Fix equations in RTL text (#4150)

crates/typst/src/layout/inline/mod.rs

@@ -102,6 +102,13 @@ type Range = std::ops::Range<usize>;
 const SPACING_REPLACE: char = ' '; // Space
 const OBJ_REPLACE: char = '\u{FFFC}'; // Object Replacement Character
 
+// Unicode BiDi control characters.
+const LTR_EMBEDDING: char = '\u{202A}';
+const RTL_EMBEDDING: char = '\u{202B}';
+const POP_EMBEDDING: char = '\u{202C}';
+const LTR_ISOLATE: char = '\u{2066}';
+const POP_ISOLATE: char = '\u{2069}';
+
 /// A paragraph representation in which children are already layouted and text
 /// is already preshaped.
 ///
@@ -206,9 +213,12 @@ impl Segment<'_> {
             Self::Box(_, frac) => {
                 (if frac { SPACING_REPLACE } else { OBJ_REPLACE }).len_utf8()
             }
-            Self::Equation(ref par_items) => {
-                par_items.iter().map(MathParItem::text).map(char::len_utf8).sum()
-            }
+            Self::Equation(ref par_items) => par_items
+                .iter()
+                .map(MathParItem::text)
+                .chain([LTR_ISOLATE, POP_ISOLATE])
+                .map(char::len_utf8)
+                .sum(),
             Self::Meta => 0,
         }
     }
@@ -227,6 +237,9 @@ enum Item<'a> {
     Frame(Frame),
     /// Metadata.
     Meta(Frame),
+    /// An item that is invisible and needs to be skipped, e.g. a Unicode
+    /// isolate.
+    Skip(char),
 }
 
 impl<'a> Item<'a> {
@@ -253,6 +266,7 @@ impl<'a> Item<'a> {
             Self::Absolute(_) | Self::Fractional(_, _) => SPACING_REPLACE.len_utf8(),
             Self::Frame(_) => OBJ_REPLACE.len_utf8(),
             Self::Meta(_) => 0,
+            Self::Skip(c) => c.len_utf8(),
         }
     }
 
@@ -263,6 +277,7 @@ impl<'a> Item<'a> {
             Self::Absolute(v) => *v,
             Self::Frame(frame) => frame.width(),
             Self::Fractional(_, _) | Self::Meta(_) => Abs::zero(),
+            Self::Skip(_) => Abs::zero(),
         }
     }
 }
@@ -452,8 +467,8 @@ fn collect<'a>(
             if dir != outer_dir {
                 // Insert "Explicit Directional Embedding".
                 match dir {
-                    Dir::LTR => full.push('\u{202A}'),
-                    Dir::RTL => full.push('\u{202B}'),
+                    Dir::LTR => full.push(LTR_EMBEDDING),
+                    Dir::RTL => full.push(RTL_EMBEDDING),
                     _ => {}
                 }
             }
@@ -466,7 +481,7 @@ fn collect<'a>(
 
             if dir != outer_dir {
                 // Insert "Pop Directional Formatting".
-                full.push('\u{202C}');
+                full.push(POP_EMBEDDING);
             }
             Segment::Text(full.len() - prev)
         } else if let Some(elem) = child.to_packed::<HElem>() {
@@ -521,7 +536,9 @@ fn collect<'a>(
                 let MathParItem::Frame(frame) = item else { continue };
                 frame.meta(styles, false);
             }
+            full.push(LTR_ISOLATE);
             full.extend(items.iter().map(MathParItem::text));
+            full.push(POP_ISOLATE);
             Segment::Equation(items)
         } else if let Some(elem) = child.to_packed::<BoxElem>() {
             let frac = elem.width(styles).is_fractional();
@@ -594,6 +611,7 @@ fn prepare<'a>(
                 }
             },
             Segment::Equation(par_items) => {
+                items.push(Item::Skip(LTR_ISOLATE));
                 for item in par_items {
                     match item {
                         MathParItem::Space(s) => items.push(Item::Absolute(s)),
@@ -603,6 +621,7 @@ fn prepare<'a>(
                         }
                     }
                 }
+                items.push(Item::Skip(POP_ISOLATE));
             }
             Segment::Box(elem, _) => {
                 if let Sizing::Fr(v) = elem.width(styles) {
@@ -1353,6 +1372,7 @@ fn commit(
             Item::Frame(frame) | Item::Meta(frame) => {
                 push(&mut offset, frame.clone());
             }
+            Item::Skip(_) => {}
         }
     }
 

