Set Rules Episode VI: Return of the Refactor

1 files changed, 112 insertions, 121 deletions

diff --git a/src/eval/node.rs b/src/eval/node.rs
index 4aacf4c1..5653beff 100644
--- a/src/eval/node.rs
+++ b/src/eval/node.rs
@@ -81,7 +81,7 @@ impl Node {
         if let Node::Block(packed) = self {
             packed
         } else {
-            let mut packer = NodePacker::new(true);
+            let mut packer = Packer::new(false);
             packer.walk(self, Styles::new());
             packer.into_block()
         }
@@ -89,7 +89,7 @@ impl Node {
 
     /// Lift to a document node, the root of the layout tree.
     pub fn into_document(self) -> DocumentNode {
-        let mut packer = NodePacker::new(false);
+        let mut packer = Packer::new(true);
         packer.walk(self, Styles::new());
         packer.into_document()
     }
@@ -126,49 +126,37 @@ impl AddAssign for Node {
 }
 
 /// Packs a [`Node`] into a flow or whole document.
-struct NodePacker {
-    /// Whether packing should produce a block-level node.
-    block: bool,
+struct Packer {
+    /// Whether this packer produces the top-level document.
+    top: bool,
     /// The accumulated page nodes.
     pages: Vec<PageNode>,
     /// The accumulated flow children.
-    flow: Vec<FlowChild>,
-    /// The common style properties of all items on the current flow.
-    flow_styles: Styles,
-    /// The kind of thing that was last added to the current flow.
-    flow_last: Last<FlowChild>,
+    flow: Builder<FlowChild>,
     /// The accumulated paragraph children.
-    par: Vec<ParChild>,
-    /// The common style properties of all items in the current paragraph.
-    par_styles: Styles,
-    /// The kind of thing that was last added to the current paragraph.
-    par_last: Last<ParChild>,
+    par: Builder<ParChild>,
 }
 
-impl NodePacker {
+impl Packer {
     /// Start a new node-packing session.
-    fn new(block: bool) -> Self {
+    fn new(top: bool) -> Self {
         Self {
-            block,
+            top,
             pages: vec![],
-            flow: vec![],
-            flow_styles: Styles::new(),
-            flow_last: Last::None,
-            par: vec![],
-            par_styles: Styles::new(),
-            par_last: Last::None,
+            flow: Builder::default(),
+            par: Builder::default(),
         }
     }
 
     /// Finish up and return the resulting flow.
     fn into_block(mut self) -> PackedNode {
-        self.finish_par();
-        FlowNode(self.flow).pack()
+        self.parbreak(None);
+        FlowNode(self.flow.children).pack()
     }
 
     /// Finish up and return the resulting document.
     fn into_document(mut self) -> DocumentNode {
-        self.pagebreak(true);
+        self.pagebreak();
         DocumentNode(self.pages)
     }
 
@@ -176,34 +164,49 @@ impl NodePacker {
     fn walk(&mut self, node: Node, styles: Styles) {
         match node {
             Node::Space => {
-                if self.is_flow_compatible(&styles) && self.is_par_compatible(&styles) {
-                    self.par_last.soft(ParChild::text(' ', styles));
-                }
+                // A text space is "soft", meaning that it can be eaten up by
+                // adjacent line breaks or explicit spacings.
+                self.par.last.soft(ParChild::text(' ', styles));
             }
             Node::Linebreak => {
-                self.par_last.hard();
+                // A line break eats up surrounding text spaces.
+                self.par.last.hard();
                 self.push_inline(ParChild::text('\n', styles));
-                self.par_last.hard();
+                self.par.last.hard();
             }
             Node::Parbreak => {
+                // An explicit paragraph break is styled according to the active
+                // styles (`Some(_)`) whereas paragraph breaks forced by
+                // incompatibility take their styles from the preceding
+                // paragraph.
                 self.parbreak(Some(styles));
             }
             Node::Pagebreak => {
-                self.pagebreak(true);
-                self.flow_styles = styles;
+                // We must set the flow styles after the page break such that an
+                // empty page created by two page breaks in a row has styles at
+                // all.
+                self.pagebreak();
+                self.flow.styles = styles;
             }
             Node::Text(text) => {
                 self.push_inline(ParChild::text(text, styles));
             }
             Node::Spacing(SpecAxis::Horizontal, kind) => {
-                self.par_last.hard();
+                // Just like a line break, explicit horizontal spacing eats up
+                // surrounding text spaces.
+                self.par.last.hard();
                 self.push_inline(ParChild::Spacing(SpacingNode { kind, styles }));
-                self.par_last.hard();
+                self.par.last.hard();
             }
             Node::Spacing(SpecAxis::Vertical, kind) => {
-                self.finish_par();
-                self.flow.push(FlowChild::Spacing(SpacingNode { kind, styles }));
-                self.flow_last.hard();
+                // Explicit vertical spacing ends the current paragraph and then
+                // discards the paragraph break.
+                self.parbreak(None);
+                self.make_flow_compatible(&styles);
+                self.flow
+                    .children
+                    .push(FlowChild::Spacing(SpacingNode { kind, styles }));
+                self.flow.last.hard();
             }
             Node::Inline(inline) => {
                 self.push_inline(ParChild::Node(inline.styled(styles)));
@@ -212,6 +215,8 @@ impl NodePacker {
                 self.push_block(block.styled(styles));
             }
             Node::Sequence(list) => {
+                // For a list of nodes, we apply the list's styles to each node
+                // individually.
                 for (node, mut inner) in list {
                     inner.apply(&styles);
                     self.walk(node, inner);
@@ -222,22 +227,22 @@ impl NodePacker {
 
