WIP

crates/typst-layout/src/math/mat.rs

@@ -220,7 +220,7 @@ fn layout_body(
     let mut x = Abs::zero();
 
     for (index, col) in cols.into_iter().enumerate() {
-        let AlignmentResult { points, width: rcol } = alignments(&col, Abs::zero());
+        let AlignmentResult { points, width: rcol, .. } = alignments(&col, None);
 
         let mut y = Abs::zero();
 

crates/typst-layout/src/math/mod.rs

@@ -98,6 +98,12 @@ pub fn layout_equation_inline(
     Ok(items)
 }
 
+pub struct EquationSizings<'a> {
+    region_size_x: Abs,
+    gaps: &'a [GapSizing<Abs>],
+    padding: &'a [GapSizing<Abs>],
+}
+
 /// Layout a block-level equation (in a flow).
 #[typst_macros::time(span = elem.span())]
 pub fn layout_equation_block(
@@ -118,9 +124,17 @@ pub fn layout_equation_block(
     let scale_style = style_for_script_scale(&ctx);
     let styles = styles.chain(&scale_style);
 
+    let gaps = elem.column_gap(styles).resolve(styles);
+    let padding = elem.column_padding(styles).resolve(styles);
+    let sizings = EquationSizings {
+        region_size_x: regions.size.x,
+        gaps: gaps.0.as_slice(),
+        padding: padding.0.as_slice(),
+    };
+
     let full_equation_builder = ctx
         .layout_into_run(&elem.body, styles)?
-        .multiline_frame_builder(styles);
+        .multiline_frame_builder(styles, Some(sizings));
     let width = full_equation_builder.size.x;
 
     let equation_builders = if BlockElem::breakable_in(styles) {

crates/typst-layout/src/math/run.rs

@@ -6,7 +6,7 @@ use typst_library::math::{EquationElem, MathSize, MEDIUM, THICK, THIN};
 use typst_library::model::ParElem;
 use unicode_math_class::MathClass;
 
-use super::{alignments, FrameFragment, MathFragment};
+use super::{alignments, EquationSizings, FrameFragment, MathFragment};
 
 const TIGHT_LEADING: Em = Em::new(0.25);
 
@@ -165,7 +165,7 @@ impl MathRun {
         if !self.is_multiline() {
             self.into_line_frame(&[], LeftRightAlternator::Right)
         } else {
-            self.multiline_frame_builder(styles).build()
+            self.multiline_frame_builder(styles, None).build()
         }
     }
 
@@ -189,12 +189,15 @@ impl MathRun {
     /// Returns a builder that lays out the [`MathFragment`]s into a possibly
     /// multi-row [`Frame`]. The rows are aligned using the same set of alignment
     /// points computed from them as a whole.
-    pub fn multiline_frame_builder(self, styles: StyleChain) -> MathRunFrameBuilder {
+    pub fn multiline_frame_builder(
+        self,
+        styles: StyleChain,
+        sizings: Option<EquationSizings>,
+    ) -> MathRunFrameBuilder {
         let rows: Vec<_> = self.rows();
         let row_count = rows.len();
 
-        let column_gap = EquationElem::column_gap_in(styles).resolve(styles);
-        let alignments = alignments(&rows, column_gap);
+        let alignments = alignments(&rows, sizings);
 
         let leading = if EquationElem::size_in(styles) >= MathSize::Text {
             ParElem::leading_in(styles)
@@ -215,15 +218,17 @@ impl MathRun {
                 size.y += leading;
             }
 
-            let mut pos = Point::with_y(size.y);
+            let mut pos = Point::new(alignments.padding.0, size.y);
             if alignments.points.is_empty() {
-                pos.x = align.position(alignments.width - sub.width());
+                pos.x += align.position(alignments.width - sub.width());
             }
-            size.x.set_max(sub.width());
+            size.x.set_max(sub.width() + alignments.padding.0);
             size.y += sub.height();
             frames.push((sub, pos));
         }
 