crates/typst/src/math/equation.rs

@@ -224,6 +224,12 @@ impl Packed<EquationElem> {
             vec![MathParItem::Frame(run.into_fragment(&ctx, styles).into_frame())]
         };
 
+        // An empty equation should have a height, so we still create a frame
+        // (which is then resized in the loop).
+        if items.is_empty() {
+            items.push(MathParItem::Frame(Frame::soft(Size::zero())));
+        }
+
         for item in &mut items {
             let MathParItem::Frame(frame) = item else { continue };
 

crates/typst/src/math/row.rs

@@ -3,7 +3,7 @@ use std::iter::once;
 use unicode_math_class::MathClass;
 
 use crate::foundations::{Resolve, StyleChain};
-use crate::layout::{Abs, AlignElem, Em, Frame, FrameKind, Point, Size};
+use crate::layout::{Abs, AlignElem, Em, Frame, Point, Size};
 use crate::math::{
     alignments, scaled_font_size, spacing, EquationElem, FrameFragment, MathContext,
     MathFragment, MathParItem, MathSize,
@@ -257,7 +257,7 @@ impl MathRun {
         let mut x = Abs::zero();
         let mut ascent = Abs::zero();
         let mut descent = Abs::zero();
-        let mut frame = Frame::new(Size::zero(), FrameKind::Soft);
+        let mut frame = Frame::soft(Size::zero());
         let mut empty = true;
 
         let finalize_frame = |frame: &mut Frame, x, ascent, descent| {
@@ -301,10 +301,8 @@ impl MathRun {
                 || (class == MathClass::Relation
                     && !iter.peek().map(is_relation).unwrap_or_default())
             {
-                let mut frame_prev = std::mem::replace(
-                    &mut frame,
-                    Frame::new(Size::zero(), FrameKind::Soft),
-                );
+                let mut frame_prev =
+                    std::mem::replace(&mut frame, Frame::soft(Size::zero()));
 
                 finalize_frame(&mut frame_prev, x, ascent, descent);
                 items.push(MathParItem::Frame(frame_prev));

tests/suite/math/equation.typ

@@ -210,3 +210,8 @@ $ <quadratic>
 // Error: 14-24 cannot reference equation without numbering
 // Hint: 14-24 you can enable equation numbering with `#set math.equation(numbering: "1.")`
 Looks at the @quadratic formula.
+
+--- issue-3696-equation-rtl ---
+#set page(width: 150pt)
+#set text(lang: "he")
+תהא סדרה $a_n$: $[a_n: 1, 1/2, 1/3, dots]$

tests/suite/math/interactions.typ

@@ -34,6 +34,26 @@ $#here[f] := #here[Hi there]$.
 // Test boxes with a baseline are respected
 #box(stroke: 0.2pt, $a #box(baseline:0.5em, stroke: 0.2pt, $a$)$)
 
+--- math-at-par-start ---
+// Test that equation at start of paragraph works fine.
+$x$ is a variable.
+
+--- math-at-par-end ---
+// Test that equation at end of paragraph works fine.
+One number is $1$
+
+--- math-at-line-start ---
+// Test math at the natural end of a line.
+#h(60pt) Number $1$ exists.
+
+--- math-at-line-end ---
+// Test math at the natural end of a line.
+#h(50pt) Number $1$ exists.
+
+--- math-consecutive ---
+// Test immediately consecutive equations.
+$x$$y$
+
 --- issue-2821-missing-fields ---
 // Issue #2821: Setting a figure's supplement to none removes the field
 #show figure.caption: it => {