LibWeb: Introduce Performance Timeline and its Performance functions

Userland/Libraries/LibWeb/PerformanceTimeline/PerformanceEntryTuple.h

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+/*
+ * Copyright (c) 2023, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/PerformanceTimeline/PerformanceEntry.h>
+
+namespace Web::PerformanceTimeline {
+
+// https://www.w3.org/TR/performance-timeline/#dfn-performance-entry-buffer-map
+struct PerformanceEntryTuple {
+    // https://www.w3.org/TR/performance-timeline/#dfn-performance-entry-buffer
+    // A performance entry buffer to store PerformanceEntry objects, that is initially empty.
+    Vector<JS::Handle<PerformanceEntry>> performance_entry_buffer;
+
+    // https://www.w3.org/TR/performance-timeline/#dfn-maxbuffersize
+    // An integer maxBufferSize, initialized to the registry value for this entry type.
+    // NOTE: The empty state represents Infinite size.
+    Optional<u64> max_buffer_size;
+
+    // https://www.w3.org/TR/performance-timeline/#dfn-availablefromtimeline
+    // A boolean availableFromTimeline, initialized to the registry value for this entry type.
+    AvailableFromTimeline available_from_timeline { AvailableFromTimeline::No };
+
+    // https://www.w3.org/TR/performance-timeline/#dfn-dropped-entries-count
+    // An integer dropped entries count that is initially 0.
+    u64 dropped_entries_count { 0 };
+
+    // https://www.w3.org/TR/performance-timeline/#dfn-determine-if-a-performance-entry-buffer-is-full
+    bool is_full()
+    {
+        // 1. Let num current entries be the size of tuple's performance entry buffer.
+        auto num_current_entries = performance_entry_buffer.size();
+
+        // 2. If num current entries is less than tuples's maxBufferSize, return false.
+        if (!max_buffer_size.has_value() || num_current_entries < max_buffer_size.value())
+            return false;
+
+        // 3. Increase tuple's dropped entries count by 1.
+        ++dropped_entries_count;
+
+        // 4. Return true.
+        return true;
+    }
+};
+
+}