Refactor flow, stack and grid layouters a bit

4 files changed, 131 insertions, 122 deletions

diff --git a/src/geom/spec.rs b/src/geom/spec.rs
index 1b8e13c2..31f93a65 100644
--- a/src/geom/spec.rs
+++ b/src/geom/spec.rs
@@ -39,6 +39,14 @@ impl<T> Spec<T> {
         Spec { x: &self.x, y: &self.y }
     }
 
+    /// Convert from `&Spec<T>` to `Spec<&<T as Deref>::Target>`.
+    pub fn as_deref(&self) -> Spec<&T::Target>
+    where
+        T: Deref,
+    {
+        Spec { x: &self.x, y: &self.y }
+    }
+
     /// Convert from `&mut Spec<T>` to `Spec<&mut T>`.
     pub fn as_mut(&mut self) -> Spec<&mut T> {
         Spec { x: &mut self.x, y: &mut self.y }
diff --git a/src/library/flow.rs b/src/library/flow.rs
index 38f1b0e7..227a7a35 100644
--- a/src/library/flow.rs
+++ b/src/library/flow.rs
@@ -190,11 +190,7 @@ impl<'a> FlowLayouter<'a> {
         let aligns = Spec::new(
             // For non-expanding paragraphs it is crucial that we align the
             // whole paragraph as it is itself aligned.
-            if node.is::<ParNode>() {
-                styles.get(ParNode::ALIGN)
-            } else {
-                Align::Left
-            },
+            styles.get(ParNode::ALIGN),
             // Vertical align node alignment is respected by the flow node.
             node.downcast::<AlignNode>()
                 .and_then(|aligned| aligned.aligns.y)
diff --git a/src/library/grid.rs b/src/library/grid.rs
index d681ae7d..59fe8486 100644
--- a/src/library/grid.rs
+++ b/src/library/grid.rs
@@ -40,12 +40,15 @@ impl Layout for GridNode {
         styles: StyleChain,
     ) -> Vec<Constrained<Arc<Frame>>> {
         // Prepare grid layout by unifying content and gutter tracks.
-        let mut layouter = GridLayouter::new(self, regions.clone(), styles);
-
-        // Determine all column sizes.
-        layouter.measure_columns(ctx);
+        let layouter = GridLayouter::new(
+            self.tracks.as_deref(),
+            self.gutter.as_deref(),
+            &self.children,
+            regions,
+            styles,
+        );
 
-        // Layout the grid row-by-row.
+        // Measure the columsna nd layout the grid row-by-row.
         layouter.layout(ctx)
     }
 }
@@ -87,9 +90,9 @@ castable! {
 }
 
 /// Performs grid layout.
-struct GridLayouter<'a> {
-    /// The children of the grid.
-    children: &'a [PackedNode],
+pub struct GridLayouter<'a> {
+    /// The  grid cells.
+    cells: &'a [PackedNode],
     /// The column tracks including gutter tracks.
     cols: Vec<TrackSizing>,
     /// The row tracks including gutter tracks.
@@ -102,7 +105,7 @@ struct GridLayouter<'a> {
     rcols: Vec<Length>,
     /// Rows in the current region.
     lrows: Vec<Row>,
-    /// Whether the grid should expand to fill the region.
+    /// Whether the grid itself should expand to fill the region.
     expand: Spec<bool>,
     /// The full height of the current region.
     full: Length,
@@ -127,19 +130,27 @@ enum Row {
 }
 
 impl<'a> GridLayouter<'a> {
-    /// Prepare grid layout by unifying content and gutter tracks.
-    fn new(grid: &'a GridNode, mut regions: Regions, styles: StyleChain<'a>) -> Self {
+    /// Create a new grid layouter.
+    ///
+    /// This prepares grid layout by unifying content and gutter tracks.
+    pub fn new(
+        tracks: Spec<&[TrackSizing]>,
+        gutter: Spec<&[TrackSizing]>,
+        cells: &'a [PackedNode],
+        regions: &Regions,
+        styles: StyleChain<'a>,
+    ) -> Self {
         let mut cols = vec![];
         let mut rows = vec![];
 
