1 files changed, 83 insertions, 0 deletions
diff --git a/src/library/layout/pad.rs b/src/library/layout/pad.rs
new file mode 100644
index 00000000..175a54f0
--- /dev/null
+++ b/src/library/layout/pad.rs
@@ -0,0 +1,83 @@
+use crate::library::prelude::*;
+
+/// Pad a node at the sides.
+#[derive(Debug, Hash)]
+pub struct PadNode {
+    /// The amount of padding.
+    pub padding: Sides<Linear>,
+    /// The child node whose sides to pad.
+    pub child: LayoutNode,
+}
+
+#[class]
+impl PadNode {
+    fn construct(_: &mut Context, args: &mut Args) -> TypResult<Template> {
+        let all = args.find()?;
+        let hor = args.named("horizontal")?;
+        let ver = args.named("vertical")?;
+        let left = args.named("left")?.or(hor).or(all).unwrap_or_default();
+        let top = args.named("top")?.or(ver).or(all).unwrap_or_default();
+        let right = args.named("right")?.or(hor).or(all).unwrap_or_default();
+        let bottom = args.named("bottom")?.or(ver).or(all).unwrap_or_default();
+        let body: LayoutNode = args.expect("body")?;
+        let padding = Sides::new(left, top, right, bottom);
+        Ok(Template::block(body.padded(padding)))
+    }
+}
+
+impl Layout for PadNode {
+    fn layout(
+        &self,
+        ctx: &mut Context,
+        regions: &Regions,
+        styles: StyleChain,
+    ) -> TypResult<Vec<Arc<Frame>>> {
+        // Layout child into padded regions.
+        let pod = regions.map(|size| shrink(size, self.padding));
+        let mut frames = self.child.layout(ctx, &pod, styles)?;
+
+        for frame in &mut frames {
+            // Apply the padding inversely such that the grown size padded
+            // yields the frame's size.
+            let padded = grow(frame.size, self.padding);
+            let padding = self.padding.resolve(padded);
+            let offset = Point::new(padding.left, padding.top);
+
+            // Grow the frame and translate everything in the frame inwards.
+            let frame = Arc::make_mut(frame);
+            frame.size = padded;
+            frame.translate(offset);
+        }
+
+        Ok(frames)
+    }
+}
+
+/// Shrink a size by padding relative to the size itself.
+fn shrink(size: Size, padding: Sides<Linear>) -> Size {
+    size - padding.resolve(size).sum_by_axis()
+}
+
+/// Grow a size by padding relative to the grown size.
+/// This is the inverse operation to `shrink()`.
+///
+/// For the horizontal axis the derivation looks as follows.
+/// (Vertical axis is analogous.)
+///
+/// Let w be the grown target width,
+///     s be given width,
+///     l be the left padding,
+///     r be the right padding,
+///     p = l + r.
+///
+/// We want that: w - l.resolve(w) - r.resolve(w) = s
+///
+/// Thus: w - l.resolve(w) - r.resolve(w) = s
+///   <=> w - p.resolve(w) = s
+///   <=> w - p.rel * w - p.abs = s
+///   <=> (1 - p.rel) * w = s + p.abs
+///   <=> w = (s + p.abs) / (1 - p.rel)
+fn grow(size: Size, padding: Sides<Linear>) -> Size {
+    size.zip(padding.sum_by_axis())
+        .map(|(s, p)| (s + p.abs).safe_div(1.0 - p.rel.get()))
+}