push::X(x, self) }
    fn push_y    (self, y: E::Unit) -> Push<E::Unit, Self> { Push::Y(y, self) }
    fn push_xy   (self, x: E::Unit, y: E::Unit) -> Push<E::Unit, Self> { Push::XY(x, y, self) }
    fn pull_x    (self, x: E::Unit) -> Pull<E::Unit, Self> { Pull::X(x, self) }
    fn pull_y    (self, y: E::Unit) -> Pull<E::Unit, Self> { Pull::Y(y, self) }
    fn pull_xy   (self, x: E::Unit, y: E::Unit) -> Pull<E::Unit, Self> { Pull::XY(x, y, self) }
    fn grow_x    (self, x: E::Unit) -> Grow<E::Unit, Self> { Grow::X(x, self) }
    fn grow_y    (self, y: E::Unit) -> Grow<E::Unit, Self> { Grow::Y(y, self) }
    fn grow_xy   (self, x: E::Unit, y: E::Unit) -> Grow<E::Unit, Self> { Grow::XY(x, y, self) }
    fn shrink_x  (self, x: E::Unit) -> Shrink<E::Unit, Self> { Shrink::X(x, self) }
    fn shrink_y  (self, y: E::Unit) -> Shrink<E::Unit, Self> { Shrink::Y(y, self) }
    fn shrink_xy (self, x: E::Unit, y: E::Unit) -> Shrink<E::Unit, Self> { Shrink::XY(x, y, self) }
    fn inset_x   (self, x: E::Unit) -> Inset<E::Unit, Self> { Inset::X(x, self) }
    fn inset_y   (self, y: E::Unit) -> Inset<E::Unit, Self> { Inset::Y(y, self) }
    fn inset_xy  (self, x: E::Unit, y: E::Unit) -> Inset<E::Unit, Self> { Inset::XY(x, y, self) }
    fn outset_x  (self, x: E::Unit) -> Outset<E::Unit, Self> { Outset::X(x, self) }
    fn outset_y  (self, y: E::Unit) -> Outset<E::Unit, Self> { Outset::Y(y, self) }
    fn outset_xy (self, x: E::Unit, y: E::Unit) -> Outset<E::Unit, Self> { Outset::XY(x, y, self) }
    fn fill_x    (self) -> Fill<E, Self> { Fill::X(self) }
    fn fill_y    (self) -> Fill<E, Self> { Fill::Y(self) }
    fn fill_xy   (self) -> Fill<E, Self> { Fill::XY(self) }
    fn debug     (self) -> DebugOverlay<E, Self> { DebugOverlay(self) }
    fn split <W: Widget<Engine = E>> (
        self, direction: Direction, amount: E::Unit, other: W
    ) -> Split<E, Self, W> { Split::new(direction, amount, self, other) }
    fn split_flip <W: Widget<Engine = E>> (
        self, direction: Direction, amount: E::Unit, other: W
    ) -> Split<E, W, Self> { Split::new(direction, amount, other, self) }
}

impl<E: Engine, W: Widget<Engine = E>> Layout<E> for W {}

pub struct DebugOverlay<E: Engine, W: Widget<Engine = E>>(pub W);

pub enum Fill<E: Engine, W: Widget<Engine = E>> { X(W), Y(W), XY(W) }

impl<E: Engine, W: Widget<Engine = E>> Fill<E, W> {
    fn inner (&self) -> &W {
        match self {
            Self::X(inner)  => &inner,
            Self::Y(inner)  => &inner,
            Self::XY(inner) => &inner,
        }
    }
}

impl<E: Engine, W: Widget<Engine = E>> Widget for Fill<E, W> {
    type Engine = E;
    fn layout (&self, to: E::Size) -> Perhaps<E::Size> {
        let area = self.inner().layout(to.into())?;
        if let Some(area) = area {
            Ok(Some(match self {
                Self::X(_)  => [to.w().into(), area.h()],
                Self::Y(_)  => [area.w(), to.h().into()],
                Self::XY(_) => [to.w().into(), to.h().into()],
            }.into()))
        } else {
            Ok(None)
        }
    }
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        self.inner().render(to)
    }
}

pub struct Layers<
    E: Engine,
    F: Send + Sync + Fn(&mut dyn FnMut(&dyn Widget<Engine = E>)->Usually<()>)->Usually<()>
>(pub F, PhantomData<E>);

