AK: Add a UTF-16 string with optimized short- and ASCII-string storage

This is a strictly UTF-16 string with some optimizations for ASCII.

* If created from a short UTF-8 or UTF-16 string that is also ASCII,
  then the string is stored in an inlined byte buffer.

* If created with a long UTF-8 or UTF-16 string that is also ASCII,
  then the string is stored in an outlined char buffer.

* If created with a short or long UTF-8 or UTF-16 string that is not
  ASCII, then the string is stored in an outlined char16 buffer.

We do not store short non-ASCII text in the inlined buffer to avoid
confusion with operations such as `length_in_code_units` and
`code_unit_at`. For example, "😀" would be stored as 4 UTF-8 bytes
in short string form. But we still want `length_in_code_units` to
be 2, and `code_unit_at(0)` to be 0xD83D.

AK/Utf16String.h

@@ -0,0 +1,117 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Badge.h>
+#include <AK/Error.h>
+#include <AK/Format.h>
+#include <AK/NonnullRefPtr.h>
+#include <AK/String.h>
+#include <AK/StringView.h>
+#include <AK/Traits.h>
+#include <AK/UnicodeUtils.h>
+#include <AK/Utf16StringBase.h>
+#include <AK/Utf16StringData.h>
+#include <AK/Utf16View.h>
+#include <AK/Utf8View.h>
+
+namespace AK {
+
+// Utf16String is a strongly owned sequence of Unicode code points encoded as UTF-16.
+//
+// The data may or may not be heap-allocated, and may or may not be reference counted. As a memory optimization, if the
+// UTF-16 string is entirely ASCII, the string is stored as 8-bit bytes.
+class [[nodiscard]] Utf16String : public Detail::Utf16StringBase {
+public:
+    using Utf16StringBase::Utf16StringBase;
+
+    explicit constexpr Utf16String(Utf16StringBase&& base)
+        : Utf16StringBase(move(base))
+    {
+    }
+
+    ALWAYS_INLINE static Utf16String from_utf8(StringView utf8_string)
+    {
+        VERIFY(Utf8View { utf8_string }.validate());
+        return from_utf8_without_validation(utf8_string);
+    }
+
+    ALWAYS_INLINE static Utf16String from_utf8(String const& utf8_string)
+    {
+        return from_utf8_without_validation(utf8_string);
+    }
+
+    ALWAYS_INLINE static ErrorOr<Utf16String> try_from_utf8(StringView utf8_string)
+    {
+        if (!Utf8View { utf8_string }.validate())
+            return Error::from_string_literal("Input was not valid UTF-8");
+        return from_utf8_without_validation(utf8_string);
+    }
+
+    ALWAYS_INLINE static Utf16String from_utf16(Utf16View const& utf16_string)
+    {
+        VERIFY(utf16_string.validate());
+        return from_utf16_without_validation(utf16_string);
+    }
+
+    ALWAYS_INLINE static ErrorOr<Utf16String> try_from_utf16(Utf16View const& utf16_string)
+    {
+        if (!utf16_string.validate())
+            return Error::from_string_literal("Input was not valid UTF-16");
+        return from_utf16_without_validation(utf16_string);
+    }
+
+    static Utf16String from_utf8_without_validation(StringView);
+    static Utf16String from_utf16_without_validation(Utf16View const&);
+    static Utf16String from_utf32(Utf32View const&);
+
+    template<typename T>
+    requires(IsOneOf<RemoveCVReference<T>, Utf16String>)
+    static Utf16String from_utf16(T&&) = delete;
+
+    template<typename T>
+    requires(IsOneOf<RemoveCVReference<T>, Utf16String>)
+    static ErrorOr<Utf16String> try_from_utf16(T&&) = delete;
+
+    template<typename T>
+    requires(IsOneOf<RemoveCVReference<T>, Utf16String>)
+    static Utf16String from_utf16_without_validation(T&&) = delete;
+
+private:
+    ALWAYS_INLINE explicit Utf16String(NonnullRefPtr<Detail::Utf16StringData const> value)
+        : Utf16StringBase(move(value))
+    {
+    }
+};
+
+template<>
+struct Formatter<Utf16String> : Formatter<FormatString> {
+    ErrorOr<void> format(FormatBuilder&, Utf16String const&);
+};
+
+template<>
+struct Traits<Utf16String> : public DefaultTraits<Utf16String> {
+    static unsigned hash(Utf16String const& s) { return s.hash(); }
+};
+
+}
+
+[[nodiscard]] ALWAYS_INLINE AK::Utf16String operator""_utf16(char const* string, size_t length)
+{
+    AK::StringView view { string, length };
+
+    ASSERT(AK::Utf8View { view }.validate());
+    return AK::Utf16String::from_utf8_without_validation(view);
+}
+
+[[nodiscard]] ALWAYS_INLINE AK::Utf16String operator""_utf16(char16_t const* string, size_t length)
+{
+    AK::Utf16View view { string, length };
+
+    ASSERT(view.validate());
+    return AK::Utf16String::from_utf16_without_validation(view);
+}