Add XMLNode::operator==/!=() for comparing XMLNode instances

Implemented to be able to test that when writing an XML document via XMLTree
and then reading back into another XMLTree the structure is equivalent as a
general API test of pbd/xml++.h to check for breakage when changing
implementation.

libs/pbd/pbd/xml++.h

@@ -101,6 +101,9 @@ public:
 
 	XMLNode& operator= (const XMLNode& other);
 
+	bool operator== (const XMLNode& other) const;
+	bool operator!= (const XMLNode& other) const;
+
 	const std::string& name() const { return _name; }
 
 	bool          is_content() const { return _is_content; }

libs/pbd/xml++.cc

@@ -289,6 +289,70 @@ XMLNode::operator= (const XMLNode& from)
 	return *this;
 }
 
+bool
+XMLNode::operator== (const XMLNode& other) const
+{
+	if (is_content () != other.is_content ()) {
+		return false;
+	}
+
+	if (is_content ()) {
+		if (content () != other.content ()) {
+			return false;
+		}
+	} else {
+		if (name () != other.name ()) {
+			return false;
+		}
+	}
+
+	XMLPropertyList const& other_properties = other.properties ();
+
+	if (_proplist.size () != other_properties.size ()) {
+		return false;
+	}
+
+	XMLPropertyConstIterator our_prop_iter = _proplist.begin();
+	XMLPropertyConstIterator other_prop_iter = other_properties.begin();
+
+	while (our_prop_iter != _proplist.end ()) {
+		XMLProperty const* our_prop = *our_prop_iter;
+		XMLProperty const* other_prop = *other_prop_iter;
+		if (our_prop->name () != other_prop->name () || our_prop->value () != other_prop->value ()) {
+			return false;
+		}
+		++our_prop_iter;
+		++other_prop_iter;
+	}
+
+	XMLNodeList const& other_children = other.children();
+
+	if (_children.size() != other_children.size()) {
+		return false;
+	}
+
+	XMLNodeConstIterator our_child_iter = _children.begin ();
+	XMLNodeConstIterator other_child_iter = other_children.begin ();
+
+	while (our_child_iter != _children.end()) {
+		XMLNode const* our_child = *our_child_iter;
+		XMLNode const* other_child = *other_child_iter;
+
+		if (*our_child != *other_child) {
+			return false;
+		}
+		++our_child_iter;
+		++other_child_iter;
+	}
+	return true;
+}
+
+bool
+XMLNode::operator!= (const XMLNode& other) const
+{
+	return !(*this == other);
+}
+
 const string&
 XMLNode::set_content(const string& c)
 {