Extract `model` module

6 files changed, 2044 insertions, 0 deletions

diff --git a/src/model/collapse.rs b/src/model/collapse.rs
new file mode 100644
index 00000000..5e7a25f8
--- /dev/null
+++ b/src/model/collapse.rs
@@ -0,0 +1,100 @@
+use super::{StyleChain, StyleVec, StyleVecBuilder};
+
+/// A wrapper around a [`StyleVecBuilder`] that allows to collapse items.
+pub struct CollapsingBuilder<'a, T> {
+    builder: StyleVecBuilder<'a, T>,
+    staged: Vec<(T, StyleChain<'a>, Option<u8>)>,
+    last: Last,
+}
+
+/// What the last non-ignorant item was.
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+enum Last {
+    Weak,
+    Destructive,
+    Supportive,
+}
+
+impl<'a, T> CollapsingBuilder<'a, T> {
+    /// Create a new style-vec builder.
+    pub fn new() -> Self {
+        Self {
+            builder: StyleVecBuilder::new(),
+            staged: vec![],
+            last: Last::Destructive,
+        }
+    }
+
+    /// Can only exist when there is at least one supportive item to its left
+    /// and to its right, with no destructive items or weak items in between to
+    /// its left and no destructive items in between to its right. There may be
+    /// ignorant items in between in both directions.
+    pub fn weak(&mut self, item: T, strength: u8, styles: StyleChain<'a>) {
+        if self.last != Last::Destructive {
+            if self.last == Last::Weak {
+                if let Some(i) = self
+                    .staged
+                    .iter()
+                    .position(|(.., prev)| prev.map_or(false, |p| p < strength))
+                {
+                    self.staged.remove(i);
+                } else {
+                    return;
+                }
+            }
+
+            self.staged.push((item, styles, Some(strength)));
+            self.last = Last::Weak;
+        }
+    }
+
+    /// Forces nearby weak items to collapse.
+    pub fn destructive(&mut self, item: T, styles: StyleChain<'a>) {
+        self.flush(false);
+        self.push(item, styles);
+        self.last = Last::Destructive;
+    }
+
+    /// Allows nearby weak items to exist.
+    pub fn supportive(&mut self, item: T, styles: StyleChain<'a>) {
+        self.flush(true);
+        self.push(item, styles);
+        self.last = Last::Supportive;
+    }
+
+    /// Has no influence on other items.
+    pub fn ignorant(&mut self, item: T, styles: StyleChain<'a>) {
+        self.staged.push((item, styles, None));
+    }
+
+    /// Iterate over the contained items.
+    pub fn items(&self) -> impl DoubleEndedIterator<Item = &T> {
+        self.builder.items().chain(self.staged.iter().map(|(item, ..)| item))
+    }
+
+    /// Return the finish style vec and the common prefix chain.
+    pub fn finish(mut self) -> (StyleVec<T>, StyleChain<'a>) {
+        self.flush(false);
+        self.builder.finish()
+    }
+
+    /// Push the staged items, filtering out weak items if `supportive` is false.
+    fn flush(&mut self, supportive: bool) {
+        for (item, styles, strength) in self.staged.drain(..) {
+            if supportive || strength.is_none() {
+                self.builder.push(item, styles);
+            }
+        }
+    }
+
+    /// Push a new item into the style vector.
+    fn push(&mut self, item: T, styles: StyleChain<'a>) {
+        self.builder.push(item, styles);
+    }
+}
+
+impl<'a, T> Default for CollapsingBuilder<'a, T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
diff --git a/src/model/content.rs b/src/model/content.rs
new file mode 100644
index 00000000..2465f0e3
--- /dev/null
+++ b/src/model/content.rs
@@ -0,0 +1,604 @@
+use std::fmt::Debug;
+use std::hash::Hash;
+use std::iter::Sum;
+use std::ops::{Add, AddAssign};
+
+use typed_arena::Arena;
+
+use super::{
+    CollapsingBuilder, Interruption, Key, Layout, LayoutNode, Show, ShowNode, StyleMap,
+    StyleVecBuilder,
+};
+use crate::diag::StrResult;
+use crate::library::layout::{FlowChild, FlowNode, PageNode, PlaceNode, Spacing};
+use crate::library::prelude::*;
+use crate::library::structure::{ListItem, ListKind, ListNode, ORDERED, UNORDERED};
+use crate::library::text::{DecoNode, ParChild, ParNode, UNDERLINE};
+use crate::util::EcoString;
+
+/// Composable representation of styled content.
+///
+/// This results from:
+/// - anything written between square brackets in Typst
+/// - any node constructor
+///
+/// Content is represented as a tree of nodes. There are two nodes of special
+/// interest:
+///
+/// 1. A `Styled` node attaches a style map to other content. For example, a
+///    single bold word could be represented as a `Styled(Text("Hello"),
+///    [TextNode::STRONG: true])` node.
+///
+/// 2. A `Sequence` node content combines other arbitrary content and is the
+///    representation of a "flow" of other nodes. So, when you write `[Hi] +
+///    [you]` in Typst, this type's [`Add`] implementation is invoked and the
+///    two [`Text`](Self::Text) nodes are combined into a single
+///    [`Sequence`](Self::Sequence) node. A sequence may contain nested
+///    sequences.
+#[derive(PartialEq, Clone, Hash)]
+pub enum Content {
+    /// A word space.
+    Space,
+    /// A forced line break. If soft (`true`), the preceding line can still be
+    /// justified, if hard (`false`) not.
+    Linebreak(bool),
+    /// Horizontal spacing.
+    Horizontal(Spacing),
+    /// Plain text.
+    Text(EcoString),
+    /// A smart quote, may be single (`false`) or double (`true`).
+    Quote(bool),
+    /// An inline-level node.
+    Inline(LayoutNode),
+    /// A paragraph break.
+    Parbreak,
+    /// A column break.
+    Colbreak,
+    /// Vertical spacing.
+    Vertical(Spacing),
+    /// A block-level node.
+    Block(LayoutNode),
+    /// An item in an unordered list.
+    List(ListItem),
+    /// An item in an ordered list.
+    Enum(ListItem),
+    /// A page break.
+    Pagebreak,
+    /// A page node.
+    Page(PageNode),
+    /// A node that can be realized with styles.
+    Show(ShowNode),
+    /// Content with attached styles.
+    Styled(Arc<(Self, StyleMap)>),
+    /// A sequence of multiple nodes.
+    Sequence(Arc<Vec<Self>>),
+}
+
+impl Content {
+    /// Create empty content.
+    pub fn new() -> Self {
+        Self::sequence(vec![])
+    }
+
+    /// Create content from an inline-level node.
+    pub fn inline<T>(node: T) -> Self
+    where
+        T: Layout + Debug + Hash + Sync + Send + 'static,
+    {
+        Self::Inline(node.pack())
+    }
+
+    /// Create content from a block-level node.
+    pub fn block<T>(node: T) -> Self
+    where
+        T: Layout + Debug + Hash + Sync + Send + 'static,
+    {
+        Self::Block(node.pack())
+    }
+
+    /// Create content from a showable node.
+    pub fn show<T>(node: T) -> Self
+    where
+        T: Show + Debug + Hash + Sync + Send + 'static,
+    {
+        Self::Show(node.pack())
+    }
+
+    /// Create a new sequence nodes from multiples nodes.
+    pub fn sequence(seq: Vec<Self>) -> Self {
+        if seq.len() == 1 {
+            seq.into_iter().next().unwrap()
+        } else {
+            Self::Sequence(Arc::new(seq))
+        }
+    }
+
+    /// Repeat this content `n` times.
+    pub fn repeat(&self, n: i64) -> StrResult<Self> {
+        let count = usize::try_from(n)
+            .map_err(|_| format!("cannot repeat this content {} times", n))?;
+
+        Ok(Self::sequence(vec![self.clone(); count]))
+    }
+
+    /// Style this content with a single style property.
