Refactor execution context 🏗

- The execution context is a lot more structured: Instead of a magic stack of arbitrary objects there are static objects for pages, stacks and paragraphs
- Page softness/keeping mechanic is now a lot simpler than before

8 files changed, 190 insertions, 226 deletions

diff --git a/src/exec/context.rs b/src/exec/context.rs
index 3b0fd897..e10c28ff 100644
--- a/src/exec/context.rs
+++ b/src/exec/context.rs
@@ -1,11 +1,11 @@
-use std::any::Any;
+use std::mem;
 use std::rc::Rc;
 
 use fontdock::FontStyle;
 
 use super::*;
 use crate::diag::{Diag, DiagSet};
-use crate::geom::{Dir, Gen, LayoutAligns, LayoutDirs, Length, Linear, Sides, Size};
+use crate::geom::{Dir, Gen, Linear, Sides, Size};
 use crate::layout::{
     Node, NodePad, NodePages, NodePar, NodeSpacing, NodeStack, NodeText, Tree,
 };
@@ -20,17 +20,15 @@ pub struct ExecContext<'a> {
     pub state: State,
     /// Execution diagnostics.
     pub diags: DiagSet,
-    /// The finished page runs.
-    runs: Vec<NodePages>,
-    /// The stack of logical groups (paragraphs and such).
-    ///
-    /// Each entry contains metadata about the group and nodes that are at the
-    /// same level as the group, which will return to `inner` once the group is
-    /// finished.
-    groups: Vec<(Box<dyn Any>, Vec<Node>)>,
-    /// The nodes in the current innermost group
-    /// (whose metadata is in `groups.last()`).
-    inner: Vec<Node>,
+    /// The tree of finished page runs.
+    tree: Tree,
+    /// Metrics of the active page.
+    page: PageData,
+    /// The content of the active stack. This may be the top-level stack for the
+    /// page or a lower one created by [`exec`](Self::exec).
+    stack: NodeStack,
+    /// The content of the active paragraph.
+    par: NodePar,
 }
 
 impl<'a> ExecContext<'a> {
@@ -38,49 +36,53 @@ impl<'a> ExecContext<'a> {
     pub fn new(env: &'a mut Env, state: State) -> Self {
         Self {
             env,
-            state,
             diags: DiagSet::new(),
-            runs: vec![],
-            groups: vec![],
-            inner: vec![],
+            tree: Tree { runs: vec![] },
+            page: PageData::new(&state, Softness::Hard),
+            stack: NodeStack::new(&state),
+            par: NodePar::new(&state),
+            state,
         }
     }
 
-    /// Finish execution and return the created layout tree.
-    pub fn finish(self) -> Pass<Tree> {
-        assert!(self.groups.is_empty(), "unfinished group");
-        Pass::new(Tree { runs: self.runs }, self.diags)
-    }
-
     /// Add a diagnostic.
     pub fn diag(&mut self, diag: Diag) {
         self.diags.insert(diag);
     }
 
-    /// Push a layout node to the active group.
+    /// Set the directions.
     ///
-    /// Spacing nodes will be handled according to their [`Softness`].
-    pub fn push(&mut self, node: impl Into<Node>) {
-        let node = node.into();
-
-        if let Node::Spacing(this) = node {
-            if this.softness == Softness::Soft && self.inner.is_empty() {
-                return;
-            }
+    /// Produces an error if the axes aligned.
+    pub fn set_dirs(&mut self, new: Gen<Option<Spanned<Dir>>>) {
+        let dirs = Gen::new(
+            new.main.map(|s| s.v).unwrap_or(self.state.dirs.main),
+            new.cross.map(|s| s.v).unwrap_or(self.state.dirs.cross),
+        );
 
-            if let Some(&Node::Spacing(other)) = self.inner.last() {
-                if this.softness > other.softness {
-                    self.inner.pop();
-                } else if this.softness == Softness::Soft {
-                    return;
-                }
+        if dirs.main.axis() != dirs.cross.axis() {
+            self.state.dirs = dirs;
+        } else {
+            for dir in new.main.iter().chain(new.cross.iter()) {
+                self.diag(error!(dir.span, "aligned axis"));
             }
         }
+    }
 
-        self.inner.push(node);
+    /// Set the font to monospace.
+    pub fn set_monospace(&mut self) {
+        let families = self.state.font.families_mut();
+        families.list.insert(0, "monospace".to_string());
+        families.flatten();
     }
 
