Inline equations linebreak at appropriate places (#2938)

Co-authored-by: David Maxwell <damaxwell@alaska.edu>
Co-authored-by: Laurenz <laurmaedje@gmail.com>

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

@@ -19,7 +19,7 @@ use crate::layout::{
     Abs, AlignElem, Axes, BoxElem, Dir, Em, FixedAlign, Fr, Fragment, Frame, HElem,
     Layout, Point, Regions, Size, Sizing, Spacing,
 };
-use crate::math::EquationElem;
+use crate::math::{EquationElem, MathParItem};
 use crate::model::{Linebreaks, ParElem};
 use crate::syntax::Span;
 use crate::text::{
@@ -61,7 +61,8 @@ pub(crate) fn layout_inline(
         };
 
         // Collect all text into one string for BiDi analysis.
-        let (text, segments, spans) = collect(children, &styles, consecutive)?;
+        let (text, segments, spans) =
+            collect(children, &mut engine, &styles, region, consecutive)?;
 
         // Perform BiDi analysis and then prepare paragraph layout by building a
         // representation on which we can do line breaking without layouting
@@ -180,7 +181,7 @@ impl<'a> Preparation<'a> {
 }
 
 /// A segment of one or multiple collapsed children.
-#[derive(Debug, Copy, Clone)]
+#[derive(Debug, Clone)]
 enum Segment<'a> {
     /// One or multiple collapsed text or text-equivalent children. Stores how
     /// long the segment is (in bytes of the full text string).
@@ -188,7 +189,7 @@ enum Segment<'a> {
     /// Horizontal spacing between other segments.
     Spacing(Spacing),
     /// A mathematical equation.
-    Equation(&'a EquationElem),
+    Equation(&'a EquationElem, Vec<MathParItem>),
     /// A box with arbitrary content.
     Box(&'a BoxElem, bool),
     /// Metadata.
@@ -201,8 +202,12 @@ impl Segment<'_> {
         match *self {
             Self::Text(len) => len,
             Self::Spacing(_) => SPACING_REPLACE.len_utf8(),
-            Self::Box(_, true) => SPACING_REPLACE.len_utf8(),
-            Self::Equation(_) | Self::Box(_, _) => OBJ_REPLACE.len_utf8(),
+            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::Meta => 0,
         }
     }
@@ -395,12 +400,14 @@ impl<'a> Line<'a> {
     }
 }
 
-/// Collect all text of the paragraph into one string. This also performs
-/// string-level preprocessing like case transformations.
+/// Collect all text of the paragraph into one string and layout equations. This
+/// also performs string-level preprocessing like case transformations.
 #[allow(clippy::type_complexity)]
 fn collect<'a>(
     children: &'a [Prehashed<Content>],
+    engine: &mut Engine<'_>,
     styles: &'a StyleChain<'a>,
+    region: Size,
     consecutive: bool,
 ) -> SourceResult<(String, Vec<(Segment<'a>, StyleChain<'a>)>, SpanMapper)> {
     let mut full = String::new();
@@ -493,8 +500,10 @@ fn collect<'a>(
             }
             Segment::Text(full.len() - prev)
         } else if let Some(elem) = child.to::<EquationElem>() {
-            full.push(OBJ_REPLACE);
-            Segment::Equation(elem)
+            let pod = Regions::one(region, Axes::splat(false));
+            let items = elem.layout_inline(engine, styles, pod)?;
+            full.extend(items.iter().map(MathParItem::text));
+            Segment::Equation(elem, items)
         } else if let Some(elem) = child.to::<BoxElem>() {
             let frac = elem.width(styles).is_fractional();
             full.push(if frac { SPACING_REPLACE } else { OBJ_REPLACE });
@@ -512,7 +521,7 @@ fn collect<'a>(
         spans.push(segment.len(), child.span());
 
         if let (Some((Segment::Text(last_len), last_styles)), Segment::Text(len)) =
-            (segments.last_mut(), segment)
+            (segments.last_mut(), &segment)
         {
             if *last_styles == styles {
                 *last_len += len;
@@ -526,8 +535,7 @@ fn collect<'a>(
     Ok((full, segments, spans))
 }
 