impl<
    E: Engine,
    F: Send + Sync + Fn(&mut dyn FnMut(&dyn Widget<Engine = E>)->Usually<()>)->Usually<()>
> Layers<E, F> {
    #[inline]
    pub fn new (build: F) -> Self {
        Self(build, Default::default())
    }
}

impl<E: Engine, F> Widget for Layers<E, F>
where
    F: Send + Sync + Fn(&mut dyn FnMut(&dyn Widget<Engine = E>)->Usually<()>)->Usually<()>
{
    type Engine = E;
    fn layout (&self, area: E::Size) -> Perhaps<E::Size> {
        let mut w: E::Unit = 0.into();
        let mut h: E::Unit = 0.into();
        (self.0)(&mut |layer| {
            if let Some(layer_area) = layer.layout(area)? {
                w = w.max(layer_area.w());
                h = h.max(layer_area.h());
            }
            Ok(())
        })?;
        Ok(Some([w, h].into()))
    }
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        if let Some(size) = self.layout(to.area().wh().into())? {
            (self.0)(&mut |layer|to.render_in(to.area().clip(size).into(), &layer))
        } else {
            Ok(())
        }
    }
}

#[derive(Copy, Clone, PartialEq)]
pub enum Direction { Up, Down, Left, Right, }
impl Direction {
    pub fn is_up    (&self) -> bool { match self { Self::Up    => true, _ => false } }
    pub fn is_down  (&self) -> bool { match self { Self::Down  => true, _ => false } }
    pub fn is_left  (&self) -> bool { match self { Self::Left  => true, _ => false } }
    pub fn is_right (&self) -> bool { match self { Self::Right => true, _ => false } }
    /// Return next direction clockwise
    pub fn cw (&self) -> Self {
        match self {
            Self::Up    => Self::Right,
            Self::Down  => Self::Left,
            Self::Left  => Self::Up,
            Self::Right => Self::Down,
        }
    }
    /// Return next direction counterclockwise
    pub fn ccw (&self) -> Self {
        match self {
            Self::Up    => Self::Left,
            Self::Down  => Self::Right,
            Self::Left  => Self::Down,
            Self::Right => Self::Up,
        }
    }
}

/// Override X and Y coordinates, aligning to corner, side, or center of area
pub enum Align<L> {
    /// Draw at center of container
    Center(L),
    /// Draw at center of X axis
    X(L),
    /// Draw at center of Y axis
    Y(L),
    /// Draw at upper left corner of contaier
    NW(L),
    /// Draw at center of upper edge of container
    N(L), 
    /// Draw at right left corner of contaier
    NE(L),
    /// Draw at center of left edge of container
    W(L), 
    /// Draw at center of right edge of container
    E(L),
    /// Draw at lower left corner of container
    SW(L),
    /// Draw at center of lower edge of container
    S(L),
    /// Draw at lower right edge of container
    SE(L)
}

impl<T> Align<T> {
    pub fn inner (&self) -> &T {
        match self {
            Self::Center(inner) => inner,
            Self::X(inner) => inner,
            Self::Y(inner) => inner,
            Self::NW(inner) => inner,
            Self::N(inner)  => inner,
            Self::NE(inner) => inner,
            Self::W(inner)  => inner,
            Self::E(inner)  => inner,
            Self::SW(inner) => inner,
            Self::S(inner)  => inner,
            Self::SE(inner) => inner,
        }
    }
}

fn align<T, N: Coordinate, R: Area<N> + From<[N;4]>> (align: &Align<T>, outer: R, inner: R) -> Option<R> {
    if outer.w() < inner.w() || outer.h() < inner.h() {
        None
    } else {
        let [ox, oy, ow, oh] = outer.xywh();
        let [ix, iy, iw, ih] = inner.xywh();
        Some(match align {
            Align::Center(_) => [ox + (ow - iw) / 2.into(), oy + (oh - ih) / 2.into(), iw, ih,].into(),
            Align::X(_)  => [ox + (ow - iw) / 2.into(), iy, iw, ih,].into(),
            Align::Y(_)  => [ix, oy + (oh - ih) / 2.into(), iw, ih,].into(),
            Align::NW(_) => [ox, oy, iw, ih,].into(),
            Align::N(_)  => [ox + (ow - iw) / 2.into(), oy, iw, ih,].into(),
            Align::NE(_) => [ox + ow - iw, oy, iw, ih,].into(),
            Align::W(_)  => [ox, oy + (oh - ih) / 2.into(), iw, ih,].into(),
            Align::E(_)  => [ox + ow - iw, oy + (oh - ih) / 2.into(), iw, ih,].into(),
            Align::SW(_) => [ox, oy + oh - ih, iw, ih,].into(),
            Align::S(_)  => [ox + (ow - iw) / 2.into(), oy + oh - ih, iw, ih,].into(),
            Align::SE(_) => [ox + ow - iw, oy + oh - ih, iw, ih,].into(),
        })
    }
}

