support optical sizes for script/scriptscript when available (#1580)

library/src/math/ctx.rs

@@ -31,6 +31,7 @@ pub struct MathContext<'a, 'b, 'v> {
     pub ttf: &'a ttf_parser::Face<'a>,
     pub table: ttf_parser::math::Table<'a>,
     pub constants: ttf_parser::math::Constants<'a>,
+    pub ssty_table: Option<ttf_parser::gsub::AlternateSubstitution<'a>>,
     pub space_width: Em,
     pub fragments: Vec<MathFragment>,
     pub local: Styles,
@@ -50,6 +51,27 @@ impl<'a, 'b, 'v> MathContext<'a, 'b, 'v> {
     ) -> Self {
         let table = font.ttf().tables().math.unwrap();
         let constants = table.constants.unwrap();
+
+        let ssty_table = font
+            .ttf()
+            .tables()
+            .gsub
+            .and_then(|gsub| {
+                gsub.features
+                    .find(ttf_parser::Tag::from_bytes(b"ssty"))
+                    .and_then(|feature| feature.lookup_indices.get(0))
+                    .and_then(|index| gsub.lookups.get(index))
+            })
+            .and_then(|ssty| {
+                ssty.subtables.get::<ttf_parser::gsub::SubstitutionSubtable>(0)
+            })
+            .and_then(|ssty| match ssty {
+                ttf_parser::gsub::SubstitutionSubtable::Alternate(alt_glyphs) => {
+                    Some(alt_glyphs)
+                }
+                _ => None,
+            });
+
         let size = TextElem::size_in(styles);
         let ttf = font.ttf();
         let space_width = ttf
@@ -66,6 +88,7 @@ impl<'a, 'b, 'v> MathContext<'a, 'b, 'v> {
             ttf: font.ttf(),
             table,
             constants,
+            ssty_table,
             space_width,
             fragments: vec![],
             local: Styles::new(),
@@ -129,21 +152,32 @@ impl<'a, 'b, 'v> MathContext<'a, 'b, 'v> {
         let text = elem.text();
         let span = elem.span();
         let mut chars = text.chars();
-        let fragment = if let Some(glyph) = chars
+        let fragment = if let Some(mut glyph) = chars
             .next()
             .filter(|_| chars.next().is_none())
             .map(|c| self.style.styled_char(c))
             .and_then(|c| GlyphFragment::try_new(self, c, span))
         {
             // A single letter that is available in the math font.
-            if self.style.size == MathSize::Display
-                && glyph.class == Some(MathClass::Large)
-            {
+            match self.style.size {
+                MathSize::Display => {
+                    if glyph.class == Some(MathClass::Large) {
                         let height = scaled!(self, display_operator_min_height);
                         glyph.stretch_vertical(self, height, Abs::zero()).into()
                     } else {
                         glyph.into()
                     }
+                }
+                MathSize::Script => {
+                    glyph.make_scriptsize(self);
+                    glyph.into()
+                }
+                MathSize::ScriptScript => {
+                    glyph.make_scriptscriptsize(self);
+                    glyph.into()
+                }
+                _ => glyph.into(),
+            }
         } else if text.chars().all(|c| c.is_ascii_digit()) {
             // Numbers aren't that difficult.
             let mut fragments = vec![];

library/src/math/fragment.rs

@@ -1,4 +1,5 @@
 use super::*;
+use ttf_parser::gsub::AlternateSet;
 
 #[derive(Debug, Clone)]
 pub enum MathFragment {
@@ -272,6 +273,25 @@ impl GlyphFragment {
         frame.meta_iter(self.meta);
         frame
     }
+
+    pub fn make_scriptsize(&mut self, ctx: &MathContext) {
+        let alt_id =
+            script_alternatives(ctx, self.id).and_then(|alts| alts.alternates.get(0));
+
+        if let Some(alt_id) = alt_id {
+            self.set_id(ctx, alt_id);
+        }
+    }
+
+    pub fn make_scriptscriptsize(&mut self, ctx: &MathContext) {
+        let alts = script_alternatives(ctx, self.id);
+        let alt_id = alts
+            .and_then(|alts| alts.alternates.get(1).or_else(|| alts.alternates.get(0)));
+
+        if let Some(alt_id) = alt_id {
+            self.set_id(ctx, alt_id);
+        }
+    }
 }
 
 impl Debug for GlyphFragment {
@@ -347,6 +367,16 @@ fn italics_correction(ctx: &MathContext, id: GlyphId) -> Option<Abs> {
     Some(ctx.table.glyph_info?.italic_corrections?.get(id)?.scaled(ctx))
 }
 
+/// Look up the script/scriptscript alternates for a glyph
+fn script_alternatives<'a>(
+    ctx: &MathContext<'a, '_, '_>,
+    id: GlyphId,
+) -> Option<AlternateSet<'a>> {
+    ctx.ssty_table.and_then(|ssty| {
+        ssty.coverage.get(id).and_then(|index| ssty.alternate_sets.get(index))
+    })
+}
+
 /// Look up the italics correction for a glyph.
 fn is_extended_shape(ctx: &MathContext, id: GlyphId) -> bool {
     ctx.table

tests/typ/math/op.typ

@@ -11,6 +11,7 @@ $  &sin x + log_2 x \
 
 ---
 // Test scripts vs limits.
+#set page(width: auto)
 #set text(font: "New Computer Modern")
 Discuss $lim_(n->oo) 1/n$ now.
 $ lim_(n->infinity) 1/n = 0 $

tests/typ/math/opticalsize.typ

@@ -0,0 +1,30 @@
+// test optical sized variants in sub/superscripts
+
+---
+
+// Test transition from script to scriptscript.
+#[
+#set text(size:20pt)
+$  e^(e^(e^(e))) $
+]
+A large number: $e^(e^(e^(e)))$. 
+
+---
+//  Test prime/double prime via scriptsize
+#let prime = [ \u{2032} ]
+#let dprime = [ \u{2033} ]
+#let tprime = [ \u{2034} ]
+$ y^dprime-2y^prime + y = 0 $
+$y^dprime-2y^prime + y = 0$
+$ y^tprime_3 + g^(prime 2) $
+
+---
+// Test prime superscript on large symbol 
+$ scripts(sum_(k in NN))^prime 1/k^2 $
+$sum_(k in NN)^prime 1/k^2$
+
+---
+// Test script-script in a fraction.
+$ 1/(x^A) $
+#[#set text(size:18pt); $1/(x^A)$] vs. #[#set text(size:14pt); $x^A$]
+