[XPath] Implement fn:serialize, fn:parse-xml, and fn:parse-xml-fragment

16211 72.0% successes
    5  0.0% skipped
 4011 17.8% failures
 2287 10.2% errors

Author: renggli

lib/src/xpath/functions/accessor.dart

@@ -1,4 +1,6 @@
 import '../../xml/nodes/attribute.dart';
+import '../../xml/nodes/document.dart';
+import '../../xml/nodes/document_fragment.dart';
 import '../../xml/nodes/element.dart';
 import '../../xml/nodes/node.dart';
 import '../../xml/nodes/processing.dart';
@@ -169,7 +171,13 @@ XPathSequence _fnSerialize(
   XPathContext context,
   XPathSequence arg, [
   Object? params,
-]) => throw UnimplementedError('fn:serialize');
+]) {
+  // TODO: Add support for serialization parameters
+  final result = arg
+      .map((item) => item is XmlNode ? item.toXmlString() : xsString.cast(item))
+      .join();
+  return XPathSequence.single(result);
+}
 
 /// https://www.w3.org/TR/xpath-functions-31/#func-parse-xml
 const fnParseXml = XPathFunctionDefinition(
@@ -185,8 +193,10 @@ const fnParseXml = XPathFunctionDefinition(
   function: _fnParseXml,
 );
 
-XPathSequence _fnParseXml(XPathContext context, String? arg) =>
-    throw UnimplementedError('fn:parse-xml');
+XPathSequence _fnParseXml(XPathContext context, String? arg) {
+  if (arg == null) return XPathSequence.empty;
+  return XPathSequence.single(XmlDocument.parse(arg));
+}
 
 /// https://www.w3.org/TR/xpath-functions-31/#func-parse-xml-fragment
 const fnParseXmlFragment = XPathFunctionDefinition(
@@ -202,8 +212,10 @@ const fnParseXmlFragment = XPathFunctionDefinition(
   function: _fnParseXmlFragment,
 );
 
-XPathSequence _fnParseXmlFragment(XPathContext context, String? arg) =>
-    throw UnimplementedError('fn:parse-xml-fragment');
-
 XmlNode _defaultToContextItem(XPathContext context) =>
     xsNode.cast(context.item);
+
+XPathSequence _fnParseXmlFragment(XPathContext context, String? arg) {
+  if (arg == null) return XPathSequence.empty;
+  return XPathSequence.single(XmlDocumentFragment.parse(arg));
+}

test/xpath/functions/accessor_test.dart

@@ -86,4 +86,49 @@ void main() {
       isXPathSequence(isEmpty),
     );
   });
+  test('fn:serialize', () {
+    expect(
+      fnSerialize(context, [
+        XPathSequence([
+          document.findAllElements('a').first,
+          'text',
+          document.findAllElements('b').first,
+        ]),
+      ]),
+      isXPathSequence(['<a>1</a>text<b>2</b>']),
+    );
+    expect(fnSerialize(context, [XPathSequence.empty]), isXPathSequence(['']));
+  });
+  test('fn:parse-xml', () {
+    expect(
+      fnParseXml(context, [const XPathSequence.single('<r><a>1</a></r>')]),
+      isXPathSequence([isA<XmlDocument>()]),
+    );
+    expect(
+      fnParseXml(context, [XPathSequence.empty]),
+      isXPathSequence(isEmpty),
+    );
+    expect(
+      () => fnParseXml(context, [const XPathSequence.single('<r>unclosed')]),
+      throwsA(isA<XmlException>()),
+    );
+  });
+  test('fn:parse-xml-fragment', () {
+    expect(
+      fnParseXmlFragment(context, [
+        const XPathSequence.single('<a>1</a><b>2</b>'),
+      ]),
+      isXPathSequence([isA<XmlDocumentFragment>()]),
+    );
+    expect(
+      fnParseXmlFragment(context, [XPathSequence.empty]),
+      isXPathSequence(isEmpty),
+    );
+    expect(
+      () => fnParseXmlFragment(context, [
+        const XPathSequence.single('<r>unclosed'),
+      ]),
+      throwsA(isA<XmlException>()),
+    );
+  });
 }