still dark; refactor and document layout crate

layout/src/transform_xy.rs

@@ -0,0 +1,41 @@
+use crate::*;
+
+/// Defines an enum that transforms its content
+/// along either the X axis, the Y axis, or both.
+///
+/// The `_Unused` variant wraps the `Engine` type
+/// using `PhantomData` to permit the double generic.
+macro_rules! transform_xy {
+    ($self:ident : $Enum:ident |$to:ident|$area:expr) => {
+        pub enum $Enum<E: Engine, T: Content<E>> { _Unused(PhantomData<E>), X(T), Y(T), XY(T) }
+        impl<E: Engine, T: Content<E>> $Enum<E, T> {
+            pub fn x (item: T) -> Self { Self::X(item) }
+            pub fn y (item: T) -> Self { Self::Y(item) }
+            pub fn xy (item: T) -> Self { Self::XY(item) }
+        }
+        impl<E: Engine, T: Content<E>> Content<E> for $Enum<E, T> {
+            fn content (&self) -> impl Content<E> {
+                match self {
+                    Self::X(item)    => item,
+                    Self::Y(item)    => item,
+                    Self::XY(item)   => item,
+                    _ => unreachable!()
+                }
+            }
+            fn area (&$self, $to: <E as Engine>::Area) -> <E as Engine>::Area {
+                $area
+            }
+        }
+    }
+}
+
+transform_xy!(self: Fill |to|{
+    let [x0, y0, wmax, hmax] = to.xywh();
+    let [x, y, w, h] = Content::area(&self.content(), to).xywh();
+    return match self {
+        Self::X(_)  => [x0, y, wmax, h],
+        Self::Y(_)  => [x, y0, w, hmax],
+        Self::XY(_) => [x0, y0, wmax, hmax],
+        _ => unreachable!()
+    }.into()
+});