-    /// Push a normal word space.
+    /// Push a layout node into the active paragraph.
+    ///
+    /// Spacing nodes will be handled according to their [`Softness`].
+    pub fn push(&mut self, node: impl Into<Node>) {
+        push(&mut self.par.children, node.into());
+    }
+
+    /// Push a word space into the active paragraph.
     pub fn push_space(&mut self) {
         let em = self.state.font.font_size();
         self.push(NodeSpacing {
@@ -89,14 +91,14 @@ impl<'a> ExecContext<'a> {
         });
     }
 
-    /// Push text into the context.
+    /// Push text into the active paragraph.
     ///
     /// The text is split into lines at newlines.
     pub fn push_text(&mut self, text: &str) {
         let mut newline = false;
         for line in text.split_terminator(is_newline) {
             if newline {
-                self.apply_linebreak();
+                self.push_linebreak();
             }
 
             let node = self.make_text_node(line.into());
@@ -105,155 +107,39 @@ impl<'a> ExecContext<'a> {
         }
     }
 
-    /// Execute a template and return the result as a stack node.
-    pub fn exec(&mut self, template: &ValueTemplate) -> Node {
-        let dirs = self.state.dirs;
-        let aligns = self.state.aligns;
-
-        self.start_group(ContentGroup);
-        self.start_par_group();
-        template.exec(self);
-        self.end_par_group();
-        let children = self.end_group::<ContentGroup>().1;
-
-        NodeStack { dirs, aligns, children }.into()
-    }
-
-    /// Start a page group based on the active page state.
-    ///
-    /// The `softness` is a hint on whether empty pages should be kept in the
-    /// output.
-    ///
-    /// This also starts an inner paragraph.
-    pub fn start_page_group(&mut self, softness: Softness) {
-        self.start_group(PageGroup {
-            size: self.state.page.size,
-            padding: self.state.page.margins(),
-            dirs: self.state.dirs,
-            aligns: self.state.aligns,
-            softness,
-        });
-        self.start_par_group();
-    }
-
-    /// End a page group, returning its [`Softness`].
-    ///
-    /// Whether the page is kept when it's empty is decided by `keep_empty`
-    /// based on its softness. If kept, the page is pushed to the finished page
-    /// runs.
-    ///
-    /// This also ends an inner paragraph.
-    pub fn end_page_group<F>(&mut self, keep_empty: F) -> Softness
-    where
-        F: FnOnce(Softness) -> bool,
-    {
-        self.end_par_group();
-        let (group, children) = self.end_group::<PageGroup>();
-        if !children.is_empty() || keep_empty(group.softness) {
-            self.runs.push(NodePages {
-                size: group.size,
-                child: NodePad {
-                    padding: group.padding,
-                    child: NodeStack {
-                        dirs: group.dirs,
-                        aligns: group.aligns,
-                        children,
-                    }
-                    .into(),
-                }
-                .into(),
-            })
-        }
-        group.softness
+    /// Apply a forced line break.
+    pub fn push_linebreak(&mut self) {
+        self.finish_par();
     }
 
-    /// Start a paragraph group based on the active text state.
-    pub fn start_par_group(&mut self) {
+    /// Apply a forced paragraph break.
+    pub fn push_parbreak(&mut self) {
         let em = self.state.font.font_size();
-        self.start_group(ParGroup {
-            dirs: self.state.dirs,
-            aligns: self.state.aligns,
-            line_spacing: self.state.par.line_spacing.resolve(em),
+        self.push_into_stack(NodeSpacing {
+            amount: self.state.par.par_spacing.resolve(em),
+            softness: Softness::Soft,
         });
     }
 
-    /// End a paragraph group and push it to its parent group if it's not empty.
-    pub fn end_par_group(&mut self) {
-        let (group, children) = self.end_group::<ParGroup>();
-        if !children.is_empty() {
-            self.push(NodePar {
-                dirs: group.dirs,
-                aligns: group.aligns,
-                line_spacing: group.line_spacing,
-                children,
-            });
-        }
+    /// Push a node directly into the stack above the paragraph. This finishes
+    /// the active paragraph and starts a new one.
+    pub fn push_into_stack(&mut self, node: impl Into<Node>) {
+        self.finish_par();
+        push(&mut self.stack.children, node.into());
     }
 
