Add `math.accent` support for `flac` and `dtls` OpenType features (#5202)

9 files changed, 109 insertions, 51 deletions

diff --git a/crates/typst-layout/src/math/accent.rs b/crates/typst-layout/src/math/accent.rs
index 9fa7a5a0..4e26502e 100644
--- a/crates/typst-layout/src/math/accent.rs
+++ b/crates/typst-layout/src/math/accent.rs
@@ -19,7 +19,12 @@ pub fn layout_accent(
     styles: StyleChain,
 ) -> SourceResult<()> {
     let cramped = style_cramped();
-    let base = ctx.layout_into_fragment(elem.base(), styles.chain(&cramped))?;
+    let mut base = ctx.layout_into_fragment(elem.base(), styles.chain(&cramped))?;
+
+    // Try to replace a glyph with its dotless variant.
+    if let MathFragment::Glyph(glyph) = &mut base {
+        glyph.make_dotless_form(ctx);
+    }
 
     // Preserve class to preserve automatic spacing.
     let base_class = base.class();
@@ -31,10 +36,17 @@ pub fn layout_accent(
         .at(scaled_font_size(ctx, styles))
         .relative_to(base.width());
 
+    let Accent(c) = elem.accent();
+    let mut glyph = GlyphFragment::new(ctx, styles, *c, elem.span());
+
+    // Try to replace accent glyph with flattened variant.
+    let flattened_base_height = scaled!(ctx, styles, flattened_accent_base_height);
+    if base.height() > flattened_base_height {
+        glyph.make_flattened_accent_form(ctx);
+    }
+
     // Forcing the accent to be at least as large as the base makes it too
     // wide in many case.
-    let Accent(c) = elem.accent();
-    let glyph = GlyphFragment::new(ctx, styles, *c, elem.span());
     let short_fall = ACCENT_SHORT_FALL.at(glyph.font_size);
     let variant = glyph.stretch_horizontal(ctx, width, short_fall);
     let accent = variant.frame;
diff --git a/crates/typst-layout/src/math/fragment.rs b/crates/typst-layout/src/math/fragment.rs
index 19a4494e..8da7c077 100644
--- a/crates/typst-layout/src/math/fragment.rs
+++ b/crates/typst-layout/src/math/fragment.rs
@@ -2,9 +2,7 @@ use std::fmt::{self, Debug, Formatter};
 
 use rustybuzz::Feature;
 use smallvec::SmallVec;
-use ttf_parser::gsub::{
-    AlternateSet, AlternateSubstitution, SingleSubstitution, SubstitutionSubtable,
-};
+use ttf_parser::gsub::{AlternateSubstitution, SingleSubstitution, SubstitutionSubtable};
 use ttf_parser::opentype_layout::LayoutTable;
 use ttf_parser::{GlyphId, Rect};
 use typst_library::foundations::StyleChain;
@@ -390,20 +388,39 @@ impl GlyphFragment {
         frame
     }
 
-    pub fn make_scriptsize(&mut self, ctx: &MathContext) {
+    pub fn make_script_size(&mut self, ctx: &MathContext) {
         let alt_id =
-            script_alternatives(ctx, self.id).and_then(|alts| alts.alternates.get(0));
+            ctx.ssty_table.as_ref().and_then(|ssty| ssty.try_apply(self.id, None));
+        if let Some(alt_id) = alt_id {
+            self.set_id(ctx, alt_id);
+        }
+    }
 
+    pub fn make_script_script_size(&mut self, ctx: &MathContext) {
+        let alt_id = ctx.ssty_table.as_ref().and_then(|ssty| {
+            // We explicitly request to apply the alternate set with value 1,
+            // as opposed to the default value in ssty, as the former
+            // corresponds to second level scripts and the latter corresponds
+            // to first level scripts.
+            ssty.try_apply(self.id, Some(1))
+                .or_else(|| ssty.try_apply(self.id, None))
+        });
         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)));
+    pub fn make_dotless_form(&mut self, ctx: &MathContext) {
+        let alt_id =
+            ctx.dtls_table.as_ref().and_then(|dtls| dtls.try_apply(self.id, None));
+        if let Some(alt_id) = alt_id {
+            self.set_id(ctx, alt_id);
+        }
+    }
 
+    pub fn make_flattened_accent_form(&mut self, ctx: &MathContext) {
+        let alt_id =
+            ctx.flac_table.as_ref().and_then(|flac| flac.try_apply(self.id, None));
         if let Some(alt_id) = alt_id {
             self.set_id(ctx, alt_id);
         }
@@ -561,16 +578,6 @@ fn accent_attach(ctx: &MathContext, id: GlyphId, font_size: Abs) -> Option<Abs>
     )
 }
 
