1 files changed, 125 insertions, 0 deletions
diff --git a/crates/typst-library/src/layout/pad.rs b/crates/typst-library/src/layout/pad.rs
new file mode 100644
index 00000000..a3d5646b
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+++ b/crates/typst-library/src/layout/pad.rs
@@ -0,0 +1,125 @@
+use crate::prelude::*;
+
+/// Adds spacing around content.
+///
+/// The spacing can be specified for each side individually, or for all sides at
+/// once by specifying a positional argument.
+///
+/// ## Example { #example }
+/// ```example
+/// #set align(center)
+///
+/// #pad(x: 16pt, image("typing.jpg"))
+/// _Typing speeds can be
+///  measured in words per minute._
+/// ```
+///
+/// Display: Padding
+/// Category: layout
+#[element(Layout)]
+pub struct PadElem {
+    /// The padding at the left side.
+    #[parse(
+        let all = args.named("rest")?.or(args.find()?);
+        let x = args.named("x")?.or(all);
+        let y = args.named("y")?.or(all);
+        args.named("left")?.or(x)
+    )]
+    pub left: Rel<Length>,
+
+    /// The padding at the top side.
+    #[parse(args.named("top")?.or(y))]
+    pub top: Rel<Length>,
+
+    /// The padding at the right side.
+    #[parse(args.named("right")?.or(x))]
+    pub right: Rel<Length>,
+
+    /// The padding at the bottom side.
+    #[parse(args.named("bottom")?.or(y))]
+    pub bottom: Rel<Length>,
+
+    /// The horizontal padding. Both `left` and `right` take precedence over
+    /// this.
+    #[external]
+    pub x: Rel<Length>,
+
+    /// The vertical padding. Both `top` and `bottom` take precedence over this.
+    #[external]
+    pub y: Rel<Length>,
+
+    /// The padding for all sides. All other parameters take precedence over
+    /// this.
+    #[external]
+    pub rest: Rel<Length>,
+
+    /// The content to pad at the sides.
+    #[required]
+    pub body: Content,
+}
+
+impl Layout for PadElem {
+    #[tracing::instrument(name = "PadElem::layout", skip_all)]
+    fn layout(
+        &self,
+        vt: &mut Vt,
+        styles: StyleChain,
+        regions: Regions,
+    ) -> SourceResult<Fragment> {
+        let sides = Sides::new(
+            self.left(styles),
+            self.top(styles),
+            self.right(styles),
+            self.bottom(styles),
+        );
+
+        // Layout child into padded regions.
+        let mut backlog = vec![];
+        let padding = sides.resolve(styles);
+        let pod = regions.map(&mut backlog, |size| shrink(size, padding));
+        let mut fragment = self.body().layout(vt, styles, pod)?;
+
+        for frame in &mut fragment {
+            // Apply the padding inversely such that the grown size padded
+            // yields the frame's size.
+            let padded = grow(frame.size(), padding);
+            let padding = padding.relative_to(padded);
+            let offset = Point::new(padding.left, padding.top);
+
+            // Grow the frame and translate everything in the frame inwards.
+            frame.set_size(padded);
+            frame.translate(offset);
+        }
+
+        Ok(fragment)
+    }
+}
+
+/// Shrink a size by padding relative to the size itself.
+fn shrink(size: Size, padding: Sides<Rel<Abs>>) -> Size {
+    size - padding.relative_to(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 the 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<Rel<Abs>>) -> Size {
+    size.zip(padding.sum_by_axis())
+        .map(|(s, p)| (s + p.abs).safe_div(1.0 - p.rel.get()))
+}