+    pub fn styled<'k, K: Key<'k>>(mut self, key: K, value: K::Value) -> Self {
+        if let Self::Styled(styled) = &mut self {
+            if let Some((_, map)) = Arc::get_mut(styled) {
+                map.apply(key, value);
+                return self;
+            }
+        }
+
+        Self::Styled(Arc::new((self, StyleMap::with(key, value))))
+    }
+
+    /// Style this content with a full style map.
+    pub fn styled_with_map(mut self, styles: StyleMap) -> Self {
+        if styles.is_empty() {
+            return self;
+        }
+
+        if let Self::Styled(styled) = &mut self {
+            if let Some((_, map)) = Arc::get_mut(styled) {
+                map.apply_map(&styles);
+                return self;
+            }
+        }
+
+        Self::Styled(Arc::new((self, styles)))
+    }
+
+    /// Underline this content.
+    pub fn underlined(self) -> Self {
+        Self::show(DecoNode::<UNDERLINE>(self))
+    }
+
+    /// Return a node that is spaced apart at top and bottom.
+    pub fn spaced(self, above: Length, below: Length) -> Self {
+        if above.is_zero() && below.is_zero() {
+            return self;
+        }
+
+        let mut seq = vec![];
+        if !above.is_zero() {
+            seq.push(Content::Vertical(above.into()));
+        }
+
+        seq.push(self);
+
+        if !below.is_zero() {
+            seq.push(Content::Vertical(below.into()));
+        }
+
+        Self::sequence(seq)
+    }
+
+    /// Layout this content into a collection of pages.
+    pub fn layout(&self, ctx: &mut Context) -> TypResult<Vec<Arc<Frame>>> {
+        let sya = Arena::new();
+        let tpa = Arena::new();
+
+        let styles = ctx.styles.clone();
+        let styles = StyleChain::with_root(&styles);
+
+        let mut builder = Builder::new(&sya, &tpa, true);
+        builder.process(ctx, self, styles)?;
+        builder.finish(ctx, styles)?;
+
+        let mut frames = vec![];
+        let (pages, shared) = builder.pages.unwrap().finish();
+
+        for (page, map) in pages.iter() {
+            let number = 1 + frames.len();
+            frames.extend(page.layout(ctx, number, map.chain(&shared))?);
+        }
+
+        Ok(frames)
+    }
+}
+
+impl Layout for Content {
+    fn layout(
+        &self,
+        ctx: &mut Context,
+        regions: &Regions,
+        styles: StyleChain,
+    ) -> TypResult<Vec<Arc<Frame>>> {
+        let sya = Arena::new();
+        let tpa = Arena::new();
+
+        let mut builder = Builder::new(&sya, &tpa, false);
+        builder.process(ctx, self, styles)?;
+        builder.finish(ctx, styles)?;
+
+        let (flow, shared) = builder.flow.finish();
+        FlowNode(flow).layout(ctx, regions, shared)
+    }
+
+    fn pack(self) -> LayoutNode {
+        match self {
+            Content::Block(node) => node,
+            other => LayoutNode::new(other),
+        }
+    }
+}
+
+impl Default for Content {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Debug for Content {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        match self {
+            Self::Space => f.pad("Space"),
+            Self::Linebreak(soft) => write!(f, "Linebreak({soft})"),
+            Self::Horizontal(kind) => write!(f, "Horizontal({kind:?})"),
+            Self::Text(text) => write!(f, "Text({text:?})"),
+            Self::Quote(double) => write!(f, "Quote({double})"),
+            Self::Inline(node) => {
+                f.write_str("Inline(")?;
+                node.fmt(f)?;
+                f.write_str(")")
+            }
+            Self::Parbreak => f.pad("Parbreak"),
+            Self::Colbreak => f.pad("Colbreak"),
+            Self::Vertical(kind) => write!(f, "Vertical({kind:?})"),
+            Self::Block(node) => {
+                f.write_str("Block(")?;
+                node.fmt(f)?;
+                f.write_str(")")
+            }
+            Self::List(item) => {
+                f.write_str("- ")?;
+                item.body.fmt(f)
+            }
+            Self::Enum(item) => {
+                if let Some(number) = item.number {
+                    write!(f, "{}", number)?;
+                }
+                f.write_str(". ")?;
+                item.body.fmt(f)
+            }
+            Self::Pagebreak => f.pad("Pagebreak"),
+            Self::Page(page) => page.fmt(f),
+            Self::Show(node) => {
+                f.write_str("Show(")?;
+                node.fmt(f)?;
+                f.write_str(")")
+            }
+            Self::Styled(styled) => {
+                let (sub, map) = styled.as_ref();
+                map.fmt(f)?;
+                sub.fmt(f)
+            }
+            Self::Sequence(seq) => f.debug_list().entries(seq.iter()).finish(),
+        }
+    }
+}
+
+impl Add for Content {
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self::Output {
+        Self::Sequence(match (self, rhs) {
+            (Self::Sequence(mut lhs), Self::Sequence(rhs)) => {
+                let mutable = Arc::make_mut(&mut lhs);
+                match Arc::try_unwrap(rhs) {
+                    Ok(vec) => mutable.extend(vec),
+                    Err(rc) => mutable.extend(rc.iter().cloned()),
+                }
+                lhs
+            }
+            (Self::Sequence(mut lhs), rhs) => {
+                Arc::make_mut(&mut lhs).push(rhs);
+                lhs
+            }
+            (lhs, Self::Sequence(mut rhs)) => {
+                Arc::make_mut(&mut rhs).insert(0, lhs);
+                rhs
+            }
+            (lhs, rhs) => Arc::new(vec![lhs, rhs]),
+        })
+    }
+}
+
+impl AddAssign for Content {
+    fn add_assign(&mut self, rhs: Self) {
+        *self = std::mem::take(self) + rhs;
+    }
+}
+
+impl Sum for Content {
+    fn sum<I: Iterator<Item = Self>>(iter: I) -> Self {
+        Self::sequence(iter.collect())
+    }
+}
+
+/// Builds a flow or page nodes from content.
+struct Builder<'a> {
+    /// An arena where intermediate style chains are stored.
+    sya: &'a Arena<StyleChain<'a>>,
+    /// An arena where intermediate content resulting from show rules is stored.
+    tpa: &'a Arena<Content>,
+    /// The already built page runs.
+    pages: Option<StyleVecBuilder<'a, PageNode>>,
+    /// The currently built list.
+    list: Option<ListBuilder<'a>>,
+    /// The currently built flow.
+    flow: CollapsingBuilder<'a, FlowChild>,
+    /// The currently built paragraph.
+    par: CollapsingBuilder<'a, ParChild>,
+    /// Whether to keep the next page even if it is empty.
+    keep_next: bool,
+}
+
+/// Builds an unordered or ordered list from items.
+struct ListBuilder<'a> {
+    styles: StyleChain<'a>,
+    kind: ListKind,
+    items: Vec<ListItem>,
+    tight: bool,
+    staged: Vec<(&'a Content, StyleChain<'a>)>,
+}
+
+impl<'a> Builder<'a> {
+    /// Prepare the builder.
+    fn new(sya: &'a Arena<StyleChain<'a>>, tpa: &'a Arena<Content>, top: bool) -> Self {
+        Self {
+            sya,
+            tpa,
+            pages: top.then(|| StyleVecBuilder::new()),
+            flow: CollapsingBuilder::new(),
+            list: None,
+            par: CollapsingBuilder::new(),
+            keep_next: true,
+        }
+    }
+
+    /// Process content.
