LibWeb: Give each Node a unique ID

We maintain a directory of ID -> Node. Nodes add themselves to this
directory when they are created, receiving a random ID. When a Node is
destroyed, it removes itself from this directory. Anyone can request a
Node from the directory by its ID using `Node::from_id()`.

We reserve the `0` ID to mean "none".

These IDs allow different processes to communicate about a given Node
over IPC, for example the DOM Inspector.

Userland/Libraries/LibWeb/DOM/Document.h

@@ -8,6 +8,7 @@
 
 #include <AK/FlyString.h>
 #include <AK/Function.h>
+#include <AK/HashMap.h>
 #include <AK/NonnullRefPtrVector.h>
 #include <AK/OwnPtr.h>
 #include <AK/String.h>

Userland/Libraries/LibWeb/DOM/Node.cpp

@@ -6,6 +6,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/IDAllocator.h>
 #include <AK/StringBuilder.h>
 #include <LibJS/AST.h>
 #include <LibJS/Runtime/FunctionObject.h>
@@ -30,10 +31,33 @@
 
 namespace Web::DOM {
 
+static IDAllocator s_node_id_allocator;
+static HashMap<i32, Node*> s_node_directory;
+
+static i32 allocate_node_id(Node* node)
+{
+    i32 id = s_node_id_allocator.allocate();
+    s_node_directory.set(id, node);
+    return id;
+}
+
+static void deallocate_node_id(i32 node_id)
+{
+    if (!s_node_directory.remove(node_id))
+        VERIFY_NOT_REACHED();
+    s_node_id_allocator.deallocate(node_id);
+}
+
+Node* Node::from_id(i32 node_id)
+{
+    return s_node_directory.get(node_id).value_or(nullptr);
+}
+
 Node::Node(Document& document, NodeType type)
     : EventTarget(static_cast<Bindings::ScriptExecutionContext&>(document))
     , m_document(&document)
     , m_type(type)
+    , m_id(allocate_node_id(this))
 {
     if (!is_document())
         m_document->ref_from_node({});
@@ -47,6 +71,8 @@ Node::~Node()
 
     if (!is_document())
         m_document->unref_from_node({});
+
+    deallocate_node_id(m_id);
 }
 
 const HTML::HTMLAnchorElement* Node::enclosing_link_element() const
@@ -477,6 +503,7 @@ void Node::set_document(Badge<Document>, Document& document)
 {
     if (m_document == &document)
         return;
+
     document.ref_from_node({});
     m_document->unref_from_node({});
     m_document = &document;

Userland/Libraries/LibWeb/DOM/Node.h

@@ -179,6 +179,9 @@ public:
     bool is_shadow_including_ancestor_of(Node const&) const;
     bool is_shadow_including_inclusive_ancestor_of(Node const&) const;
 
+    i32 id() const { return m_id; }
+    static Node* from_id(i32 node_id);
+
 protected:
     Node(Document&, NodeType);
 
@@ -187,6 +190,8 @@ protected:
     NodeType m_type { NodeType::INVALID };
     bool m_needs_style_update { false };
     bool m_child_needs_style_update { false };
+
+    i32 m_id;
 };
 
 }