Move `CellGrid` from `typst-layout` to `typst-library` (#5585)

3 files changed, 1776 insertions, 1 deletions

diff --git a/crates/typst-library/src/layout/grid.rs b/crates/typst-library/src/layout/grid/mod.rs
index 2e1e9abc..e46440fb 100644
--- a/crates/typst-library/src/layout/grid.rs
+++ b/crates/typst-library/src/layout/grid/mod.rs
@@ -1,3 +1,5 @@
+pub mod resolve;
+
 use std::num::NonZeroUsize;
 use std::sync::Arc;
 
diff --git a/crates/typst-library/src/layout/grid/resolve.rs b/crates/typst-library/src/layout/grid/resolve.rs
new file mode 100644
index 00000000..adaff1c1
--- /dev/null
+++ b/crates/typst-library/src/layout/grid/resolve.rs
@@ -0,0 +1,1773 @@
+use std::num::NonZeroUsize;
+use std::sync::Arc;
+
+use ecow::eco_format;
+use typst_library::diag::{
+    bail, At, Hint, HintedStrResult, HintedString, SourceResult, Trace, Tracepoint,
+};
+use typst_library::engine::Engine;
+use typst_library::foundations::{Content, Fold, Packed, Smart, StyleChain};
+use typst_library::introspection::Locator;
+use typst_library::layout::{
+    Abs, Alignment, Axes, Celled, GridCell, GridChild, GridElem, GridItem, Length,
+    OuterHAlignment, OuterVAlignment, Rel, ResolvedCelled, Sides, Sizing,
+};
+use typst_library::model::{TableCell, TableChild, TableElem, TableItem};
+use typst_library::text::TextElem;
+use typst_library::visualize::{Paint, Stroke};
+use typst_library::Dir;
+
+use typst_syntax::Span;
+use typst_utils::NonZeroExt;
+
+/// Convert a grid to a cell grid.
+#[typst_macros::time(span = elem.span())]
+pub fn grid_to_cellgrid<'a>(
+    elem: &Packed<GridElem>,
+    engine: &mut Engine,
+    locator: Locator<'a>,
+    styles: StyleChain,
+) -> SourceResult<CellGrid<'a>> {
+    let inset = elem.inset(styles);
+    let align = elem.align(styles);
+    let columns = elem.columns(styles);
+    let rows = elem.rows(styles);
+    let column_gutter = elem.column_gutter(styles);
+    let row_gutter = elem.row_gutter(styles);
+    let fill = elem.fill(styles);
+    let stroke = elem.stroke(styles);
+
+    let tracks = Axes::new(columns.0.as_slice(), rows.0.as_slice());
+    let gutter = Axes::new(column_gutter.0.as_slice(), row_gutter.0.as_slice());
+    // Use trace to link back to the grid when a specific cell errors
+    let tracepoint = || Tracepoint::Call(Some(eco_format!("grid")));
+    let resolve_item = |item: &GridItem| grid_item_to_resolvable(item, styles);
+    let children = elem.children().iter().map(|child| match child {
+        GridChild::Header(header) => ResolvableGridChild::Header {
+            repeat: header.repeat(styles),
+            span: header.span(),
+            items: header.children().iter().map(resolve_item),
+        },
+        GridChild::Footer(footer) => ResolvableGridChild::Footer {
+            repeat: footer.repeat(styles),
+            span: footer.span(),
+            items: footer.children().iter().map(resolve_item),
+        },
+        GridChild::Item(item) => {
+            ResolvableGridChild::Item(grid_item_to_resolvable(item, styles))
+        }
+    });
+    CellGrid::resolve(
+        tracks,
+        gutter,
+        locator,
+        children,
+        fill,
+        align,
+        &inset,
+        &stroke,
+        engine,
+        styles,
+        elem.span(),
+    )
+    .trace(engine.world, tracepoint, elem.span())
+}
+
+/// Convert a table to a cell grid.
+#[typst_macros::time(span = elem.span())]
+pub fn table_to_cellgrid<'a>(
+    elem: &Packed<TableElem>,
+    engine: &mut Engine,
+    locator: Locator<'a>,
+    styles: StyleChain,
+) -> SourceResult<CellGrid<'a>> {
+    let inset = elem.inset(styles);
+    let align = elem.align(styles);
+    let columns = elem.columns(styles);
+    let rows = elem.rows(styles);
+    let column_gutter = elem.column_gutter(styles);
+    let row_gutter = elem.row_gutter(styles);
+    let fill = elem.fill(styles);
+    let stroke = elem.stroke(styles);
+
+    let tracks = Axes::new(columns.0.as_slice(), rows.0.as_slice());
+    let gutter = Axes::new(column_gutter.0.as_slice(), row_gutter.0.as_slice());
+    // Use trace to link back to the table when a specific cell errors
+    let tracepoint = || Tracepoint::Call(Some(eco_format!("table")));
+    let resolve_item = |item: &TableItem| table_item_to_resolvable(item, styles);
+    let children = elem.children().iter().map(|child| match child {
+        TableChild::Header(header) => ResolvableGridChild::Header {
+            repeat: header.repeat(styles),
+            span: header.span(),
+            items: header.children().iter().map(resolve_item),
+        },
+        TableChild::Footer(footer) => ResolvableGridChild::Footer {
+            repeat: footer.repeat(styles),
+            span: footer.span(),
+            items: footer.children().iter().map(resolve_item),
+        },
+        TableChild::Item(item) => {
+            ResolvableGridChild::Item(table_item_to_resolvable(item, styles))
+        }
+    });
+    CellGrid::resolve(
+        tracks,
+        gutter,
+        locator,
+        children,
+        fill,
+        align,
+        &inset,
+        &stroke,
+        engine,
+        styles,
+        elem.span(),
+    )
+    .trace(engine.world, tracepoint, elem.span())
+}
+
+fn grid_item_to_resolvable(
+    item: &GridItem,
+    styles: StyleChain,
+) -> ResolvableGridItem<Packed<GridCell>> {
+    match item {
+        GridItem::HLine(hline) => ResolvableGridItem::HLine {
+            y: hline.y(styles),
+            start: hline.start(styles),
+            end: hline.end(styles),
+            stroke: hline.stroke(styles),
+            span: hline.span(),
+            position: match hline.position(styles) {
+                OuterVAlignment::Top => LinePosition::Before,
+                OuterVAlignment::Bottom => LinePosition::After,
+            },
+        },
+        GridItem::VLine(vline) => ResolvableGridItem::VLine {
+            x: vline.x(styles),
+            start: vline.start(styles),
+            end: vline.end(styles),
+            stroke: vline.stroke(styles),
+            span: vline.span(),
+            position: match vline.position(styles) {
+                OuterHAlignment::Left if TextElem::dir_in(styles) == Dir::RTL => {
+                    LinePosition::After
+                }
+                OuterHAlignment::Right if TextElem::dir_in(styles) == Dir::RTL => {
+                    LinePosition::Before
+                }
+                OuterHAlignment::Start | OuterHAlignment::Left => LinePosition::Before,
+                OuterHAlignment::End | OuterHAlignment::Right => LinePosition::After,
+            },
+        },
+        GridItem::Cell(cell) => ResolvableGridItem::Cell(cell.clone()),
+    }
+}
+
+fn table_item_to_resolvable(
+    item: &TableItem,
+    styles: StyleChain,
+) -> ResolvableGridItem<Packed<TableCell>> {
+    match item {
+        TableItem::HLine(hline) => ResolvableGridItem::HLine {
+            y: hline.y(styles),
+            start: hline.start(styles),
+            end: hline.end(styles),
+            stroke: hline.stroke(styles),
+            span: hline.span(),
+            position: match hline.position(styles) {
+                OuterVAlignment::Top => LinePosition::Before,
+                OuterVAlignment::Bottom => LinePosition::After,
+            },
+        },
+        TableItem::VLine(vline) => ResolvableGridItem::VLine {
+            x: vline.x(styles),
+            start: vline.start(styles),
+            end: vline.end(styles),
+            stroke: vline.stroke(styles),
+            span: vline.span(),
+            position: match vline.position(styles) {
+                OuterHAlignment::Left if TextElem::dir_in(styles) == Dir::RTL => {
+                    LinePosition::After
+                }
+                OuterHAlignment::Right if TextElem::dir_in(styles) == Dir::RTL => {
+                    LinePosition::Before
+                }
+                OuterHAlignment::Start | OuterHAlignment::Left => LinePosition::Before,
+                OuterHAlignment::End | OuterHAlignment::Right => LinePosition::After,
+            },
+        },
+        TableItem::Cell(cell) => ResolvableGridItem::Cell(cell.clone()),
+    }
+}
+
+impl ResolvableCell for Packed<TableCell> {
+    fn resolve_cell<'a>(
+        mut self,
+        x: usize,
+        y: usize,
+        fill: &Option<Paint>,
+        align: Smart<Alignment>,
+        inset: Sides<Option<Rel<Length>>>,
+        stroke: Sides<Option<Option<Arc<Stroke<Abs>>>>>,
+        breakable: bool,
+        locator: Locator<'a>,
+        styles: StyleChain,
+    ) -> Cell<'a> {
+        let cell = &mut *self;
+        let colspan = cell.colspan(styles);
+        let rowspan = cell.rowspan(styles);
+        let breakable = cell.breakable(styles).unwrap_or(breakable);
+        let fill = cell.fill(styles).unwrap_or_else(|| fill.clone());
+
+        let cell_stroke = cell.stroke(styles);
+        let stroke_overridden =
+            cell_stroke.as_ref().map(|side| matches!(side, Some(Some(_))));
+
+        // Using a typical 'Sides' fold, an unspecified side loses to a
+        // specified side. Additionally, when both are specified, an inner
+        // None wins over the outer Some, and vice-versa. When both are
+        // specified and Some, fold occurs, which, remarkably, leads to an Arc
+        // clone.