impl<E: Engine, T: Widget<Engine = E>> Widget for Align<T> {
    type Engine = E;
    fn layout (&self, outer_area: E::Size) -> Perhaps<E::Size> {
        self.inner().layout(outer_area)
    }
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        let outer_area = to.area();
        Ok(if let Some(inner_size) = self.layout(outer_area.wh().into())? {
            let inner_area = outer_area.clip(inner_size);
            if let Some(aligned) = align(&self, outer_area.into(), inner_area.into()) {
                to.render_in(aligned, self.inner())?
            } else {
                ()
            }
        } else {
            ()
        })
    }
}

/// Enforce fixed size of drawing area
pub enum Fixed<U: Coordinate, T> {
    /// Enforce fixed width
    X(U, T),
    /// Enforce fixed height
    Y(U, T),
    /// Enforce fixed width and height
    XY(U, U, T),
}
impl<N: Coordinate, T> Fixed<N, T> {
    pub fn inner (&self) -> &T {
        match self { Self::X(_, i) => i, Self::Y(_, i) => i, Self::XY(_, _, i) => i, }
    }
}
impl<E: Engine, T: Widget<Engine = E>> Widget for Fixed<E::Unit, T> {
    type Engine = E;
    fn layout (&self, to: E::Size) -> Perhaps<E::Size> {
        Ok(match self {
            Self::X(w, _) =>
                if to.w() >= *w { Some([*w, to.h()].into()) } else { None },
            Self::Y(h, _) =>
                if to.h() >= *h { Some([to.w(), *h].into()) } else { None },
            Self::XY(w, h, _)
                => if to.w() >= *w && to.h() >= *h { Some([*w, *h].into()) } else { None },
        })
    }
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        // 🡘  🡙 ←🡙→
        if let Some(size) = self.layout(to.area().wh().into())? {
            to.render_in(to.area().clip(size).into(), self.inner())
        } else {
            Ok(())
        }
    }
}

/// Enforce minimum size of drawing area
pub enum Min<U: Coordinate, T> {
    /// Enforce minimum width
    X(U, T),
    /// Enforce minimum height
    Y(U, T),
    /// Enforce minimum width and height
    XY(U, U, T),
}
impl<N: Coordinate, T> Min<N, T> {
    pub fn inner (&self) -> &T {
        match self { Self::X(_, i) => i, Self::Y(_, i) => i, Self::XY(_, _, i) => i, }
    }
}
impl<E: Engine, T: Widget<Engine = E>> Widget for Min<E::Unit, T> {
    type Engine = E;
    fn layout (&self, to: E::Size) -> Perhaps<E::Size> {
        Ok(self.inner().layout(to)?.map(|to|match *self {
            Self::X(w, _)     => [to.w().max(w), to.h()],
            Self::Y(h, _)     => [to.w(), to.h().max(h)],
            Self::XY(w, h, _) => [to.w().max(w), to.h().max(h)],
        }.into()))
    }
    // TODO: 🡘  🡙 ←🡙→
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        Ok(self.layout(to.area().wh().into())?
            .map(|size|to.render_in(to.area().clip(size).into(), self.inner()))
            .transpose()?.unwrap_or(()))
    }
}