-    /// Start a layouting group.
-    ///
-    /// All further calls to [`push`](Self::push) will collect nodes for this
-    /// group. The given metadata will be returned alongside the collected nodes
-    /// in a matching call to [`end_group`](Self::end_group).
-    fn start_group<T: 'static>(&mut self, meta: T) {
-        self.groups.push((Box::new(meta), std::mem::take(&mut self.inner)));
-    }
-
-    /// End a layouting group started with [`start_group`](Self::start_group).
-    ///
-    /// This returns the stored metadata and the collected nodes.
-    #[track_caller]
-    fn end_group<T: 'static>(&mut self) -> (T, Vec<Node>) {
-        if let Some(&Node::Spacing(spacing)) = self.inner.last() {
-            if spacing.softness == Softness::Soft {
-                self.inner.pop();
-            }
-        }
-
-        let (any, outer) = self.groups.pop().expect("no pushed group");
-        let group = *any.downcast::<T>().expect("bad group type");
-        (group, std::mem::replace(&mut self.inner, outer))
-    }
-
-    /// Set the directions if they would apply to different axes, producing an
-    /// appropriate error otherwise.
-    pub fn set_dirs(&mut self, new: Gen<Option<Spanned<Dir>>>) {
-        let dirs = Gen::new(
-            new.main.map(|s| s.v).unwrap_or(self.state.dirs.main),
-            new.cross.map(|s| s.v).unwrap_or(self.state.dirs.cross),
-        );
-
-        if dirs.main.axis() != dirs.cross.axis() {
-            self.state.dirs = dirs;
-        } else {
-            for dir in new.main.iter().chain(new.cross.iter()) {
-                self.diag(error!(dir.span, "aligned axis"));
-            }
-        }
-    }
+    /// Execute a template and return the result as a stack node.
+    pub fn exec(&mut self, template: &ValueTemplate) -> NodeStack {
+        let prev_par = mem::replace(&mut self.par, NodePar::new(&self.state));
+        let prev_stack = mem::replace(&mut self.stack, NodeStack::new(&self.state));
 
-    /// Set the font to monospace.
-    pub fn apply_monospace(&mut self) {
-        let families = self.state.font.families_mut();
-        families.list.insert(0, "monospace".to_string());
-        families.flatten();
-    }
+        template.exec(self);
+        let stack = self.finish_stack();
 
-    /// Apply a forced line break.
-    pub fn apply_linebreak(&mut self) {
-        self.end_par_group();
-        self.start_par_group();
-    }
+        self.par = prev_par;
+        self.stack = prev_stack;
 
-    /// Apply a forced paragraph break.
-    pub fn apply_parbreak(&mut self) {
-        self.end_par_group();
-        let em = self.state.font.font_size();
-        self.push(NodeSpacing {
-            amount: self.state.par.par_spacing.resolve(em),
-            softness: Softness::Soft,
-        });
-        self.start_par_group();
+        stack
     }
 
     /// Construct a text node from the given string based on the active text
@@ -282,35 +168,113 @@ impl<'a> ExecContext<'a> {
             variant,
         }
     }
+
+    /// Finish the active paragraph.
+    fn finish_par(&mut self) {
+        let mut par = mem::replace(&mut self.par, NodePar::new(&self.state));
+        trim(&mut par.children);
+
+        if !par.children.is_empty() {
+            self.stack.children.push(par.into());
+        }
+    }
+
+    /// Finish the active stack.
+    fn finish_stack(&mut self) -> NodeStack {
+        self.finish_par();
+
+        let mut stack = mem::replace(&mut self.stack, NodeStack::new(&self.state));
+        trim(&mut stack.children);
+
+        stack
+    }
+
+    /// Finish the active page.
+    pub fn finish_page(&mut self, keep: bool, new_softnes: Softness) {
+        let stack = self.finish_stack();
+        let data = mem::replace(&mut self.page, PageData::new(&self.state, new_softnes));
+        if !stack.children.is_empty() || (keep && data.softness == Softness::Hard) {
+            self.tree.runs.push(NodePages {
+                size: data.size,
+                child: NodePad {
+                    padding: data.padding,
+                    child: stack.into(),
+                }
+                .into(),
+            });
+        }
+    }
+
+    /// Finish execution and return the created layout tree.
+    pub fn finish(mut self) -> Pass<Tree> {
+        self.finish_page(true, Softness::Soft);
+        Pass::new(self.tree, self.diags)
+    }
 }
 
-/// Defines how an item interacts with surrounding items.
-#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
-pub enum Softness {
-    /// A soft item can be skipped in some circumstances.
-    Soft,
-    /// A hard item is always retained.
-    Hard,
+/// Push a node into a list, taking care of spacing softness.
+fn push(nodes: &mut Vec<Node>, node: Node) {
+    if let Node::Spacing(spacing) = node {
+        if spacing.softness == Softness::Soft && nodes.is_empty() {
+            return;
+        }
+
+        if let Some(&Node::Spacing(other)) = nodes.last() {
+            if spacing.softness > other.softness {
+                nodes.pop();
+            } else if spacing.softness == Softness::Soft {
+                return;
+            }
+        }
+    }
+
+    nodes.push(node);
+}
+
+/// Remove trailing soft spacing from a node list.
+fn trim(nodes: &mut Vec<Node>) {
+    if let Some(&Node::Spacing(spacing)) = nodes.last() {
+        if spacing.softness == Softness::Soft {
+            nodes.pop();
+        }
+    }
 }
 