+        //
+        // In the end, we flatten because, for layout purposes, an unspecified
+        // cell stroke is the same as specifying 'none', so we equate the two
+        // concepts.
+        let stroke = cell_stroke.fold(stroke).map(Option::flatten);
+        cell.push_x(Smart::Custom(x));
+        cell.push_y(Smart::Custom(y));
+        cell.push_fill(Smart::Custom(fill.clone()));
+        cell.push_align(match align {
+            Smart::Custom(align) => {
+                Smart::Custom(cell.align(styles).map_or(align, |inner| inner.fold(align)))
+            }
+            // Don't fold if the table is using outer alignment. Use the
+            // cell's alignment instead (which, in the end, will fold with
+            // the outer alignment when it is effectively displayed).
+            Smart::Auto => cell.align(styles),
+        });
+        cell.push_inset(Smart::Custom(
+            cell.inset(styles).map_or(inset, |inner| inner.fold(inset)),
+        ));
+        cell.push_stroke(
+            // Here we convert the resolved stroke to a regular stroke, however
+            // with resolved units (that is, 'em' converted to absolute units).
+            // We also convert any stroke unspecified by both the cell and the
+            // outer stroke ('None' in the folded stroke) to 'none', that is,
+            // all sides are present in the resulting Sides object accessible
+            // by show rules on table cells.
+            stroke.as_ref().map(|side| {
+                Some(side.as_ref().map(|cell_stroke| {
+                    Arc::new((**cell_stroke).clone().map(Length::from))
+                }))
+            }),
+        );
+        cell.push_breakable(Smart::Custom(breakable));
+        Cell {
+            body: self.pack(),
+            locator,
+            fill,
+            colspan,
+            rowspan,
+            stroke,
+            stroke_overridden,
+            breakable,
+        }
+    }
+
+    fn x(&self, styles: StyleChain) -> Smart<usize> {
+        (**self).x(styles)
+    }
+
+    fn y(&self, styles: StyleChain) -> Smart<usize> {
+        (**self).y(styles)
+    }
+
+    fn colspan(&self, styles: StyleChain) -> NonZeroUsize {
+        (**self).colspan(styles)
+    }
+
+    fn rowspan(&self, styles: StyleChain) -> NonZeroUsize {
+        (**self).rowspan(styles)
+    }
+
+    fn span(&self) -> Span {
+        Packed::span(self)
+    }
+}
+
+impl ResolvableCell for Packed<GridCell> {
+    fn resolve_cell<'a>(
+        mut self,
+        x: usize,
+        y: usize,
+        fill: &Option<Paint>,
+        align: Smart<Alignment>,
+        inset: Sides<Option<Rel<Length>>>,
+        stroke: Sides<Option<Option<Arc<Stroke<Abs>>>>>,
+        breakable: bool,
+        locator: Locator<'a>,
+        styles: StyleChain,
+    ) -> Cell<'a> {
+        let cell = &mut *self;
+        let colspan = cell.colspan(styles);
+        let rowspan = cell.rowspan(styles);
+        let breakable = cell.breakable(styles).unwrap_or(breakable);
+        let fill = cell.fill(styles).unwrap_or_else(|| fill.clone());
+
+        let cell_stroke = cell.stroke(styles);
+        let stroke_overridden =
+            cell_stroke.as_ref().map(|side| matches!(side, Some(Some(_))));
+
+        // Using a typical 'Sides' fold, an unspecified side loses to a
+        // specified side. Additionally, when both are specified, an inner
+        // None wins over the outer Some, and vice-versa. When both are
+        // specified and Some, fold occurs, which, remarkably, leads to an Arc
+        // clone.
+        //
+        // In the end, we flatten because, for layout purposes, an unspecified
+        // cell stroke is the same as specifying 'none', so we equate the two
+        // concepts.
+        let stroke = cell_stroke.fold(stroke).map(Option::flatten);
+        cell.push_x(Smart::Custom(x));
+        cell.push_y(Smart::Custom(y));
+        cell.push_fill(Smart::Custom(fill.clone()));
+        cell.push_align(match align {
+            Smart::Custom(align) => {
+                Smart::Custom(cell.align(styles).map_or(align, |inner| inner.fold(align)))
+            }
+            // Don't fold if the grid is using outer alignment. Use the
+            // cell's alignment instead (which, in the end, will fold with
+            // the outer alignment when it is effectively displayed).
+            Smart::Auto => cell.align(styles),
+        });
+        cell.push_inset(Smart::Custom(
+            cell.inset(styles).map_or(inset, |inner| inner.fold(inset)),
+        ));
+        cell.push_stroke(
+            // Here we convert the resolved stroke to a regular stroke, however
+            // with resolved units (that is, 'em' converted to absolute units).
+            // We also convert any stroke unspecified by both the cell and the
+            // outer stroke ('None' in the folded stroke) to 'none', that is,
+            // all sides are present in the resulting Sides object accessible
+            // by show rules on grid cells.
+            stroke.as_ref().map(|side| {
+                Some(side.as_ref().map(|cell_stroke| {
+                    Arc::new((**cell_stroke).clone().map(Length::from))
+                }))
+            }),
+        );
+        cell.push_breakable(Smart::Custom(breakable));
+        Cell {
+            body: self.pack(),
+            locator,
+            fill,
+            colspan,
+            rowspan,
+            stroke,
+            stroke_overridden,
+            breakable,
+        }
+    }
+
+    fn x(&self, styles: StyleChain) -> Smart<usize> {
+        (**self).x(styles)
+    }
+
+    fn y(&self, styles: StyleChain) -> Smart<usize> {
+        (**self).y(styles)
+    }
+
+    fn colspan(&self, styles: StyleChain) -> NonZeroUsize {
+        (**self).colspan(styles)
+    }
+
+    fn rowspan(&self, styles: StyleChain) -> NonZeroUsize {
+        (**self).rowspan(styles)
+    }
+
+    fn span(&self) -> Span {
+        Packed::span(self)
+    }
+}
+
+/// Represents an explicit grid line (horizontal or vertical) specified by the
+/// user.
+pub struct Line {
+    /// The index of the track after this line. This will be the index of the
+    /// row a horizontal line is above of, or of the column right after a
+    /// vertical line.
+    ///
+    /// Must be within `0..=tracks.len()` (where `tracks` is either `grid.cols`
+    /// or `grid.rows`, ignoring gutter tracks, as appropriate).
+    pub index: usize,
+    /// The index of the track at which this line starts being drawn.
+    /// This is the first column a horizontal line appears in, or the first row
+    /// a vertical line appears in.
+    ///
+    /// Must be within `0..tracks.len()` minus gutter tracks.
+    pub start: usize,
+    /// The index after the last track through which the line is drawn.
+    /// Thus, the line is drawn through tracks `start..end` (note that `end` is
+    /// exclusive).