+    fn process(
+        &mut self,
+        ctx: &mut Context,
+        content: &'a Content,
+        styles: StyleChain<'a>,
+    ) -> TypResult<()> {
+        if let Some(builder) = &mut self.list {
+            match content {
+                Content::Space => {
+                    builder.staged.push((content, styles));
+                    return Ok(());
+                }
+                Content::Parbreak => {
+                    builder.staged.push((content, styles));
+                    return Ok(());
+                }
+                Content::List(item) if builder.kind == UNORDERED => {
+                    builder.tight &=
+                        builder.staged.iter().all(|&(t, _)| *t != Content::Parbreak);
+                    builder.staged.clear();
+                    builder.items.push(item.clone());
+                    return Ok(());
+                }
+                Content::Enum(item) if builder.kind == ORDERED => {
+                    builder.tight &=
+                        builder.staged.iter().all(|&(t, _)| *t != Content::Parbreak);
+                    builder.staged.clear();
+                    builder.items.push(item.clone());
+                    return Ok(());
+                }
+                _ => self.finish_list(ctx)?,
+            }
+        }
+
+        match content {
+            Content::Space => {
+                self.par.weak(ParChild::Text(' '.into()), 0, styles);
+            }
+            Content::Linebreak(soft) => {
+                let c = if *soft { '\u{2028}' } else { '\n' };
+                self.par.destructive(ParChild::Text(c.into()), styles);
+            }
+            Content::Horizontal(kind) => {
+                let child = ParChild::Spacing(*kind);
+                if kind.is_fractional() {
+                    self.par.destructive(child, styles);
+                } else {
+                    self.par.ignorant(child, styles);
+                }
+            }
+            Content::Quote(double) => {
+                self.par.supportive(ParChild::Quote(*double), styles);
+            }
+            Content::Text(text) => {
+                self.par.supportive(ParChild::Text(text.clone()), styles);
+            }
+            Content::Inline(node) => {
+                self.par.supportive(ParChild::Node(node.clone()), styles);
+            }
+            Content::Parbreak => {
+                self.finish_par(styles);
+                self.flow.weak(FlowChild::Parbreak, 1, styles);
+            }
+            Content::Colbreak => {
+                self.finish_par(styles);
+                self.flow.destructive(FlowChild::Colbreak, styles);
+            }
+            Content::Vertical(kind) => {
+                self.finish_par(styles);
+                let child = FlowChild::Spacing(*kind);
+                if kind.is_fractional() {
+                    self.flow.destructive(child, styles);
+                } else {
+                    self.flow.ignorant(child, styles);
+                }
+            }
+            Content::Block(node) => {
+                self.finish_par(styles);
+                let child = FlowChild::Node(node.clone());
+                if node.is::<PlaceNode>() {
+                    self.flow.ignorant(child, styles);
+                } else {
+                    self.flow.supportive(child, styles);
+                }
+                self.finish_par(styles);
+            }
+            Content::List(item) => {
+                self.list = Some(ListBuilder {
+                    styles,
+                    kind: UNORDERED,
+                    items: vec![item.clone()],
+                    tight: true,
+                    staged: vec![],
+                });
+            }
+            Content::Enum(item) => {
+                self.list = Some(ListBuilder {
+                    styles,
+                    kind: ORDERED,
+                    items: vec![item.clone()],
+                    tight: true,
+                    staged: vec![],
+                });
+            }
+            Content::Pagebreak => {
+                self.finish_page(ctx, true, true, styles)?;
+            }
+            Content::Page(page) => {
+                self.finish_page(ctx, false, false, styles)?;
+                if let Some(pages) = &mut self.pages {
+                    pages.push(page.clone(), styles);
+                }
+            }
+            Content::Show(node) => {
+                let id = node.id();
+                let realized = match styles.realize(ctx, node)? {
+                    Some(content) => content,
+                    None => node.realize(ctx, styles)?,
+                };
+                let content = node.finalize(ctx, styles, realized)?;
+                let stored = self.tpa.alloc(content);
+                self.process(ctx, stored, styles.unscoped(id))?;
+            }
+            Content::Styled(styled) => {
+                let (sub, map) = styled.as_ref();
+                let stored = self.sya.alloc(styles);
+                let styles = map.chain(stored);
+
+                let interruption = map.interruption();
+                match interruption {
+                    Some(Interruption::Page) => {
+                        self.finish_page(ctx, false, true, styles)?
+                    }
+                    Some(Interruption::Par) => self.finish_par(styles),
+                    None => {}
+                }
+
+                self.process(ctx, sub, styles)?;
+
+                match interruption {
+                    Some(Interruption::Page) => {
+                        self.finish_page(ctx, true, false, styles)?
+                    }
+                    Some(Interruption::Par) => self.finish_par(styles),
+                    None => {}
+                }
+            }
+            Content::Sequence(seq) => {
+                for sub in seq.iter() {
+                    self.process(ctx, sub, styles)?;
+                }
+            }
+        }
+
+        Ok(())
+    }
+
+    /// Finish the currently built paragraph.
+    fn finish_par(&mut self, styles: StyleChain<'a>) {
+        let (mut par, shared) = std::mem::take(&mut self.par).finish();
+        if !par.is_empty() {
+            // Paragraph indent should only apply if the paragraph starts with
+            // text and follows directly after another paragraph.
+            let indent = shared.get(ParNode::INDENT);
+            if !indent.is_zero()
+                && par
+                    .items()
+                    .find_map(|child| match child {
+                        ParChild::Spacing(_) => None,
+                        ParChild::Text(_) | ParChild::Quote(_) => Some(true),
+                        ParChild::Node(_) => Some(false),
+                    })
+                    .unwrap_or_default()
+                && self
+                    .flow
+                    .items()
+                    .rev()
+                    .find_map(|child| match child {
+                        FlowChild::Leading => None,
+                        FlowChild::Parbreak => None,
+                        FlowChild::Node(node) => Some(node.is::<ParNode>()),
+                        FlowChild::Spacing(_) => Some(false),
+                        FlowChild::Colbreak => Some(false),
+                    })
+                    .unwrap_or_default()
+            {
+                par.push_front(ParChild::Spacing(indent.into()));
+            }
+
+            let node = ParNode(par).pack();
+            self.flow.supportive(FlowChild::Node(node), shared);
+        }
+        self.flow.weak(FlowChild::Leading, 0, styles);
+    }
+
+    /// Finish the currently built list.
+    fn finish_list(&mut self, ctx: &mut Context) -> TypResult<()> {
+        let ListBuilder { styles, kind, items, tight, staged } = match self.list.take() {
+            Some(list) => list,
+            None => return Ok(()),
+        };
+
+        let content = match kind {
+            UNORDERED => Content::show(ListNode::<UNORDERED> { start: 1, tight, items }),
+            ORDERED | _ => Content::show(ListNode::<ORDERED> { start: 1, tight, items }),
+        };
+
+        let stored = self.tpa.alloc(content);
+        self.process(ctx, stored, styles)?;
+        for (content, styles) in staged {
+            self.process(ctx, content, styles)?;
+        }
+
+        Ok(())
+    }
+
+    /// Finish the currently built page run.
+    fn finish_page(
+        &mut self,
+        ctx: &mut Context,
+        keep_last: bool,
+        keep_next: bool,
+        styles: StyleChain<'a>,
+    ) -> TypResult<()> {
+        self.finish_list(ctx)?;
+        self.finish_par(styles);
+        if let Some(pages) = &mut self.pages {
+            let (flow, shared) = std::mem::take(&mut self.flow).finish();
+            if !flow.is_empty() || (keep_last && self.keep_next) {
+                let styles = if flow.is_empty() { styles } else { shared };
+                let node = PageNode(FlowNode(flow).pack());
+                pages.push(node, styles);
+            }
+        }
+        self.keep_next = keep_next;
+        Ok(())
+    }
+
+    /// Finish everything.
+    fn finish(&mut self, ctx: &mut Context, styles: StyleChain<'a>) -> TypResult<()> {
+        self.finish_page(ctx, true, false, styles)
+    }
+}
diff --git a/src/model/layout.rs b/src/model/layout.rs
new file mode 100644
index 00000000..da6290ae
--- /dev/null
+++ b/src/model/layout.rs
@@ -0,0 +1,383 @@
+//! Layouting infrastructure.
+
+use std::any::Any;
+use std::fmt::{self, Debug, Formatter};
+use std::hash::Hash;
+use std::sync::Arc;
+
+use super::{Barrier, Resolve, StyleChain};
+use crate::diag::TypResult;
+use crate::eval::{RawAlign, RawLength};
+use crate::frame::{Element, Frame, Geometry};
+use crate::geom::{Align, Length, Paint, Point, Relative, Sides, Size, Spec, Stroke};
+use crate::library::graphics::MoveNode;
+use crate::library::layout::{AlignNode, PadNode};
+use crate::util::Prehashed;
+use crate::Context;
+
+/// A node that can be layouted into a sequence of regions.