-/// 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 whether a glyph is an extended shape.
 fn is_extended_shape(ctx: &MathContext, id: GlyphId) -> bool {
     ctx.table
@@ -662,10 +669,11 @@ pub enum GlyphwiseSubsts<'a> {
 }
 
 impl<'a> GlyphwiseSubsts<'a> {
-    pub fn new(gsub: LayoutTable<'a>, feature: Feature) -> Option<Self> {
+    pub fn new(gsub: Option<LayoutTable<'a>>, feature: Feature) -> Option<Self> {
+        let gsub = gsub?;
         let table = gsub
             .features
-            .find(ttf_parser::Tag(feature.tag.0))
+            .find(feature.tag)
             .and_then(|feature| feature.lookup_indices.get(0))
             .and_then(|index| gsub.lookups.get(index))?;
         let table = table.subtables.get::<SubstitutionSubtable>(0)?;
@@ -680,7 +688,11 @@ impl<'a> GlyphwiseSubsts<'a> {
         }
     }
 
-    pub fn try_apply(&self, glyph_id: GlyphId) -> Option<GlyphId> {
+    pub fn try_apply(
+        &self,
+        glyph_id: GlyphId,
+        alt_value: Option<u32>,
+    ) -> Option<GlyphId> {
         match self {
             Self::Single(single) => match single {
                 SingleSubstitution::Format1 { coverage, delta } => coverage
@@ -694,11 +706,11 @@ impl<'a> GlyphwiseSubsts<'a> {
                 .coverage
                 .get(glyph_id)
                 .and_then(|idx| alternate.alternate_sets.get(idx))
-                .and_then(|set| set.alternates.get(*value as u16)),
+                .and_then(|set| set.alternates.get(alt_value.unwrap_or(*value) as u16)),
         }
     }
 
     pub fn apply(&self, glyph_id: GlyphId) -> GlyphId {
-        self.try_apply(glyph_id).unwrap_or(glyph_id)
+        self.try_apply(glyph_id, None).unwrap_or(glyph_id)
     }
 }
diff --git a/crates/typst-layout/src/math/mod.rs b/crates/typst-layout/src/math/mod.rs
index b3dde977..32059cef 100644
--- a/crates/typst-layout/src/math/mod.rs
+++ b/crates/typst-layout/src/math/mod.rs
@@ -13,7 +13,8 @@ mod stretch;
 mod text;
 mod underover;
 
-use ttf_parser::gsub::SubstitutionSubtable;
+use rustybuzz::Feature;
+use ttf_parser::Tag;
 use typst_library::diag::{bail, SourceResult};
 use typst_library::engine::Engine;
 use typst_library::foundations::{Content, NativeElement, Packed, Resolve, StyleChain};
@@ -369,7 +370,9 @@ struct MathContext<'a, 'v, 'e> {
     ttf: &'a ttf_parser::Face<'a>,
     table: ttf_parser::math::Table<'a>,
     constants: ttf_parser::math::Constants<'a>,
-    ssty_table: Option<ttf_parser::gsub::AlternateSubstitution<'a>>,
+    dtls_table: Option<GlyphwiseSubsts<'a>>,
+    flac_table: Option<GlyphwiseSubsts<'a>>,
+    ssty_table: Option<GlyphwiseSubsts<'a>>,
     glyphwise_tables: Option<Vec<GlyphwiseSubsts<'a>>>,
     space_width: Em,
     // Mutable.
@@ -389,26 +392,17 @@ impl<'a, 'v, 'e> MathContext<'a, 'v, 'e> {
         let gsub_table = font.ttf().tables().gsub;
         let constants = math_table.constants.unwrap();
 
-        let ssty_table = gsub_table
-            .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::<SubstitutionSubtable>(0))
-            .and_then(|ssty| match ssty {
-                SubstitutionSubtable::Alternate(alt_glyphs) => Some(alt_glyphs),
-                _ => None,
-            });
+        let feat = |tag: &[u8; 4]| {
+            GlyphwiseSubsts::new(gsub_table, Feature::new(Tag::from_bytes(tag), 0, ..))
+        };
 
         let features = features(styles);
-        let glyphwise_tables = gsub_table.map(|gsub| {
+        let glyphwise_tables = Some(
             features
                 .into_iter()
-                .filter_map(|feature| GlyphwiseSubsts::new(gsub, feature))
-                .collect()
-        });
+                .filter_map(|feature| GlyphwiseSubsts::new(gsub_table, feature))
+                .collect(),
+        );
 
