LibJS: Add the Set built-in object

Userland/Libraries/LibJS/Runtime/Set.h

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/HashTable.h>
+#include <LibJS/Runtime/BigInt.h>
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Object.h>
+#include <LibJS/Runtime/Value.h>
+
+namespace JS {
+
+struct ValueTraits : public Traits<Value> {
+    static unsigned hash(Value value)
+    {
+        VERIFY(!value.is_empty());
+        if (value.is_string())
+            return value.as_string().string().hash();
+
+        if (value.is_bigint())
+            return value.as_bigint().big_integer().hash();
+
+        return u64_hash(value.encoded()); // FIXME: Is this the best way to hash pointers, doubles & ints?
+    }
+    static bool equals(const Value a, const Value b)
+    {
+        return same_value_zero(a, b);
+    }
+};
+
+class Set : public Object {
+    JS_OBJECT(Set, Object);
+
+public:
+    static Set* create(GlobalObject&);
+
+    explicit Set(Object& prototype);
+    virtual ~Set() override;
+
+    static Set* typed_this(VM&, GlobalObject&);
+
+    HashTable<Value, ValueTraits> const& values() const { return m_values; };
+    HashTable<Value, ValueTraits>& values() { return m_values; };
+
+private:
+    HashTable<Value, ValueTraits> m_values; // FIXME: Replace with a HashTable that maintains a linked list of insertion order for correct iteration order
+};
+
+}