4 files changed, 87 insertions, 0 deletions

diff --git a/Userland/Libraries/LibWeb/DOM/Element.h b/Userland/Libraries/LibWeb/DOM/Element.h
index c6bc944399..8f7edebc00 100644
--- a/Userland/Libraries/LibWeb/DOM/Element.h
+++ b/Userland/Libraries/LibWeb/DOM/Element.h
@@ -52,6 +52,7 @@ public:
     String get_attribute(const FlyString& name) const { return attribute(name); }
     ExceptionOr<void> set_attribute(const FlyString& name, const String& value);
     void remove_attribute(const FlyString& name);
+    size_t attribute_list_size() const { return m_attributes.size(); }
 
     template<typename Callback>
     void for_each_attribute(Callback callback) const
diff --git a/Userland/Libraries/LibWeb/DOM/Node.cpp b/Userland/Libraries/LibWeb/DOM/Node.cpp
index 2730a47223..b5367692eb 100644
--- a/Userland/Libraries/LibWeb/DOM/Node.cpp
+++ b/Userland/Libraries/LibWeb/DOM/Node.cpp
@@ -774,4 +774,88 @@ bool Node::is_same_node(Node const* other_node) const
     return this == other_node;
 }
 
+// https://dom.spec.whatwg.org/#dom-node-isequalnode
+bool Node::is_equal_node(Node const* other_node) const
+{
+    // The isEqualNode(otherNode) method steps are to return true if otherNode is non-null and this equals otherNode; otherwise false.
+    if (!other_node)
+        return false;
+
+    // Fast path for testing a node against itself.
+    if (this == other_node)
+        return true;
+
+    // A node A equals a node B if all of the following conditions are true:
+
+    // A and B implement the same interfaces.
+    if (node_name() != other_node->node_name())
+        return false;
+
+    // The following are equal, switching on the interface A implements:
+    switch (node_type()) {
+    case (u16)NodeType::DOCUMENT_TYPE_NODE: {
+        // Its name, public ID, and system ID.
+        auto& this_doctype = verify_cast<DocumentType>(*this);
+        auto& other_doctype = verify_cast<DocumentType>(*other_node);
+        if (this_doctype.name() != other_doctype.name()
+            || this_doctype.public_id() != other_doctype.public_id()
+            || this_doctype.system_id() != other_doctype.system_id())
+            return false;
+        break;
+    }
+    case (u16)NodeType::ELEMENT_NODE: {
+        // Its namespace, namespace prefix, local name, and its attribute list’s size.
+        auto& this_element = verify_cast<Element>(*this);
+        auto& other_element = verify_cast<Element>(*other_node);
+        if (this_element.namespace_() != other_element.namespace_()
+            || this_element.prefix() != other_element.prefix()
+            || this_element.local_name() != other_element.local_name()
+            || this_element.attribute_list_size() != other_element.attribute_list_size())
+            return false;
+        // If A is an element, each attribute in its attribute list has an attribute that equals an attribute in B’s attribute list.
+        bool has_same_attributes = true;
+        this_element.for_each_attribute([&](auto& name, auto& value) {
+            if (other_element.get_attribute(name) != value)
+                has_same_attributes = false;
+        });
+        if (!has_same_attributes)
+            return false;
+        break;
+    }
+    case (u16)NodeType::COMMENT_NODE:
+    case (u16)NodeType::TEXT_NODE: {
+        // Its data.
+        auto& this_cdata = verify_cast<CharacterData>(*this);
+        auto& other_cdata = verify_cast<CharacterData>(*other_node);
+        if (this_cdata.data() != other_cdata.data())
+            return false;
+        break;
+    }
+    case (u16)NodeType::PROCESSING_INSTRUCTION_NODE:
+    case (u16)NodeType::ATTRIBUTE_NODE:
+        TODO();
+    default:
+        break;
+    }
+
+    // A and B have the same number of children.
+    size_t this_child_count = child_count();
+    size_t other_child_count = other_node->child_count();
+    if (this_child_count != other_child_count)
+        return false;
+
+    // Each child of A equals the child of B at the identical index.
+    // FIXME: This can be made nicer. child_at_index() is O(n).
+    for (size_t i = 0; i < this_child_count; ++i) {
+        auto* this_child = child_at_index(i);
+        auto* other_child = other_node->child_at_index(i);
+        VERIFY(this_child);
+        VERIFY(other_child);
+        if (!this_child->is_equal_node(other_child))
+            return false;
+    }
+
+    return true;
+}
+
 }
diff --git a/Userland/Libraries/LibWeb/DOM/Node.h b/Userland/Libraries/LibWeb/DOM/Node.h
index eb2f6747f1..22266eeafa 100644
--- a/Userland/Libraries/LibWeb/DOM/Node.h
+++ b/Userland/Libraries/LibWeb/DOM/Node.h
@@ -187,6 +187,7 @@ public:
     void string_replace_all(String const&);
 
     bool is_same_node(Node const*) const;
+    bool is_equal_node(Node const*) const;
 
 protected:
     Node(Document&, NodeType);
diff --git a/Userland/Libraries/LibWeb/DOM/Node.idl b/Userland/Libraries/LibWeb/DOM/Node.idl
index 419793432f..a1a15d6baa 100644
--- a/Userland/Libraries/LibWeb/DOM/Node.idl
+++ b/Userland/Libraries/LibWeb/DOM/Node.idl
@@ -26,6 +26,7 @@ interface Node : EventTarget {
     [ImplementedAs=pre_remove] Node removeChild(Node child);
     [ImplementedAs=clone_node_binding] Node cloneNode(optional boolean deep = false);
     boolean contains(Node? other);
+    boolean isEqualNode(Node? otherNode);
     boolean isSameNode(Node? otherNode);
 
     const unsigned short ELEMENT_NODE = 1;