+    ///
+    /// Must be within `1..=tracks.len()` minus gutter tracks.
+    /// `None` indicates the line should go all the way to the end.
+    pub end: Option<NonZeroUsize>,
+    /// The line's stroke. This is `None` when the line is explicitly used to
+    /// override a previously specified line.
+    pub stroke: Option<Arc<Stroke<Abs>>>,
+    /// The line's position in relation to the track with its index.
+    pub position: LinePosition,
+}
+
+/// A repeatable grid header. Starts at the first row.
+pub struct Header {
+    /// The index after the last row included in this header.
+    pub end: usize,
+}
+
+/// A repeatable grid footer. Stops at the last row.
+pub struct Footer {
+    /// The first row included in this footer.
+    pub start: usize,
+}
+
+/// A possibly repeatable grid object.
+/// It still exists even when not repeatable, but must not have additional
+/// considerations by grid layout, other than for consistency (such as making
+/// a certain group of rows unbreakable).
+pub enum Repeatable<T> {
+    Repeated(T),
+    NotRepeated(T),
+}
+
+impl<T> Repeatable<T> {
+    /// Gets the value inside this repeatable, regardless of whether
+    /// it repeats.
+    pub fn unwrap(&self) -> &T {
+        match self {
+            Self::Repeated(repeated) => repeated,
+            Self::NotRepeated(not_repeated) => not_repeated,
+        }
+    }
+
+    /// Returns `Some` if the value is repeated, `None` otherwise.
+    pub fn as_repeated(&self) -> Option<&T> {
+        match self {
+            Self::Repeated(repeated) => Some(repeated),
+            Self::NotRepeated(_) => None,
+        }
+    }
+}
+
+/// Used for cell-like elements which are aware of their final properties in
+/// the table, and may have property overrides.
+pub trait ResolvableCell {
+    /// Resolves the cell's fields, given its coordinates and default grid-wide
+    /// fill, align, inset and stroke properties, plus the expected value of
+    /// the `breakable` field.
+    /// Returns a final Cell.
+    #[allow(clippy::too_many_arguments)]
+    fn resolve_cell<'a>(
+        self,
+        x: usize,
+        y: usize,
+        fill: &Option<Paint>,
+        align: Smart<Alignment>,
+        inset: Sides<Option<Rel<Length>>>,
+        stroke: Sides<Option<Option<Arc<Stroke<Abs>>>>>,
+        breakable: bool,
+        locator: Locator<'a>,
+        styles: StyleChain,
+    ) -> Cell<'a>;
+
+    /// Returns this cell's column override.
+    fn x(&self, styles: StyleChain) -> Smart<usize>;
+
+    /// Returns this cell's row override.
+    fn y(&self, styles: StyleChain) -> Smart<usize>;
+
+    /// The amount of columns spanned by this cell.
+    fn colspan(&self, styles: StyleChain) -> NonZeroUsize;
+
+    /// The amount of rows spanned by this cell.
+    fn rowspan(&self, styles: StyleChain) -> NonZeroUsize;
+
+    /// The cell's span, for errors.
+    fn span(&self) -> Span;
+}
+
+/// A grid item, possibly affected by automatic cell positioning. Can be either
+/// a line or a cell.
+pub enum ResolvableGridItem<T: ResolvableCell> {
+    /// A horizontal line in the grid.
+    HLine {
+        /// The row above which the horizontal line is drawn.
+        y: Smart<usize>,
+        start: usize,
+        end: Option<NonZeroUsize>,
+        stroke: Option<Arc<Stroke<Abs>>>,
+        /// The span of the corresponding line element.
+        span: Span,
+        /// The line's position. "before" here means on top of row `y`, while
+        /// "after" means below it.
+        position: LinePosition,
+    },
+    /// A vertical line in the grid.
+    VLine {
+        /// The column before which the vertical line is drawn.
+        x: Smart<usize>,
+        start: usize,
+        end: Option<NonZeroUsize>,
+        stroke: Option<Arc<Stroke<Abs>>>,
+        /// The span of the corresponding line element.
+        span: Span,
+        /// The line's position. "before" here means to the left of column `x`,
+        /// while "after" means to its right (both considering LTR).
+        position: LinePosition,
+    },
+    /// A cell in the grid.
+    Cell(T),
+}
+
+/// Represents a cell in CellGrid, to be laid out by GridLayouter.
+pub struct Cell<'a> {
+    /// The cell's body.
+    pub body: Content,
+    /// The cell's locator.
+    pub locator: Locator<'a>,
+    /// The cell's fill.
+    pub fill: Option<Paint>,
+    /// The amount of columns spanned by the cell.
+    pub colspan: NonZeroUsize,
+    /// The amount of rows spanned by the cell.
+    pub rowspan: NonZeroUsize,
+    /// The cell's stroke.
+    ///
+    /// We use an Arc to avoid unnecessary space usage when all sides are the
+    /// same, or when the strokes come from a common source.
+    pub stroke: Sides<Option<Arc<Stroke<Abs>>>>,
+    /// Which stroke sides were explicitly overridden by the cell, over the
+    /// grid's global stroke setting.
+    ///
+    /// This is used to define whether or not this cell's stroke sides should
+    /// have priority over adjacent cells' stroke sides, if those don't
+    /// override their own stroke properties (and thus have less priority when
+    /// defining with which stroke to draw grid lines around this cell).
+    pub stroke_overridden: Sides<bool>,
+    /// Whether rows spanned by this cell can be placed in different pages.
+    /// By default, a cell spanning only fixed-size rows is unbreakable, while
+    /// a cell spanning at least one `auto`-sized row is breakable.
+    pub breakable: bool,
+}
+
+impl<'a> Cell<'a> {
+    /// Create a simple cell given its body and its locator.
+    pub fn new(body: Content, locator: Locator<'a>) -> Self {
+        Self {
+            body,
+            locator,
+            fill: None,
+            colspan: NonZeroUsize::ONE,
+            rowspan: NonZeroUsize::ONE,
+            stroke: Sides::splat(None),
+            stroke_overridden: Sides::splat(false),
+            breakable: true,
+        }
+    }
+}
+
+/// Indicates whether the line should be drawn before or after the track with
+/// its index. This is mostly only relevant when gutter is used, since, then,
+/// the position after a track is not the same as before the next
+/// non-gutter track.
+#[derive(Copy, Clone, PartialEq, Eq)]
+pub enum LinePosition {
+    /// The line should be drawn before its track (e.g. hline on top of a row).
+    Before,
+    /// The line should be drawn after its track (e.g. hline below a row).
+    After,
+}
+
+/// A grid entry.
+pub enum Entry<'a> {
+    /// An entry which holds a cell.
+    Cell(Cell<'a>),
+    /// An entry which is merged with another cell.
+    Merged {
+        /// The index of the cell this entry is merged with.
+        parent: usize,
+    },
+}
+
+impl<'a> Entry<'a> {
+    /// Obtains the cell inside this entry, if this is not a merged cell.
+    fn as_cell(&self) -> Option<&Cell<'a>> {
+        match self {
+            Self::Cell(cell) => Some(cell),
+            Self::Merged { .. } => None,
+        }
+    }
+}
+
+/// Any grid child, which can be either a header or an item.
+pub enum ResolvableGridChild<T: ResolvableCell, I> {
+    Header { repeat: bool, span: Span, items: I },
+    Footer { repeat: bool, span: Span, items: I },
+    Item(ResolvableGridItem<T>),
+}
+
+/// A grid of cells, including the columns, rows, and cell data.
+pub struct CellGrid<'a> {
+    /// The grid cells.
+    pub entries: Vec<Entry<'a>>,
+    /// The column tracks including gutter tracks.
+    pub cols: Vec<Sizing>,
+    /// The row tracks including gutter tracks.
+    pub rows: Vec<Sizing>,
+    /// The vertical lines before each column, or on the end border.
+    /// Gutter columns are not included.
+    /// Contains up to 'cols_without_gutter.len() + 1' vectors of lines.
+    pub vlines: Vec<Vec<Line>>,
+    /// The horizontal lines on top of each row, or on the bottom border.
+    /// Gutter rows are not included.
+    /// Contains up to 'rows_without_gutter.len() + 1' vectors of lines.
+    pub hlines: Vec<Vec<Line>>,
+    /// The repeatable header of this grid.
