Support base IRIs in writer.

Closes #157

src/N3Writer.js

@@ -55,6 +55,7 @@ export default class N3Writer {
     if (!(/triple|quad/i).test(options.format)) {
       this._lineMode = false;
       this._graph = DEFAULTGRAPH;
+      this._baseIRI = options.baseIRI;
       this._prefixIRIs = Object.create(null);
       options.prefixes && this.addPrefixes(options.prefixes);
     }
@@ -145,8 +146,11 @@ export default class N3Writer {
         entity = this.list(this._lists[entity.value]);
       return 'id' in entity ? entity.id : `_:${entity.value}`;
     }
-    // Escape special characters
     let iri = entity.value;
+    // Use relative IRIs if requested and possible
+    if (this._baseIRI && iri.startsWith(this._baseIRI))
+      iri = iri.substr(this._baseIRI.length);
+    // Escape special characters
     if (escape.test(iri))
       iri = iri.replace(escapeAll, characterReplacer);
     // Try to represent the IRI as prefixed name

test/N3Writer-test.js

@@ -337,6 +337,18 @@ describe('Writer', () => {
       });
     });
 
+    it('uses a base IRI when given', done => {
+      const writer = new Writer({ baseIRI: 'http://example.org/foo/' });
+      writer.addQuad(new Quad(
+        new NamedNode('http://example.org/foo/'),
+        new NamedNode('http://example.org/foo/#b'),
+        new NamedNode('http://example.org/foo/cdeFgh/ijk')));
+      writer.end((error, output) => {
+        output.should.equal('<> <#b> <cdeFgh/ijk>.\n');
+        done(error);
+      });
+    });
+
     it('should accept triples with separated components', done => {
       const writer = new Writer();
       writer.addQuad(new NamedNode('a'), new NamedNode('b'), new NamedNode('c'));