Honour user alternateNumber in GSUB Type 3 substitutions

src/opentype/GPOSProcessor.js

@@ -305,8 +305,8 @@ export default class GPOSProcessor extends OTProcessor {
     return { x, y };
   }
 
-  applyFeatures(userFeatures, glyphs, advances) {
-    super.applyFeatures(userFeatures, glyphs, advances);
+  applyFeatures(featureTags, glyphs, advances, userFeatures) {
+    super.applyFeatures(featureTags, glyphs, advances, userFeatures);
 
     for (var i = 0; i < this.glyphs.length; i++) {
       this.fixCursiveAttachment(i);

src/opentype/GSUBProcessor.js

@@ -61,8 +61,17 @@ export default class GSUBProcessor extends OTProcessor {
       case 3: { // Alternate Substitution
         let index = this.coverageIndex(table.coverage);
         if (index !== -1) {
-          let USER_INDEX = 0; // TODO
-          this.glyphIterator.cur.id = table.alternateSet.get(index)[USER_INDEX];
+          // Honour the user's 1-based alternate index for the current feature
+          // (e.g. `nalt: 2` selects the second alternate). Fall back to the
+          // first alternate when no value was given or the value isn't a
+          // positive integer.
+          let alt = this.userFeatures && this.userFeatures[this.currentFeature];
+          let userIndex = Number.isInteger(alt) && alt > 0 ? alt - 1 : 0;
+          let alternates = table.alternateSet.get(index);
+          if (userIndex >= alternates.length) {
+            userIndex = 0;
+          }
+          this.glyphIterator.cur.id = alternates[userIndex];
           return true;
         }
 

src/opentype/OTLayoutEngine.js

@@ -43,10 +43,16 @@ export default class OTLayoutEngine {
     this.plan = new ShapingPlan(this.font, script, glyphRun.direction);
     this.shaper.plan(this.plan, this.glyphInfos, glyphRun.features);
 
-    // Assign chosen features to output glyph run
-    for (let key in this.plan.allFeatures) {
-      glyphRun.features[key] = true;
-    }
+    // Build the output features object: every shaper-chosen feature marked
+    // `true`, then overlay the user's original values so alternateNumber
+    // selections (e.g. `nalt: 2` for Type 3 lookups) survive instead of being
+    // clobbered to `true`. The caller's input object is left untouched.
+    glyphRun.features = {
+      ...Object.fromEntries(
+        Object.keys(this.plan.allFeatures).map(k => [k, true])
+      ),
+      ...glyphRun.features
+    };
   }
 
   substitute(glyphRun) {

src/opentype/OTProcessor.js

@@ -179,8 +179,9 @@ export default class OTProcessor {
     });
   }
 
-  applyFeatures(userFeatures, glyphs, advances) {
-    let lookups = this.lookupsForFeatures(userFeatures);
+  applyFeatures(featureTags, glyphs, advances, userFeatures) {
+    this.userFeatures = userFeatures;
+    let lookups = this.lookupsForFeatures(featureTags);
     this.applyLookups(lookups, glyphs, advances);
   }
 

src/opentype/ShapingPlan.js

@@ -17,6 +17,11 @@ export default class ShapingPlan {
     this.stages = [];
     this.globalFeatures = {};
     this.allFeatures = {};
+    // Snapshot of the user's original features object — preserved for
+    // Type 3 (Alternate Substitution) lookups that need the per-feature
+    // alternate index (1-based; e.g. `nalt: 2` selects the second
+    // alternate). Set in setFeatureOverrides.
+    this.userFeatures = null;
   }
 
   /**
@@ -76,6 +81,7 @@ export default class ShapingPlan {
     if (Array.isArray(features)) {
       this.add(features);
     } else if (typeof features === 'object') {
+      this.userFeatures = features;
       for (let tag in features) {
         if (features[tag]) {
           this.add(tag);
@@ -111,7 +117,7 @@ export default class ShapingPlan {
         }
 
       } else if (stage.length > 0) {
-        processor.applyFeatures(stage, glyphs, positions);
+        processor.applyFeatures(stage, glyphs, positions, this.userFeatures);
       }
     }
   }

test/shaping.js

@@ -582,4 +582,23 @@ describe('shaping', function () {
       test('SHBALI-2/12', 'NotoSans/NotoSansBalinese-Regular.ttf', "ᬓ᭄ᭅᬸ", '23+2275|162+0|60@0,-1000+0');
     });
   });
+
+  describe('alternate substitution (GSUB Type 3) honours user-supplied alternateNumber', function () {
+    // FiraSans's aalt lookup carries multiple alternates per coverage entry.
+    // For 'A' (glyph 3) the alternate set is [1078 'ordfeminine', 764 'a.sc'].
+    // Without honouring the user-supplied alternateNumber, every aalt request
+    // resolves to the first entry — matching HarfBuzz's `aalt=2` requires
+    // selecting the second.
+    let font = fontkit.openSync(new URL('data/FiraSans/FiraSans-Regular.ttf', import.meta.url));
+
+    it('selects the first alternate when aalt is `true` (or 1)', function () {
+      let { glyphs } = font.layout('A', { aalt: true });
+      assert.deepEqual(glyphs.map(g => g.id), [1078]);
+    });
+
+    it('selects the second alternate when aalt is 2', function () {
+      let { glyphs } = font.layout('A', { aalt: 2 });
+      assert.deepEqual(glyphs.map(g => g.id), [764]);
+    });
+  });
 });