1 files changed, 188 insertions, 0 deletions
diff --git a/library/src/text/shift.rs b/library/src/text/shift.rs
new file mode 100644
index 00000000..e5f142dd
--- /dev/null
+++ b/library/src/text/shift.rs
@@ -0,0 +1,188 @@
+use typst::model::SequenceNode;
+use typst::util::EcoString;
+
+use super::{variant, SpaceNode, TextNode, TextSize};
+use crate::prelude::*;
+
+/// Sub or superscript text.
+///
+/// The text is rendered smaller and its baseline is raised. To provide the best
+/// typography possible, we first try to transform the text to superscript
+/// codepoints. If that fails, we fall back to rendering shrunk normal letters
+/// in a raised way.
+#[derive(Debug, Hash)]
+pub struct ShiftNode<const S: ScriptKind>(pub Content);
+
+/// Shift the text into superscript.
+pub type SuperNode = ShiftNode<SUPERSCRIPT>;
+
+/// Shift the text into subscript.
+pub type SubNode = ShiftNode<SUBSCRIPT>;
+
+#[node(Show)]
+impl<const S: ScriptKind> ShiftNode<S> {
+    /// Whether to prefer the dedicated sub- and superscript characters of the
+    /// font.
+    pub const TYPOGRAPHIC: bool = true;
+    /// The baseline shift for synthetic sub- and superscripts.
+    pub const BASELINE: Length =
+        Em::new(if S == SUPERSCRIPT { -0.5 } else { 0.2 }).into();
+    /// The font size for synthetic sub- and superscripts.
+    pub const SIZE: TextSize = TextSize(Em::new(0.6).into());
+
+    fn construct(_: &mut Vm, args: &mut Args) -> SourceResult<Content> {
+        Ok(Self(args.expect("body")?).pack())
+    }
+}
+
+impl<const S: ScriptKind> Show for ShiftNode<S> {
+    fn unguard_parts(&self, _: Selector) -> Content {
+        Self(self.0.clone()).pack()
+    }
+
+    fn field(&self, name: &str) -> Option<Value> {
+        match name {
+            "body" => Some(Value::Content(self.0.clone())),
+            _ => None,
+        }
+    }
+
+    fn realize(
+        &self,
+        world: Tracked<dyn World>,
+        styles: StyleChain,
+    ) -> SourceResult<Content> {
+        let mut transformed = None;
+        if styles.get(Self::TYPOGRAPHIC) {
+            if let Some(text) = search_text(&self.0, S) {
+                if is_shapable(world, &text, styles) {
+                    transformed = Some(TextNode(text).pack());
+                }
+            }
+        };
+
+        Ok(transformed.unwrap_or_else(|| {
+            let mut map = StyleMap::new();
+            map.set(TextNode::BASELINE, styles.get(Self::BASELINE));
+            map.set(TextNode::SIZE, styles.get(Self::SIZE));
+            self.0.clone().styled_with_map(map)
+        }))
+    }
+}
+
+/// Find and transform the text contained in `content` to the given script kind
+/// if and only if it only consists of `Text`, `Space`, and `Empty` leaf nodes.
+fn search_text(content: &Content, mode: ScriptKind) -> Option<EcoString> {
+    if content.is_empty() {
+        Some(EcoString::new())
+    } else if content.is::<SpaceNode>() {
+        Some(' '.into())
+    } else if let Some(text) = content.downcast::<TextNode>() {
+        if let Some(sup) = convert_script(&text.0, mode) {
+            return Some(sup);
+        }
+        None
+    } else if let Some(seq) = content.downcast::<SequenceNode>() {
+        let mut full = EcoString::new();
+        for item in seq.0.iter() {
+            match search_text(item, mode) {
+                Some(text) => full.push_str(&text),
+                None => return None,
+            }
+        }
+        Some(full)
+    } else {
+        None
+    }
+}
+
+/// Checks whether the first retrievable family contains all code points of the
+/// given string.
+fn is_shapable(world: Tracked<dyn World>, text: &str, styles: StyleChain) -> bool {
+    for family in styles.get(TextNode::FAMILY).0.iter() {
+        if let Some(font) = world
+            .book()
+            .select(family.as_str(), variant(styles))
+            .and_then(|id| world.font(id))
+        {
+            return text.chars().all(|c| font.ttf().glyph_index(c).is_some());
+        }
+    }
+
+    false
+}
+
+/// Convert a string to sub- or superscript codepoints if all characters
+/// can be mapped to such a codepoint.
+fn convert_script(text: &str, mode: ScriptKind) -> Option<EcoString> {
+    let mut result = EcoString::with_capacity(text.len());
+    let converter = match mode {
+        SUPERSCRIPT => to_superscript_codepoint,
+        SUBSCRIPT | _ => to_subscript_codepoint,
+    };
+
+    for c in text.chars() {
+        match converter(c) {
+            Some(c) => result.push(c),
+            None => return None,
+        }
+    }
+
+    Some(result)
+}
+
+/// Convert a character to its corresponding Unicode superscript.
+fn to_superscript_codepoint(c: char) -> Option<char> {
+    char::from_u32(match c {
+        '0' => 0x2070,
+        '1' => 0x00B9,
+        '2' => 0x00B2,
+        '3' => 0x00B3,
+        '4' ..= '9' => 0x2070 + (c as u32 + 4 - '4' as u32),
+        '+' => 0x207A,
+        '-' => 0x207B,
+        '=' => 0x207C,
+        '(' => 0x207D,
+        ')' => 0x207E,
+        'n' => 0x207F,
+        'i' => 0x2071,
+        ' ' => 0x0020,
+        _ => return None,
+    })
+}
+
+/// Convert a character to its corresponding Unicode subscript.
+fn to_subscript_codepoint(c: char) -> Option<char> {
+    char::from_u32(match c {
+        '0' => 0x2080,
+        '1' ..= '9' => 0x2080 + (c as u32 - '0' as u32),
+        '+' => 0x208A,
+        '-' => 0x208B,
+        '=' => 0x208C,
+        '(' => 0x208D,
+        ')' => 0x208E,
+        'a' => 0x2090,
+        'e' => 0x2091,
+        'o' => 0x2092,
+        'x' => 0x2093,
+        'h' => 0x2095,
+        'k' => 0x2096,
+        'l' => 0x2097,
+        'm' => 0x2098,
+        'n' => 0x2099,
+        'p' => 0x209A,
+        's' => 0x209B,
+        't' => 0x209C,
+        ' ' => 0x0020,
+        _ => return None,
+    })
+}
+
+/// A category of script.
+pub type ScriptKind = usize;
+
+/// Text that is rendered smaller and raised, also known as superior.
+const SUPERSCRIPT: ScriptKind = 0;
+
+/// Text that is rendered smaller and lowered, also known as inferior.
+const SUBSCRIPT: ScriptKind = 1;