LibWeb: Implement a slow but functional HTMLCollection :^)

HTMLCollection is an awkward legacy interface from the DOM spec.

It provides a live view of a DOM subtree, with some kind of filtering
that determines which elements are part of the collection.

We now return HTMLCollection objects from these APIs:

- getElementsByClassName()
- getElementsByName()
- getElementsByTagName()

This initial implementation does not do any kind of caching, since that
is quite a tricky problem, and there will be plenty of time for tricky
problems later on when the engine is more mature.

Userland/Libraries/LibWeb/DOM/HTMLCollection.h

@@ -0,0 +1,64 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/FlyString.h>
+#include <AK/Function.h>
+#include <AK/Noncopyable.h>
+#include <LibWeb/Bindings/Wrappable.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::DOM {
+
+// NOTE: HTMLCollection is in the DOM namespace because it's part of the DOM specification.
+
+// This class implements a live, filtered view of a DOM subtree.
+// When constructing an HTMLCollection, you provide a root node + a filter.
+// The filter is a simple Function object that answers the question
+// "is this Element part of the collection?"
+
+// FIXME: HTMLCollection currently does no caching. It will re-filter on every access!
+//        We should teach it how to cache results. The main challenge is invalidating
+//        these caches, since this needs to happen on various kinds of DOM mutation.
+
+class HTMLCollection
+    : public RefCounted<HTMLCollection>
+    , public Bindings::Wrappable {
+    AK_MAKE_NONCOPYABLE(HTMLCollection);
+    AK_MAKE_NONMOVABLE(HTMLCollection);
+
+public:
+    using WrapperType = Bindings::HTMLCollectionWrapper;
+
+    static NonnullRefPtr<HTMLCollection> create(ParentNode& root, Function<bool(Element const&)> filter)
+    {
+        return adopt(*new HTMLCollection(root, move(filter)));
+    }
+
+    ~HTMLCollection();
+
+    size_t length();
+    Element* item(size_t index);
+    Element* named_item(FlyString const& name);
+
+    Vector<NonnullRefPtr<Element>> collect_matching_elements();
+
+protected:
+    HTMLCollection(ParentNode& root, Function<bool(Element const&)> filter);
+
+private:
+    NonnullRefPtr<ParentNode> m_root;
+    Function<bool(Element const&)> m_filter;
+};
+
+}
+
+namespace Web::Bindings {
+
+HTMLCollectionWrapper* wrap(JS::GlobalObject&, DOM::HTMLCollection&);
+
+}