/// Enforce maximum size of drawing area
pub enum Max<U: Coordinate, T> {
    /// Enforce maximum width
    X(U, T),
    /// Enforce maximum height
    Y(U, T),
    /// Enforce maximum width and height
    XY(U, U, T),
}

impl<N: Coordinate, T> Max<N, T> {
    fn inner (&self) -> &T {
        match self { Self::X(_, i) => i, Self::Y(_, i) => i, Self::XY(_, _, i) => i, }
    }
}

impl<E: Engine, T: Widget<Engine = E>> Widget for Max<E:: Unit, T> {
    type Engine = E;
    fn layout (&self, to: E::Size) -> Perhaps<E::Size> {
        Ok(self.inner().layout(to)?.map(|to|match *self {
            Self::X(w, _)     => [to.w().min(w), to.h()],
            Self::Y(h, _)     => [to.w(), to.h().min(h)],
            Self::XY(w, h, _) => [to.w().min(w), to.h().min(h)],
        }.into()))
    }
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        Ok(self.layout(to.area().wh().into())?
            .map(|size|to.render_in(to.area().clip(size).into(), self.inner()))
            .transpose()?.unwrap_or(()))
    }
}

/// Expand drawing area
pub enum Grow<N: Coordinate, T> {
    /// Increase width
    X(N, T),
    /// Increase height
    Y(N, T),
    /// Increase width and height
    XY(N, N, T)
}

impl<N: Coordinate, T> Grow<N, T> {
    fn inner (&self) -> &T {
        match self { Self::X(_, i) => i, Self::Y(_, i) => i, Self::XY(_, _, i) => i, }
    }
}

impl<E: Engine, T: Widget<Engine = E>> Widget for Grow<E::Unit, T> {
    type Engine = E;
    fn layout (&self, to: E::Size) -> Perhaps<E::Size> {
        Ok(self.inner().layout(to)?.map(|to|match *self {
            Self::X(w, _)     => [to.w() + w, to.h()],
            Self::Y(h, _)     => [to.w(), to.h() + h],
            Self::XY(w, h, _) => [to.w() + w, to.h() + h],
        }.into()))
    }
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        Ok(self.layout(to.area().wh().into())?
            .map(|size|to.render_in(to.area().clip(size).into(), self.inner()))
            .transpose()?.unwrap_or(()))
    }
}

/// Shrink drawing area
pub enum Shrink<N: Coordinate, T> {
    /// Decrease width
    X(N, T),
    /// Decrease height
    Y(N, T),
    /// Decrease width and height
    XY(N, N, T),
}

impl<N: Coordinate, T: Widget> Shrink<N, T> {
    fn inner (&self) -> &T {
        match self { Self::X(_, i) => i, Self::Y(_, i) => i, Self::XY(_, _, i) => i, }
    }
}

impl<E: Engine, T: Widget<Engine = E>> Widget for Shrink<E::Unit, T> {
    type Engine = E;
    fn layout (&self, to: E::Size) -> Perhaps<E::Size> {
        Ok(self.inner().layout(to)?.map(|to|match *self {
            Self::X(w, _)     => [
                if to.w() > w { to.w() - w } else { 0.into() },
                to.h()
            ],
            Self::Y(h, _)     => [
                to.w(),
                if to.h() > h { to.h() - h } else { 0.into() }
            ],
            Self::XY(w, h, _) => [
                if to.w() > w { to.w() - w } else { 0.into() },
                if to.h() > h { to.h() - h } else { 0.into() }
            ]
        }.into()))
    }
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        Ok(self.layout(to.area().wh().into())?
            .map(|size|to.render_in(to.area().clip(size).into(), self.inner()))
            .transpose()?.unwrap_or(()))
    }
}

/// Shrink from each side
pub enum Inset<N: Coordinate, T> {
    /// Decrease width
    X(N, T),
    /// Decrease height
    Y(N, T),
    /// Decrease width and height
    XY(N, N, T),
}

impl<N: Coordinate, T: Widget> Inset<N, T> {
    pub fn inner (&self) -> &T {
        match self { Self::X(_, i) => i, Self::Y(_, i) => i, Self::XY(_, _, i) => i, }
    }
}

