From 326fd3380151551538d6f245f884d174551bdf0f Mon Sep 17 00:00:00 2001
From: Eric Biedert <github@ericbiedert.de>
Date: Mon, 26 Aug 2024 17:26:50 +0200
Subject: [PATCH] Don't number empty regions of broken block equations (#4836)

---
 crates/typst/src/math/equation.rs                 |   9 ++++++++-
 .../issue-4829-math-pagebreaking-wrong-number.png | Bin 0 -> 394 bytes
 tests/ref/math-equation-number-empty.png          | Bin 0 -> 116 bytes
 tests/suite/math/equation.typ                     |   4 ++++
 tests/suite/math/multiline.typ                    |  12 ++++++++++++
 5 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 tests/ref/issue-4829-math-pagebreaking-wrong-number.png
 create mode 100644 tests/ref/math-equation-number-empty.png

diff --git a/crates/typst/src/math/equation.rs b/crates/typst/src/math/equation.rs
index df7cc0216..208540cd0 100644
--- a/crates/typst/src/math/equation.rs
+++ b/crates/typst/src/math/equation.rs
@@ -332,8 +332,10 @@ fn layout_equation_block(
         .layout_into_run(elem, styles)?
         .multiline_frame_builder(&ctx, styles);
     let width = full_equation_builder.size.x;
+    let can_break =
+        BlockElem::breakable_in(styles) && full_equation_builder.frames.len() > 1;
 
-    let equation_builders = if BlockElem::breakable_in(styles) {
+    let equation_builders = if can_break {
         let mut rows = full_equation_builder.frames.into_iter().peekable();
         let mut equation_builders = vec![];
         let mut last_first_pos = Point::zero();
@@ -415,9 +417,14 @@ fn layout_equation_block(
     };
 
     // Add equation numbers to each equation region.
+    let region_count = equation_builders.len();
     let frames = equation_builders
         .into_iter()
         .map(|builder| {
+            if builder.frames.is_empty() && region_count > 1 {
+                // Don't number empty regions, but do number empty equations.
+                return builder.build();
+            }
             add_equation_number(
                 builder,
                 number.clone(),
diff --git a/tests/ref/issue-4829-math-pagebreaking-wrong-number.png b/tests/ref/issue-4829-math-pagebreaking-wrong-number.png
new file mode 100644
index 0000000000000000000000000000000000000000..1db8a5110a3d748bceb72e66dc5cf9ecd7c0002d
GIT binary patch
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diff --git a/tests/ref/math-equation-number-empty.png b/tests/ref/math-equation-number-empty.png
new file mode 100644
index 0000000000000000000000000000000000000000..9311f1147ab376893dd87e12b5d78e50b82b2471
GIT binary patch
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literal 0
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diff --git a/tests/suite/math/equation.typ b/tests/suite/math/equation.typ
index 2345de549..6cdf7b7ed 100644
--- a/tests/suite/math/equation.typ
+++ b/tests/suite/math/equation.typ
@@ -256,6 +256,10 @@ $ q &= sum_k ln A \
   ],
 )
 
+--- math-equation-number-empty ---
+// Test numbering on empty equations.
+#math.equation(numbering: "1", block: true, [])
+
 --- issue-4187-alignment-point-affects-row-height ---
 // In this bug, a row of "-" only should have a very small height; but
 // after adding an alignment point "&", the row gains a larger height.
diff --git a/tests/suite/math/multiline.typ b/tests/suite/math/multiline.typ
index 35f10ea41..b6d970f36 100644
--- a/tests/suite/math/multiline.typ
+++ b/tests/suite/math/multiline.typ
@@ -133,3 +133,15 @@ $ a &+ b + & c \
 --- issue-1948-math-text-break ---
 // Test text with linebreaks in math.
 $ x := "a\nb\nc\nd\ne" $
+
+
+--- issue-4829-math-pagebreaking-wrong-number ---
+// Test numbering of empty regions of broken equations.
+#set page(height: 5em)
+#set math.equation(numbering: "1")
+#show math.equation: set block(breakable: true)
+
+#rect(height: 1.5em)
+
+$ a + b \
+  a + b $