+///
+/// Layout return one frame per used region alongside constraints that define
+/// whether the result is reusable in other regions.
+pub trait Layout: 'static {
+    /// Layout this node into the given regions, producing constrained frames.
+    fn layout(
+        &self,
+        ctx: &mut Context,
+        regions: &Regions,
+        styles: StyleChain,
+    ) -> TypResult<Vec<Arc<Frame>>>;
+
+    /// Convert to a packed node.
+    fn pack(self) -> LayoutNode
+    where
+        Self: Debug + Hash + Sized + Sync + Send + 'static,
+    {
+        LayoutNode::new(self)
+    }
+}
+
+/// A sequence of regions to layout into.
+#[derive(Debug, Clone, Hash)]
+pub struct Regions {
+    /// The (remaining) size of the first region.
+    pub first: Size,
+    /// The base size for relative sizing.
+    pub base: Size,
+    /// The height of followup regions. The width is the same for all regions.
+    pub backlog: Vec<Length>,
+    /// The height of the final region that is repeated once the backlog is
+    /// drained. The width is the same for all regions.
+    pub last: Option<Length>,
+    /// Whether nodes should expand to fill the regions instead of shrinking to
+    /// fit the content.
+    pub expand: Spec<bool>,
+}
+
+impl Regions {
+    /// Create a new region sequence with exactly one region.
+    pub fn one(size: Size, base: Size, expand: Spec<bool>) -> Self {
+        Self {
+            first: size,
+            base,
+            backlog: vec![],
+            last: None,
+            expand,
+        }
+    }
+
+    /// Create a new sequence of same-size regions that repeats indefinitely.
+    pub fn repeat(size: Size, base: Size, expand: Spec<bool>) -> Self {
+        Self {
+            first: size,
+            base,
+            backlog: vec![],
+            last: Some(size.y),
+            expand,
+        }
+    }
+
+    /// Create new regions where all sizes are mapped with `f`.
+    ///
+    /// Note that since all regions must have the same width, the width returned
+    /// by `f` is ignored for the backlog and the final region.
+    pub fn map<F>(&self, mut f: F) -> Self
+    where
+        F: FnMut(Size) -> Size,
+    {
+        let x = self.first.x;
+        Self {
+            first: f(self.first),
+            base: f(self.base),
+            backlog: self.backlog.iter().map(|&y| f(Size::new(x, y)).y).collect(),
+            last: self.last.map(|y| f(Size::new(x, y)).y),
+            expand: self.expand,
+        }
+    }
+
+    /// Whether the first region is full and a region break is called for.
+    pub fn is_full(&self) -> bool {
+        Length::zero().fits(self.first.y) && !self.in_last()
+    }
+
+    /// Whether the first region is the last usable region.
+    ///
+    /// If this is true, calling `next()` will have no effect.
+    pub fn in_last(&self) -> bool {
+        self.backlog.len() == 0 && self.last.map_or(true, |height| self.first.y == height)
+    }
+
+    /// Advance to the next region if there is any.
+    pub fn next(&mut self) {
+        if let Some(height) = (!self.backlog.is_empty())
+            .then(|| self.backlog.remove(0))
+            .or(self.last)
+        {
+            self.first.y = height;
+            self.base.y = height;
+        }
+    }
+
+    /// An iterator that returns the sizes of the first and all following
+    /// regions, equivalently to what would be produced by calling
+    /// [`next()`](Self::next) repeatedly until all regions are exhausted.
+    /// This iterater may be infinite.
+    pub fn iter(&self) -> impl Iterator<Item = Size> + '_ {
+        let first = std::iter::once(self.first);
+        let backlog = self.backlog.iter();
+        let last = self.last.iter().cycle();
+        first.chain(backlog.chain(last).map(|&h| Size::new(self.first.x, h)))
+    }
+}
+
+/// A type-erased layouting node with a precomputed hash.
+#[derive(Clone, Hash)]
+pub struct LayoutNode(Arc<Prehashed<dyn Bounds>>);
+
+impl LayoutNode {
+    /// Pack any layoutable node.
+    pub fn new<T>(node: T) -> Self
+    where
+        T: Layout + Debug + Hash + Sync + Send + 'static,
+    {
+        Self(Arc::new(Prehashed::new(node)))
+    }
+
+    /// Check whether the contained node is a specific layout node.
+    pub fn is<T: 'static>(&self) -> bool {
+        (**self.0).as_any().is::<T>()
+    }
+
+    /// A barrier for the node.
+    pub fn barrier(&self) -> Barrier {
+        (**self.0).barrier()
+    }
+
+    /// Try to downcast to a specific layout node.
+    pub fn downcast<T>(&self) -> Option<&T>
+    where
+        T: Layout + Debug + Hash + 'static,
+    {
+        (**self.0).as_any().downcast_ref()
+    }
+
+    /// Force a size for this node.
+    pub fn sized(self, sizing: Spec<Option<Relative<RawLength>>>) -> Self {
+        if sizing.any(Option::is_some) {
+            SizedNode { sizing, child: self }.pack()
+        } else {
+            self
+        }
+    }
+
+    /// Fill the frames resulting from a node.
+    pub fn filled(self, fill: Paint) -> Self {
+        FillNode { fill, child: self }.pack()
+    }
+
+    /// Stroke the frames resulting from a node.
+    pub fn stroked(self, stroke: Stroke) -> Self {
+        StrokeNode { stroke, child: self }.pack()
+    }
+
+    /// Set alignments for this node.
+    pub fn aligned(self, aligns: Spec<Option<RawAlign>>) -> Self {
+        if aligns.any(Option::is_some) {
+            AlignNode { aligns, child: self }.pack()
+        } else {
+            self
+        }
+    }
+
+    /// Pad this node at the sides.
+    pub fn padded(self, padding: Sides<Relative<RawLength>>) -> Self {
+        if !padding.left.is_zero()
+            || !padding.top.is_zero()
+            || !padding.right.is_zero()
+            || !padding.bottom.is_zero()
+        {
+            PadNode { padding, child: self }.pack()
+        } else {
+            self
+        }
+    }
+
+    /// Transform this node's contents without affecting layout.
+    pub fn moved(self, delta: Spec<Relative<RawLength>>) -> Self {
+        if delta.any(|r| !r.is_zero()) {
+            MoveNode { delta, child: self }.pack()
+        } else {
+            self
+        }
+    }
+}
+
+impl Layout for LayoutNode {
+    fn layout(
+        &self,
+        ctx: &mut Context,
+        regions: &Regions,
+        styles: StyleChain,
+    ) -> TypResult<Vec<Arc<Frame>>> {
+        ctx.query((self, regions, styles), |ctx, (node, regions, styles)| {
+            node.0.layout(ctx, regions, node.barrier().chain(&styles))
+        })
+        .clone()
+    }
+
+    fn pack(self) -> LayoutNode {
+        self
+    }
+}
+
+impl Default for LayoutNode {
+    fn default() -> Self {
+        EmptyNode.pack()
+    }
+}
+
+impl Debug for LayoutNode {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+impl PartialEq for LayoutNode {
+    fn eq(&self, other: &Self) -> bool {
+        self.0.eq(&other.0)
+    }
+}
+
+trait Bounds: Layout + Debug + Sync + Send + 'static {
+    fn as_any(&self) -> &dyn Any;
+    fn barrier(&self) -> Barrier;
+}
+
+impl<T> Bounds for T
+where
+    T: Layout + Debug + Hash + Sync + Send + 'static,
+{
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+
+    fn barrier(&self) -> Barrier {
+        Barrier::new::<T>()
+    }
+}
+
+/// A layout node that produces an empty frame.
+///
+/// The packed version of this is returned by [`PackedNode::default`].
