1 files changed, 150 insertions, 0 deletions
diff --git a/src/library/elements/heading.rs b/src/library/elements/heading.rs
new file mode 100644
index 00000000..a67f4f24
--- /dev/null
+++ b/src/library/elements/heading.rs
@@ -0,0 +1,150 @@
+use crate::library::prelude::*;
+use crate::library::text::{FontFamily, TextNode};
+
+/// A section heading.
+#[derive(Debug, Hash)]
+pub struct HeadingNode {
+    /// The logical nesting depth of the section, starting from one. In the
+    /// default style, this controls the text size of the heading.
+    pub level: usize,
+    /// The heading's contents.
+    pub body: Template,
+}
+
+#[class]
+impl HeadingNode {
+    /// The heading's font family. Just the normal text family if `auto`.
+    pub const FAMILY: Leveled<Smart<FontFamily>> = Leveled::Value(Smart::Auto);
+    /// The color of text in the heading. Just the normal text color if `auto`.
+    pub const FILL: Leveled<Smart<Paint>> = Leveled::Value(Smart::Auto);
+    /// The size of text in the heading.
+    pub const SIZE: Leveled<Linear> = Leveled::Mapping(|level| {
+        let upscale = (1.6 - 0.1 * level as f64).max(0.75);
+        Relative::new(upscale).into()
+    });
+    /// Whether text in the heading is strengthend.
+    pub const STRONG: Leveled<bool> = Leveled::Value(true);
+    /// Whether text in the heading is emphasized.
+    pub const EMPH: Leveled<bool> = Leveled::Value(false);
+    /// Whether the heading is underlined.
+    pub const UNDERLINE: Leveled<bool> = Leveled::Value(false);
+    /// The extra padding above the heading.
+    pub const ABOVE: Leveled<Length> = Leveled::Value(Length::zero());
+    /// The extra padding below the heading.
+    pub const BELOW: Leveled<Length> = Leveled::Value(Length::zero());
+    /// Whether the heading is block-level.
+    pub const BLOCK: Leveled<bool> = Leveled::Value(true);
+
+    fn construct(_: &mut Context, args: &mut Args) -> TypResult<Template> {
+        Ok(Template::show(Self {
+            body: args.expect("body")?,
+            level: args.named("level")?.unwrap_or(1),
+        }))
+    }
+}
+
+impl Show for HeadingNode {
+    fn show(&self, ctx: &mut Context, styles: StyleChain) -> TypResult<Template> {
+        macro_rules! resolve {
+            ($key:expr) => {
+                styles.get_cloned($key).resolve(ctx, self.level)?
+            };
+        }
+
+        // Resolve the user recipe.
+        let mut body = styles
+            .show(self, ctx, [
+                Value::Int(self.level as i64),
+                Value::Template(self.body.clone()),
+            ])?
+            .unwrap_or_else(|| self.body.clone());
+
+        let mut map = StyleMap::new();
+        map.set(TextNode::SIZE, resolve!(Self::SIZE));
+
+        if let Smart::Custom(family) = resolve!(Self::FAMILY) {
+            map.set(
+                TextNode::FAMILY,
+                std::iter::once(family)
+                    .chain(styles.get_ref(TextNode::FAMILY).iter().cloned())
+                    .collect(),
+            );
+        }
+
+        if let Smart::Custom(fill) = resolve!(Self::FILL) {
+            map.set(TextNode::FILL, fill);
+        }
+
+        if resolve!(Self::STRONG) {
+            map.set(TextNode::STRONG, true);
+        }
+
+        if resolve!(Self::EMPH) {
+            map.set(TextNode::EMPH, true);
+        }
+
+        let mut seq = vec![];
+        if resolve!(Self::UNDERLINE) {
+            body = body.underlined();
+        }
+
+        let above = resolve!(Self::ABOVE);
+        if !above.is_zero() {
+            seq.push(Template::Vertical(above.into()));
+        }
+
+        seq.push(body);
+
+        let below = resolve!(Self::BELOW);
+        if !below.is_zero() {
+            seq.push(Template::Vertical(below.into()));
+        }
+
+        let mut template = Template::sequence(seq).styled_with_map(map);
+        if resolve!(Self::BLOCK) {
+            template = Template::block(template);
+        }
+
+        Ok(template)
+    }
+}
+
+/// Either the value or a closure mapping to the value.
+#[derive(Debug, Clone, PartialEq, Hash)]
+pub enum Leveled<T> {
+    /// A bare value.
+    Value(T),
+    /// A simple mapping from a heading level to a value.
+    Mapping(fn(usize) -> T),
+    /// A closure mapping from a heading level to a value.
+    Func(Func, Span),
+}
+
+impl<T: Cast> Leveled<T> {
+    /// Resolve the value based on the level.
+    pub fn resolve(self, ctx: &mut Context, level: usize) -> TypResult<T> {
+        Ok(match self {
+            Self::Value(value) => value,
+            Self::Mapping(mapping) => mapping(level),
+            Self::Func(func, span) => {
+                let args = Args::from_values(span, [Value::Int(level as i64)]);
+                func.call(ctx, args)?.cast().at(span)?
+            }
+        })
+    }
+}
+
+impl<T: Cast> Cast<Spanned<Value>> for Leveled<T> {
+    fn is(value: &Spanned<Value>) -> bool {
+        matches!(&value.v, Value::Func(_)) || T::is(&value.v)
+    }
+
+    fn cast(value: Spanned<Value>) -> StrResult<Self> {
+        match value.v {
+            Value::Func(v) => Ok(Self::Func(v, value.span)),
+            v => T::cast(v)
+                .map(Self::Value)
+                .map_err(|msg| with_alternative(msg, "function")),
+        }
+    }
+}