-/// Prepare paragraph layout by shaping the whole paragraph and layouting all
-/// contained inline-level content.
+/// Prepare paragraph layout by shaping the whole paragraph.
 fn prepare<'a>(
     engine: &mut Engine,
     children: &'a [Prehashed<Content>],
@@ -566,11 +574,16 @@ fn prepare<'a>(
                     items.push(Item::Fractional(v, None));
                 }
             },
-            Segment::Equation(equation) => {
-                let pod = Regions::one(region, Axes::splat(false));
-                let mut frame = equation.layout(engine, styles, pod)?.into_frame();
-                frame.translate(Point::with_y(TextElem::baseline_in(styles)));
-                items.push(Item::Frame(frame));
+            Segment::Equation(_, par_items) => {
+                for item in par_items {
+                    match item {
+                        MathParItem::Space(s) => items.push(Item::Absolute(s)),
+                        MathParItem::Frame(mut frame) => {
+                            frame.translate(Point::with_y(TextElem::baseline_in(styles)));
+                            items.push(Item::Frame(frame));
+                        }
+                    }
+                }
             }
             Segment::Box(elem, _) => {
                 if let Sizing::Fr(v) = elem.width(styles) {

crates/typst/src/math/ctx.rs

@@ -140,6 +140,11 @@ impl<'a, 'b, 'v> MathContext<'a, 'b, 'v> {
         self.fragments.extend(fragments);
     }
 
+    pub fn layout_root(&mut self, elem: &dyn LayoutMath) -> SourceResult<MathRow> {
+        let row = self.layout_fragments(elem)?;
+        Ok(MathRow::new(row))
+    }
+
     pub fn layout_fragment(
         &mut self,
         elem: &dyn LayoutMath,

crates/typst/src/math/equation.rs

@@ -8,13 +8,14 @@ use crate::foundations::{
 };
 use crate::introspection::{Count, Counter, CounterUpdate, Locatable};
 use crate::layout::{
-    Abs, Align, AlignElem, Axes, Dir, Em, FixedAlign, Fragment, Layout, Point, Regions,
-    Size,
+    Abs, Align, AlignElem, Axes, Dir, Em, FixedAlign, Fragment, Frame, Layout, Point,
+    Regions, Size,
 };
 use crate::math::{LayoutMath, MathContext};
 use crate::model::{Numbering, Outlinable, ParElem, Refable, Supplement};
+use crate::syntax::Span;
 use crate::text::{
-    families, variant, FontFamily, FontList, FontWeight, Lang, LocalName, Region,
+    families, variant, Font, FontFamily, FontList, FontWeight, Lang, LocalName, Region,
     TextElem,
 };
 use crate::util::{option_eq, NonZeroExt, Numeric};
@@ -136,70 +137,47 @@ impl Finalize for EquationElem {
     }
 }
 
-impl Layout for EquationElem {
-    #[typst_macros::time(name = "math.equation", span = self.span())]
-    fn layout(
+/// Layouted items suitable for placing in a paragraph.
+#[derive(Debug, Clone)]
+pub enum MathParItem {
+    Space(Abs),
+    Frame(Frame),
+}
+
+impl MathParItem {
+    /// The text representation of this item.
+    pub fn text(&self) -> char {
+        match self {
+            MathParItem::Space(_) => ' ',        // Space
+            MathParItem::Frame(_) => '\u{FFFC}', // Object Replacement Character
+        }
+    }
+}
+
+impl EquationElem {
+    pub fn layout_inline(
         &self,
-        engine: &mut Engine,
+        engine: &mut Engine<'_>,
         styles: StyleChain,
         regions: Regions,
-    ) -> SourceResult<Fragment> {
-        const NUMBER_GUTTER: Em = Em::new(0.5);
-
-        let block = self.block(styles);
+    ) -> SourceResult<Vec<MathParItem>> {
+        assert!(!self.block(styles));
 