+#[derive(Debug, Hash)]
+struct EmptyNode;
+
+impl Layout for EmptyNode {
+    fn layout(
+        &self,
+        _: &mut Context,
+        regions: &Regions,
+        _: StyleChain,
+    ) -> TypResult<Vec<Arc<Frame>>> {
+        Ok(vec![Arc::new(Frame::new(
+            regions.expand.select(regions.first, Size::zero()),
+        ))])
+    }
+}
+
+/// Fix the size of a node.
+#[derive(Debug, Hash)]
+struct SizedNode {
+    /// How to size the node horizontally and vertically.
+    sizing: Spec<Option<Relative<RawLength>>>,
+    /// The node to be sized.
+    child: LayoutNode,
+}
+
+impl Layout for SizedNode {
+    fn layout(
+        &self,
+        ctx: &mut Context,
+        regions: &Regions,
+        styles: StyleChain,
+    ) -> TypResult<Vec<Arc<Frame>>> {
+        // The "pod" is the region into which the child will be layouted.
+        let pod = {
+            // Resolve the sizing to a concrete size.
+            let size = self
+                .sizing
+                .resolve(styles)
+                .zip(regions.base)
+                .map(|(s, b)| s.map(|v| v.relative_to(b)))
+                .unwrap_or(regions.first);
+
+            // Select the appropriate base and expansion for the child depending
+            // on whether it is automatically or relatively sized.
+            let is_auto = self.sizing.map_is_none();
+            let base = is_auto.select(regions.base, size);
+            let expand = regions.expand | !is_auto;
+
+            Regions::one(size, base, expand)
+        };
+
+        // Layout the child.
+        let mut frames = self.child.layout(ctx, &pod, styles)?;
+
+        // Ensure frame size matches regions size if expansion is on.
+        let frame = &mut frames[0];
+        let target = regions.expand.select(regions.first, frame.size);
+        Arc::make_mut(frame).resize(target, Align::LEFT_TOP);
+
+        Ok(frames)
+    }
+}
+
+/// Fill the frames resulting from a node.
+#[derive(Debug, Hash)]
+struct FillNode {
+    /// How to fill the frames resulting from the `child`.
+    fill: Paint,
+    /// The node to fill.
+    child: LayoutNode,
+}
+
+impl Layout for FillNode {
+    fn layout(
+        &self,
+        ctx: &mut Context,
+        regions: &Regions,
+        styles: StyleChain,
+    ) -> TypResult<Vec<Arc<Frame>>> {
+        let mut frames = self.child.layout(ctx, regions, styles)?;
+        for frame in &mut frames {
+            let shape = Geometry::Rect(frame.size).filled(self.fill);
+            Arc::make_mut(frame).prepend(Point::zero(), Element::Shape(shape));
+        }
+        Ok(frames)
+    }
+}
+
+/// Stroke the frames resulting from a node.
+#[derive(Debug, Hash)]
+struct StrokeNode {
+    /// How to stroke the frames resulting from the `child`.
+    stroke: Stroke,
+    /// The node to stroke.
+    child: LayoutNode,
+}
+
+impl Layout for StrokeNode {
+    fn layout(
+        &self,
+        ctx: &mut Context,
+        regions: &Regions,
+        styles: StyleChain,
+    ) -> TypResult<Vec<Arc<Frame>>> {
+        let mut frames = self.child.layout(ctx, regions, styles)?;
+        for frame in &mut frames {
+            let shape = Geometry::Rect(frame.size).stroked(self.stroke);
+            Arc::make_mut(frame).prepend(Point::zero(), Element::Shape(shape));
+        }
+        Ok(frames)
+    }
+}
diff --git a/src/model/mod.rs b/src/model/mod.rs
new file mode 100644
index 00000000..df39207f
--- /dev/null
+++ b/src/model/mod.rs
@@ -0,0 +1,14 @@
+//! Structured representation of styled content.
+
+#[macro_use]
+mod styles;
+mod collapse;
+mod content;
+mod layout;
+mod show;
+
+pub use collapse::*;
+pub use content::*;
+pub use layout::*;
+pub use show::*;
+pub use styles::*;
diff --git a/src/model/show.rs b/src/model/show.rs
new file mode 100644
index 00000000..5f76ba19
--- /dev/null
+++ b/src/model/show.rs
@@ -0,0 +1,112 @@
+use std::any::{Any, TypeId};
+use std::fmt::{self, Debug, Formatter};
+use std::hash::Hash;
+use std::sync::Arc;
+
+use super::{Content, StyleChain};
+use crate::diag::TypResult;
+use crate::eval::Dict;
+use crate::util::Prehashed;
+use crate::Context;
+
+/// A node that can be realized given some styles.
+pub trait Show: 'static {
+    /// Encode this node into a dictionary.
+    fn encode(&self) -> Dict;
+
+    /// The base recipe for this node that is executed if there is no
+    /// user-defined show rule.
+    fn realize(&self, ctx: &mut Context, styles: StyleChain) -> TypResult<Content>;
+
+    /// Finalize this node given the realization of a base or user recipe. Use
+    /// this for effects that should work even in the face of a user-defined
+    /// show rule, for example:
+    /// - Application of general settable properties
+    /// - Attaching things like semantics to a heading
+    ///
+    /// Defaults to just the realized content.
+    #[allow(unused_variables)]
+    fn finalize(
+        &self,
+        ctx: &mut Context,
+        styles: StyleChain,
+        realized: Content,
+    ) -> TypResult<Content> {
+        Ok(realized)
+    }
+
+    /// Convert to a packed show node.
+    fn pack(self) -> ShowNode
+    where
+        Self: Debug + Hash + Sized + Sync + Send + 'static,
+    {
+        ShowNode::new(self)
+    }
+}
+
+/// A type-erased showable node with a precomputed hash.
+#[derive(Clone, Hash)]
+pub struct ShowNode(Arc<Prehashed<dyn Bounds>>);
+
+impl ShowNode {
+    /// Pack any showable node.
+    pub fn new<T>(node: T) -> Self
+    where
+        T: Show + Debug + Hash + Sync + Send + 'static,
+    {
+        Self(Arc::new(Prehashed::new(node)))
+    }
+
+    /// The type id of this node.
+    pub fn id(&self) -> TypeId {
+        self.0.as_any().type_id()
+    }
+}
+
+impl Show for ShowNode {
+    fn encode(&self) -> Dict {
+        self.0.encode()
+    }
+
+    fn realize(&self, ctx: &mut Context, styles: StyleChain) -> TypResult<Content> {
+        self.0.realize(ctx, styles)
+    }
+
+    fn finalize(
+        &self,
+        ctx: &mut Context,
+        styles: StyleChain,
+        realized: Content,
+    ) -> TypResult<Content> {
+        self.0.finalize(ctx, styles, realized)
+    }
+
+    fn pack(self) -> ShowNode {
+        self
+    }
+}
+
+impl Debug for ShowNode {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+impl PartialEq for ShowNode {
+    fn eq(&self, other: &Self) -> bool {
+        self.0.eq(&other.0)
+    }
+}
+
+trait Bounds: Show + Debug + Sync + Send + 'static {
+    fn as_any(&self) -> &dyn Any;
+}
+
+impl<T> Bounds for T
+where
+    T: Show + Debug + Hash + Sync + Send + 'static,
+{
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+}
diff --git a/src/model/styles.rs b/src/model/styles.rs
new file mode 100644
index 00000000..2195568d
--- /dev/null
+++ b/src/model/styles.rs
@@ -0,0 +1,831 @@
+use std::any::{Any, TypeId};
+use std::fmt::{self, Debug, Formatter};
+use std::hash::Hash;
+use std::marker::PhantomData;
+use std::sync::Arc;
+
+use super::{Content, Layout, Show, ShowNode};
+use crate::diag::{At, TypResult};
+use crate::eval::{Args, Func, Node, Smart, Value};
+use crate::geom::{Numeric, Relative, Sides, Spec};
+use crate::library::layout::PageNode;
+use crate::library::text::{FontFamily, ParNode, TextNode};
+use crate::syntax::Span;
+use crate::util::Prehashed;
+use crate::Context;
+
+/// A map of style properties.