+    pub header: Option<Repeatable<Header>>,
+    /// The repeatable footer of this grid.
+    pub footer: Option<Repeatable<Footer>>,
+    /// Whether this grid has gutters.
+    pub has_gutter: bool,
+}
+
+impl<'a> CellGrid<'a> {
+    /// Generates the cell grid, given the tracks and cells.
+    pub fn new(
+        tracks: Axes<&[Sizing]>,
+        gutter: Axes<&[Sizing]>,
+        cells: impl IntoIterator<Item = Cell<'a>>,
+    ) -> Self {
+        let entries = cells.into_iter().map(Entry::Cell).collect();
+        Self::new_internal(tracks, gutter, vec![], vec![], None, None, entries)
+    }
+
+    /// Resolves and positions all cells in the grid before creating it.
+    /// Allows them to keep track of their final properties and positions
+    /// and adjust their fields accordingly.
+    /// Cells must implement Clone as they will be owned. Additionally, they
+    /// must implement Default in order to fill positions in the grid which
+    /// weren't explicitly specified by the user with empty cells.
+    #[allow(clippy::too_many_arguments)]
+    pub fn resolve<T, C, I>(
+        tracks: Axes<&[Sizing]>,
+        gutter: Axes<&[Sizing]>,
+        locator: Locator<'a>,
+        children: C,
+        fill: &Celled<Option<Paint>>,
+        align: &Celled<Smart<Alignment>>,
+        inset: &Celled<Sides<Option<Rel<Length>>>>,
+        stroke: &ResolvedCelled<Sides<Option<Option<Arc<Stroke>>>>>,
+        engine: &mut Engine,
+        styles: StyleChain,
+        span: Span,
+    ) -> SourceResult<Self>
+    where
+        T: ResolvableCell + Default,
+        I: Iterator<Item = ResolvableGridItem<T>>,
+        C: IntoIterator<Item = ResolvableGridChild<T, I>>,
+        C::IntoIter: ExactSizeIterator,
+    {
+        let mut locator = locator.split();
+
+        // Number of content columns: Always at least one.
+        let c = tracks.x.len().max(1);
+
+        // Lists of lines.
+        // Horizontal lines are only pushed later to be able to check for row
+        // validity, since the amount of rows isn't known until all items were
+        // analyzed in the for loop below.
+        // We keep their spans so we can report errors later.
+        // The additional boolean indicates whether the hline had an automatic
+        // 'y' index, and is used to change the index of hlines at the top of a
+        // header or footer.
+        let mut pending_hlines: Vec<(Span, Line, bool)> = vec![];
+
+        // For consistency, only push vertical lines later as well.
+        let mut pending_vlines: Vec<(Span, Line)> = vec![];
+        let has_gutter = gutter.any(|tracks| !tracks.is_empty());
+
+        let mut header: Option<Header> = None;
+        let mut repeat_header = false;
+
+        // Stores where the footer is supposed to end, its span, and the
+        // actual footer structure.
+        let mut footer: Option<(usize, Span, Footer)> = None;
+        let mut repeat_footer = false;
+
+        // Resolves the breakability of a cell. Cells that span at least one
+        // auto-sized row or gutter are considered breakable.
+        let resolve_breakable = |y, rowspan| {
+            let auto = Sizing::Auto;
+            let zero = Sizing::Rel(Rel::zero());
+            tracks
+                .y
+                .iter()
+                .chain(std::iter::repeat(tracks.y.last().unwrap_or(&auto)))
+                .skip(y)
+                .take(rowspan)
+                .any(|row| row == &Sizing::Auto)
+                || gutter
+                    .y
+                    .iter()
+                    .chain(std::iter::repeat(gutter.y.last().unwrap_or(&zero)))
+                    .skip(y)
+                    .take(rowspan - 1)
+                    .any(|row_gutter| row_gutter == &Sizing::Auto)
+        };
+
+        // We can't just use the cell's index in the 'cells' vector to
+        // determine its automatic position, since cells could have arbitrary
+        // positions, so the position of a cell in 'cells' can differ from its
+        // final position in 'resolved_cells' (see below).
+        // Therefore, we use a counter, 'auto_index', to determine the position
+        // of the next cell with (x: auto, y: auto). It is only stepped when
+        // a cell with (x: auto, y: auto), usually the vast majority, is found.
+        let mut auto_index: usize = 0;
+
+        // We have to rebuild the grid to account for arbitrary positions.
+        // Create at least 'children.len()' positions, since there could be at
+        // least 'children.len()' cells (if no explicit lines were specified),
+        // even though some of them might be placed in arbitrary positions and
+        // thus cause the grid to expand.
+        // Additionally, make sure we allocate up to the next multiple of 'c',
+        // since each row will have 'c' cells, even if the last few cells
+        // weren't explicitly specified by the user.
+        // We apply '% c' twice so that the amount of cells potentially missing
+        // is zero when 'children.len()' is already a multiple of 'c' (thus
+        // 'children.len() % c' would be zero).
+        let children = children.into_iter();
+        let Some(child_count) = children.len().checked_add((c - children.len() % c) % c)
+        else {
+            bail!(span, "too many cells or lines were given")
+        };
+        let mut resolved_cells: Vec<Option<Entry>> = Vec::with_capacity(child_count);
+        for child in children {
+            let mut is_header = false;
+            let mut is_footer = false;
+            let mut child_start = usize::MAX;
+            let mut child_end = 0;
+            let mut child_span = Span::detached();
+            let mut start_new_row = false;
+            let mut first_index_of_top_hlines = usize::MAX;
+            let mut first_index_of_non_top_hlines = usize::MAX;
+
+            let (header_footer_items, simple_item) = match child {
+                ResolvableGridChild::Header { repeat, span, items, .. } => {
+                    if header.is_some() {
+                        bail!(span, "cannot have more than one header");
+                    }
+
+                    is_header = true;
+                    child_span = span;
+                    repeat_header = repeat;
+
+                    // If any cell in the header is automatically positioned,
+                    // have it skip to the next row. This is to avoid having a
+                    // header after a partially filled row just add cells to
+                    // that row instead of starting a new one.
+                    // FIXME: Revise this approach when headers can start from
+                    // arbitrary rows.
+                    start_new_row = true;
+
+                    // Any hlines at the top of the header will start at this
+                    // index.
+                    first_index_of_top_hlines = pending_hlines.len();
+
+                    (Some(items), None)
+                }
+                ResolvableGridChild::Footer { repeat, span, items, .. } => {
+                    if footer.is_some() {
+                        bail!(span, "cannot have more than one footer");
+                    }
+
+                    is_footer = true;
+                    child_span = span;
+                    repeat_footer = repeat;
+
+                    // If any cell in the footer is automatically positioned,
+                    // have it skip to the next row. This is to avoid having a
+                    // footer after a partially filled row just add cells to
+                    // that row instead of starting a new one.
+                    start_new_row = true;
+
+                    // Any hlines at the top of the footer will start at this
+                    // index.
+                    first_index_of_top_hlines = pending_hlines.len();
+
+                    (Some(items), None)
+                }
+                ResolvableGridChild::Item(item) => (None, Some(item)),
+            };
+
+            let items = header_footer_items
+                .into_iter()
+                .flatten()
+                .chain(simple_item.into_iter());
+            for item in items {
+                let cell = match item {
+                    ResolvableGridItem::HLine {
+                        y,
+                        start,
+                        end,
+                        stroke,
+                        span,
+                        position,
+                    } => {
+                        let has_auto_y = y.is_auto();
+                        let y = y.unwrap_or_else(|| {
+                            // Avoid placing the hline inside consecutive
+                            // rowspans occupying all columns, as it'd just
+                            // disappear, at least when there's no column
+                            // gutter.
+                            skip_auto_index_through_fully_merged_rows(
+                                &resolved_cells,
+                                &mut auto_index,
+                                c,
+                            );
+
+                            // When no 'y' is specified for the hline, we place
+                            // it under the latest automatically positioned
+                            // cell.
+                            // The current value of the auto index is always
+                            // the index of the latest automatically positioned
+                            // cell placed plus one (that's what we do in
+                            // 'resolve_cell_position'), so we subtract 1 to
+                            // get that cell's index, and place the hline below
+                            // its row. The exception is when the auto_index is
+                            // 0, meaning no automatically positioned cell was
+                            // placed yet. In that case, we place the hline at
+                            // the top of the table.