/// Grow on each side
pub enum Outset<N: Coordinate, T> {
    /// Increase width
    X(N, T),
    /// Increase height
    Y(N, T),
    /// Increase width and height
    XY(N, N, T),
}

impl<N: Coordinate, T: Widget> Outset<N, T> {
    pub fn inner (&self) -> &T {
        match self { Self::X(_, i) => i, Self::Y(_, i) => i, Self::XY(_, _, i) => i, }
    }
}

impl<E: Engine, T: Widget<Engine = E>> Widget for Inset<E::Unit, T> {
    type Engine = E;
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        match *self {
            Self::X(x, ref inner) =>
                (inner as &dyn Widget<Engine = E>).shrink_x(x).push_x(x),
            Self::Y(y, ref inner) =>
                (inner as &dyn Widget<Engine = E>).shrink_y(y).push_y(y),
            Self::XY(x, y, ref inner) =>
                (inner as &dyn Widget<Engine = E>).shrink_xy(x, y).push_xy(x, y)
        }.render(to)
    }
}

impl<E: Engine, T: Widget<Engine = E>> Widget for Outset<E::Unit, T> {
    type Engine = E;
    fn layout (&self, to: E::Size) -> Perhaps<E::Size> {
        match *self {
            Self::X(x, ref inner) =>
                (inner as &dyn Widget<Engine = E>).grow_x(x + x),
            Self::Y(y, ref inner) =>
                (inner as &dyn Widget<Engine = E>).grow_y(y + y),
            Self::XY(x, y, ref inner) =>
                (inner as &dyn Widget<Engine = E>).grow_xy(x + x, y + y),
        }.layout(to)
    }
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        match *self {
            Self::X(x, ref inner) =>
                (inner as &dyn Widget<Engine = E>).push_x(x),
            Self::Y(y, ref inner) =>
                (inner as &dyn Widget<Engine = E>).push_y(y),
            Self::XY(x, y, ref inner) =>
                (inner as &dyn Widget<Engine = E>).push_xy(x, y),
        }.render(to)
    }
}

/// Move origin point of drawing area
pub enum Push<N: Coordinate, T: Widget> {
    /// Move origin to the right
    X(N, T),
    /// Move origin downwards
    Y(N, T),
    /// Move origin to the right and downwards
    XY(N, N, T),
}

impl<N: Coordinate, T: Widget> Push<N, T> {
    pub fn inner (&self) -> &T {
        match self { Self::X(_, i) => i, Self::Y(_, i) => i, Self::XY(_, _, i) => i, }
    }
    pub fn x (&self) -> N {
        match self { Self::X(x, _) => *x, Self::Y(_, _) => N::default(), Self::XY(x, _, _) => *x }
    }
    pub fn y (&self) -> N {
        match self { Self::X(_, _) => N::default(), Self::Y(y, _) => *y, Self::XY(_, y, _) => *y }
    }
}

impl<E: Engine, T: Widget<Engine = E>> Widget for Push<E::Unit, T> {
    type Engine = E;
    fn layout (&self, to: E::Size) -> Perhaps<E::Size> {
        self.inner().layout(to)
    }
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        let area = to.area();
        Ok(self.layout(area.wh().into())?
            .map(|size|to.render_in(match *self {
                Self::X(x, _)     => [area.x() + x, area.y(),     size.w(), size.h()],
                Self::Y(y, _)     => [area.x(),     area.y() + y, size.w(), size.h()],
                Self::XY(x, y, _) => [area.x() + x, area.y() + y, size.w(), size.h()],
            }.into(), self.inner())).transpose()?.unwrap_or(()))
    }
}

/// Move origin point of drawing area
pub enum Pull<N: Coordinate, T: Widget> {
    /// Move origin to the right
    X(N, T),
    /// Move origin downwards
    Y(N, T),
    /// Move origin to the right and downwards
    XY(N, N, T),
}

impl<N: Coordinate, T: Widget> Pull<N, T> {
    pub fn inner (&self) -> &T {
        match self { Self::X(_, i) => i, Self::Y(_, i) => i, Self::XY(_, _, i) => i, }
    }
    pub fn x (&self) -> N {
        match self { Self::X(x, _) => *x, Self::Y(_, _) => N::default(), Self::XY(x, _, _) => *x }
    }
    pub fn y (&self) -> N {
        match self { Self::X(_, _) => N::default(), Self::Y(y, _) => *y, Self::XY(_, y, _) => *y }
    }
}

