From 298ec6a12ad5c743e74e6744a00681720cc1fa5b Mon Sep 17 00:00:00 2001
From: Timothy Flynn <trflynn89@pm.me>
Date: Wed, 6 Aug 2025 06:57:58 -0400
Subject: [PATCH] AK: Ensure StringBuilder encodes U+10000 as 2 UTF-16 code
 units

---
 AK/StringBuilder.cpp         |  2 +-
 Tests/AK/TestUtf16String.cpp | 24 +++++++++++++++++++++---
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/AK/StringBuilder.cpp b/AK/StringBuilder.cpp
index 622060acfa2..8b0eed5b44d 100644
--- a/AK/StringBuilder.cpp
+++ b/AK/StringBuilder.cpp
@@ -370,7 +370,7 @@ void StringBuilder::append_code_point(u32 code_point)
     if (m_mode == Mode::UTF16) {
         (void)(will_append(2));
 
-        if (code_point <= UnicodeUtils::FIRST_SUPPLEMENTARY_PLANE_CODE_POINT) {
+        if (code_point < UnicodeUtils::FIRST_SUPPLEMENTARY_PLANE_CODE_POINT) {
             auto code_unit = static_cast<char16_t>(code_point);
             m_buffer.append(&code_unit, sizeof(code_unit));
             return;
diff --git a/Tests/AK/TestUtf16String.cpp b/Tests/AK/TestUtf16String.cpp
index 94f1081eb91..3d21098c859 100644
--- a/Tests/AK/TestUtf16String.cpp
+++ b/Tests/AK/TestUtf16String.cpp
@@ -343,6 +343,7 @@ TEST_CASE(formatted)
             u"ABCDEFGHIJKLMNOPQRSTUVWXYZ"sv,
             u"\xd83d\xde00"sv,
             u"abcdefghijklmnopqrstuvwxyz"sv,
+            u"\xd800\xdc00"sv,
             u"ABCDEFGHIJKLMNOPQRSTUVWXYZ"sv,
             u"🍕"sv,
             u"abcdefghijklmnopqrstuvwxyz"sv,
@@ -354,9 +355,9 @@ TEST_CASE(formatted)
         EXPECT(!string.is_ascii());
         EXPECT(!string.has_long_ascii_storage());
         EXPECT(!string.has_short_ascii_storage());
-        EXPECT_EQ(string.length_in_code_units(), 174uz);
-        EXPECT_EQ(string.length_in_code_points(), 172uz);
-        EXPECT_EQ(string, u"abcdefghijklmnopqrstuvwxyz--ABCDEFGHIJKLMNOPQRSTUVWXYZ--😀--abcdefghijklmnopqrstuvwxyz--ABCDEFGHIJKLMNOPQRSTUVWXYZ--🍕--abcdefghijklmnopqrstuvwxyz--ABCDEFGHIJKLMNOPQRSTUVWXYZ"sv);
+        EXPECT_EQ(string.length_in_code_units(), 178uz);
+        EXPECT_EQ(string.length_in_code_points(), 175uz);
+        EXPECT_EQ(string, u"abcdefghijklmnopqrstuvwxyz--ABCDEFGHIJKLMNOPQRSTUVWXYZ--😀--abcdefghijklmnopqrstuvwxyz--𐀀--ABCDEFGHIJKLMNOPQRSTUVWXYZ--🍕--abcdefghijklmnopqrstuvwxyz--ABCDEFGHIJKLMNOPQRSTUVWXYZ"sv);
     }
 }
 
@@ -415,6 +416,23 @@ TEST_CASE(repeated)
     EXPECT_DEATH("Creating a string from an invalid code point", (void)Utf16String::repeated(0xffffffff, 1));
 }
 
+TEST_CASE(from_string_builder)
+{
+    StringBuilder builder(StringBuilder::Mode::UTF16);
+    builder.append_code_point('a');
+    builder.append_code_point('b');
+    builder.append_code_point(0x1f600);
+    builder.append_code_point(0x10000);
+    builder.append_code_point(0x1f355);
+    builder.append_code_point('c');
+    builder.append_code_point('d');
+
+    auto string = builder.to_utf16_string();
+    EXPECT_EQ(string.length_in_code_units(), 10uz);
+    EXPECT_EQ(string.length_in_code_points(), 7uz);
+    EXPECT_EQ(string, "ab😀𐀀🍕cd"sv);
+}
+
 TEST_CASE(from_ipc_stream)
 {
     {