AK: Implement a flyweight string for Utf16String

Utf16FlyString more or less works exactly the same as FlyString. It will
store the raw encoded data of the string instance. If the string is a
short ASCII string, Utf16FlyString holds the ShortString bytes; else,
Utf16FlyString holds a pointer to the Utf16StringData.

AK/Utf16FlyString.cpp

@@ -0,0 +1,110 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/HashTable.h>
+#include <AK/Singleton.h>
+#include <AK/Utf16FlyString.h>
+
+namespace AK {
+
+struct Utf16FlyStringTableHashTraits : public Traits<Detail::Utf16StringData const*> {
+    static u32 hash(Detail::Utf16StringData const* string) { return string->hash(); }
+    static bool equals(Detail::Utf16StringData const* a, Detail::Utf16StringData const* b) { return *a == *b; }
+};
+
+static auto& all_utf16_fly_strings()
+{
+    static Singleton<HashTable<Detail::Utf16StringData const*, Utf16FlyStringTableHashTraits>> table;
+    return *table;
+}
+
+namespace Detail {
+
+void did_destroy_utf16_fly_string_data(Badge<Detail::Utf16StringData>, Detail::Utf16StringData const& data)
+{
+    all_utf16_fly_strings().remove(&data);
+}
+
+}
+
+template<typename ViewType>
+Optional<Utf16FlyString> Utf16FlyString::create_fly_string_from_cache(ViewType const& string)
+{
+    if (string.is_empty())
+        return {};
+
+    if constexpr (IsSame<ViewType, StringView>) {
+        if (string.length() <= Detail::MAX_SHORT_STRING_BYTE_COUNT && string.is_ascii())
+            return Utf16String::from_utf8_without_validation(string);
+    } else {
+        if (string.length_in_code_units() <= Detail::MAX_SHORT_STRING_BYTE_COUNT && string.is_ascii())
+            return Utf16String::from_utf16_without_validation(string);
+    }
+
+    if (auto it = all_utf16_fly_strings().find(string.hash(), [&](auto const& entry) { return *entry == string; }); it != all_utf16_fly_strings().end())
+        return Utf16FlyString { Detail::Utf16StringBase(**it) };
+
+    return {};
+}
+
+Utf16FlyString Utf16FlyString::from_utf8(StringView string)
+{
+    if (auto result = create_fly_string_from_cache(string); result.has_value())
+        return result.release_value();
+    return Utf16String::from_utf8(string);
+}
+
+Utf16FlyString Utf16FlyString::from_utf8_without_validation(StringView string)
+{
+    if (auto result = create_fly_string_from_cache(string); result.has_value())
+        return result.release_value();
+    return Utf16String::from_utf8_without_validation(string);
+}
+
+Utf16FlyString Utf16FlyString::from_utf16(Utf16View const& string)
+{
+    if (auto result = create_fly_string_from_cache(string); result.has_value())
+        return result.release_value();
+    return Utf16String::from_utf16(string);
+}
+
+Utf16FlyString Utf16FlyString::from_utf16_without_validation(Utf16View const& string)
+{
+    if (auto result = create_fly_string_from_cache(string); result.has_value())
+        return result.release_value();
+    return Utf16String::from_utf16_without_validation(string);
+}
+
+Utf16FlyString::Utf16FlyString(Utf16String const& string)
+{
+    if (string.has_short_ascii_storage()) {
+        m_data = string;
+        return;
+    }
+
+    auto const* data = string.data({});
+
+    if (data->is_fly_string()) {
+        m_data = string;
+        return;
+    }
+
+    if (auto it = all_utf16_fly_strings().find(data); it == all_utf16_fly_strings().end()) {
+        m_data = string;
+
+        all_utf16_fly_strings().set(data);
+        data->mark_as_fly_string({});
+    } else {
+        m_data.set_data({}, *it);
+    }
+}
+
+size_t Utf16FlyString::number_of_utf16_fly_strings()
+{
+    return all_utf16_fly_strings().size();
+}
+
+}