Fix: number-align should align line/number baselines

crates/typst/src/math/equation.rs

@@ -389,42 +389,30 @@ fn add_equation_number(
     region_size_x: Abs,
     full_number_width: Abs,
 ) -> Frame {
-    let first = equation_builder
-        .frames
-        .first()
-        .map_or((equation_builder.size, Point::zero()), |(frame, point)| {
-            (frame.size(), *point)
-        });
-    let last = equation_builder
-        .frames
-        .last()
-        .map_or((equation_builder.size, Point::zero()), |(frame, point)| {
-            (frame.size(), *point)
-        });
+    let first = equation_builder.frames.first().map_or(
+        (equation_builder.size, Point::zero(), Abs::zero()),
+        |(baseline, frame, point)| (frame.size(), *point, *baseline),
+    );
+    let last = equation_builder.frames.last().map_or(
+        (equation_builder.size, Point::zero(), Abs::zero()),
+        |(baseline, frame, point)| (frame.size(), *point, *baseline),
+    );
+    let line_count = equation_builder.frames.len();
     let mut equation = equation_builder.build();
 
     let width = if region_size_x.is_finite() {
         region_size_x
     } else {
         equation.width() + 2.0 * full_number_width
     };
-    let height = match number_align.y {
-        FixedAlignment::Start => {
-            let (size, point) = first;
-            let excess_above = (number.height() - size.y) / 2.0 - point.y;
-            equation.height() + Abs::zero().max(excess_above)
-        }
-        FixedAlignment::Center => equation.height().max(number.height()),
-        FixedAlignment::End => {
-            let (size, point) = last;
-            let excess_below =
-                (number.height() + size.y) / 2.0 - equation.height() + point.y;
-            equation.height() + Abs::zero().max(excess_below)
-        }
-    };
-    let resizing_offset = equation.resize(
-        Size::new(width, height),
-        Axes::<FixedAlignment>::new(equation_align, number_align.y.inv()),
+    let resizing_offset = resize_equation(
+        &mut equation,
+        &number,
+        number_align,
+        equation_align,
+        width,
+        line_count,
+        [first, last],
     );
     equation.translate(Point::with_x(match (equation_align, number_align.x) {
         (FixedAlignment::Start, FixedAlignment::Start) => full_number_width,
@@ -437,19 +425,57 @@ fn add_equation_number(
         FixedAlignment::End => equation.width() - number.width(),
         _ => unreachable!(),
     };
-    let dh = |h1: Abs, h2: Abs| (h1 - h2) / 2.0;
+    let align_baseline = |(_, point, baseline): (_, Point, Abs), number: &Frame| {
+        resizing_offset.y + point.y + baseline - number.baseline()
+    };
     let y = match number_align.y {
-        FixedAlignment::Start => {
-            let (size, point) = first;
-            resizing_offset.y + point.y + dh(size.y, number.height())
-        }
-        FixedAlignment::Center => dh(equation.height(), number.height()),
-        FixedAlignment::End => {
-            let (size, point) = last;
-            resizing_offset.y + point.y + dh(size.y, number.height())
-        }
+        FixedAlignment::Start => align_baseline(first, &number),
+        FixedAlignment::Center if line_count < 2 => align_baseline(first, &number),
+        FixedAlignment::Center => (equation.height() - number.height()) / 2.0,
+        FixedAlignment::End => align_baseline(last, &number),
     };
 