+                            //
+                            // Exceptionally, the hline will be placed before
+                            // the minimum auto index if the current auto index
+                            // from previous iterations is smaller than the
+                            // minimum it should have for the current grid
+                            // child. Effectively, this means that a hline at
+                            // the start of a header will always appear above
+                            // that header's first row. Similarly for footers.
+                            auto_index
+                                .checked_sub(1)
+                                .map_or(0, |last_auto_index| last_auto_index / c + 1)
+                        });
+                        if end.is_some_and(|end| end.get() < start) {
+                            bail!(span, "line cannot end before it starts");
+                        }
+                        let line = Line { index: y, start, end, stroke, position };
+
+                        // Since the amount of rows is dynamic, delay placing
+                        // hlines until after all cells were placed so we can
+                        // properly verify if they are valid. Note that we
+                        // can't place hlines even if we already know they
+                        // would be in a valid row, since it's possible that we
+                        // pushed pending hlines in the same row as this one in
+                        // previous iterations, and we need to ensure that
+                        // hlines from previous iterations are pushed to the
+                        // final vector of hlines first - the order of hlines
+                        // must be kept, as this matters when determining which
+                        // one "wins" in case of conflict. Pushing the current
+                        // hline before we push pending hlines later would
+                        // change their order!
+                        pending_hlines.push((span, line, has_auto_y));
+                        continue;
+                    }
+                    ResolvableGridItem::VLine {
+                        x,
+                        start,
+                        end,
+                        stroke,
+                        span,
+                        position,
+                    } => {
+                        let x = x.unwrap_or_else(|| {
+                            // When no 'x' is specified for the vline, we place
+                            // it after the latest automatically positioned
+                            // cell.
+                            // The current value of the auto index is always
+                            // the index of the latest automatically positioned
+                            // cell placed plus one (that's what we do in
+                            // 'resolve_cell_position'), so we subtract 1 to
+                            // get that cell's index, and place the vline after
+                            // its column. The exception is when the auto_index
+                            // is 0, meaning no automatically positioned cell
+                            // was placed yet. In that case, we place the vline
+                            // to the left of the table.
+                            //
+                            // Exceptionally, a vline is also placed to the
+                            // left of the table if we should start a new row
+                            // for the next automatically positioned cell.
+                            // For example, this means that a vline at
+                            // the beginning of a header will be placed to its
+                            // left rather than after the previous
+                            // automatically positioned cell. Same for footers.
+                            auto_index
+                                .checked_sub(1)
+                                .filter(|_| !start_new_row)
+                                .map_or(0, |last_auto_index| last_auto_index % c + 1)
+                        });
+                        if end.is_some_and(|end| end.get() < start) {
+                            bail!(span, "line cannot end before it starts");
+                        }
+                        let line = Line { index: x, start, end, stroke, position };
+
+                        // For consistency with hlines, we only push vlines to
+                        // the final vector of vlines after processing every
+                        // cell.
+                        pending_vlines.push((span, line));
+                        continue;
+                    }
+                    ResolvableGridItem::Cell(cell) => cell,
+                };
+                let cell_span = cell.span();
+                let colspan = cell.colspan(styles).get();
+                let rowspan = cell.rowspan(styles).get();
+                // Let's calculate the cell's final position based on its
+                // requested position.
+                let resolved_index = {
+                    let cell_x = cell.x(styles);
+                    let cell_y = cell.y(styles);
+                    resolve_cell_position(
+                        cell_x,
+                        cell_y,
+                        colspan,
+                        rowspan,
+                        &resolved_cells,
+                        &mut auto_index,
+                        &mut start_new_row,
+                        c,
+                    )
+                    .at(cell_span)?
+                };
+                let x = resolved_index % c;
+                let y = resolved_index / c;
+
+                if colspan > c - x {
+                    bail!(
+                        cell_span,
+                        "cell's colspan would cause it to exceed the available column(s)";
+                        hint: "try placing the cell in another position or reducing its colspan"
+                    )
+                }
+
+                let Some(largest_index) = c
+                    .checked_mul(rowspan - 1)
+                    .and_then(|full_rowspan_offset| {
+                        resolved_index.checked_add(full_rowspan_offset)
+                    })
+                    .and_then(|last_row_pos| last_row_pos.checked_add(colspan - 1))
+                else {
+                    bail!(
+                        cell_span,
+                        "cell would span an exceedingly large position";
+                        hint: "try reducing the cell's rowspan or colspan"
+                    )
+                };
+
+                // Let's resolve the cell so it can determine its own fields
+                // based on its final position.
+                let cell = cell.resolve_cell(
+                    x,
+                    y,
+                    &fill.resolve(engine, styles, x, y)?,
+                    align.resolve(engine, styles, x, y)?,
+                    inset.resolve(engine, styles, x, y)?,
+                    stroke.resolve(engine, styles, x, y)?,
+                    resolve_breakable(y, rowspan),
+                    locator.next(&cell_span),
+                    styles,
+                );
+
+                if largest_index >= resolved_cells.len() {
+                    // Ensure the length of the vector of resolved cells is
+                    // always a multiple of 'c' by pushing full rows every
+                    // time. Here, we add enough absent positions (later
+                    // converted to empty cells) to ensure the last row in the
+                    // new vector length is completely filled. This is
+                    // necessary so that those positions, even if not
+                    // explicitly used at the end, are eventually susceptible
+                    // to show rules and receive grid styling, as they will be
+                    // resolved as empty cells in a second loop below.
+                    let Some(new_len) = largest_index
+                        .checked_add(1)
+                        .and_then(|new_len| new_len.checked_add((c - new_len % c) % c))
+                    else {
+                        bail!(cell_span, "cell position too large")
+                    };
+
+                    // Here, the cell needs to be placed in a position which
+                    // doesn't exist yet in the grid (out of bounds). We will
+                    // add enough absent positions for this to be possible.
+                    // They must be absent as no cells actually occupy them
+                    // (they can be overridden later); however, if no cells
+                    // occupy them as we finish building the grid, then such
+                    // positions will be replaced by empty cells.
+                    resolved_cells.resize_with(new_len, || None);
+                }
+
+                // The vector is large enough to contain the cell, so we can
+                // just index it directly to access the position it will be
+                // placed in. However, we still need to ensure we won't try to
+                // place a cell where there already is one.
+                let slot = &mut resolved_cells[resolved_index];
+                if slot.is_some() {
+                    bail!(
+                        cell_span,
+                        "attempted to place a second cell at column {x}, row {y}";
+                        hint: "try specifying your cells in a different order"
+                    );
+                }
+
+                *slot = Some(Entry::Cell(cell));
+
+                // Now, if the cell spans more than one row or column, we fill
+                // the spanned positions in the grid with Entry::Merged
+                // pointing to the original cell as its parent.
+                for rowspan_offset in 0..rowspan {
+                    let spanned_y = y + rowspan_offset;
+                    let first_row_index = resolved_index + c * rowspan_offset;
+                    for (colspan_offset, slot) in resolved_cells[first_row_index..]
+                        [..colspan]
+                        .iter_mut()
+                        .enumerate()
+                    {
+                        let spanned_x = x + colspan_offset;
+                        if spanned_x == x && spanned_y == y {
+                            // This is the parent cell.
+                            continue;
+                        }
+                        if slot.is_some() {
+                            bail!(
+                                cell_span,
+                                "cell would span a previously placed cell at column {spanned_x}, row {spanned_y}";
+                                hint: "try specifying your cells in a different order or reducing the cell's rowspan or colspan"
+                            )
+                        }
+                        *slot = Some(Entry::Merged { parent: resolved_index });
+                    }
+                }
+
+                if is_header || is_footer {
+                    // Ensure each cell in a header or footer is fully
+                    // contained within it.
+                    child_start = child_start.min(y);
+                    child_end = child_end.max(y + rowspan);
+
+                    if start_new_row && child_start <= auto_index.div_ceil(c) {
+                        // No need to start a new row as we already include
+                        // the row of the next automatically positioned cell in
+                        // the header or footer.
+                        start_new_row = false;
+                    }
+
+                    if !start_new_row {
+                        // From now on, upcoming hlines won't be at the top of
+                        // the child, as the first automatically positioned
+                        // cell was placed.