     /// Insert an inline-level element into the current paragraph.
     fn push_inline(&mut self, child: ParChild) {
-        if let Some(child) = self.par_last.any() {
-            self.push_inline_impl(child);
+        if let Some(child) = self.par.last.any() {
+            self.push_coalescing(child);
         }
 
         // The node must be both compatible with the current page and the
         // current paragraph.
         self.make_flow_compatible(child.styles());
         self.make_par_compatible(child.styles());
-        self.push_inline_impl(child);
-        self.par_last = Last::Any;
+        self.push_coalescing(child);
+        self.par.last.any();
     }
 
     /// Push a paragraph child, coalescing text nodes with compatible styles.
-    fn push_inline_impl(&mut self, child: ParChild) {
+    fn push_coalescing(&mut self, child: ParChild) {
         if let ParChild::Text(right) = &child {
-            if let Some(ParChild::Text(left)) = self.par.last_mut() {
+            if let Some(ParChild::Text(left)) = self.par.children.last_mut() {
                 if left.styles.compatible(&right.styles, TextNode::has_property) {
                     left.text.push_str(&right.text);
                     return;
@@ -245,137 +250,122 @@ impl NodePacker {
             }
         }
 
-        self.par.push(child);
+        self.par.children.push(child);
     }
 
     /// Insert a block-level element into the current flow.
     fn push_block(&mut self, node: PackedNode) {
-        let mut is_placed = false;
-        if let Some(placed) = node.downcast::<PlacedNode>() {
-            is_placed = true;
-
-            // This prevents paragraph spacing after the placed node if it
-            // is completely out-of-flow.
-            if placed.out_of_flow() {
-                self.flow_last = Last::None;
-            }
-        }
+        let placed = node.is::<PlacedNode>();
 
         self.parbreak(None);
         self.make_flow_compatible(&node.styles);
-
-        if let Some(child) = self.flow_last.any() {
-            self.flow.push(child);
-        }
-
-        self.flow.push(FlowChild::Node(node));
+        self.flow.children.extend(self.flow.last.any());
+        self.flow.children.push(FlowChild::Node(node));
         self.parbreak(None);
 
-        // This prevents paragraph spacing between the placed node and
-        // the paragraph below it.
-        if is_placed {
-            self.flow_last = Last::None;
+        // Prevent paragraph spacing between the placed node and the paragraph
+        // below it.
+        if placed {
+            self.flow.last.hard();
         }
     }
 
     /// Advance to the next paragraph.
     fn parbreak(&mut self, break_styles: Option<Styles>) {
-        let styles = break_styles.unwrap_or_else(|| self.par_styles.clone());
-        self.finish_par();
-
-        // Insert paragraph spacing.
-        self.flow_last.soft(FlowChild::Parbreak(styles));
-    }
+        // Erase any styles that will be inherited anyway.
+        let Builder { mut children, styles, .. } = mem::take(&mut self.par);
+        for child in &mut children {
+            child.styles_mut().erase(&styles);
+        }
 
-    fn finish_par(&mut self) {
-        let mut children = mem::take(&mut self.par);
-        let styles = mem::take(&mut self.par_styles);
-        self.par_last = Last::None;
+        // For explicit paragraph breaks, `break_styles` is already `Some(_)`.
+        // For page breaks due to incompatibility, we fall back to the styles
+        // of the preceding paragraph.
+        let break_styles = break_styles.unwrap_or_else(|| styles.clone());
 