     equation.push_frame(Point::new(x, y), number);
     equation
 }
+
+fn resize_equation(
+    equation: &mut Frame,
+    number: &Frame,
+    number_align: Axes<FixedAlignment>,
+    equation_align: FixedAlignment,
+    width: Abs,
+    line_count: usize,
+    [first, last]: [(Axes<Abs>, Point, Abs); 2],
+) -> Point {
+    let excess_above = Abs::zero().max({
+        let (.., baseline) = first;
+        number.baseline() - baseline
+    });
+    let excess_below = Abs::zero().max({
+        let (size, .., baseline) = last;
+        (number.height() - number.baseline()) - (size.y - baseline)
+    });
+    let height = match number_align.y {
+        FixedAlignment::Start => equation.height() + excess_above,
+        FixedAlignment::Center if line_count < 2 => {
+            equation.height() + excess_above + excess_below
+        }
+        FixedAlignment::Center => equation.height().max(number.height()),
+        FixedAlignment::End => equation.height() + excess_below,
+    };
+
+    if matches!(number_align.y, FixedAlignment::Center if line_count < 2) {
+        let resizing_offset = equation.resize(
+            Size::new(width, height),
+            Axes::<FixedAlignment>::new(equation_align, FixedAlignment::Start),
+        );
+        equation.translate(Point::with_y(excess_above));
+        resizing_offset + Point::with_y(excess_above)
+    } else {
+        equation.resize(
+            Size::new(width, height),
+            Axes::<FixedAlignment>::new(equation_align, number_align.y.inv()),
+        )
+    }
+}

crates/typst/src/math/row.rs

@@ -174,7 +174,7 @@ impl MathRun {
         };
 
         let align = AlignElem::alignment_in(styles).resolve(styles).x;
-        let mut frames: Vec<(Frame, Point)> = vec![];
+        let mut frames: Vec<(Abs, Frame, Point)> = vec![];
         let mut size = Size::zero();
         for (i, row) in rows.into_iter().enumerate() {
             if i == row_count - 1 && row.0.is_empty() {
@@ -192,7 +192,7 @@ impl MathRun {
             }
             size.x.set_max(sub.width());
             size.y += sub.height();
-            frames.push((sub, pos));
+            frames.push((sub.baseline(), sub, pos));
         }
 
         MathRunFrameBuilder { size, frames }
@@ -375,16 +375,16 @@ impl Iterator for LeftRightAlternator {
 pub struct MathRunFrameBuilder {
     /// The size of the resulting frame.
     pub size: Size,
-    /// Sub frames for each row, and the positions where they should be pushed into
-    /// the resulting frame.
-    pub frames: Vec<(Frame, Point)>,
+    /// Each row: the baseline, the frame, and the positions where they should
+    /// be pushed into the resulting frame.
+    pub frames: Vec<(Abs, Frame, Point)>,
 }
 
 impl MathRunFrameBuilder {
     /// Consumes the builder and returns a [`Frame`].
     pub fn build(self) -> Frame {
         let mut frame = Frame::soft(self.size);
-        for (sub, pos) in self.frames.into_iter() {
+        for (_, sub, pos) in self.frames.into_iter() {
             frame.push_frame(pos, sub);
         }
         frame

tests/suite/math/equation.typ

@@ -152,6 +152,16 @@ $ a + b = c $
 // Error: 52-67 expected `start`, `left`, `right`, or `end`, found center
 #set math.equation(numbering: "(1)", number-align: center + bottom)
 
+--- math-equation-number-align-monoline ---
+#set math.equation(numbering: "(1)")
+$ p = sum_k k ln a $
+
+#set math.equation(numbering: "(1)", number-align: top)
+$ p = sum_k k ln a $
+
+#set math.equation(numbering: "(1)", number-align: bottom)
+$ p = sum_k k ln a $
+
 --- math-equation-number-align-multiline ---
 #set math.equation(numbering: "(1)")
 
@@ -163,13 +173,17 @@ $ p &= ln a b \
 
 $ p &= ln a b \
     &= ln a + ln b $
+$ q &= sum_k k ln a \
+    &= sum_k ln A $
 
 --- math-equation-number-align-multiline-bottom ---
 #show math.equation: set align(left)
 #set math.equation(numbering: "(1)", number-align: bottom)
 
-$ q &= ln sqrt(a b) \
-    &= 1/2 (ln a + ln b) $
+$ p &= ln a b \
+    &= ln a + ln b $
+$ q &= sum_k ln A \
+    &= sum_k k ln a $
 
 --- math-equation-number-align-multiline-expand ---
 // Tests that if the numbering's layout box vertically exceeds the box of