         // Number of content columns: Always at least one.
-        let c = grid.tracks.x.len().max(1);
+        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 = grid.children.len();
-            let given = grid.tracks.y.len();
+            let len = cells.len();
+            let given = tracks.y.len();
             let needed = len / c + (len % c).clamp(0, 1);
             given.max(needed)
         };
@@ -152,14 +163,14 @@ impl<'a> GridLayouter<'a> {
 
         // Collect content and gutter columns.
         for x in 0 .. c {
-            cols.push(get_or(&grid.tracks.x, x, auto));
-            cols.push(get_or(&grid.gutter.x, x, zero));
+            cols.push(get_or(tracks.x, x, auto));
+            cols.push(get_or(gutter.x, x, zero));
         }
 
         // Collect content and gutter rows.
         for y in 0 .. r {
-            rows.push(get_or(&grid.tracks.y, y, auto));
-            rows.push(get_or(&grid.gutter.y, y, zero));
+            rows.push(get_or(tracks.y, y, auto));
+            rows.push(get_or(gutter.y, y, zero));
         }
 
         // Remove superfluous gutter tracks.
@@ -173,10 +184,11 @@ impl<'a> GridLayouter<'a> {
 
         // We use the regions for auto row measurement. Since at that moment,
         // columns are already sized, we can enable horizontal expansion.
+        let mut regions = regions.clone();
         regions.expand = Spec::new(true, false);
 
         Self {
-            children: &grid.children,
+            cells,
             cols,
             rows,
             regions,
@@ -192,6 +204,32 @@ impl<'a> GridLayouter<'a> {
         }
     }
 
+    /// Determines the columns sizes and then layouts the grid row-by-row.
+    pub fn layout(mut self, ctx: &mut LayoutContext) -> Vec<Constrained<Arc<Frame>>> {
+        self.measure_columns(ctx);
+
+        for y in 0 .. self.rows.len() {
+            // Skip to next region if current one is full, but only for content
+            // rows, not for gutter rows.
+            if y % 2 == 0 && self.regions.is_full() {
+                self.finish_region(ctx);
+            }
+
+            match self.rows[y] {
+                TrackSizing::Auto => self.layout_auto_row(ctx, y),
+                TrackSizing::Linear(v) => self.layout_linear_row(ctx, v, y),
+                TrackSizing::Fractional(v) => {
+                    self.cts.exact.y = Some(self.full);
+                    self.lrows.push(Row::Fr(v, y));
+                    self.fr += v;
+                }
+            }
+        }
+
+        self.finish_region(ctx);
+        self.finished
+    }
+
     /// Determine all column sizes.
     fn measure_columns(&mut self, ctx: &mut LayoutContext) {
         enum Case {
@@ -354,30 +392,6 @@ impl<'a> GridLayouter<'a> {
         }
     }
 