-        // No empty paragraphs.
+        // We don't want empty paragraphs.
         if !children.is_empty() {
-            // Erase any styles that will be inherited anyway.
-            for child in &mut children {
-                child.styles_mut().erase(&styles);
-            }
-
-            if let Some(child) = self.flow_last.any() {
-                self.flow.push(child);
-            }
-
             // The paragraph's children are all compatible with the page, so the
             // paragraph is too, meaning we don't need to check or intersect
             // anything here.
-            let node = ParNode(children).pack().styled(styles);
-            self.flow.push(FlowChild::Node(node));
+            let par = ParNode(children).pack().styled(styles);
+            self.flow.children.extend(self.flow.last.any());
+            self.flow.children.push(FlowChild::Node(par));
         }
+
+        // Insert paragraph spacing.
+        self.flow.last.soft(FlowChild::Break(break_styles));
     }
 
     /// Advance to the next page.
-    fn pagebreak(&mut self, keep: bool) {
-        if self.block {
-            return;
-        }
-
-        self.finish_par();
-
-        let styles = mem::take(&mut self.flow_styles);
-        let mut children = mem::take(&mut self.flow);
-        self.flow_last = Last::None;
+    fn pagebreak(&mut self) {
+        if self.top {
+            self.parbreak(None);
 
-        if keep || !children.is_empty() {
-            // Erase any styles that will be inherited anyway.
+            // Take the flow and erase any styles that will be inherited anyway.
+            let Builder { mut children, styles, .. } = mem::take(&mut self.flow);
             for child in &mut children {
                 child.styles_mut().erase(&styles);
             }
 
-            let node = PageNode { node: FlowNode(children).pack(), styles };
-            self.pages.push(node);
+            let flow = FlowNode(children).pack();
+            let page = PageNode { child: flow, styles };
+            self.pages.push(page);
         }
     }
 
     /// Break to a new paragraph if the `styles` contain paragraph styles that
     /// are incompatible with the current paragraph.
     fn make_par_compatible(&mut self, styles: &Styles) {
-        if self.par.is_empty() {
-            self.par_styles = styles.clone();
+        if self.par.children.is_empty() {
+            self.par.styles = styles.clone();
             return;
         }
 
-        if !self.is_par_compatible(styles) {
+        if !self.par.styles.compatible(&styles, ParNode::has_property) {
             self.parbreak(None);
-            self.par_styles = styles.clone();
+            self.par.styles = styles.clone();
             return;
         }
 
-        self.par_styles.intersect(&styles);
+        self.par.styles.intersect(&styles);
     }
 
     /// Break to a new page if the `styles` contain page styles that are
     /// incompatible with the current flow.
     fn make_flow_compatible(&mut self, styles: &Styles) {
-        if self.flow.is_empty() && self.par.is_empty() {
-            self.flow_styles = styles.clone();
+        if self.flow.children.is_empty() && self.par.children.is_empty() {
+            self.flow.styles = styles.clone();
             return;
         }
 
-        if !self.is_flow_compatible(styles) {
-            self.pagebreak(false);
-            self.flow_styles = styles.clone();
+        if self.top && !self.flow.styles.compatible(&styles, PageNode::has_property) {
+            self.pagebreak();
+            self.flow.styles = styles.clone();
             return;
         }
 
-        self.flow_styles.intersect(styles);
+        self.flow.styles.intersect(styles);
     }
+}
 
-    /// Whether the given styles are compatible with the current page.
-    fn is_par_compatible(&self, styles: &Styles) -> bool {
-        self.par_styles.compatible(&styles, ParNode::has_property)
-    }
+/// Container for building a flow or paragraph.
+struct Builder<T> {
+    /// The intersection of the style properties of all `children`.
+    styles: Styles,
+    /// The accumulated flow or paragraph children.
+    children: Vec<T>,
+    /// The kind of thing that was last added.
+    last: Last<T>,
+}
 
-    /// Whether the given styles are compatible with the current flow.
-    fn is_flow_compatible(&self, styles: &Styles) -> bool {
-        self.block || self.flow_styles.compatible(&styles, PageNode::has_property)
+impl<T> Default for Builder<T> {
+    fn default() -> Self {
+        Self {
+            styles: Styles::new(),
+            children: vec![],
+            last: Last::None,
+        }
     }
 }
 
@@ -383,6 +373,7 @@ impl NodePacker {
 enum Last<N> {
     None,
     Any,
+    Hard,
     Soft(N),
 }
 
@@ -401,6 +392,6 @@ impl<N> Last<N> {
     }
 
     fn hard(&mut self) {
-        *self = Self::None;
+        *self = Self::Hard;
     }
 }