-/// A group for a page run.
 #[derive(Debug)]
-struct PageGroup {
-    dirs: LayoutDirs,
-    aligns: LayoutAligns,
+struct PageData {
     size: Size,
     padding: Sides<Linear>,
     softness: Softness,
 }
 
-/// A group for generic content.
-#[derive(Debug)]
-struct ContentGroup;
+impl PageData {
+    fn new(state: &State, softness: Softness) -> Self {
+        Self {
+            size: state.page.size,
+            padding: state.page.margins(),
+            softness,
+        }
+    }
+}
 
-/// A group for a paragraph.
-#[derive(Debug)]
-struct ParGroup {
-    dirs: LayoutDirs,
-    aligns: LayoutAligns,
-    line_spacing: Length,
+impl NodeStack {
+    fn new(state: &State) -> Self {
+        Self {
+            dirs: state.dirs,
+            aligns: state.aligns,
+            children: vec![],
+        }
+    }
+}
+
+impl NodePar {
+    fn new(state: &State) -> Self {
+        let em = state.font.font_size();
+        Self {
+            dirs: state.dirs,
+            aligns: state.aligns,
+            line_spacing: state.par.line_spacing.resolve(em),
+            children: vec![],
+        }
+    }
 }
diff --git a/src/exec/mod.rs b/src/exec/mod.rs
index 58b5cdc0..45abca02 100644
--- a/src/exec/mod.rs
+++ b/src/exec/mod.rs
@@ -31,12 +31,19 @@ pub fn exec(
     state: State,
 ) -> Pass<layout::Tree> {
     let mut ctx = ExecContext::new(env, state);
-    ctx.start_page_group(Softness::Hard);
     tree.exec_with_map(&mut ctx, &map);
-    ctx.end_page_group(|s| s == Softness::Hard);
     ctx.finish()
 }
 
+/// Defines how an item interacts with surrounding items.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+pub enum Softness {
+    /// A soft item can be skipped in some circumstances.
+    Soft,
+    /// A hard item is always retained.
+    Hard,
+}
+
 /// Execute a node.
 ///
 /// This manipulates active styling and document state and produces layout
@@ -68,8 +75,8 @@ impl ExecWithMap for Node {
         match self {
             Node::Text(text) => ctx.push_text(text),
             Node::Space => ctx.push_space(),
-            Node::Linebreak => ctx.apply_linebreak(),
-            Node::Parbreak => ctx.apply_parbreak(),
+            Node::Linebreak => ctx.push_linebreak(),
+            Node::Parbreak => ctx.push_parbreak(),
             Node::Strong => ctx.state.font.strong ^= true,
             Node::Emph => ctx.state.font.emph ^= true,
             Node::Heading(heading) => heading.exec_with_map(ctx, map),
@@ -87,7 +94,7 @@ impl ExecWithMap for NodeHeading {
         ctx.state.font.strong = true;
 
         self.contents.exec_with_map(ctx, map);
-        ctx.apply_parbreak();
+        ctx.push_parbreak();
 
         ctx.state = prev;
     }
@@ -96,7 +103,7 @@ impl ExecWithMap for NodeHeading {
 impl Exec for NodeRaw {
     fn exec(&self, ctx: &mut ExecContext) {
         let prev = Rc::clone(&ctx.state.font.families);
-        ctx.apply_monospace();
+        ctx.set_monospace();
 
         let em = ctx.state.font.font_size();
         let line_spacing = ctx.state.par.line_spacing.resolve(em);
@@ -116,7 +123,7 @@ impl Exec for NodeRaw {
         }
 
         if self.block {
-            ctx.apply_parbreak();
+            ctx.push_parbreak();
         }
 
         // This is wrapped in a fixed node to make sure the stack fits to its
@@ -133,7 +140,7 @@ impl Exec for NodeRaw {
         });
 
         if self.block {
-            ctx.apply_parbreak();
+            ctx.push_parbreak();
         }
 
         ctx.state.font.families = prev;
@@ -153,7 +160,7 @@ impl Exec for Value {
                 // For values which can't be shown "naturally", we print
                 // the representation in monospace.
                 let prev = Rc::clone(&ctx.state.font.families);
-                ctx.apply_monospace();
+                ctx.set_monospace();
                 ctx.push_text(&pretty(other));
                 ctx.state.font.families = prev;
             }
diff --git a/src/library/align.rs b/src/library/align.rs
index 07566f2d..98feb3ef 100644
--- a/src/library/align.rs
+++ b/src/library/align.rs
@@ -89,15 +89,14 @@ pub fn align(ctx: &mut EvalContext, args: &mut ValueArgs) -> Value {
             }
         }
 