         let ttf = font.ttf();
         let space_width = ttf
@@ -422,10 +416,12 @@ impl<'a, 'v, 'e> MathContext<'a, 'v, 'e> {
             locator,
             region: Region::new(base, Axes::splat(false)),
             font,
-            ttf: font.ttf(),
+            ttf,
             table: math_table,
             constants,
-            ssty_table,
+            dtls_table: feat(b"dtls"),
+            flac_table: feat(b"flac"),
+            ssty_table: feat(b"ssty"),
             glyphwise_tables,
             space_width,
             fragments: vec![],
diff --git a/crates/typst-layout/src/math/text.rs b/crates/typst-layout/src/math/text.rs
index df80b45a..86d871a2 100644
--- a/crates/typst-layout/src/math/text.rs
+++ b/crates/typst-layout/src/math/text.rs
@@ -26,20 +26,25 @@ pub fn layout_text(
     let span = elem.span();
     let mut chars = text.chars();
     let math_size = EquationElem::size_in(styles);
-
+    let mut dtls = ctx.dtls_table.is_some();
     let fragment: MathFragment = if let Some(mut glyph) = chars
         .next()
         .filter(|_| chars.next().is_none())
+        .map(|c| dtls_char(c, &mut dtls))
         .map(|c| styled_char(styles, c, true))
         .and_then(|c| GlyphFragment::try_new(ctx, styles, c, span))
     {
         // A single letter that is available in the math font.
+        if dtls {
+            glyph.make_dotless_form(ctx);
+        }
+
         match math_size {
             MathSize::Script => {
-                glyph.make_scriptsize(ctx);
+                glyph.make_script_size(ctx);
             }
             MathSize::ScriptScript => {
-                glyph.make_scriptscriptsize(ctx);
+                glyph.make_script_script_size(ctx);
             }
             _ => (),
         }
@@ -342,3 +347,16 @@ fn greek_exception(
         _ => return None,
     })
 }
+
+/// Switch dotless character to non dotless character for use of the dtls
+/// OpenType feature.
+pub fn dtls_char(c: char, dtls: &mut bool) -> char {
+    match (c, *dtls) {
+        ('ı', true) => 'i',
+        ('ȷ', true) => 'j',
+        _ => {
+            *dtls = false;
+            c
+        }
+    }
+}
diff --git a/crates/typst-library/src/symbols/sym.rs b/crates/typst-library/src/symbols/sym.rs
index 606e44ea..42f47715 100644
--- a/crates/typst-library/src/symbols/sym.rs
+++ b/crates/typst-library/src/symbols/sym.rs
@@ -991,5 +991,5 @@ pub(crate) const SYM: &[(&str, Symbol)] = typst_macros::symbols! {
     kelvin: 'K',
     Re: 'ℜ',
     Im: 'ℑ',
-    dotless: [i: '𝚤', j: '𝚥'],
+    dotless: [i: 'ı', j: 'ȷ'],
 };
diff --git a/tests/ref/math-accent-dotless.png b/tests/ref/math-accent-dotless.png
new file mode 100644
index 00000000..81eb4fa2
--- /dev/null
+++ b/tests/ref/math-accent-dotless.png
diff --git a/tests/ref/math-style-dotless.png b/tests/ref/math-style-dotless.png
new file mode 100644
index 00000000..6f4063c6
--- /dev/null
+++ b/tests/ref/math-style-dotless.png
diff --git a/tests/suite/math/accent.typ b/tests/suite/math/accent.typ
index 87ed8158..5be4f576 100644
--- a/tests/suite/math/accent.typ
+++ b/tests/suite/math/accent.typ
@@ -35,3 +35,10 @@ $tilde(sum), tilde(sum, size: #50%), accent(H, hat, size: #200%)$
 --- math-accent-sized-script ---
 // Test accent size in script size.
 $tilde(U, size: #1.1em), x^tilde(U, size: #1.1em), sscript(tilde(U, size: #1.1em))$
+
+--- math-accent-dotless ---
+// Test dotless glyph variants.
+#let test(c) = $grave(#c), acute(sans(#c)), hat(frak(#c)), tilde(mono(#c)),
+  macron(bb(#c)), dot(cal(#c)), diaer(upright(#c)), breve(bold(#c)),
+  circle(bold(upright(#c))), caron(upright(sans(#c))), arrow(bold(frak(#c)))$
+$test(i) \ test(j)$
diff --git a/tests/suite/math/style.typ b/tests/suite/math/style.typ
index 09ddd3c1..e21cd4fd 100644
--- a/tests/suite/math/style.typ
+++ b/tests/suite/math/style.typ
@@ -12,6 +12,19 @@ $A, italic(A), upright(A), bold(A), bold(upright(A)), \
  bb("hello") + bold(cal("world")), \
  mono("SQRT")(x) wreath mono(123 + 456)$
 
+--- math-style-dotless ---
+// Test styling dotless i and j.
+$ dotless.i dotless.j,
+  upright(dotless.i) upright(dotless.j),
+  sans(dotless.i) sans(dotless.j),
+  bold(dotless.i) bold(dotless.j),
+  bb(dotless.i) bb(dotless.j),
+  cal(dotless.i) cal(dotless.j),
+  frak(dotless.i) frak(dotless.j),
+  mono(dotless.i) mono(dotless.j), 
+  bold(frak(dotless.i)) upright(sans(dotless.j)),
+  italic(bb(dotless.i)) frak(sans(dotless.j)) $
+
 --- math-style-exceptions ---
 // Test a few style exceptions.
 $h, bb(N), cal(R), Theta, italic(Theta), sans(Theta), sans(italic(Theta)) \