+#[derive(Default, Clone, PartialEq, Hash)]
+pub struct StyleMap(Vec<Entry>);
+
+impl StyleMap {
+    /// Create a new, empty style map.
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Whether this map contains no styles.
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    /// Create a style map from a single property-value pair.
+    pub fn with<'a, K: Key<'a>>(key: K, value: K::Value) -> Self {
+        let mut styles = Self::new();
+        styles.set(key, value);
+        styles
+    }
+
+    /// Set an inner value for a style property.
+    ///
+    /// If the property needs folding and the value is already contained in the
+    /// style map, `self` contributes the outer values and `value` is the inner
+    /// one.
+    pub fn set<'a, K: Key<'a>>(&mut self, key: K, value: K::Value) {
+        self.0.push(Entry::Property(Property::new(key, value)));
+    }
+
+    /// Set an inner value for a style property if it is `Some(_)`.
+    pub fn set_opt<'a, K: Key<'a>>(&mut self, key: K, value: Option<K::Value>) {
+        if let Some(value) = value {
+            self.set(key, value);
+        }
+    }
+
+    /// Set a font family composed of a preferred family and existing families
+    /// from a style chain.
+    pub fn set_family(&mut self, preferred: FontFamily, existing: StyleChain) {
+        self.set(
+            TextNode::FAMILY,
+            std::iter::once(preferred)
+                .chain(existing.get(TextNode::FAMILY).iter().cloned())
+                .collect(),
+        );
+    }
+
+    /// Set a show rule recipe for a node.
+    pub fn set_recipe<T: Node>(&mut self, func: Func, span: Span) {
+        self.0.push(Entry::Recipe(Recipe::new::<T>(func, span)));
+    }
+
+    /// Whether the map contains a style property for the given key.
+    pub fn contains<'a, K: Key<'a>>(&self, _: K) -> bool {
+        self.0
+            .iter()
+            .filter_map(|entry| entry.property())
+            .any(|property| property.key == TypeId::of::<K>())
+    }
+
+    /// Make `self` the first link of the `tail` chain.
+    ///
+    /// The resulting style chain contains styles from `self` as well as
+    /// `tail`. The ones from `self` take precedence over the ones from
+    /// `tail`. For folded properties `self` contributes the inner value.
+    pub fn chain<'a>(&'a self, tail: &'a StyleChain<'a>) -> StyleChain<'a> {
+        if self.is_empty() {
+            *tail
+        } else {
+            StyleChain { head: &self.0, tail: Some(tail) }
+        }
+    }
+
+    /// Set an outer value for a style property.
+    ///
+    /// If the property needs folding and the value is already contained in the
+    /// style map, `self` contributes the inner values and `value` is the outer
+    /// one.
+    ///
+    /// Like [`chain`](Self::chain) or [`apply_map`](Self::apply_map), but with
+    /// only a single property.
+    pub fn apply<'a, K: Key<'a>>(&mut self, key: K, value: K::Value) {
+        self.0.insert(0, Entry::Property(Property::new(key, value)));
+    }
+
+    /// Apply styles from `tail` in-place. The resulting style map is equivalent
+    /// to the style chain created by `self.chain(StyleChain::new(tail))`.
+    ///
+    /// This is useful over `chain` when you want to combine two maps, but you
+    /// still need an owned map without a lifetime.
+    pub fn apply_map(&mut self, tail: &Self) {
+        self.0.splice(0 .. 0, tail.0.iter().cloned());
+    }
+
+    /// Mark all contained properties as _scoped_. This means that they only
+    /// apply to the first descendant node (of their type) in the hierarchy and
+    /// not its children, too. This is used by [constructors](Node::construct).
+    pub fn scoped(mut self) -> Self {
+        for entry in &mut self.0 {
+            if let Entry::Property(property) = entry {
+                property.scoped = true;
+            }
+        }
+        self
+    }
+
+    /// The highest-level kind of of structure the map interrupts.
+    pub fn interruption(&self) -> Option<Interruption> {
+        self.0
+            .iter()
+            .filter_map(|entry| entry.property())
+            .filter_map(|property| property.interruption())
+            .max()
+    }
+}
+
+impl Debug for StyleMap {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        for entry in self.0.iter().rev() {
+            writeln!(f, "{:?}", entry)?;
+        }
+        Ok(())
+    }
+}
+
+/// An entry for a single style property, recipe or barrier.
+#[derive(Clone, PartialEq, Hash)]
+enum Entry {
+    /// A style property originating from a set rule or constructor.
+    Property(Property),
+    /// A barrier for scoped styles.
+    Barrier(TypeId, &'static str),
+    /// A show rule recipe.
+    Recipe(Recipe),
+}
+
+impl Entry {
+    /// If this is a property, return it.
+    fn property(&self) -> Option<&Property> {
+        match self {
+            Self::Property(property) => Some(property),
+            _ => None,
+        }
+    }
+
+    /// If this is a recipe, return it.
+    fn recipe(&self) -> Option<&Recipe> {
+        match self {
+            Self::Recipe(recipe) => Some(recipe),
+            _ => None,
+        }
+    }
+}
+
+impl Debug for Entry {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        f.write_str("#[")?;
+        match self {
+            Self::Property(property) => property.fmt(f)?,
+            Self::Recipe(recipe) => recipe.fmt(f)?,
+            Self::Barrier(_, name) => write!(f, "Barrier for {name}")?,
+        }
+        f.write_str("]")
+    }
+}
+
+/// A style property originating from a set rule or constructor.
+#[derive(Clone, Hash)]
+struct Property {
+    /// The type id of the property's [key](Key).
+    key: TypeId,
+    /// The type id of the node the property belongs to.
+    node: TypeId,
+    /// The name of the property.
+    name: &'static str,
+    /// The property's value.
+    value: Arc<Prehashed<dyn Bounds>>,
+    /// Whether the property should only affects the first node down the
+    /// hierarchy. Used by constructors.
+    scoped: bool,
+}
+
+impl Property {
+    /// Create a new property from a key-value pair.
+    fn new<'a, K: Key<'a>>(_: K, value: K::Value) -> Self {
+        Self {
+            key: TypeId::of::<K>(),
+            node: K::node(),
+            name: K::NAME,
+            value: Arc::new(Prehashed::new(value)),
+            scoped: false,
+        }
+    }
+
+    /// What kind of structure the property interrupts.
+    fn interruption(&self) -> Option<Interruption> {
+        if self.is_of::<PageNode>() {
+            Some(Interruption::Page)
+        } else if self.is_of::<ParNode>() {
+            Some(Interruption::Par)
+        } else {
+            None
+        }
+    }
+
+    /// Access the property's value if it is of the given key.
+    fn downcast<'a, K: Key<'a>>(&'a self) -> Option<&'a K::Value> {
+        if self.key == TypeId::of::<K>() {
+            (**self.value).as_any().downcast_ref()
+        } else {
+            None
+        }
+    }
+
+    /// Whether this property belongs to the node `T`.
+    fn is_of<T: Node>(&self) -> bool {
+        self.node == TypeId::of::<T>()
+    }
+}
+
+impl Debug for Property {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        write!(f, "{} = {:?}", self.name, self.value)?;
+        if self.scoped {
+            write!(f, " [scoped]")?;
+        }
+        Ok(())
+    }
+}
+
+impl PartialEq for Property {
+    fn eq(&self, other: &Self) -> bool {
+        self.key == other.key
+            && self.value.eq(&other.value)
+            && self.scoped == other.scoped
+    }
+}
+
+trait Bounds: Debug + Sync + Send + 'static {
+    fn as_any(&self) -> &dyn Any;
+}
+
+impl<T> Bounds for T
+where
+    T: Debug + Sync + Send + 'static,
+{
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+}
+
+/// Style property keys.
+///
+/// This trait is not intended to be implemented manually, but rather through
+/// the `#[node]` proc-macro.
+pub trait Key<'a>: Copy + 'static {
+    /// The unfolded type which this property is stored as in a style map. For
+    /// example, this is [`Toggle`](crate::geom::Length) for the
+    /// [`STRONG`](TextNode::STRONG) property.
