fix: direction getter incorrectly matching -- inside literal values (#565)

* Initial plan

* Fix direction getter for literals with '--' in value and add tests

Co-authored-by: rubensworks <440384+rubensworks@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: rubensworks <440384+rubensworks@users.noreply.github.com>
Co-authored-by: Jesse Wright <63333554+jeswr@users.noreply.github.com>

Author: 3 people

src/N3DataFactory.js

@@ -95,10 +95,11 @@ export class Literal extends Term {
 
   // ### The direction of this literal
   get direction() {
-    // Find the last double dash (e.g., '"abc"@en-us--ltr')
+    // Find the last double dash after the closing quote (e.g., '"abc"@en-us--ltr')
     const id = this.id;
-    const atPos = id.lastIndexOf('--') + 2;
-    return atPos > 1 && atPos < id.length ? id.substr(atPos).toLowerCase() : '';
+    const endPos = id.lastIndexOf('"');
+    const dirPos = id.lastIndexOf('--');
+    return dirPos > endPos && dirPos + 2 < id.length ? id.substr(dirPos + 2).toLowerCase() : '';
   }
 
   // ### The datatype IRI of this literal

test/Literal-test.js

@@ -621,6 +621,48 @@ describe('Literal', () => {
     });
   });
 
+  describe('A Literal instance created from a string containing "--" with a language tag', () => {
+    let literal;
+    beforeAll(() => { literal = new Literal('"bla bla -- more bla bla"@en'); });
+
+    it('should have the text value as value', () => {
+      expect(literal).toHaveProperty('value', 'bla bla -- more bla bla');
+    });
+
+    it('should have the language tag as language', () => {
+      expect(literal).toHaveProperty('language', 'en');
+    });
+
+    it('should have the empty string as direction', () => {
+      expect(literal).toHaveProperty('direction', '');
+    });
+
+    it('should have rdf:langString as datatype', () => {
+      expect(literal.datatype.value).toBe('http://www.w3.org/1999/02/22-rdf-syntax-ns#langString');
+    });
+  });
+
+  describe('A Literal instance created from a string containing "--" with a language tag and direction', () => {
+    let literal;
+    beforeAll(() => { literal = new Literal('"bla bla -- more bla bla"@en--ltr'); });
+
+    it('should have the text value as value', () => {
+      expect(literal).toHaveProperty('value', 'bla bla -- more bla bla');
+    });
+
+    it('should have the language tag as language', () => {
+      expect(literal).toHaveProperty('language', 'en');
+    });
+
+    it('should have the direction as direction', () => {
+      expect(literal).toHaveProperty('direction', 'ltr');
+    });
+
+    it('should have rdf:dirLangString as datatype', () => {
+      expect(literal.datatype.value).toBe('http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString');
+    });
+  });
+
   describe('A Literal instance created from the empty string with a datatype', () => {
     let literal;
     beforeAll(() => { literal = new Literal('""^^http://example.org/types#type'); });

test/N3Writer-test.js

@@ -83,6 +83,12 @@ describe('Writer', () => {
             '<a> <b> "cde"@en-us--ltr.\n'),
     );
 
+    it(
+        'should serialize a literal containing "--" with a language',
+        shouldSerialize(['a', 'b', '"bla bla -- more bla bla"@en'],
+            '<a> <b> "bla bla -- more bla bla"@en.\n'),
+    );
+
     // e.g. http://vocab.getty.edu/aat/300264727.ttl
     it(
       'should serialize a literal with an artificial language',