         // Find a math font.
-        let variant = variant(styles);
-        let world = engine.world;
-        let Some(font) = families(styles).find_map(|family| {
-            let id = world.book().select(family, variant)?;
-            let font = world.font(id)?;
-            let _ = font.ttf().tables().math?.constants?;
-            Some(font)
-        }) else {
-            bail!(self.span(), "current font does not support math");
+        let font = find_math_font(engine, styles, self.span())?;
+
+        let mut ctx = MathContext::new(engine, styles, regions, &font, false);
+        let rows = ctx.layout_root(self)?;
+
+        let mut items = if rows.row_count() == 1 {
+            rows.into_par_items()
+        } else {
+            vec![MathParItem::Frame(rows.into_fragment(&ctx).into_frame())]
         };
 
-        let mut ctx = MathContext::new(engine, styles, regions, &font, block);
-        let mut frame = ctx.layout_frame(self)?;
+        for item in &mut items {
+            let MathParItem::Frame(frame) = item else { continue };
 
-        if block {
-            if let Some(numbering) = self.numbering(styles) {
-                let pod = Regions::one(regions.base(), Axes::splat(false));
-                let counter = Counter::of(Self::elem())
-                    .display(self.span(), Some(numbering), false)
-                    .layout(engine, styles, pod)?
-                    .into_frame();
-
-                let full_counter_width = counter.width() + NUMBER_GUTTER.resolve(styles);
-                let width = if regions.size.x.is_finite() {
-                    regions.size.x
-                } else {
-                    frame.width() + 2.0 * full_counter_width
-                };
-
-                let height = frame.height().max(counter.height());
-                let align = AlignElem::alignment_in(styles).resolve(styles).x;
-                frame.resize(Size::new(width, height), Axes::splat(align));
-
-                let dir = TextElem::dir_in(styles);
-                let offset = match (align, dir) {
-                    (FixedAlign::Start, Dir::RTL) => full_counter_width,
-                    (FixedAlign::End, Dir::LTR) => -full_counter_width,
-                    _ => Abs::zero(),
-                };
-                frame.translate(Point::with_x(offset));
-
-                let x = if dir.is_positive() {
-                    frame.width() - counter.width()
-                } else {
-                    Abs::zero()
-                };
-                let y = (frame.height() - counter.height()) / 2.0;
-
-                frame.push_frame(Point::new(x, y), counter)
-            }
-        } else {
             let font_size = TextElem::size_in(styles);
             let slack = ParElem::leading_in(styles) * 0.7;
             let top_edge = TextElem::top_edge_in(styles).resolve(font_size, &font, None);
@@ -210,6 +188,67 @@ impl Layout for EquationElem {
             let descent = bottom_edge.max(frame.descent() - slack);
             frame.translate(Point::with_y(ascent - frame.baseline()));
             frame.size_mut().y = ascent + descent;
+
+            // Apply metadata.
+            frame.meta(styles, false);
+        }
+
+        Ok(items)
+    }
+}
+
+impl Layout for EquationElem {
+    #[typst_macros::time(name = "math.equation", span = self.span())]
+    fn layout(
+        &self,
+        engine: &mut Engine,
+        styles: StyleChain,
+        regions: Regions,
+    ) -> SourceResult<Fragment> {
+        const NUMBER_GUTTER: Em = Em::new(0.5);
+
+        assert!(self.block(styles));
+
+        // Find a math font.
+        let font = find_math_font(engine, styles, self.span())?;
+
+        let mut ctx = MathContext::new(engine, styles, regions, &font, true);
+        let mut frame = ctx.layout_frame(self)?;
+
+        if let Some(numbering) = self.numbering(styles) {
+            let pod = Regions::one(regions.base(), Axes::splat(false));
+            let counter = Counter::of(Self::elem())
+                .display(self.span(), Some(numbering), false)
+                .layout(engine, styles, pod)?
+                .into_frame();
+
+            let full_counter_width = counter.width() + NUMBER_GUTTER.resolve(styles);
+            let width = if regions.size.x.is_finite() {
+                regions.size.x
+            } else {
+                frame.width() + 2.0 * full_counter_width
+            };
+
+            let height = frame.height().max(counter.height());
+            let align = AlignElem::alignment_in(styles).resolve(styles).x;
+            frame.resize(Size::new(width, height), Axes::splat(align));
+
+            let dir = TextElem::dir_in(styles);
+            let offset = match (align, dir) {
+                (FixedAlign::Start, Dir::RTL) => full_counter_width,
+                (FixedAlign::End, Dir::LTR) => -full_counter_width,
+                _ => Abs::zero(),
+            };
+            frame.translate(Point::with_x(offset));
+
+            let x = if dir.is_positive() {
+                frame.width() - counter.width()
+            } else {
+                Abs::zero()
+            };
+            let y = (frame.height() - counter.height()) / 2.0;
+
+            frame.push_frame(Point::new(x, y), counter)
         }
 
