LibUnicode: Implement locale-aware MORE_ABOVE special casing

Author: https://github.com/trflynn89
Commit: https://github.com/SerenityOS/serenity/commit/436faf9fd97
Pull-request: https://github.com/SerenityOS/serenity/pull/9841
Reviewed-by: https://github.com/IdanHo
Reviewed-by: https://github.com/linusg ✅

Tests/LibUnicode/TestUnicodeCharacterTypes.cpp

@@ -198,6 +198,51 @@ TEST_CASE(to_unicode_lowercase_special_casing_i)
     EXPECT_EQ(result, "\u0131a\u0307"sv);
 }
 
+TEST_CASE(to_unicode_lowercase_special_casing_more_above)
+{
+    // LATIN CAPITAL LETTER I
+    auto result = Unicode::to_unicode_lowercase_full("I"sv, "en"sv);
+    EXPECT_EQ(result, "i"sv);
+
+    result = Unicode::to_unicode_lowercase_full("I"sv, "lt"sv);
+    EXPECT_EQ(result, "i"sv);
+
+    // LATIN CAPITAL LETTER J
+    result = Unicode::to_unicode_lowercase_full("J"sv, "en"sv);
+    EXPECT_EQ(result, "j"sv);
+
+    result = Unicode::to_unicode_lowercase_full("J"sv, "lt"sv);
+    EXPECT_EQ(result, "j"sv);
+
+    // LATIN CAPITAL LETTER I WITH OGONEK
+    result = Unicode::to_unicode_lowercase_full("\u012e"sv, "en"sv);
+    EXPECT_EQ(result, "\u012f"sv);
+
+    result = Unicode::to_unicode_lowercase_full("\u012e"sv, "lt"sv);
+    EXPECT_EQ(result, "\u012f"sv);
+
+    // LATIN CAPITAL LETTER I followed by COMBINING GRAVE ACCENT
+    result = Unicode::to_unicode_lowercase_full("I\u0300"sv, "en"sv);
+    EXPECT_EQ(result, "i\u0300"sv);
+
+    result = Unicode::to_unicode_lowercase_full("I\u0300"sv, "lt"sv);
+    EXPECT_EQ(result, "i\u0307\u0300"sv);
+
+    // LATIN CAPITAL LETTER J followed by COMBINING GRAVE ACCENT
+    result = Unicode::to_unicode_lowercase_full("J\u0300"sv, "en"sv);
+    EXPECT_EQ(result, "j\u0300"sv);
+
+    result = Unicode::to_unicode_lowercase_full("J\u0300"sv, "lt"sv);
+    EXPECT_EQ(result, "j\u0307\u0300"sv);
+
+    // LATIN CAPITAL LETTER I WITH OGONEK followed by COMBINING GRAVE ACCENT
+    result = Unicode::to_unicode_lowercase_full("\u012e\u0300"sv, "en"sv);
+    EXPECT_EQ(result, "\u012f\u0300"sv);
+
+    result = Unicode::to_unicode_lowercase_full("\u012e\u0300"sv, "lt"sv);
+    EXPECT_EQ(result, "\u012f\u0307\u0300"sv);
+}
+
 TEST_CASE(to_unicode_uppercase_unconditional_special_casing)
 {
     // LATIN SMALL LETTER SHARP S

Userland/Libraries/LibUnicode/CharacterTypes.cpp

@@ -115,6 +115,26 @@ static bool is_final_code_point(Utf8View const& string, size_t index, size_t byt
     return true;
 }
 
+static bool is_followed_by_combining_class_above(Utf8View const& string, size_t index, size_t byte_length)
+{
+    // C is followed by a character of combining class 230 (Above) with no intervening character of combining class 0 or 230 (Above).
+    auto following_view = ((index + byte_length) < string.byte_length())
+        ? string.substring_view(index + byte_length)
+        : Utf8View {};
+
+    for (auto code_point : following_view) {
+        auto unicode_data = Detail::unicode_data_for_code_point(code_point);
+        if (!unicode_data.has_value())
+            return false;
+        if (unicode_data->canonical_combining_class == 0)
+            return false;
+        if (unicode_data->canonical_combining_class == 230)
+            return true;
+    }
+
+    return false;
+}
+
 static SpecialCasing const* find_matching_special_case(Utf8View const& string, Optional<StringView> locale, size_t index, size_t byte_length, UnicodeData const& unicode_data)
 {
     auto requested_locale = Locale::None;
@@ -149,6 +169,11 @@ static SpecialCasing const* find_matching_special_case(Utf8View const& string, O
                 return special_casing;
             break;
 
+        case Condition::MoreAbove:
+            if (is_followed_by_combining_class_above(string, index, byte_length))
+                return special_casing;
+            break;
+
         default:
             break;
         }