[XPath] Implement fn:generate-id

16185 71.9% successes
    5  0.0% skipped
 3922 17.4% failures
 2402 10.7% errors

Author: renggli

lib/src/xpath/functions/node.dart

@@ -176,8 +176,10 @@ const fnGenerateId = XPathFunctionDefinition(
 
 XPathSequence _fnGenerateId(XPathContext context, [XmlNode? node]) {
   if (node == null) return XPathSequence.emptyString;
-  // Fall back to UnimplementedError for now, test checks exception type.
-  throw UnimplementedError('fn:generate-id');
+  final id = identityHashCode(
+    node,
+  ).toRadixString(16).toUpperCase().padLeft(8, '0');
+  return XPathSequence.single('autoId$id');
 }
 
 /// https://www.w3.org/TR/xpath-functions-31/#func-root

test/xpath/functions/node_test.dart

@@ -56,6 +56,13 @@ void main() {
       isXPathSequence(['/r/a']),
     );
   });
+  test('fn:generate-id', () {
+    final ids = document.descendants
+        .map((node) => fnGenerateId(context, [XPathSequence.single(node)]))
+        .map((sequence) => sequence.single)
+        .toList();
+    expect(ids, unorderedEquals(ids.toSet()));
+  });
   group('integration', () {
     final xml = XmlDocument.parse('<r><a>1</a><b>2<c/>3</b></r>');
     test('last()', () {