         // Apply metadata.
@@ -316,3 +355,21 @@ impl LayoutMath for EquationElem {
         self.body().layout_math(ctx)
     }
 }
+
+fn find_math_font(
+    engine: &mut Engine<'_>,
+    styles: StyleChain,
+    span: Span,
+) -> SourceResult<Font> {
+    let variant = variant(styles);
+    let world = engine.world;
+    let Some(font) = families(styles).find_map(|family| {
+        let id = world.book().select(family, variant)?;
+        let font = world.font(id)?;
+        let _ = font.ttf().tables().math?.constants?;
+        Some(font)
+    }) else {
+        bail!(span, "current font does not support math");
+    };
+    Ok(font)
+}

crates/typst/src/math/row.rs

@@ -3,10 +3,10 @@ use std::iter::once;
 use unicode_math_class::MathClass;
 
 use crate::foundations::Resolve;
-use crate::layout::{Abs, AlignElem, Em, FixedAlign, Frame, Point, Size};
+use crate::layout::{Abs, AlignElem, Em, FixedAlign, Frame, FrameKind, Point, Size};
 use crate::math::{
     alignments, spacing, AlignmentResult, FrameFragment, MathContext, MathFragment,
-    MathSize, Scaled,
+    MathParItem, MathSize, Scaled,
 };
 use crate::model::ParElem;
 