+                        first_index_of_non_top_hlines =
+                            first_index_of_non_top_hlines.min(pending_hlines.len());
+                    }
+                }
+            }
+
+            if (is_header || is_footer) && child_start == usize::MAX {
+                // Empty header/footer: consider the header/footer to be
+                // at the next empty row after the latest auto index.
+                auto_index = find_next_empty_row(&resolved_cells, auto_index, c);
+                child_start = auto_index.div_ceil(c);
+                child_end = child_start + 1;
+
+                if resolved_cells.len() <= c * child_start {
+                    // Ensure the automatically chosen row actually exists.
+                    resolved_cells.resize_with(c * (child_start + 1), || None);
+                }
+            }
+
+            if is_header {
+                if child_start != 0 {
+                    bail!(
+                        child_span,
+                        "header must start at the first row";
+                        hint: "remove any rows before the header"
+                    );
+                }
+
+                header = Some(Header {
+                    // Later on, we have to correct this number in case there
+                    // is gutter. But only once all cells have been analyzed
+                    // and the header has fully expanded in the fixup loop
+                    // below.
+                    end: child_end,
+                });
+            }
+
+            if is_footer {
+                // Only check if the footer is at the end later, once we know
+                // the final amount of rows.
+                footer = Some((
+                    child_end,
+                    child_span,
+                    Footer {
+                        // Later on, we have to correct this number in case there
+                        // is gutter, but only once all cells have been analyzed
+                        // and the header's and footer's exact boundaries are
+                        // known. That is because the gutter row immediately
+                        // before the footer might not be included as part of
+                        // the footer if it is contained within the header.
+                        start: child_start,
+                    },
+                ));
+            }
+
+            if is_header || is_footer {
+                let amount_hlines = pending_hlines.len();
+                for (_, top_hline, has_auto_y) in pending_hlines
+                    .get_mut(
+                        first_index_of_top_hlines
+                            ..first_index_of_non_top_hlines.min(amount_hlines),
+                    )
+                    .unwrap_or(&mut [])
+                {
+                    if *has_auto_y {
+                        // Move this hline to the top of the child, as it was
+                        // placed before the first automatically positioned cell
+                        // and had an automatic index.
+                        top_hline.index = child_start;
+                    }
+                }
+
+                // Next automatically positioned cell goes under this header.
+                // FIXME: Consider only doing this if the header has any fully
+                // automatically positioned cells. Otherwise,
+                // `resolve_cell_position` should be smart enough to skip
+                // upcoming headers.
+                // Additionally, consider that cells with just an 'x' override
+                // could end up going too far back and making previous
+                // non-header rows into header rows (maybe they should be
+                // placed at the first row that is fully empty or something).
+                // Nothing we can do when both 'x' and 'y' were overridden, of
+                // course.
+                // None of the above are concerns for now, as headers must
+                // start at the first row.
+                auto_index = auto_index.max(c * child_end);
+            }
+        }
+
+        // If the user specified cells occupying less rows than the given rows,
+        // we shall expand the grid so that it has at least the given amount of
+        // rows.
+        let Some(expected_total_cells) = c.checked_mul(tracks.y.len()) else {
+            bail!(span, "too many rows were specified");
+        };
+        let missing_cells = expected_total_cells.saturating_sub(resolved_cells.len());
+
+        // Fixup phase (final step in cell grid generation):
+        // 1. Replace absent entries by resolved empty cells, and produce a
+        // vector of 'Entry' from 'Option<Entry>'.
+        // 2. Add enough empty cells to the end of the grid such that it has at
+        // least the given amount of rows.
+        // 3. If any cells were added to the header's rows after the header's
+        // creation, ensure the header expands enough to accommodate them
+        // across all of their spanned rows. Same for the footer.
+        // 4. If any cells before the footer try to span it, error.
+        let resolved_cells = resolved_cells
+            .into_iter()
+            .chain(std::iter::repeat_with(|| None).take(missing_cells))
+            .enumerate()
+            .map(|(i, cell)| {
+                if let Some(cell) = cell {
+                    if let Some(parent_cell) = cell.as_cell() {
+                        if let Some(header) = &mut header
+                        {
+                            let y = i / c;
+                            if y < header.end {
+                                // Ensure the header expands enough such that
+                                // all cells inside it, even those added later,
+                                // are fully contained within the header.
+                                // FIXME: check if start < y < end when start can
+                                // be != 0.
+                                // FIXME: when start can be != 0, decide what
+                                // happens when a cell after the header placed
+                                // above it tries to span the header (either
+                                // error or expand upwards).
+                                header.end = header.end.max(y + parent_cell.rowspan.get());
+                            }
+                        }
+
+                        if let Some((end, footer_span, footer)) = &mut footer {
+                            let x = i % c;
+                            let y = i / c;
+                            let cell_end = y + parent_cell.rowspan.get();
+                            if y < footer.start && cell_end > footer.start {
+                                // Don't allow a cell before the footer to span
+                                // it. Surely, we could move the footer to
+                                // start at where this cell starts, so this is
+                                // more of a design choice, as it's unlikely
+                                // for the user to intentionally include a cell
+                                // before the footer spanning it but not
+                                // being repeated with it.
+                                bail!(
+                                    *footer_span,
+                                    "footer would conflict with a cell placed before it at column {x} row {y}";
+                                    hint: "try reducing that cell's rowspan or moving the footer"
+                                );
+                            }
+                            if y >= footer.start && y < *end {
+                                // Expand the footer to include all rows
+                                // spanned by this cell, as it is inside the
+                                // footer.
+                                *end = (*end).max(cell_end);
+                            }
+                        }
+                    }
+
+                    Ok(cell)
+                } else {
+                    let x = i % c;
+                    let y = i / c;
+
+                    // Ensure all absent entries are affected by show rules and
+                    // grid styling by turning them into resolved empty cells.
+                    let new_cell = T::default().resolve_cell(
+                        x,
+                        y,
+                        &fill.resolve(engine, styles, x, y)?,
+                        align.resolve(engine, styles, x, y)?,
+                        inset.resolve(engine, styles, x, y)?,
+                        stroke.resolve(engine, styles, x, y)?,
+                        resolve_breakable(y, 1),
+                        locator.next(&()),
+                        styles,
+                    );
+                    Ok(Entry::Cell(new_cell))
+                }
+            })
+            .collect::<SourceResult<Vec<Entry>>>()?;
+
+        // Populate the final lists of lines.
+        // For each line type (horizontal or vertical), we keep a vector for
+        // every group of lines with the same index.
+        let mut vlines: Vec<Vec<Line>> = vec![];
+        let mut hlines: Vec<Vec<Line>> = vec![];
+        let row_amount = resolved_cells.len().div_ceil(c);
+
+        for (line_span, line, _) in pending_hlines {
+            let y = line.index;
+            if y > row_amount {
+                bail!(line_span, "cannot place horizontal line at invalid row {y}");
+            }
+            if y == row_amount && line.position == LinePosition::After {
+                bail!(
+                    line_span,
+                    "cannot place horizontal line at the 'bottom' position of the bottom border (y = {y})";
+                    hint: "set the line's position to 'top' or place it at a smaller 'y' index"
+                );
+            }
+            let line = if line.position == LinePosition::After
+                && (!has_gutter || y + 1 == row_amount)
+            {
+                // Just place the line on top of the next row if
+                // there's no gutter and the line should be placed
+                // after the one with given index.
+                //
+                // Note that placing after the last row is also the same as
+                // just placing on the grid's bottom border, even with
+                // gutter.
+                Line {
+                    index: y + 1,
+                    position: LinePosition::Before,
+                    ..line
+                }
+            } else {
+                line
+            };
+            let y = line.index;
+
+            if hlines.len() <= y {
+                hlines.resize_with(y + 1, Vec::new);
+            }
+            hlines[y].push(line);
+        }
+
+        for (line_span, line) in pending_vlines {
+            let x = line.index;
+            if x > c {
+                bail!(line_span, "cannot place vertical line at invalid column {x}");
+            }
+            if x == c && line.position == LinePosition::After {
+                bail!(
+                    line_span,
+                    "cannot place vertical line at the 'end' position of the end border (x = {c})";
+                    hint: "set the line's position to 'start' or place it at a smaller 'x' index"
+                );
+            }
+            let line =
+                if line.position == LinePosition::After && (!has_gutter || x + 1 == c) {
+                    // Just place the line before the next column if
+                    // there's no gutter and the line should be placed
+                    // after the one with given index.