+    type Value: Debug + Clone + Hash + Sync + Send + 'static;
+
+    /// The folded type of value that is returned when reading this property
+    /// from a style chain. For example, this is [`bool`] for the
+    /// [`STRONG`](TextNode::STRONG) property. For non-copy, non-folding
+    /// properties this is a reference type.
+    type Output;
+
+    /// The name of the property, used for debug printing.
+    const NAME: &'static str;
+
+    /// The type id of the node this property belongs to.
+    fn node() -> TypeId;
+
+    /// Compute an output value from a sequence of values belong to this key,
+    /// folding if necessary.
+    fn get(
+        chain: StyleChain<'a>,
+        values: impl Iterator<Item = &'a Self::Value>,
+    ) -> Self::Output;
+}
+
+/// A property that is resolved with other properties from the style chain.
+pub trait Resolve {
+    /// The type of the resolved output.
+    type Output;
+
+    /// Resolve the value using the style chain.
+    fn resolve(self, styles: StyleChain) -> Self::Output;
+}
+
+impl<T: Resolve> Resolve for Option<T> {
+    type Output = Option<T::Output>;
+
+    fn resolve(self, styles: StyleChain) -> Self::Output {
+        self.map(|v| v.resolve(styles))
+    }
+}
+
+impl<T: Resolve> Resolve for Smart<T> {
+    type Output = Smart<T::Output>;
+
+    fn resolve(self, styles: StyleChain) -> Self::Output {
+        self.map(|v| v.resolve(styles))
+    }
+}
+
+impl<T: Resolve> Resolve for Spec<T> {
+    type Output = Spec<T::Output>;
+
+    fn resolve(self, styles: StyleChain) -> Self::Output {
+        self.map(|v| v.resolve(styles))
+    }
+}
+
+impl<T: Resolve> Resolve for Sides<T> {
+    type Output = Sides<T::Output>;
+
+    fn resolve(self, styles: StyleChain) -> Self::Output {
+        Sides {
+            left: self.left.resolve(styles),
+            right: self.right.resolve(styles),
+            top: self.top.resolve(styles),
+            bottom: self.bottom.resolve(styles),
+        }
+    }
+}
+
+impl<T> Resolve for Relative<T>
+where
+    T: Resolve + Numeric,
+    <T as Resolve>::Output: Numeric,
+{
+    type Output = Relative<<T as Resolve>::Output>;
+
+    fn resolve(self, styles: StyleChain) -> Self::Output {
+        self.map(|abs| abs.resolve(styles))
+    }
+}
+
+/// A property that is folded to determine its final value.
+pub trait Fold {
+    /// The type of the folded output.
+    type Output;
+
+    /// Fold this inner value with an outer folded value.
+    fn fold(self, outer: Self::Output) -> Self::Output;
+}
+
+impl<T> Fold for Option<T>
+where
+    T: Fold,
+    T::Output: Default,
+{
+    type Output = Option<T::Output>;
+
+    fn fold(self, outer: Self::Output) -> Self::Output {
+        self.map(|inner| inner.fold(outer.unwrap_or_default()))
+    }
+}
+
+impl<T> Fold for Smart<T>
+where
+    T: Fold,
+    T::Output: Default,
+{
+    type Output = Smart<T::Output>;
+
+    fn fold(self, outer: Self::Output) -> Self::Output {
+        self.map(|inner| inner.fold(outer.unwrap_or_default()))
+    }
+}
+
+/// A show rule recipe.
+#[derive(Clone, PartialEq, Hash)]
+struct Recipe {
+    /// The affected node.
+    node: TypeId,
+    /// The name of the affected node.
+    name: &'static str,
+    /// The function that defines the recipe.
+    func: Func,
+    /// The span to report all erros with.
+    span: Span,
+}
+
+impl Recipe {
+    /// Create a new recipe for the node `T`.
+    fn new<T: Node>(func: Func, span: Span) -> Self {
+        Self {
+            node: TypeId::of::<T>(),
+            name: std::any::type_name::<T>(),
+            func,
+            span,
+        }
+    }
+}
+
+impl Debug for Recipe {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        write!(f, "Recipe for {} from {:?}", self.name, self.span)
+    }
+}
+
+/// A style chain barrier.
+///
+/// Barriers interact with [scoped](StyleMap::scoped) styles: A scoped style
+/// can still be read through a single barrier (the one of the node it
+/// _should_ apply to), but a second barrier will make it invisible.
+#[derive(Clone, PartialEq, Hash)]
+pub struct Barrier(Entry);
+
+impl Barrier {
+    /// Create a new barrier for the layout node `T`.
+    pub fn new<T: Layout>() -> Self {
+        Self(Entry::Barrier(
+            TypeId::of::<T>(),
+            std::any::type_name::<T>(),
+        ))
+    }
+
+    /// Make this barrier the first link of the `tail` chain.
+    pub fn chain<'a>(&'a self, tail: &'a StyleChain) -> StyleChain<'a> {
+        // We have to store a full `Entry` enum inside the barrier because
+        // otherwise the `slice::from_ref` trick below won't work.
+        // Unfortunately, that also means we have to somehow extract the id
+        // here.
+        let id = match self.0 {
+            Entry::Barrier(id, _) => id,
+            _ => unreachable!(),
+        };
+
+        if tail
+            .entries()
+            .filter_map(Entry::property)
+            .any(|p| p.scoped && p.node == id)
+        {
+            StyleChain {
+                head: std::slice::from_ref(&self.0),
+                tail: Some(tail),
+            }
+        } else {
+            *tail
+        }
+    }
+}
+
+impl Debug for Barrier {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+/// Determines whether a style could interrupt some composable structure.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+pub enum Interruption {
+    /// The style forces a paragraph break.
+    Par,
+    /// The style forces a page break.
+    Page,
+}
+
+/// A chain of style maps, similar to a linked list.
+///
+/// A style chain allows to combine properties from multiple style maps in a
+/// node hierarchy in a non-allocating way. Rather than eagerly merging the
+/// maps, each access walks the hierarchy from the innermost to the outermost
+/// map, trying to find a match and then folding it with matches further up the
+/// chain.
+#[derive(Default, Clone, Copy, Hash)]
+pub struct StyleChain<'a> {
+    /// The first link of this chain.
+    head: &'a [Entry],
+    /// The remaining links in the chain.
+    tail: Option<&'a Self>,
+}
+
+impl<'a> StyleChain<'a> {
+    /// Create a new, empty style chain.
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Start a new style chain with a root map.
+    pub fn with_root(root: &'a StyleMap) -> Self {
+        Self { head: &root.0, tail: None }
+    }
+
+    /// Get the output value of a style property.
+    ///
+    /// Returns the property's default value if no map in the chain contains an
+    /// entry for it. Also takes care of resolving and folding and returns
+    /// references where applicable.
+    pub fn get<K: Key<'a>>(self, key: K) -> K::Output {
+        K::get(self, self.values(key))
+    }
+
+    /// Realize a node with a user recipe.
+    pub fn realize(
+        self,
+        ctx: &mut Context,
+        node: &ShowNode,
+    ) -> TypResult<Option<Content>> {
+        let id = node.id();
+        if let Some(recipe) = self
+            .entries()
+            .filter_map(Entry::recipe)
+            .find(|recipe| recipe.node == id)
+        {
+            let dict = node.encode();
+            let args = Args::from_values(recipe.span, [Value::Dict(dict)]);
+            Ok(Some(recipe.func.call(ctx, args)?.cast().at(recipe.span)?))
+        } else {
+            Ok(None)
+        }
+    }
+}
+
+impl<'a> StyleChain<'a> {
+    /// Return the chain, but without the trailing scoped property for the given
+    /// `node`. This is a 90% hack fix for show node constructor scoping.
+    pub(super) fn unscoped(mut self, node: TypeId) -> Self {
+        while self
+            .head
+            .last()
+            .and_then(Entry::property)
+            .map_or(false, |p| p.scoped && p.node == node)
+        {
+            let len = self.head.len();
+            self.head = &self.head[.. len - 1]
+        }
+        self
+    }
+
+    /// Remove the last link from the chain.