impl<E: Engine, T: Widget<Engine = E>> Widget for Pull<E::Unit, T> {
    type Engine = E;
    fn layout (&self, to: E::Size) -> Perhaps<E::Size> {
        self.inner().layout(to)
    }
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        let area = to.area();
        Ok(self.layout(area.wh().into())?
            .map(|size|to.render_in(match *self {
                Self::X(x, _)     => [area.x().minus(x), area.y(),          size.w(), size.h()],
                Self::Y(y, _)     => [area.x(),     area.y().minus(y),      size.w(), size.h()],
                Self::XY(x, y, _) => [area.x().minus(x), area.y().minus(y), size.w(), size.h()],
            }.into(), self.inner())).transpose()?.unwrap_or(()))
    }
}

pub struct Stack<
    E: Engine,
    F: Send + Sync + Fn(&mut dyn FnMut(&dyn Widget<Engine = E>)->Usually<()>)->Usually<()>
>(pub F, pub Direction, PhantomData<E>);

impl<
    E: Engine,
    F: Send + Sync + Fn(&mut dyn FnMut(&dyn Widget<Engine = E>)->Usually<()>)->Usually<()>
> Stack<E, F> {
    #[inline] pub fn new (direction: Direction, build: F) -> Self {
        Self(build, direction, Default::default())
    }
    #[inline] pub fn right (build: F) -> Self {
        Self::new(Direction::Right, build)
    }
    #[inline] pub fn down (build: F) -> Self {
        Self::new(Direction::Down, build)
    }
}

impl<E: Engine, F> Widget for Stack<E, F>
where
    F: Send + Sync + Fn(&mut dyn FnMut(&dyn Widget<Engine = E>)->Usually<()>)->Usually<()>
{
    type Engine = E;
    fn layout (&self, to: E::Size) -> Perhaps<E::Size> {
        let mut w = 0.into();
        let mut h = 0.into();
        match self.1 {
            Direction::Down => {
                (self.0)(&mut |component| {
                    if h >= to.h() { return Ok(()) }
                    let size = component.push_y(h).max_y(to.h() - h).layout(to)?;
                    if let Some([width, height]) = size.map(|size|size.wh()) {
                        h = h + height.into();
                        if width > w { w = width; }
                    }
                    Ok(())
                })?;
            },
            Direction::Right => {
                (self.0)(&mut |component| {
                    if w >= to.w() { return Ok(()) }
                    let size = component.push_x(w).max_x(to.w() - w).layout(to)?;
                    if let Some([width, height]) = size.map(|size|size.wh()) {
                        w = w + width.into();
                        if height > h { h = height }
                    }
                    Ok(())
                })?;
            },
            _ => todo!()
        };
        Ok(Some([w, h].into()))
    }
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        let area = to.area();
        let mut w = 0.into();
        let mut h = 0.into();
        match self.1 {
            Direction::Down => {
                (self.0)(&mut |component| {
                    if h >= area.h() { return Ok(()) }
                    let item = component.push_y(h).max_y(area.h() - h);
                    let size = item.layout(area.wh().into())?;
                    if let Some([width, height]) = size.map(|size|size.wh()) {
                        item.render(to)?;
                        h = h + height;
                        if width > w { w = width }
                    };
                    Ok(())
                })?;
            },
            Direction::Right => {
                (self.0)(&mut |component| {
                    if w >= area.w() { return Ok(()) }
                    let item = component.push_x(w).max_x(area.w() - w);
                    let size = item.layout(area.wh().into())?;
                    if let Some([width, height]) = size.map(|size|size.wh()) {
                        item.render(to)?;
                        w = width + w;
                        if height > h { h = height }
                    };
                    Ok(())
                })?;
            },
            _ => todo!()
        };
        Ok(())
    }
}

#[macro_export] macro_rules! lay {
    ($($expr:expr),* $(,)?) => { Layers::new(move|add|{ $(add(&$expr)?;)* Ok(()) }) }
}
#[macro_export] macro_rules! col {
    ($($expr:expr),* $(,)?) => { Stack::down(move|add|{ $(add(&$expr)?;)* Ok(()) }) };
    ($pat:pat in $collection:expr => $item:expr) => {
        Stack::down(move |add|{
            for $pat in $collection { add(&$item)?; }
            Ok(())
        })
    }
}
#[macro_export] macro_rules! col_up {
    ($($expr:expr),* $(,)?) => { Stack::down(move|add|{ $(add(&$expr)?;)* Ok(()) }) };
    ($pat:pat in $collection:expr => $item:expr) => {
        Stack::up(move |add|{
            for $pat in $collection { add(&$item)?; }
            Ok(())
        })
    }
}
#[macro_export] macro_rules! row {
    ($($expr:expr),* $(,)?) => { Stack::right(move|add|{ $(add(&$expr)?;)* Ok(()) }) };
    ($pat:pat in $collection:expr => $item:expr) => {
        Stack::right(move |add|{
            for $pat in $collection { add(&$item)?; }
            Ok(())
        })
    }
}