-    /// Layout the grid row-by-row.
-    fn layout(mut self, ctx: &mut LayoutContext) -> Vec<Constrained<Arc<Frame>>> {
-        for y in 0 .. self.rows.len() {
-            // Skip to next region if current one is full, but only for content
-            // rows, not for gutter rows.
-            if y % 2 == 0 && self.regions.is_full() {
-                self.finish_region(ctx);
-            }
-
-            match self.rows[y] {
-                TrackSizing::Auto => self.layout_auto_row(ctx, y),
-                TrackSizing::Linear(v) => self.layout_linear_row(ctx, v, y),
-                TrackSizing::Fractional(v) => {
-                    self.cts.exact.y = Some(self.full);
-                    self.lrows.push(Row::Fr(v, y));
-                    self.fr += v;
-                }
-            }
-        }
-
-        self.finish_region(ctx);
-        self.finished
-    }
-
     /// Layout a row with automatic height. Such a row may break across multiple
     /// regions.
     fn layout_auto_row(&mut self, ctx: &mut LayoutContext, y: usize) {
@@ -599,7 +613,7 @@ impl<'a> GridLayouter<'a> {
         // 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.children.get((y / 2) * c + x / 2)
+            self.cells.get((y / 2) * c + x / 2)
         } else {
             None
         }
diff --git a/src/library/stack.rs b/src/library/stack.rs
index 394b0a38..0ff8e973 100644
--- a/src/library/stack.rs
+++ b/src/library/stack.rs
@@ -32,7 +32,29 @@ impl Layout for StackNode {
         regions: &Regions,
         styles: StyleChain,
     ) -> Vec<Constrained<Arc<Frame>>> {
-        StackLayouter::new(self, regions.clone(), styles).layout(ctx)
+        let mut layouter = StackLayouter::new(self.dir, regions);
+
+        // Spacing to insert before the next node.
+        let mut deferred = None;
+
+        for child in &self.children {
+            match child {
+                StackChild::Spacing(kind) => {
+                    layouter.layout_spacing(*kind);
+                    deferred = None;
+                }
+                StackChild::Node(node) => {
+                    if let Some(kind) = deferred {
+                        layouter.layout_spacing(kind);
+                    }
+
+                    layouter.layout_node(ctx, node, styles);
+                    deferred = self.spacing;
+                }
+            }
+        }
+
+        layouter.finish()
     }
 }
 
@@ -65,29 +87,23 @@ castable! {
 }
 
 /// Performs stack layout.
-struct StackLayouter<'a> {
-    /// The children of the stack.
-    children: &'a [StackChild],
+pub struct StackLayouter {
     /// The stacking direction.
     dir: Dir,
     /// The axis of the stacking direction.
     axis: SpecAxis,
-    /// The spacing between non-spacing children.
-    spacing: Option<SpacingKind>,
     /// The regions to layout children into.
     regions: Regions,
-    /// The inherited styles.
-    styles: StyleChain<'a>,
-    /// Whether the stack should expand to fill the region.
+    /// Whether the stack itself should expand to fill the region.
     expand: Spec<bool>,
-    /// The full size of `regions.current` that was available before we started
-    /// subtracting.
+    /// The full size of the current region that was available at the start.
     full: Size,
     /// The generic size used by the frames for the current region.
     used: Gen<Length>,
     /// The sum of fractional ratios in the current region.
     fr: Fractional,
-    /// Spacing and layouted nodes.
+    /// Already layouted items whose exact positions are not yet known due to
+    /// fractional spacing.
     items: Vec<StackItem>,
     /// Finished frames for previous regions.
     finished: Vec<Constrained<Arc<Frame>>>,
@@ -103,24 +119,21 @@ enum StackItem {
     Frame(Arc<Frame>, Align),
 }
 
-impl<'a> StackLayouter<'a> {
+impl StackLayouter {
     /// Create a new stack layouter.
-    fn new(stack: &'a StackNode, mut regions: Regions, styles: StyleChain<'a>) -> Self {
-        let dir = stack.dir;
+    pub fn new(dir: Dir, regions: &Regions) -> Self {
         let axis = dir.axis();
         let expand = regions.expand;
         let full = regions.current;
 
         // Disable expansion along the block axis for children.
+        let mut regions = regions.clone();
         regions.expand.set(axis, false);
 
         Self {
-            children: &stack.children,
             dir,
             axis,
-            spacing: stack.spacing,
             regions,
-            styles,
             expand,
             full,
             used: Gen::zero(),
@@ -130,67 +143,43 @@ impl<'a> StackLayouter<'a> {
         }
     }
 