+                    //
+                    // Note that placing after the last column is also the
+                    // same as just placing on the grid's end border, even
+                    // with gutter.
+                    Line {
+                        index: x + 1,
+                        position: LinePosition::Before,
+                        ..line
+                    }
+                } else {
+                    line
+                };
+            let x = line.index;
+
+            if vlines.len() <= x {
+                vlines.resize_with(x + 1, Vec::new);
+            }
+            vlines[x].push(line);
+        }
+
+        let header = header
+            .map(|mut header| {
+                // Repeat the gutter below a header (hence why we don't
+                // subtract 1 from the gutter case).
+                // Don't do this if there are no rows under the header.
+                if has_gutter {
+                    // - 'header.end' is always 'last y + 1'. The header stops
+                    // before that row.
+                    // - Therefore, '2 * header.end' will be 2 * (last y + 1),
+                    // which is the adjusted index of the row before which the
+                    // header stops, meaning it will still stop right before it
+                    // even with gutter thanks to the multiplication below.
+                    // - This means that it will span all rows up to
+                    // '2 * (last y + 1) - 1 = 2 * last y + 1', which equates
+                    // to the index of the gutter row right below the header,
+                    // which is what we want (that gutter spacing should be
+                    // repeated across pages to maintain uniformity).
+                    header.end *= 2;
+
+                    // If the header occupies the entire grid, ensure we don't
+                    // include an extra gutter row when it doesn't exist, since
+                    // the last row of the header is at the very bottom,
+                    // therefore '2 * last y + 1' is not a valid index.
+                    let row_amount = (2 * row_amount).saturating_sub(1);
+                    header.end = header.end.min(row_amount);
+                }
+                header
+            })
+            .map(|header| {
+                if repeat_header {
+                    Repeatable::Repeated(header)
+                } else {
+                    Repeatable::NotRepeated(header)
+                }
+            });
+
+        let footer = footer
+            .map(|(footer_end, footer_span, mut footer)| {
+                if footer_end != row_amount {
+                    bail!(footer_span, "footer must end at the last row");
+                }
+
+                let header_end =
+                    header.as_ref().map(Repeatable::unwrap).map(|header| header.end);
+
+                if has_gutter {
+                    // Convert the footer's start index to post-gutter coordinates.
+                    footer.start *= 2;
+
+                    // Include the gutter right before the footer, unless there is
+                    // none, or the gutter is already included in the header (no
+                    // rows between the header and the footer).
+                    if header_end.map_or(true, |header_end| header_end != footer.start) {
+                        footer.start = footer.start.saturating_sub(1);
+                    }
+                }
+
+                if header_end.is_some_and(|header_end| header_end > footer.start) {
+                    bail!(footer_span, "header and footer must not have common rows");
+                }
+
+                Ok(footer)
+            })
+            .transpose()?
+            .map(|footer| {
+                if repeat_footer {
+                    Repeatable::Repeated(footer)
+                } else {
+                    Repeatable::NotRepeated(footer)
+                }
+            });
+
+        Ok(Self::new_internal(
+            tracks,
+            gutter,
+            vlines,
+            hlines,
+            header,
+            footer,
+            resolved_cells,
+        ))
+    }
+
+    /// Generates the cell grid, given the tracks and resolved entries.
+    pub fn new_internal(
+        tracks: Axes<&[Sizing]>,
+        gutter: Axes<&[Sizing]>,
+        vlines: Vec<Vec<Line>>,
+        hlines: Vec<Vec<Line>>,
+        header: Option<Repeatable<Header>>,
+        footer: Option<Repeatable<Footer>>,
+        entries: Vec<Entry<'a>>,
+    ) -> Self {
+        let mut cols = vec![];
+        let mut rows = vec![];
+
+        // Number of content columns: Always at least one.
+        let c = tracks.x.len().max(1);
+
+        // Number of content rows: At least as many as given, but also at least
+        // as many as needed to place each item.
+        let r = {
+            let len = entries.len();
+            let given = tracks.y.len();
+            let needed = len / c + (len % c).clamp(0, 1);
+            given.max(needed)
+        };
+
+        let has_gutter = gutter.any(|tracks| !tracks.is_empty());
+        let auto = Sizing::Auto;
+        let zero = Sizing::Rel(Rel::zero());
+        let get_or = |tracks: &[_], idx, default| {
+            tracks.get(idx).or(tracks.last()).copied().unwrap_or(default)
+        };
+
+        // Collect content and gutter columns.
+        for x in 0..c {
+            cols.push(get_or(tracks.x, x, auto));
+            if has_gutter {
+                cols.push(get_or(gutter.x, x, zero));
+            }
+        }
+
+        // Collect content and gutter rows.
+        for y in 0..r {
+            rows.push(get_or(tracks.y, y, auto));
+            if has_gutter {
+                rows.push(get_or(gutter.y, y, zero));
+            }
+        }
+
+        // Remove superfluous gutter tracks.
+        if has_gutter {
+            cols.pop();
+            rows.pop();
+        }
+
+        Self {
+            cols,
+            rows,
+            entries,
+            vlines,
+            hlines,
+            header,
+            footer,
+            has_gutter,
+        }
+    }
+
+    /// Get the grid entry in column `x` and row `y`.
+    ///
+    /// Returns `None` if it's a gutter cell.
+    #[track_caller]
+    pub fn entry(&self, x: usize, y: usize) -> Option<&Entry<'a>> {
+        assert!(x < self.cols.len());
+        assert!(y < self.rows.len());
+
+        if self.has_gutter {
+            // Even columns and rows are children, odd ones are gutter.
+            if x % 2 == 0 && y % 2 == 0 {
+                let c = 1 + self.cols.len() / 2;
+                self.entries.get((y / 2) * c + x / 2)
+            } else {
+                None
+            }
+        } else {
+            let c = self.cols.len();
+            self.entries.get(y * c + x)
+        }
+    }
+
+    /// Get the content of the cell in column `x` and row `y`.
+    ///
+    /// Returns `None` if it's a gutter cell or merged position.
+    #[track_caller]
+    pub fn cell(&self, x: usize, y: usize) -> Option<&Cell<'a>> {
+        self.entry(x, y).and_then(Entry::as_cell)
+    }
+
+    /// Returns the position of the parent cell of the grid entry at the given
+    /// position. It is guaranteed to have a non-gutter, non-merged cell at
+    /// the returned position, due to how the grid is built.
+    /// - If the entry at the given position is a cell, returns the given
+    ///   position.
+    /// - If it is a merged cell, returns the parent cell's position.
+    /// - If it is a gutter cell, returns None.
+    #[track_caller]
+    pub fn parent_cell_position(&self, x: usize, y: usize) -> Option<Axes<usize>> {
+        self.entry(x, y).map(|entry| match entry {
+            Entry::Cell(_) => Axes::new(x, y),
+            Entry::Merged { parent } => {
+                let c = if self.has_gutter {
+                    1 + self.cols.len() / 2
+                } else {
+                    self.cols.len()
+                };
+                let factor = if self.has_gutter { 2 } else { 1 };
+                Axes::new(factor * (*parent % c), factor * (*parent / c))
+            }
+        })
+    }
+
+    /// Returns the position of the actual parent cell of a merged position,
+    /// even if the given position is gutter, in which case we return the
+    /// parent of the nearest adjacent content cell which could possibly span
+    /// the given gutter position. If the given position is not a gutter cell,
+    /// then this function will return the same as `parent_cell_position` would.
+    /// If the given position is a gutter cell, but no cell spans it, returns
+    /// `None`.
+    ///
+    /// This is useful for lines. A line needs to check if a cell next to it
+    /// has a stroke override - even at a gutter position there could be a
+    /// stroke override, since a cell could be merged with two cells at both
+    /// ends of the gutter cell (e.g. to its left and to its right), and thus
+    /// that cell would impose a stroke under the gutter. This function allows
+    /// getting the position of that cell (which spans the given gutter
+    /// position, if it is gutter), if it exists; otherwise returns None (it's
+    /// gutter and no cell spans it).
+    #[track_caller]
+    pub fn effective_parent_cell_position(
+        &self,
+        x: usize,
+        y: usize,
+    ) -> Option<Axes<usize>> {
+        if self.has_gutter {
+            // If (x, y) is a gutter cell, we skip it (skip a gutter column and
+            // row) to the nearest adjacent content cell, in the direction
+            // which merged cells grow toward (increasing x and increasing y),
+            // such that we can verify if that adjacent cell is merged with the
+            // gutter cell by checking if its parent would come before (x, y).
