Type six, format two

.prettierrc

@@ -0,0 +1,6 @@
+{
+  "useTabs": false,
+  "tabWidth": 2,
+  "semi": true,
+  "trailingComma": "es5"
+}

src/opentype/tables/advanced/lookups/gsub/lookup-type-6.js

@@ -5,6 +5,7 @@ import {
   SubstLookupRecord,
 } from "./gsub-lookup.js";
 import { CoverageTable } from "../../shared/coverage.js";
+import { ClassDefinition } from "../../shared/class.js";
 
 class LookupType6 extends LookupType {
   type = 6;
@@ -82,6 +83,36 @@ class LookupType6 extends LookupType {
     return new CoverageTable(p);
   }
 
+  getBacktrackClassDef() {
+    if (this.format !== 2)
+      throw new Error(
+        `lookup type 6.${this.format} does not use class definitions.`
+      );
+    let p = this.parser;
+    p.currentPosition = this.start + this.backtrackClassDefOffset;
+    return new ClassDefinition(p);
+  }
+
+  getInputClassDef() {
+    if (this.format !== 2)
+      throw new Error(
+        `lookup type 6.${this.format} does not use class definitions.`
+      );
+    let p = this.parser;
+    p.currentPosition = this.start + this.inputClassDefOffset;
+    return new ClassDefinition(p);
+  }
+
+  getLookaheadClassDef() {
+    if (this.format !== 2)
+      throw new Error(
+        `lookup type 6.${this.format} does not use class definitions.`
+      );
+    let p = this.parser;
+    p.currentPosition = this.start + this.lookaheadClassDefOffset;
+    return new ClassDefinition(p);
+  }
+
   getSubstLookupRecord(index) {
     return this.substLookupRecords?.[index];
   }

testing/node/gsub/issue-153/gsub.test.js

@@ -115,4 +115,59 @@ describe("GSUB 6.2 checks", () => {
       substitutionCount: 1,
     });
   });
+
+  test("getInputClassDef returns class definitions for i and j", () => {
+    const { GSUB } = font.opentype.tables;
+    const lookup = GSUB.getLookup(3);
+    const subtable = lookup.getSubTable(0);
+
+    const inputClassDef = subtable.getInputClassDef();
+
+    expect(inputClassDef.classFormat).toBe(2);
+
+    expect(inputClassDef.classRangeRecords).toEqual([
+      { startGlyphID: 272, endGlyphID: 272, class: 2 }, // i
+      { startGlyphID: 288, endGlyphID: 288, class: 1 }, // j
+    ]);
+  });
+
+  test("getBacktrackClassDef returns no class definitions", () => {
+    const { GSUB } = font.opentype.tables;
+    const lookup = GSUB.getLookup(3);
+    const subtable = lookup.getSubTable(0);
+
+    const backtrackClassDef = subtable.getBacktrackClassDef();
+
+    // Bangers has no backtrack chars for this lookup
+    expect(backtrackClassDef.classRangeRecords).toEqual([]);
+  });
+
+  test("getLookaheadClassDef returns multiple definitions", () => {
+    const { GSUB } = font.opentype.tables;
+    const lookup = GSUB.getLookup(3);
+    const subtable = lookup.getSubTable(0);
+
+    const lookaheadClassDef = subtable.getLookaheadClassDef();
+
+    // Bangers lookahead:
+    // uni0308
+    // uni0307
+    // gravecomb
+    // acutecomb
+    // uni030B
+    // uni0302
+    // uni030C
+    // uni0306
+    // uni030A
+    // tildecomb
+    // uni0304
+    // hookabovecomb
+    // uni030F
+    // uni0312
+    expect(lookaheadClassDef.classRangeRecords).toEqual([
+      { startGlyphID: 525, endGlyphID: 529, class: 1 },
+      { startGlyphID: 531, endGlyphID: 538, class: 1 },
+      { startGlyphID: 540, endGlyphID: 540, class: 1 },
+    ]);
+  });
 });