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.cpp

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/DOM/Element.h>
+#include <LibWeb/DOM/HTMLCollection.h>
+#include <LibWeb/DOM/ParentNode.h>
+
+namespace Web::DOM {
+
+HTMLCollection::HTMLCollection(ParentNode& root, Function<bool(Element const&)> filter)
+    : m_root(root)
+    , m_filter(move(filter))
+{
+}
+
+HTMLCollection::~HTMLCollection()
+{
+}
+
+Vector<NonnullRefPtr<Element>> HTMLCollection::collect_matching_elements()
+{
+    Vector<NonnullRefPtr<Element>> elements;
+    m_root->for_each_in_inclusive_subtree_of_type<Element>([&](auto& element) {
+        if (m_filter(element))
+            elements.append(element);
+        return IterationDecision::Continue;
+    });
+    return elements;
+}
+
+size_t HTMLCollection::length()
+{
+    return collect_matching_elements().size();
+}
+
+Element* HTMLCollection::item(size_t index)
+{
+    auto elements = collect_matching_elements();
+    if (index >= elements.size())
+        return nullptr;
+    return elements[index];
+}
+
+Element* HTMLCollection::named_item(FlyString const& name)
+{
+    if (name.is_null())
+        return nullptr;
+    auto elements = collect_matching_elements();
+    // First look for an "id" attribute match
+    if (auto it = elements.find_if([&](auto& entry) { return entry->attribute(HTML::AttributeNames::id) == name; }); it != elements.end())
+        return *it;
+    // Then look for a "name" attribute match
+    if (auto it = elements.find_if([&](auto& entry) { return entry->name() == name; }); it != elements.end())
+        return *it;
+    return nullptr;
+}
+
+}