+    fn pop(&mut self) {
+        *self = self.tail.copied().unwrap_or_default();
+    }
+
+    /// Build a style map from the suffix (all links beyond the `len`) of the
+    /// chain.
+    fn suffix(self, len: usize) -> StyleMap {
+        let mut suffix = StyleMap::new();
+        let take = self.links().count().saturating_sub(len);
+        for link in self.links().take(take) {
+            suffix.0.splice(0 .. 0, link.iter().cloned());
+        }
+        suffix
+    }
+
+    /// Iterate over all values for the given property in the chain.
+    fn values<K: Key<'a>>(self, _: K) -> Values<'a, K> {
+        Values {
+            entries: self.entries(),
+            depth: 0,
+            key: PhantomData,
+        }
+    }
+
+    /// Iterate over the entries of the chain.
+    fn entries(self) -> Entries<'a> {
+        Entries {
+            inner: [].as_slice().iter(),
+            links: self.links(),
+        }
+    }
+
+    /// Iterate over the links of the chain.
+    fn links(self) -> Links<'a> {
+        Links(Some(self))
+    }
+}
+
+impl Debug for StyleChain<'_> {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        for entry in self.entries() {
+            writeln!(f, "{:?}", entry)?;
+        }
+        Ok(())
+    }
+}
+
+impl PartialEq for StyleChain<'_> {
+    fn eq(&self, other: &Self) -> bool {
+        let as_ptr = |s| s as *const _;
+        self.head.as_ptr() == other.head.as_ptr()
+            && self.head.len() == other.head.len()
+            && self.tail.map(as_ptr) == other.tail.map(as_ptr)
+    }
+}
+
+/// An iterator over the values in a style chain.
+struct Values<'a, K> {
+    entries: Entries<'a>,
+    depth: usize,
+    key: PhantomData<K>,
+}
+
+impl<'a, K: Key<'a>> Iterator for Values<'a, K> {
+    type Item = &'a K::Value;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        while let Some(entry) = self.entries.next() {
+            match entry {
+                Entry::Property(property) => {
+                    if let Some(value) = property.downcast::<K>() {
+                        if !property.scoped || self.depth <= 1 {
+                            return Some(value);
+                        }
+                    }
+                }
+                Entry::Barrier(id, _) => {
+                    self.depth += (*id == K::node()) as usize;
+                }
+                Entry::Recipe(_) => {}
+            }
+        }
+
+        None
+    }
+}
+
+/// An iterator over the entries in a style chain.
+struct Entries<'a> {
+    inner: std::slice::Iter<'a, Entry>,
+    links: Links<'a>,
+}
+
+impl<'a> Iterator for Entries<'a> {
+    type Item = &'a Entry;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        loop {
+            if let Some(entry) = self.inner.next_back() {
+                return Some(entry);
+            }
+
+            match self.links.next() {
+                Some(next) => self.inner = next.iter(),
+                None => return None,
+            }
+        }
+    }
+}
+
+/// An iterator over the links of a style chain.
+struct Links<'a>(Option<StyleChain<'a>>);
+
+impl<'a> Iterator for Links<'a> {
+    type Item = &'a [Entry];
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let StyleChain { head, tail } = self.0?;
+        self.0 = tail.copied();
+        Some(head)
+    }
+}
+
+/// A sequence of items with associated styles.
+#[derive(Hash)]
+pub struct StyleVec<T> {
+    items: Vec<T>,
+    maps: Vec<(StyleMap, usize)>,
+}
+
+impl<T> StyleVec<T> {
+    /// Whether there are any items in the sequence.
+    pub fn is_empty(&self) -> bool {
+        self.items.is_empty()
+    }
+
+    /// Number of items in the sequence.
+    pub fn len(&self) -> usize {
+        self.items.len()
+    }
+
+    /// Iterate over the contained maps. Note that zipping this with `items()`
+    /// does not yield the same result as calling `iter()` because this method
+    /// only returns maps once that are shared by consecutive items. This method
+    /// is designed for use cases where you want to check, for example, whether
+    /// any of the maps fulfills a specific property.
+    pub fn maps(&self) -> impl Iterator<Item = &StyleMap> {
+        self.maps.iter().map(|(map, _)| map)
+    }
+
+    /// Iterate over the contained items.
+    pub fn items(&self) -> std::slice::Iter<'_, T> {
+        self.items.iter()
+    }
+
+    /// Iterate over the contained items and associated style maps.
+    pub fn iter(&self) -> impl Iterator<Item = (&T, &StyleMap)> + '_ {
+        let styles = self
+            .maps
+            .iter()
+            .flat_map(|(map, count)| std::iter::repeat(map).take(*count));
+        self.items().zip(styles)
+    }
+
+    /// Insert an element in the front. The element will share the style of the
+    /// current first element.
+    ///
+    /// This method has no effect if the vector is empty.
+    pub fn push_front(&mut self, item: T) {
+        if !self.maps.is_empty() {
+            self.items.insert(0, item);
+            self.maps[0].1 += 1;
+        }
+    }
+}
+
+impl<T> Default for StyleVec<T> {
+    fn default() -> Self {
+        Self { items: vec![], maps: vec![] }
+    }
+}
+
+impl<T: Debug> Debug for StyleVec<T> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        f.debug_list()
+            .entries(self.iter().map(|(item, map)| {
+                crate::util::debug(|f| {
+                    map.fmt(f)?;
+                    item.fmt(f)
+                })
+            }))
+            .finish()
+    }
+}
+
+/// Assists in the construction of a [`StyleVec`].
+pub struct StyleVecBuilder<'a, T> {
+    items: Vec<T>,
+    chains: Vec<(StyleChain<'a>, usize)>,
+}
+
+impl<'a, T> StyleVecBuilder<'a, T> {
+    /// Create a new style-vec builder.
+    pub fn new() -> Self {
+        Self { items: vec![], chains: vec![] }
+    }
+
+    /// Push a new item into the style vector.
+    pub fn push(&mut self, item: T, styles: StyleChain<'a>) {
+        self.items.push(item);
+
+        if let Some((prev, count)) = self.chains.last_mut() {
+            if *prev == styles {
+                *count += 1;
+                return;
+            }
+        }
+
+        self.chains.push((styles, 1));
+    }
+
+    /// Access the last item mutably and its chain by value.
+    pub fn last_mut(&mut self) -> Option<(&mut T, StyleChain<'a>)> {
+        let item = self.items.last_mut()?;
+        let chain = self.chains.last()?.0;
+        Some((item, chain))
+    }
+
+    /// Iterate over the contained items.
+    pub fn items(&self) -> std::slice::Iter<'_, T> {
+        self.items.iter()
+    }
+
+    /// Finish building, returning a pair of two things:
+    /// - a style vector of items with the non-shared styles
+    /// - a shared prefix chain of styles that apply to all items
+    pub fn finish(self) -> (StyleVec<T>, StyleChain<'a>) {
+        let mut iter = self.chains.iter();
+        let mut trunk = match iter.next() {
+            Some(&(chain, _)) => chain,
+            None => return Default::default(),
+        };
+
+        let mut shared = trunk.links().count();
+        for &(mut chain, _) in iter {
+            let len = chain.links().count();
+            if len < shared {
+                for _ in 0 .. shared - len {
+                    trunk.pop();
+                }
+                shared = len;
+            } else if len > shared {
+                for _ in 0 .. len - shared {
+                    chain.pop();
+                }
+            }
+
+            while shared > 0 && chain != trunk {
+                trunk.pop();
+                chain.pop();
+                shared -= 1;
+            }
+        }
+
+        let maps = self
+            .chains
+            .into_iter()
+            .map(|(chain, count)| (chain.suffix(shared), count))
+            .collect();
+
+        (StyleVec { items: self.items, maps }, trunk)
+    }
+}
+
+impl<'a, T> Default for StyleVecBuilder<'a, T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}