+        size.x += alignments.padding.1;
+
         MathRunFrameBuilder { size, frames }
     }
 

crates/typst-layout/src/math/shared.rs

@@ -1,10 +1,12 @@
 use ttf_parser::math::MathValue;
 use typst_library::foundations::{Style, StyleChain};
-use typst_library::layout::{Abs, Em, FixedAlignment, Frame, Point, Size, VAlignment};
-use typst_library::math::{EquationElem, MathSize};
+use typst_library::layout::{
+    Abs, Em, FixedAlignment, Fr, Frame, Point, Size, VAlignment,
+};
+use typst_library::math::{EquationElem, GapSizing, MathSize};
 use typst_utils::LazyHash;
 
-use super::{LeftRightAlternator, MathContext, MathFragment, MathRun};
+use super::{EquationSizings, LeftRightAlternator, MathContext, MathFragment, MathRun};
 
 macro_rules! scaled {
     ($ctx:expr, $styles:expr, text: $text:ident, display: $display:ident $(,)?) => {
@@ -118,7 +120,7 @@ pub fn stack(
     baseline: usize,
     alternator: LeftRightAlternator,
 ) -> Frame {
-    let AlignmentResult { points, width } = alignments(&rows, Abs::zero());
+    let AlignmentResult { points, width, .. } = alignments(&rows, None);
     let rows: Vec<_> = rows
         .into_iter()
         .map(|row| row.into_line_frame(&points, alternator))
@@ -148,8 +150,15 @@ pub fn stack(
     frame
 }
 
-/// Determine the positions of the alignment points, according to the input rows combined.
-pub fn alignments(rows: &[MathRun], gap: Abs) -> AlignmentResult {
+pub struct AlignmentResult {
+    pub points: Vec<Abs>,
+    pub width: Abs,
+    pub padding: (Abs, Abs),
+}
+
+/// Determine the positions of the alignment points, according to the input
+/// rows combined.
+pub fn alignments(rows: &[MathRun], sizings: Option<EquationSizings>) -> AlignmentResult {
     let mut widths = Vec::<Abs>::new();
 
     let mut pending_width = Abs::zero();
@@ -159,9 +168,6 @@ pub fn alignments(rows: &[MathRun], gap: Abs) -> AlignmentResult {
 
         for fragment in row.iter() {
             if matches!(fragment, MathFragment::Align) {
-                if alignment_index > 0 && alignment_index % 2 == 0 {
-                    width += gap;
-                }
                 if alignment_index < widths.len() {
                     widths[alignment_index].set_max(width);
                 } else {
@@ -182,18 +188,80 @@ pub fn alignments(rows: &[MathRun], gap: Abs) -> AlignmentResult {
         }
     }
 
+    if widths.is_empty() {
+        widths.push(pending_width);
+        let padding = add_gaps(&mut widths, sizings);
+        return AlignmentResult { width: pending_width, points: vec![], padding };
+    }
+
+    let padding = add_gaps(&mut widths, sizings);
     let mut points = widths;
     for i in 1..points.len() {
         let prev = points[i - 1];
         points[i] += prev;
     }
     AlignmentResult {
-        width: points.last().copied().unwrap_or(pending_width),
+        width: points.last().copied().unwrap(),
         points,
+        padding,
     }
 }
 