+            // Otherwise, no cell is merged with this gutter cell, and we
+            // return None.
+            self.parent_cell_position(x + x % 2, y + y % 2)
+                .filter(|&parent| parent.x <= x && parent.y <= y)
+        } else {
+            self.parent_cell_position(x, y)
+        }
+    }
+
+    /// Checks if the track with the given index is gutter.
+    /// Does not check if the index is a valid track.
+    #[inline]
+    pub fn is_gutter_track(&self, index: usize) -> bool {
+        self.has_gutter && index % 2 == 1
+    }
+
+    /// Returns the effective colspan of a cell, considering the gutters it
+    /// might span if the grid has gutters.
+    #[inline]
+    pub fn effective_colspan_of_cell(&self, cell: &Cell) -> usize {
+        if self.has_gutter {
+            2 * cell.colspan.get() - 1
+        } else {
+            cell.colspan.get()
+        }
+    }
+
+    /// Returns the effective rowspan of a cell, considering the gutters it
+    /// might span if the grid has gutters.
+    #[inline]
+    pub fn effective_rowspan_of_cell(&self, cell: &Cell) -> usize {
+        if self.has_gutter {
+            2 * cell.rowspan.get() - 1
+        } else {
+            cell.rowspan.get()
+        }
+    }
+}
+
+/// Given a cell's requested x and y, the vector with the resolved cell
+/// positions, the `auto_index` counter (determines the position of the next
+/// `(auto, auto)` cell) and the amount of columns in the grid, returns the
+/// final index of this cell in the vector of resolved cells.
+///
+/// The `start_new_row` parameter is used to ensure that, if this cell is
+/// fully automatically positioned, it should start a new, empty row. This is
+/// useful for headers and footers, which must start at their own rows, without
+/// interference from previous cells.
+#[allow(clippy::too_many_arguments)]
+fn resolve_cell_position(
+    cell_x: Smart<usize>,
+    cell_y: Smart<usize>,
+    colspan: usize,
+    rowspan: usize,
+    resolved_cells: &[Option<Entry>],
+    auto_index: &mut usize,
+    start_new_row: &mut bool,
+    columns: usize,
+) -> HintedStrResult<usize> {
+    // Translates a (x, y) position to the equivalent index in the final cell vector.
+    // Errors if the position would be too large.
+    let cell_index = |x, y: usize| {
+        y.checked_mul(columns)
+            .and_then(|row_index| row_index.checked_add(x))
+            .ok_or_else(|| HintedString::from(eco_format!("cell position too large")))
+    };
+    match (cell_x, cell_y) {
+        // Fully automatic cell positioning. The cell did not
+        // request a coordinate.
+        (Smart::Auto, Smart::Auto) => {
+            // Let's find the first available position starting from the
+            // automatic position counter, searching in row-major order.
+            let mut resolved_index = *auto_index;
+            if *start_new_row {
+                resolved_index =
+                    find_next_empty_row(resolved_cells, resolved_index, columns);
+
+                // Next cell won't have to start a new row if we just did that,
+                // in principle.
+                *start_new_row = false;
+            } else {
+                while let Some(Some(_)) = resolved_cells.get(resolved_index) {
+                    // Skip any non-absent cell positions (`Some(None)`) to
+                    // determine where this cell will be placed. An out of
+                    // bounds position (thus `None`) is also a valid new
+                    // position (only requires expanding the vector).
+                    resolved_index += 1;
+                }
+            }
+
+            // Ensure the next cell with automatic position will be
+            // placed after this one (maybe not immediately after).
+            //
+            // The calculation below also affects the position of the upcoming
+            // automatically-positioned lines.
+            *auto_index = if colspan == columns {
+                // The cell occupies all columns, so no cells can be placed
+                // after it until all of its rows have been spanned.
+                resolved_index + colspan * rowspan
+            } else {
+                // The next cell will have to be placed at least after its
+                // spanned columns.
+                resolved_index + colspan
+            };
+
+            Ok(resolved_index)
+        }
+        // Cell has chosen at least its column.
+        (Smart::Custom(cell_x), cell_y) => {
+            if cell_x >= columns {
+                return Err(HintedString::from(eco_format!(
+                    "cell could not be placed at invalid column {cell_x}"
+                )));
+            }
+            if let Smart::Custom(cell_y) = cell_y {
+                // Cell has chosen its exact position.
+                cell_index(cell_x, cell_y)
+            } else {
+                // Cell has only chosen its column.
+                // Let's find the first row which has that column available.
+                let mut resolved_y = 0;
+                while let Some(Some(_)) =
+                    resolved_cells.get(cell_index(cell_x, resolved_y)?)
+                {
+                    // Try each row until either we reach an absent position
+                    // (`Some(None)`) or an out of bounds position (`None`),
+                    // in which case we'd create a new row to place this cell in.
+                    resolved_y += 1;
+                }
+                cell_index(cell_x, resolved_y)
+            }
+        }
+        // Cell has only chosen its row, not its column.
+        (Smart::Auto, Smart::Custom(cell_y)) => {
+            // Let's find the first column which has that row available.
+            let first_row_pos = cell_index(0, cell_y)?;
+            let last_row_pos = first_row_pos
+                .checked_add(columns)
+                .ok_or_else(|| eco_format!("cell position too large"))?;
+
+            (first_row_pos..last_row_pos)
+                .find(|possible_index| {
+                    // Much like in the previous cases, we skip any occupied
+                    // positions until we either reach an absent position
+                    // (`Some(None)`) or an out of bounds position (`None`),
+                    // in which case we can just expand the vector enough to
+                    // place this cell. In either case, we found an available
+                    // position.
+                    !matches!(resolved_cells.get(*possible_index), Some(Some(_)))
+                })
+                .ok_or_else(|| {
+                    eco_format!(
+                        "cell could not be placed in row {cell_y} because it was full"
+                    )
+                })
+                .hint("try specifying your cells in a different order")
+        }
+    }
+}
+
+/// Computes the index of the first cell in the next empty row in the grid,
+/// starting with the given initial index.
+fn find_next_empty_row(
+    resolved_cells: &[Option<Entry>],
+    initial_index: usize,
+    columns: usize,
+) -> usize {
+    let mut resolved_index = initial_index.next_multiple_of(columns);
+    while resolved_cells
+        .get(resolved_index..resolved_index + columns)
+        .is_some_and(|row| row.iter().any(Option::is_some))
+    {
+        // Skip non-empty rows.
+        resolved_index += columns;
+    }
+
+    resolved_index
+}
+
+/// Fully merged rows under the cell of latest auto index indicate rowspans
+/// occupying all columns, so we skip the auto index until the shortest rowspan
+/// ends, such that, in the resulting row, we will be able to place an
+/// automatically positioned cell - and, in particular, hlines under it. The
+/// idea is that an auto hline will be placed after the shortest such rowspan.
+/// Otherwise, the hline would just be placed under the first row of those
+/// rowspans and disappear (except at the presence of column gutter).
+fn skip_auto_index_through_fully_merged_rows(
+    resolved_cells: &[Option<Entry>],
+    auto_index: &mut usize,
+    columns: usize,
+) {
+    // If the auto index isn't currently at the start of a row, that means
+    // there's still at least one auto position left in the row, ignoring
+    // cells with manual positions, so we wouldn't have a problem in placing
+    // further cells or, in this case, hlines here.
+    if *auto_index % columns == 0 {
+        while resolved_cells
+            .get(*auto_index..*auto_index + columns)
+            .is_some_and(|row| {
+                row.iter().all(|entry| matches!(entry, Some(Entry::Merged { .. })))
+            })
+        {
+            *auto_index += columns;
+        }
+    }
+}
diff --git a/crates/typst-library/src/layout/mod.rs b/crates/typst-library/src/layout/mod.rs
index b54d6906..574a2830 100644
--- a/crates/typst-library/src/layout/mod.rs
+++ b/crates/typst-library/src/layout/mod.rs
@@ -12,7 +12,7 @@ mod em;
 mod fr;
 mod fragment;
 mod frame;
-mod grid;
+pub mod grid;
 mod hide;
 #[path = "layout.rs"]
 mod layout_;