-    /// Layout all children.
-    fn layout(mut self, ctx: &mut LayoutContext) -> Vec<Constrained<Arc<Frame>>> {
-        // Spacing to insert before the next node.
-        let mut deferred = None;
-
-        for child in self.children {
-            match *child {
-                StackChild::Spacing(kind) => {
-                    self.layout_spacing(kind);
-                    deferred = None;
-                }
-                StackChild::Node(ref node) => {
-                    if let Some(kind) = deferred {
-                        self.layout_spacing(kind);
-                    }
-
-                    if self.regions.is_full() {
-                        self.finish_region();
-                    }
-
-                    self.layout_node(ctx, node);
-                    deferred = self.spacing;
-                }
-            }
-        }
-
-        self.finish_region();
-        self.finished
-    }
-
-    /// Layout spacing.
-    fn layout_spacing(&mut self, spacing: SpacingKind) {
+    /// Add spacing along the spacing direction.
+    pub fn layout_spacing(&mut self, spacing: SpacingKind) {
         match spacing {
-            SpacingKind::Linear(v) => self.layout_absolute(v),
+            SpacingKind::Linear(v) => {
+                // Resolve the linear and limit it to the remaining space.
+                let resolved = v.resolve(self.regions.base.get(self.axis));
+                let remaining = self.regions.current.get_mut(self.axis);
+                let limited = resolved.min(*remaining);
+                *remaining -= limited;
+                self.used.main += limited;
+                self.items.push(StackItem::Absolute(resolved));
+            }
             SpacingKind::Fractional(v) => {
-                self.items.push(StackItem::Fractional(v));
                 self.fr += v;
+                self.items.push(StackItem::Fractional(v));
             }
         }
     }
 
-    /// Layout absolute spacing.
-    fn layout_absolute(&mut self, amount: Linear) {
-        // Resolve the linear, limiting it to the remaining available space.
-        let remaining = self.regions.current.get_mut(self.axis);
-        let resolved = amount.resolve(self.regions.base.get(self.axis));
-        let limited = resolved.min(*remaining);
-        *remaining -= limited;
-        self.used.main += limited;
-        self.items.push(StackItem::Absolute(resolved));
-    }
+    /// Layout an arbitrary node.
+    pub fn layout_node(
+        &mut self,
+        ctx: &mut LayoutContext,
+        node: &PackedNode,
+        styles: StyleChain,
+    ) {
+        if self.regions.is_full() {
+            self.finish_region();
+        }
 
-    /// Layout a node.
-    fn layout_node(&mut self, ctx: &mut LayoutContext, node: &PackedNode) {
         // Align nodes' block-axis alignment is respected by the stack node.
         let align = node
             .downcast::<AlignNode>()
             .and_then(|node| node.aligns.get(self.axis))
             .unwrap_or(self.dir.start().into());
 
-        let frames = node.layout(ctx, &self.regions, self.styles);
+        let frames = node.layout(ctx, &self.regions, styles);
         let len = frames.len();
         for (i, frame) in frames.into_iter().enumerate() {
             // Grow our size, shrink the region and save the frame for later.
@@ -206,8 +195,8 @@ impl<'a> StackLayouter<'a> {
         }
     }
 
-    /// Finish the frame for one region.
-    fn finish_region(&mut self) {
+    /// Advance to the next region.
+    pub fn finish_region(&mut self) {
         // Determine the size of the stack in this region dependening on whether
         // the region expands.
         let used = self.used.to_spec(self.axis);
@@ -228,12 +217,8 @@ impl<'a> StackLayouter<'a> {
         // Place all frames.
         for item in self.items.drain(..) {
             match item {
-                StackItem::Absolute(v) => {
-                    cursor += v;
-                }
-                StackItem::Fractional(v) => {
-                    cursor += v.resolve(self.fr, remaining);
-                }
+                StackItem::Absolute(v) => cursor += v,
+                StackItem::Fractional(v) => cursor += v.resolve(self.fr, remaining),
                 StackItem::Frame(frame, align) => {
                     if self.dir.is_positive() {
                         ruler = ruler.max(align);
@@ -270,4 +255,10 @@ impl<'a> StackLayouter<'a> {
         self.fr = Fractional::zero();
         self.finished.push(output.constrain(cts));
     }
+
+    /// Finish layouting and return the resulting frames.
+    pub fn finish(mut self) -> Vec<Constrained<Arc<Frame>>> {
+        self.finish_region();
+        self.finished
+    }
 }