-pub struct AlignmentResult {
-    pub points: Vec<Abs>,
-    pub width: Abs,
+/// Inserts gaps between columns given by the alignments.
+fn add_gaps(widths: &mut [Abs], sizings: Option<EquationSizings>) -> (Abs, Abs) {
+    let Some(sizings) = sizings else {
+        return (Abs::zero(), Abs::zero());
+    };
+
+    // Padding to be returned.
+    let mut padding = [Abs::zero(), Abs::zero()];
+
+    // Number of gaps between columns.
+    let len = widths.len();
+    let ngaps = len.div_ceil(2).saturating_sub(1);
+
+    // Discard excess gaps or repeat the last gap to match the number of gaps.
+    let mut gaps = sizings.gaps.to_vec();
+    gaps.truncate(ngaps);
+    if let Some(last_gap) = gaps.last().copied() {
+        gaps.extend(std::iter::repeat_n(last_gap, ngaps.saturating_sub(gaps.len())));
+    }
+
+    // Sum of fractions of all fractional gaps.
+    let mut fr = Fr::zero();
+
+    // Resolve the size of all relative gaps and compute the sum of all
+    // fractional gaps.
+    let region_width = sizings.region_size_x;
+    for (i, gap) in gaps.iter().enumerate() {
+        match gap {
+            GapSizing::Rel(v) => widths[1 + i * 2] += v.relative_to(region_width),
+            GapSizing::Fr(v) => fr += *v,
+        }
+    }
+    for (i, gap) in sizings.padding.iter().enumerate() {
+        match gap {
+            GapSizing::Rel(v) => padding[i] = v.relative_to(region_width),
+            GapSizing::Fr(v) => fr += *v,
+        }
+    }
+
+    // Size that is not used by fixed-size gaps.
+    let remaining = region_width - (widths.iter().sum::<Abs>() + padding.iter().sum());
+
+    // Distribute remaining space to fractional gaps.
+    if !remaining.approx_empty() {
+        for (i, gap) in gaps.iter().enumerate() {
+            if let GapSizing::Fr(v) = gap {
+                widths[1 + i * 2] += v.share(fr, remaining);
+            }
+        }
+        for (i, gap) in sizings.padding.iter().enumerate() {
+            if let GapSizing::Fr(v) = gap {
+                padding[i] = v.share(fr, remaining);
+            }
+        }
+    }
+
+    (padding[0], padding[1])
 }

crates/typst-library/src/math/equation.rs

@@ -1,25 +1,24 @@
 use std::num::NonZeroUsize;
 
-use typst_utils::NonZeroExt;
+use smallvec::{smallvec, SmallVec};
+use typst_utils::{NonZeroExt, Numeric};
 use unicode_math_class::MathClass;
 
-use crate::diag::SourceResult;
+use crate::diag::{bail, HintedStrResult, SourceResult};
 use crate::engine::Engine;
 use crate::foundations::{
-    elem, Content, NativeElement, Packed, Show, ShowSet, Smart, StyleChain, Styles,
-    Synthesize,
+    cast, elem, Array, Content, NativeElement, Packed, Resolve, Show, ShowSet, Smart,
+    StyleChain, Styles, Synthesize, Value,
 };
 use crate::introspection::{Count, Counter, CounterUpdate, Locatable};
 use crate::layout::{
-    AlignElem, Alignment, BlockElem, Em, InlineElem, Length, OuterHAlignment,
-    SpecificAlignment, VAlignment,
+    Abs, AlignElem, Alignment, BlockElem, Fr, InlineElem, Length, OuterHAlignment, Rel,
+    Spacing, SpecificAlignment, VAlignment,
 };
 use crate::math::{MathSize, MathVariant};
 use crate::model::{Numbering, Outlinable, ParLine, Refable, Supplement};
 use crate::text::{FontFamily, FontList, FontWeight, LocalName, TextElem};
 
-const DEFAULT_COL_GAP: Em = Em::new(1.0);
-
 /// A mathematical equation.
 ///
 /// Can be displayed inline with text or as a separate block. An equation
@@ -111,8 +110,14 @@ pub struct EquationElem {
     ///   0   &= 0 & &"no" \
     ///   1+1 &= 2 & &"maybe" $
     /// ```
-    #[default(DEFAULT_COL_GAP.into())]
-    pub column_gap: Length,
+    #[default(Fr::one().into())]
+    #[borrowed]
+    pub column_gap: GapSizings,
+
+    ///
+    #[default(Fr::one().into())]
+    #[borrowed]
+    pub column_padding: PaddingSizings,
 
