From aafa09e7a5b5a9353d1a36d7877fd1d5039cd79f Mon Sep 17 00:00:00 2001
From: Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
Date: Fri, 7 Apr 2023 00:03:15 +0300
Subject: [PATCH] LibWeb: Implement "traverse the history by delta" for
 traversables

---
 .../LibWeb/HTML/TraversableNavigable.cpp      | 28 +++++++++++++++++++
 .../LibWeb/HTML/TraversableNavigable.h        |  1 +
 2 files changed, 29 insertions(+)

diff --git a/Userland/Libraries/LibWeb/HTML/TraversableNavigable.cpp b/Userland/Libraries/LibWeb/HTML/TraversableNavigable.cpp
index ae7ecd3ff96..2d46cf44a83 100644
--- a/Userland/Libraries/LibWeb/HTML/TraversableNavigable.cpp
+++ b/Userland/Libraries/LibWeb/HTML/TraversableNavigable.cpp
@@ -470,4 +470,32 @@ void TraversableNavigable::clear_the_forward_session_history()
     }
 }
 
+// https://html.spec.whatwg.org/multipage/browsing-the-web.html#traverse-the-history-by-a-delta
+void TraversableNavigable::traverse_the_history_by_delta(int delta)
+{
+    // FIXME: 1. Let sourceSnapshotParams and initiatorToCheck be null.
+
+    // FIXME: 2. If sourceDocument is given, then:
+
+    // 3. Append the following session history traversal steps to traversable:
+
+    // 1. Let allSteps be the result of getting all used history steps for traversable.
+    auto all_steps = get_all_used_history_steps();
+
+    // 2. Let currentStepIndex be the index of traversable's current session history step within allSteps.
+    auto current_step_index = *all_steps.find_first_index(current_session_history_step());
+
+    // 3. Let targetStepIndex be currentStepIndex plus delta
+    auto target_step_index = current_step_index + delta;
+
+    // 4. If allSteps[targetStepIndex] does not exist, then abort these steps.
+    if (target_step_index >= all_steps.size()) {
+        return;
+    }
+
+    // 5. Apply the history step allSteps[targetStepIndex] to traversable, with checkForUserCancelation set to true,
+    //    sourceSnapshotParams set to sourceSnapshotParams, and initiatorToCheck set to initiatorToCheck.
+    apply_the_history_step(all_steps[target_step_index]);
+}
+
 }
diff --git a/Userland/Libraries/LibWeb/HTML/TraversableNavigable.h b/Userland/Libraries/LibWeb/HTML/TraversableNavigable.h
index 68be0956426..810e2e44591 100644
--- a/Userland/Libraries/LibWeb/HTML/TraversableNavigable.h
+++ b/Userland/Libraries/LibWeb/HTML/TraversableNavigable.h
@@ -42,6 +42,7 @@ public:
     Vector<JS::Handle<Navigable>> get_all_navigables_whose_current_session_history_entry_will_change_or_reload(int) const;
     Vector<int> get_all_used_history_steps() const;
     void clear_the_forward_session_history();
+    void traverse_the_history_by_delta(int delta);
 
 private:
     TraversableNavigable();