Class-based math spacing

library/src/math/mod.rs

@@ -9,6 +9,7 @@ mod group;
 mod matrix;
 mod root;
 mod script;
+mod spacing;
 mod stretch;
 mod style;
 
@@ -59,6 +60,10 @@ pub fn define(scope: &mut Scope) {
     scope.def_func::<FrakNode>("frak");
     scope.def_func::<MonoNode>("mono");
     scope.def_func::<BbNode>("bb");
+    scope.define("thin", HNode::strong(THIN).pack());
+    scope.define("med", HNode::strong(MEDIUM).pack());
+    scope.define("thick", HNode::strong(THICK).pack());
+    scope.define("quad", HNode::strong(QUAD).pack());
 }
 
 /// # Math

library/src/math/spacing.rs

@@ -0,0 +1,36 @@
+use super::*;
+
+pub(super) const ZERO: Em = Em::zero();
+pub(super) const THIN: Em = Em::new(1.0 / 6.0);
+pub(super) const MEDIUM: Em = Em::new(2.0 / 9.0);
+pub(super) const THICK: Em = Em::new(5.0 / 18.0);
+pub(super) const QUAD: Em = Em::new(1.0);
+
+/// Determine the spacing between two fragments in a given style.
+pub(super) fn spacing(left: MathClass, right: MathClass, style: MathStyle) -> Em {
+    use MathClass::*;
+    let script = style.size <= MathSize::Script;
+    match (left, right) {
+        // No spacing before punctuation; thin spacing after punctuation, unless
+        // in script size.
+        (_, Punctuation) => ZERO,
+        (Punctuation, _) if !script => THIN,
+
+        // No spacing after opening delimiters and before closing delimiters.
+        (Opening, _) | (_, Closing) => ZERO,
+
+        // Thick spacing around relations, unless followed by a another relation
+        // or in script size.
+        (Relation, Relation) => ZERO,
+        (Relation, _) | (_, Relation) if !script => THICK,
+
+        // Medium spacing around binary operators, unless in script size.
+        (Vary | Binary, _) | (_, Vary | Binary) if !script => MEDIUM,
+
+        // Thin spacing around large operators, unless next to a delimiter.
+        (Large, Opening | Fence) | (Closing | Fence, Large) => ZERO,
+        (Large, _) | (_, Large) => THIN,
+
+        _ => ZERO,
+    }
+}