     /// The contents of the equation.
     #[required]
@@ -203,7 +208,6 @@ impl ShowSet for Packed<EquationElem> {
             out.set(BlockElem::set_breakable(false));
             out.set(ParLine::set_numbering(None));
             out.set(EquationElem::set_size(MathSize::Display));
-            out.set(EquationElem::set_column_gap(self.column_gap(styles)));
         } else {
             out.set(EquationElem::set_size(MathSize::Text));
         }
@@ -262,3 +266,101 @@ impl Outlinable for Packed<EquationElem> {
         Content::empty()
     }
 }
+
+/// Gap sizing definitions.
+#[derive(Debug, Clone, Eq, PartialEq, Hash)]
+pub struct GapSizings<T: Numeric = Length>(pub SmallVec<[GapSizing<T>; 1]>);
+
+impl<T: Into<Spacing>> From<T> for GapSizings {
+    fn from(spacing: T) -> Self {
+        Self(smallvec![GapSizing::from(spacing)])
+    }
+}
+
+impl Resolve for &GapSizings {
+    type Output = GapSizings<Abs>;
+
+    fn resolve(self, styles: StyleChain) -> Self::Output {
+        Self::Output {
+            0: self.0.iter().map(|v| v.resolve(styles)).collect(),
+        }
+    }
+}
+
+cast! {
+    GapSizings,
+    self => self.0.into_value(),
+    v: GapSizing => Self(smallvec![v]),
+    v: Array => Self(v.into_iter().map(Value::cast).collect::<HintedStrResult<_>>()?),
+}
+
+/// Padding sizing definitions.
+#[derive(Debug, Clone, Eq, PartialEq, Hash)]
+pub struct PaddingSizings<T: Numeric = Length>(pub SmallVec<[GapSizing<T>; 2]>);
+
+impl<T: Into<Spacing>> From<T> for PaddingSizings {
+    fn from(spacing: T) -> Self {
+        let spacing = spacing.into();
+        Self(smallvec![GapSizing::from(spacing), GapSizing::from(spacing)])
+    }
+}
+
+impl Resolve for &PaddingSizings {
+    type Output = PaddingSizings<Abs>;
+
+    fn resolve(self, styles: StyleChain) -> Self::Output {
+        Self::Output {
+            0: self.0.iter().map(|v| v.resolve(styles)).collect(),
+        }
+    }
+}
+
+cast! {
+    PaddingSizings,
+    self => self.0.into_value(),
+    v: GapSizing => Self(smallvec![v, v]),
+    v: Array => match v.as_slice() {
+        [start, end] => Self(smallvec![start.clone().cast()?, end.clone().cast()?]),
+        _ => bail!("expected 2 sizings, found {}", v.len()),
+    },
+}
+
+/// Defines how to size a gap along an axis.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+pub enum GapSizing<T: Numeric = Length> {
+    /// A size specified in absolute terms and relative to the parent's size.
+    Rel(Rel<T>),
+    /// A size specified as a fraction of the remaining free space in the
+    /// parent.
+    Fr(Fr),
+}
+
+impl Resolve for GapSizing {
+    type Output = GapSizing<Abs>;
+
+    fn resolve(self, styles: StyleChain) -> Self::Output {
+        match self {
+            Self::Rel(rel) => Self::Output::Rel(rel.resolve(styles)),
+            Self::Fr(fr) => Self::Output::Fr(fr),
+        }
+    }
+}
+
+impl<T: Into<Spacing>> From<T> for GapSizing {
+    fn from(spacing: T) -> Self {
+        match spacing.into() {
+            Spacing::Rel(rel) => Self::Rel(rel),
+            Spacing::Fr(fr) => Self::Fr(fr),
+        }
+    }
+}
+
+cast! {
+    GapSizing,
+    self => match self {
+        Self::Rel(rel) => rel.into_value(),
+        Self::Fr(fr) => fr.into_value(),
+    },
+    v: Rel<Length> => Self::Rel(v),
+    v: Fr => Self::Fr(v),
+}