@@ -103,6 +103,19 @@ impl MathRow {
             .collect()
     }
 
+    pub fn row_count(&self) -> usize {
+        let mut count =
+            1 + self.0.iter().filter(|f| matches!(f, MathFragment::Linebreak)).count();
+
+        // A linebreak at the very end does not introduce an extra row.
+        if let Some(f) = self.0.last() {
+            if matches!(f, MathFragment::Linebreak) {
+                count -= 1
+            }
+        }
+        count
+    }
+
     pub fn ascent(&self) -> Abs {
         self.iter().map(MathFragment::ascent).max().unwrap_or_default()
     }
@@ -239,6 +252,85 @@ impl MathRow {
         frame.size_mut().x = x;
         frame
     }
+
+    pub fn into_par_items(self) -> Vec<MathParItem> {
+        let mut items = vec![];
+
+        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 finalize_frame = |frame: &mut Frame, x, ascent, descent| {
+            frame.set_size(Size::new(x, ascent + descent));
+            frame.set_baseline(Abs::zero());
+            frame.translate(Point::with_y(ascent));
+        };
+
+        let mut space_is_visible = false;
+
+        let is_relation =
+            |f: &MathFragment| matches!(f.class(), Some(MathClass::Relation));
+        let is_space = |f: &MathFragment| {
+            matches!(f, MathFragment::Space(_) | MathFragment::Spacing(_))
+        };
+
+        let mut iter = self.0.into_iter().peekable();
+        while let Some(fragment) = iter.next() {
+            if space_is_visible {
+                match fragment {
+                    MathFragment::Space(s) | MathFragment::Spacing(s) => {
+                        items.push(MathParItem::Space(s));
+                        continue;
+                    }
+                    _ => {}
+                }
+            }
+
+            let class = fragment.class();
+            let y = fragment.ascent();
+
+            ascent.set_max(y);
+            descent.set_max(fragment.descent());
+
+            let pos = Point::new(x, -y);
+            x += fragment.width();
+            frame.push_frame(pos, fragment.into_frame());
+
+            if class == Some(MathClass::Binary)
+                || (class == Some(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),
+                );
+
+                finalize_frame(&mut frame_prev, x, ascent, descent);
+                items.push(MathParItem::Frame(frame_prev));
+
+                x = Abs::zero();
+                ascent = Abs::zero();
+                descent = Abs::zero();
+
+                space_is_visible = true;
+                if let Some(f_next) = iter.peek() {
+                    if !is_space(f_next) {
+                        items.push(MathParItem::Space(Abs::zero()));
+                    }
+                }
+            } else {
+                space_is_visible = false;
+            }
+        }
+
+        if !frame.is_empty() {
+            finalize_frame(&mut frame, x, ascent, descent);
+            items.push(MathParItem::Frame(frame));
+        }
+
+        items
+    }
 }
 
 impl<T: Into<MathFragment>> From<T> for MathRow {

tests/typ/math/delimited.typ

@@ -2,6 +2,7 @@
 
 ---
 // Test automatic matching.
+#set page(width:122pt)
 $ (a) + {b/2} + abs(a)/2 + (b) $
 $f(x/2) < zeta(c^2 + abs(a + b/2))$
 

tests/typ/math/linebreak.typ

@@ -0,0 +1,50 @@
+// Test inline equation line breaking.
+
+---
+// Basic breaking after binop, rel
+#let hrule(x) = box(line(length: x))
+#hrule(45pt)$e^(pi i)+1 = 0$\
+#hrule(55pt)$e^(pi i)+1 = 0$\
+#hrule(70pt)$e^(pi i)+1 = 0$
+
+---
+// LR groups prevent linbreaking.
+#let hrule(x) = box(line(length: x))
+#hrule(76pt)$a+b$\
+#hrule(74pt)$(a+b)$\
+#hrule(74pt)$paren.l a+b paren.r$
+
+---
+// Multiline yet inline does not linebreak
+#let hrule(x) = box(line(length: x))
+#hrule(80pt)$a + b \ c + d$\
+
+---
+// A single linebreak at the end still counts as one line.
+#let hrule(x) = box(line(length: x))
+#hrule(60pt)$e^(pi i)+1 = 0\ $
+
+---
+// Inline, in a box, doesn't linebreak.
+#let hrule(x) = box(line(length: x))
+#hrule(80pt)#box($a+b$)
+
+---
+// A relation followed by a relation doesn't linebreak
+#let hrule(x) = box(line(length: x))
+#hrule(70pt)$a < = b$\
+#hrule(74pt)$a < = b$
+
+---
+// Page breaks can happen after a relation even if there is no 
+// explicit space.
+#let hrule(x) = box(line(length: x))
+#hrule(90pt)$<;$\
+#hrule(95pt)$<;$\
+#hrule(90pt)$<)$\
+#hrule(95pt)$<)$
+
+---
+// Verify empty rows are handled ok.
+$ $\
+Nothing: $ $, just empty.

tests/typ/math/spacing.typ

@@ -3,7 +3,7 @@
 ---
 // Test spacing cases.
 $ä, +, c, (, )$ \
-$=), (+), {times}$
+$=), (+), {times}$ \
 $⟧<⟦, abs(-), [=$ \
 $a=b, a==b$ \
 $-a, +a$ \