From be9c975b2e9da68fadd83f6a7f88c58837ad5556 Mon Sep 17 00:00:00 2001
From: Sam Atkins <atkinssj@serenityos.org>
Date: Fri, 11 Aug 2023 20:20:09 +0100
Subject: [PATCH] LibWeb: Implement algorithms for determining an indicated
 element

---
 Userland/Libraries/LibWeb/DOM/Document.cpp | 66 ++++++++++++++++++++++
 Userland/Libraries/LibWeb/DOM/Document.h   |  5 ++
 2 files changed, 71 insertions(+)

diff --git a/Userland/Libraries/LibWeb/DOM/Document.cpp b/Userland/Libraries/LibWeb/DOM/Document.cpp
index 4915601fda..b5f1d02151 100644
--- a/Userland/Libraries/LibWeb/DOM/Document.cpp
+++ b/Userland/Libraries/LibWeb/DOM/Document.cpp
@@ -1730,6 +1730,72 @@ void Document::set_target_element(Element* element)
         m_layout_root->set_needs_display();
 }
 
+// https://html.spec.whatwg.org/multipage/browsing-the-web.html#the-indicated-part-of-the-document
+Document::IndicatedPart Document::determine_the_indicated_part() const
+{
+    // For an HTML document document, the following processing model must be followed to determine its indicated part:
+
+    // 1. Let fragment be document's URL's fragment.
+    auto fragment = url().raw_fragment();
+
+    // 2. If fragment is the empty string, then return the special value top of the document.
+    if (fragment.is_empty())
+        return Document::TopOfTheDocument {};
+
+    // 3. Let potentialIndicatedElement be the result of finding a potential indicated element given document and fragment.
+    auto potential_indicated_element = find_a_potential_indicated_element(fragment);
+
+    // 4. If potentialIndicatedElement is not null, then return potentialIndicatedElement.
+    if (potential_indicated_element)
+        return potential_indicated_element;
+
+    // 5. Let fragmentBytes be the result of percent-decoding fragment.
+    // 6. Let decodedFragment be the result of running UTF-8 decode without BOM on fragmentBytes.
+    // NOTE: 5 and 6 are done as a single step in `AK::URL::fragment()`.
+    auto decoded_fragment = url().fragment();
+
+    // 7. Set potentialIndicatedElement to the result of finding a potential indicated element given document and decodedFragment.
+    potential_indicated_element = find_a_potential_indicated_element(decoded_fragment);
+
+    // 8. If potentialIndicatedElement is not null, then return potentialIndicatedElement.
+    if (potential_indicated_element)
+        return potential_indicated_element;
+
+    // 9. If decodedFragment is an ASCII case-insensitive match for the string top, then return the top of the document.
+    if (Infra::is_ascii_case_insensitive_match(decoded_fragment, "top"sv))
+        return Document::TopOfTheDocument {};
+
+    // 10. Return null.
+    return nullptr;
+}
+
+// https://html.spec.whatwg.org/multipage/browsing-the-web.html#find-a-potential-indicated-element
+Element* Document::find_a_potential_indicated_element(DeprecatedString fragment) const
+{
+    // To find a potential indicated element given a Document document and a string fragment, run these steps:
+
+    // 1. If there is an element in the document tree whose root is document and that has an ID equal to
+    //    fragment, then return the first such element in tree order.
+    if (auto element = get_element_by_id(fragment))
+        return const_cast<Element*>(element.ptr());
+
+    // 2. If there is an a element in the document tree whose root is document that has a name attribute
+    //    whose value is equal to fragment, then return the first such element in tree order.
+    Element* element_with_name;
+    root().for_each_in_subtree_of_type<Element>([&](Element const& element) {
+        if (element.name() == fragment) {
+            element_with_name = const_cast<Element*>(&element);
+            return IterationDecision::Break;
+        }
+        return IterationDecision::Continue;
+    });
+    if (element_with_name)
+        return element_with_name;
+
+    // 3. Return null.
+    return nullptr;
+}
+
 DeprecatedString Document::ready_state() const
 {
     switch (m_readiness) {
diff --git a/Userland/Libraries/LibWeb/DOM/Document.h b/Userland/Libraries/LibWeb/DOM/Document.h
index 0bb6f76770..a25f43e4df 100644
--- a/Userland/Libraries/LibWeb/DOM/Document.h
+++ b/Userland/Libraries/LibWeb/DOM/Document.h
@@ -524,6 +524,11 @@ private:
     void queue_intersection_observer_task();
     void queue_an_intersection_observer_entry(IntersectionObserver::IntersectionObserver&, HighResolutionTime::DOMHighResTimeStamp time, JS::NonnullGCPtr<Geometry::DOMRectReadOnly> root_bounds, JS::NonnullGCPtr<Geometry::DOMRectReadOnly> bounding_client_rect, JS::NonnullGCPtr<Geometry::DOMRectReadOnly> intersection_rect, bool is_intersecting, double intersection_ratio, JS::NonnullGCPtr<Element> target);
 
+    struct TopOfTheDocument { };
+    using IndicatedPart = Variant<Element*, TopOfTheDocument>;
+    IndicatedPart determine_the_indicated_part() const;
+    Element* find_a_potential_indicated_element(DeprecatedString fragment) const;
+
     OwnPtr<CSS::StyleComputer> m_style_computer;
     JS::GCPtr<CSS::StyleSheetList> m_style_sheets;
     JS::GCPtr<Node> m_hovered_node;