1 files changed, 63 insertions, 0 deletions
diff --git a/crates/typst-library/src/math/align.rs b/crates/typst-library/src/math/align.rs
new file mode 100644
index 00000000..aee89a89
--- /dev/null
+++ b/crates/typst-library/src/math/align.rs
@@ -0,0 +1,63 @@
+use super::*;
+
+/// A math alignment point: `&`, `&&`.
+///
+/// Display: Alignment Point
+/// Category: math
+#[element(LayoutMath)]
+pub struct AlignPointElem {}
+
+impl LayoutMath for AlignPointElem {
+    #[tracing::instrument(skip(ctx))]
+    fn layout_math(&self, ctx: &mut MathContext) -> SourceResult<()> {
+        ctx.push(MathFragment::Align);
+        Ok(())
+    }
+}
+
+pub(super) struct AlignmentResult {
+    pub points: Vec<Abs>,
+    pub width: Abs,
+}
+
+/// Determine the position of the alignment points.
+pub(super) fn alignments(rows: &[MathRow]) -> AlignmentResult {
+    let mut widths = Vec::<Abs>::new();
+
+    let mut pending_width = Abs::zero();
+    for row in rows {
+        let mut width = Abs::zero();
+        let mut alignment_index = 0;
+
+        for fragment in row.iter() {
+            if matches!(fragment, MathFragment::Align) {
+                if alignment_index < widths.len() {
+                    widths[alignment_index].set_max(width);
+                } else {
+                    widths.push(width.max(pending_width));
+                }
+                width = Abs::zero();
+                alignment_index += 1;
+            } else {
+                width += fragment.width();
+            }
+        }
+        if widths.is_empty() {
+            pending_width.set_max(width);
+        } else if alignment_index < widths.len() {
+            widths[alignment_index].set_max(width);
+        } else {
+            widths.push(width.max(pending_width));
+        }
+    }
+
+    let mut points = widths;
+    for i in 1..points.len() {
+        let prev = points[i - 1];
+        points[i] += prev;
+    }
+    AlignmentResult {
+        width: points.last().copied().unwrap_or(pending_width),
+        points,
+    }
+}