-        // If `had_center` wasn't flushed by now, it's the only argument and then we
-        // default to applying it to the cross axis.
+        // If `had_center` wasn't flushed by now, it's the only argument and
+        // then we default to applying it to the cross axis.
         if had_center {
             ctx.state.aligns.cross = Align::Center;
         }
 
         if ctx.state.aligns.main != snapshot.aligns.main {
-            ctx.end_par_group();
-            ctx.start_par_group();
+            ctx.push_linebreak();
         }
 
         if let Some(body) = &body {
diff --git a/src/library/pad.rs b/src/library/pad.rs
index 0bf93093..ba28bf8e 100644
--- a/src/library/pad.rs
+++ b/src/library/pad.rs
@@ -29,7 +29,7 @@ pub fn pad(ctx: &mut EvalContext, args: &mut ValueArgs) -> Value {
     Value::template("pad", move |ctx| {
         let snapshot = ctx.state.clone();
 
-        let child = ctx.exec(&body);
+        let child = ctx.exec(&body).into();
         ctx.push(NodePad { padding, child });
 
         ctx.state = snapshot;
diff --git a/src/library/page.rs b/src/library/page.rs
index 4ab92b31..963ab913 100644
--- a/src/library/page.rs
+++ b/src/library/page.rs
@@ -83,25 +83,20 @@ pub fn page(ctx: &mut EvalContext, args: &mut ValueArgs) -> Value {
         }
 
         ctx.set_dirs(Gen::new(main, cross));
+        ctx.finish_page(false, Softness::Hard);
 
-        let mut softness = ctx.end_page_group(|_| false);
         if let Some(body) = &body {
             // TODO: Restrict body to a single page?
-            ctx.start_page_group(Softness::Hard);
             body.exec(ctx);
-            ctx.end_page_group(|s| s == Softness::Hard);
-            softness = Softness::Soft;
             ctx.state = snapshot;
+            ctx.finish_page(true, Softness::Soft);
         }
-
-        ctx.start_page_group(softness);
     })
 }
 
 /// `pagebreak`: Start a new page.
 pub fn pagebreak(_: &mut EvalContext, _: &mut ValueArgs) -> Value {
     Value::template("pagebreak", move |ctx| {
-        ctx.end_page_group(|_| true);
-        ctx.start_page_group(Softness::Hard);
+        ctx.finish_page(true, Softness::Hard);
     })
 }
diff --git a/src/library/shapes.rs b/src/library/shapes.rs
index 9c466318..254a2285 100644
--- a/src/library/shapes.rs
+++ b/src/library/shapes.rs
@@ -31,7 +31,7 @@ pub fn box_(ctx: &mut EvalContext, args: &mut ValueArgs) -> Value {
 
         ctx.set_dirs(Gen::new(main, cross));
 
-        let child = ctx.exec(&body);
+        let child = ctx.exec(&body).into();
         let fixed = NodeFixed { width, height, child };
         if let Some(color) = color {
             ctx.push(NodeBackground {
diff --git a/src/library/spacing.rs b/src/library/spacing.rs
index 624890e9..3cc718b2 100644
--- a/src/library/spacing.rs
+++ b/src/library/spacing.rs
@@ -25,9 +25,7 @@ fn spacing(ctx: &mut EvalContext, args: &mut ValueArgs, axis: SpecAxis) -> Value
             let amount = linear.resolve(ctx.state.font.font_size());
             let spacing = NodeSpacing { amount, softness: Softness::Hard };
             if axis == ctx.state.dirs.main.axis() {
-                ctx.end_par_group();
-                ctx.push(spacing);
-                ctx.start_par_group();
+                ctx.push_into_stack(spacing);
             } else {
                 ctx.push(spacing);
             }
diff --git a/tests/typ/library/page.typ b/tests/typ/library/page.typ
index ac222e04..5123b876 100644
--- a/tests/typ/library/page.typ
+++ b/tests/typ/library/page.typ
@@ -50,6 +50,7 @@ Right to left!
 #page[First]
 #page[Second]
 #pagebreak()
+#pagebreak()
 Fourth
 #page[]
 Sixth