Parse math fonts only once

src/font.rs

@@ -6,6 +6,8 @@ use std::fmt::{self, Debug, Formatter};
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
 
+use once_cell::sync::OnceCell;
+use rex::font::MathFont;
 use serde::{Deserialize, Serialize};
 use ttf_parser::{name_id, GlyphId, PlatformId, Tag};
 use unicode_segmentation::UnicodeSegmentation;
@@ -246,6 +248,8 @@ pub struct Face {
     ttf: rustybuzz::Face<'static>,
     /// The faces metrics.
     metrics: FaceMetrics,
+    /// The parsed ReX math font.
+    math: OnceCell<Option<MathFont>>,
 }
 
 impl Face {
@@ -263,7 +267,13 @@ impl Face {
         let ttf = rustybuzz::Face::from_slice(slice, index)?;
         let metrics = FaceMetrics::from_ttf(&ttf);
 
-        Some(Self { buffer, index, ttf, metrics })
+        Some(Self {
+            buffer,
+            index,
+            ttf,
+            metrics,
+            math: OnceCell::new(),
+        })
     }
 
     /// The underlying buffer.
@@ -293,6 +303,11 @@ impl Face {
         &self.metrics
     }
 
+    /// Access the math font, if any.
+    pub fn math(&self) -> Option<&MathFont> {
+        self.math.get_or_init(|| MathFont::parse(self.buffer()).ok()).as_ref()
+    }
+
     /// Convert from font units to an em length.
     pub fn to_em(&self, units: impl Into<f64>) -> Em {
         Em::from_units(units, self.units_per_em())

src/library/math/rex.rs

@@ -35,13 +35,13 @@ impl Layout for RexNode {
             .at(span)?;
 
         // Prepare the font.
-        let data = ctx.fonts.get(face_id).buffer();
-        let font = MathFont::parse(data)
-            .map_err(|_| "failed to parse math font")
+        let face = ctx.fonts.get(face_id);
+        let ctx = face
+            .math()
+            .and_then(FontContext::new)
+            .ok_or("font is not suitable for math")
             .at(span)?;
 
-        let ctx = FontContext::new(&font).ok_or("failed to parse math font").at(span)?;
-
         // Layout the formula.
         let em = styles.get(TextNode::SIZE);
         let style = if self.display { Style::Display } else { Style::Text };

tests/typ/math/basic.typ

@@ -15,6 +15,12 @@ $[ \sum_{k=0}^n k = \frac{n(n+1)}{2} ]$
 // Test that blackboard style looks nice.
 $[ f: \mathbb{N} \rightarrow \mathbb{R} ]$
 
+---
+#set math(family: "IBM Plex Sans")
+
+// Error: 1-4 font is not suitable for math
+$a$
+
 ---
 // Error: 1-10 expected '}' found EOF
 $\sqrt{x$