AK: Add Utf16StringBuilder formatting helpers

Add vformat support for Utf16StringBuilder and expose appendff and
try_appendff helpers on the builder itself. This lets callers build a
Utf16String on the purpose-built UTF-16 path even when using formatting.

Use the new API for Utf16String::formatted and add AK coverage for
formatting directly into a Utf16StringBuilder.

AK/Format.cpp

@@ -17,6 +17,8 @@
 #include <AK/StringBuilder.h>
 #include <AK/StringConversions.h>
 #include <AK/Time.h>
+#include <AK/Utf16String.h>
+#include <AK/Utf16StringBuilder.h>
 #include <math.h>
 #include <pthread.h>
 #include <stdio.h>
@@ -823,6 +825,14 @@ ErrorOr<void> vformat(StringBuilder& builder, StringView fmtstr, TypeErasedForma
     return {};
 }
 
+ErrorOr<void> vformat(Utf16StringBuilder& builder, StringView fmtstr, TypeErasedFormatParams& params)
+{
+    StringBuilder string_builder(StringBuilder::Mode::UTF16);
+    TRY(vformat(string_builder, fmtstr, params));
+    builder.append(string_builder.to_utf16_string());
+    return {};
+}
+
 void StandardFormatter::parse(TypeErasedFormatParams& params, FormatParser& parser)
 {
     if ("<^>"sv.contains(parser.peek(1))) {

AK/Format.h

@@ -616,6 +616,7 @@ struct Formatter<Checked<T>> : Formatter<T> {
 };
 
 ErrorOr<void> vformat(StringBuilder&, StringView fmtstr, TypeErasedFormatParams&);
+ErrorOr<void> vformat(Utf16StringBuilder&, StringView fmtstr, TypeErasedFormatParams&);
 
 void vout(FILE*, StringView fmtstr, TypeErasedFormatParams&, bool newline = false);
 

AK/Utf16String.h

@@ -16,6 +16,7 @@
 #include <AK/Traits.h>
 #include <AK/UnicodeUtils.h>
 #include <AK/Utf16StringBase.h>
+#include <AK/Utf16StringBuilder.h>
 #include <AK/Utf16StringData.h>
 #include <AK/Utf16View.h>
 #include <AK/Utf8View.h>
@@ -113,12 +114,12 @@ public:
     template<typename... Parameters>
     ALWAYS_INLINE static Utf16String formatted(CheckedFormatString<Parameters...>&& format, Parameters const&... parameters)
     {
-        StringBuilder builder(StringBuilder::Mode::UTF16);
+        Utf16StringBuilder builder;
 
         VariadicFormatParams<AllowDebugOnlyFormatters::No, Parameters...> variadic_format_parameters { parameters... };
         MUST(vformat(builder, format.view(), variadic_format_parameters));
 
-        return builder.to_utf16_string();
+        return builder.to_string();
     }
 
     template<Integral T>

AK/Utf16StringBuilder.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/Badge.h>
+#include <AK/Format.h>
 #include <AK/Forward.h>
 #include <AK/Optional.h>
 #include <AK/Span.h>
@@ -85,6 +86,20 @@ public:
     void append_repeated_ascii(char, size_t);
     void append_repeated(Utf16View const&, size_t);
 
+    template<typename... Parameters>
+    ErrorOr<void> try_appendff(CheckedFormatString<Parameters...>&& fmtstr, Parameters const&... parameters)
+    {
+        VariadicFormatParams<AllowDebugOnlyFormatters::No, Parameters...> variadic_format_parameters { parameters... };
+        return vformat(*this, fmtstr.view(), variadic_format_parameters);
+    }
+
+    template<typename... Parameters>
+    void appendff(CheckedFormatString<Parameters...>&& fmtstr, Parameters const&... parameters)
+    {
+        VariadicFormatParams<AllowDebugOnlyFormatters::No, Parameters...> variadic_format_parameters { parameters... };
+        MUST(vformat(*this, fmtstr.view(), variadic_format_parameters));
+    }
+
     [[nodiscard]] Utf16String to_string();
 
     [[nodiscard]] Utf16View view() const;

Tests/AK/TestUtf16StringBuilder.cpp

@@ -108,3 +108,14 @@ TEST_CASE(reuse_after_creating_long_utf16_string)
     EXPECT_EQ(builder.view(), "ok"sv);
     EXPECT_EQ(builder.to_string(), "ok"sv);
 }
+
+TEST_CASE(formatted_append)
+{
+    Utf16StringBuilder builder;
+    builder.appendff("{} {} {}", "hello"sv, Utf16View { u"\u263A"sv }, 42);
+    MUST(builder.try_appendff(" {}", Utf16String::formatted("{}", 3.14)));
+
+    auto string = builder.to_string();
+    EXPECT_EQ(string, u"hello \u263A 42 3.14"sv);
+    EXPECT(!string.is_ascii());
+}