/// A binary split with fixed proportion
pub struct Split<E: Engine, A: Widget<Engine = E>, B: Widget<Engine = E>>(
    pub Direction, pub E::Unit, A, B, PhantomData<E>
);

impl<E: Engine, A: Widget<Engine = E>, B: Widget<Engine = E>> Split<E, A, B> {
    pub fn new (direction: Direction, proportion: E::Unit, a: A, b: B) -> Self {
        Self(direction, proportion, a, b, Default::default())
    }
    pub fn up (proportion: E::Unit, a: A, b: B) -> Self {
        Self(Direction::Up, proportion, a, b, Default::default())
    }
    pub fn down (proportion: E::Unit, a: A, b: B) -> Self {
        Self(Direction::Down, proportion, a, b, Default::default())
    }
    pub fn left (proportion: E::Unit, a: A, b: B) -> Self {
        Self(Direction::Left, proportion, a, b, Default::default())
    }
    pub fn right (proportion: E::Unit, a: A, b: B) -> Self {
        Self(Direction::Right, proportion, a, b, Default::default())
    }
}

impl<E: Engine, A: Widget<Engine = E>, B: Widget<Engine = E>> Widget for Split<E, A, B> {
    type Engine = E;
    fn layout (&self, to: E::Size) -> Perhaps<E::Size> {
        Ok(Some(to))
    }
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        let (a, b) = to.area().split_fixed(self.0, self.1);
        to.render_in(a.into(), &self.2)?;
        to.render_in(b.into(), &self.3)?;
        Ok(())
    }
}

/// A widget that tracks its render width and height
pub struct Measure<E: Engine>(PhantomData<E>, AtomicUsize, AtomicUsize);

impl<E: Engine> Clone for Measure<E> {
    fn clone (&self) -> Self {
        Self(
            Default::default(),
            AtomicUsize::from(self.1.load(Ordering::Relaxed)),
            AtomicUsize::from(self.2.load(Ordering::Relaxed)),
        )
    }
}

impl<E: Engine> std::fmt::Debug for Measure<E> {
    fn fmt (&self, f: &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error> {
        f.debug_struct("Measure")
            .field("width",  &self.0)
            .field("height", &self.1)
            .finish()
    }
}

impl<E: Engine> Measure<E> {
    pub fn w (&self) -> usize { self.1.load(Ordering::Relaxed) }
    pub fn h (&self) -> usize { self.2.load(Ordering::Relaxed) }
    pub fn wh (&self) -> [usize;2] { [self.w(), self.h()] }
    pub fn set_w (&self, w: impl Into<usize>) { self.1.store(w.into(), Ordering::Relaxed) }
    pub fn set_h (&self, h: impl Into<usize>) { self.2.store(h.into(), Ordering::Relaxed) }
    pub fn set_wh (&self, w: impl Into<usize>, h: impl Into<usize>) { self.set_w(w); self.set_h(h); }
    pub fn new () -> Self { Self(PhantomData::default(), 0.into(), 0.into()) }
    pub fn format (&self) -> String { format!("{}x{}", self.w(), self.h()) }
}

impl<E: Engine> Widget for Measure<E> {
    type Engine = E;
    fn layout (&self, _: E::Size) -> Perhaps<E::Size> {
        Ok(Some([0u16.into(), 0u16.into()].into()))
    }
    fn render (&self, to: &mut E::Output) -> Usually<()> {
        self.set_w(to.area().w());
        self.set_h(to.area().h());
        Ok(())
    }
}

/// A scrollable area.
pub struct Scroll<
    E: Engine,
    F: Send + Sync + Fn(&mut dyn FnMut(&dyn Widget<Engine = E>)->Usually<()>)->Usually<()>
>(pub F, pub Direction, pub u64, PhantomData<E>);