From 2c48c8d7a10c89fe15c0e29083ab3f3b39585423 Mon Sep 17 00:00:00 2001
From: Laurenz <laurmaedje@gmail.com>
Date: Sun, 22 Jan 2023 13:31:16 +0100
Subject: [PATCH] Multi-line alignment with `&`

---
 library/src/math/align.rs | 60 +++++++++++++++++++++++++++++++++++++++
 library/src/math/mod.rs   |  1 +
 2 files changed, 61 insertions(+)
 create mode 100644 library/src/math/align.rs

diff --git a/library/src/math/align.rs b/library/src/math/align.rs
new file mode 100644
index 000000000..82b461e9d
--- /dev/null
+++ b/library/src/math/align.rs
@@ -0,0 +1,60 @@
+use super::*;
+
+/// # Alignment Point
+/// A math alignment point: `&`, `&&`.
+///
+/// ## Parameters
+/// - index: usize (positional, required)
+///   The alignment point's index.
+///
+/// ## Category
+/// math
+#[func]
+#[capable(LayoutMath)]
+#[derive(Debug, Hash)]
+pub struct AlignPointNode;
+
+#[node]
+impl AlignPointNode {}
+
+impl LayoutMath for AlignPointNode {
+    fn layout_math(&self, ctx: &mut MathContext) -> SourceResult<()> {
+        ctx.push(MathFragment::Align);
+        Ok(())
+    }
+}
+
+/// Determine the position of the alignment points.
+pub(super) fn alignments(rows: &[MathRow]) -> Vec<Abs> {
+    let count = rows
+        .iter()
+        .map(|row| {
+            row.0
+                .iter()
+                .filter(|fragment| matches!(fragment, MathFragment::Align))
+                .count()
+        })
+        .max()
+        .unwrap_or(0);
+
+    let mut points = vec![Abs::zero(); count];
+    for current in 0..count {
+        for row in rows {
+            let mut x = Abs::zero();
+            let mut i = 0;
+            for fragment in &row.0 {
+                if matches!(fragment, MathFragment::Align) {
+                    if i < current {
+                        x = points[i];
+                    } else if i == current {
+                        points[i].set_max(x);
+                    }
+                    i += 1;
+                }
+                x += fragment.width();
+            }
+        }
+    }
+
+    points
+}
diff --git a/library/src/math/mod.rs b/library/src/math/mod.rs
index de954fbf5..35e005d09 100644
--- a/library/src/math/mod.rs
+++ b/library/src/math/mod.rs
@@ -3,6 +3,7 @@
 #[macro_use]
 mod ctx;
 mod accent;
+mod align;
 mod atom;
 mod frac;
 mod group;