LibJS: Add a simple reference-counted UTF-16 string

To help alleviate memory usage when creating and copying large strings,
create a simple wrapper around a Vector<u16> to reference count UTF-16
strings.

Userland/Libraries/LibJS/Runtime/Utf16String.cpp

@@ -0,0 +1,120 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/StringView.h>
+#include <AK/Utf16View.h>
+#include <LibJS/Runtime/Utf16String.h>
+
+namespace JS {
+namespace Detail {
+
+static NonnullRefPtr<Utf16StringImpl> the_empty_utf16_string()
+{
+    static NonnullRefPtr<Utf16StringImpl> empty_string = Utf16StringImpl::create();
+    return empty_string;
+}
+
+Utf16StringImpl::Utf16StringImpl(Vector<u16> string)
+    : m_string(move(string))
+{
+}
+
+NonnullRefPtr<Utf16StringImpl> Utf16StringImpl::create()
+{
+    return adopt_ref(*new Utf16StringImpl());
+}
+
+NonnullRefPtr<Utf16StringImpl> Utf16StringImpl::create(Vector<u16> string)
+{
+    return adopt_ref(*new Utf16StringImpl(move(string)));
+}
+
+NonnullRefPtr<Utf16StringImpl> Utf16StringImpl::create(StringView const& string)
+{
+    return create(AK::utf8_to_utf16(string));
+}
+
+NonnullRefPtr<Utf16StringImpl> Utf16StringImpl::create(Utf16View const& view)
+{
+    Vector<u16> string;
+    string.ensure_capacity(view.length_in_code_units());
+    string.append(view.data(), view.length_in_code_units());
+    return create(move(string));
+}
+
+Vector<u16> const& Utf16StringImpl::string() const
+{
+    return m_string;
+}
+
+Utf16View Utf16StringImpl::view() const
+{
+    return Utf16View { m_string };
+}
+
+}
+
+Utf16String::Utf16String()
+    : m_string(Detail::the_empty_utf16_string())
+{
+}
+
+Utf16String::Utf16String(Vector<u16> string)
+    : m_string(Detail::Utf16StringImpl::create(move(string)))
+{
+}
+
+Utf16String::Utf16String(StringView const& string)
+    : m_string(Detail::Utf16StringImpl::create(move(string)))
+{
+}
+
+Utf16String::Utf16String(Utf16View const& string)
+    : m_string(Detail::Utf16StringImpl::create(move(string)))
+{
+}
+
+Vector<u16> const& Utf16String::string() const
+{
+    return m_string->string();
+}
+
+Utf16View Utf16String::view() const
+{
+    return m_string->view();
+}
+
+Utf16View Utf16String::substring_view(size_t code_unit_offset, size_t code_unit_length) const
+{
+    return view().substring_view(code_unit_offset, code_unit_length);
+}
+
+Utf16View Utf16String::substring_view(size_t code_unit_offset) const
+{
+    return view().substring_view(code_unit_offset);
+}
+
+String Utf16String::to_utf8() const
+{
+    return view().to_utf8(Utf16View::AllowInvalidCodeUnits::Yes);
+}
+
+u16 Utf16String::code_unit_at(size_t index) const
+{
+    return view().code_unit_at(index);
+}
+
+size_t Utf16String::length_in_code_units() const
+{
+    return view().length_in_code_units();
+}
+
+bool Utf16String::is_empty() const
+{
+    return view().is_empty();
+}
+
+}