LibUnicode: Fall back to GMT offset when a time zone name is unavailable

The following table in TR-35 includes a web of fall back rules when the
requested time zone style is unavailable:
https://unicode.org/reports/tr35/tr35-dates.html#dfst-zone

Conveniently, the subset of styles supported by ECMA-402 (and therefore
LibUnicode) all either fall back to GMT offset or to a style that is
unsupported but itself falls back to GMT offset.

Tests/LibUnicode/TestUnicodeDateTimeFormat.cpp

@@ -35,31 +35,32 @@ TEST_CASE(time_zone_name)
         TestData { "en"sv, Unicode::CalendarPatternStyle::Short, "America/Los_Angeles"sv, "PDT"sv },
 
         TestData { "ar"sv, Unicode::CalendarPatternStyle::Long, "America/Los_Angeles"sv, "توقيت المحيط الهادي الصيفي"sv },
-        // The "ar" locale does not have a short name for PDT. LibUnicode will need to fall back to GMT offset when we have that data.
+        TestData { "ar"sv, Unicode::CalendarPatternStyle::Short, "America/Los_Angeles"sv, "غرينتش-٨"sv },
 
         TestData { "en"sv, Unicode::CalendarPatternStyle::Long, "America/Vancouver"sv, "Pacific Daylight Time"sv },
         TestData { "en"sv, Unicode::CalendarPatternStyle::Short, "America/Vancouver"sv, "PDT"sv },
 
         TestData { "ar"sv, Unicode::CalendarPatternStyle::Long, "America/Vancouver"sv, "توقيت المحيط الهادي الصيفي"sv },
-        // The "ar" locale does not have a short name for PDT. LibUnicode will need to fall back to GMT offset when we have that data.
+        TestData { "ar"sv, Unicode::CalendarPatternStyle::Short, "America/Vancouver"sv, "غرينتش-٨"sv },
 
         TestData { "en"sv, Unicode::CalendarPatternStyle::Long, "Europe/London"sv, "Greenwich Mean Time"sv },
         TestData { "en"sv, Unicode::CalendarPatternStyle::Short, "Europe/London"sv, "GMT"sv },
 
         TestData { "ar"sv, Unicode::CalendarPatternStyle::Long, "Europe/London"sv, "توقيت غرينتش"sv },
-        // The "ar" locale does not have a short name for GMT. LibUnicode will need to fall back to GMT offset when we have that data.
+        TestData { "ar"sv, Unicode::CalendarPatternStyle::Short, "Europe/London"sv, "غرينتش"sv },
 
         TestData { "en"sv, Unicode::CalendarPatternStyle::Long, "Africa/Accra"sv, "Greenwich Mean Time"sv },
         TestData { "en"sv, Unicode::CalendarPatternStyle::Short, "Africa/Accra"sv, "GMT"sv },
 
         TestData { "ar"sv, Unicode::CalendarPatternStyle::Long, "Africa/Accra"sv, "توقيت غرينتش"sv },
-        // The "ar" locale does not have a short name for GMT. LibUnicode will need to fall back to GMT offset when we have that data.
+        TestData { "ar"sv, Unicode::CalendarPatternStyle::Short, "Africa/Accra"sv, "غرينتش"sv },
     };
 
+    constexpr auto jan_1_2022 = AK::Time::from_seconds(1640995200); // Saturday, January 1, 2022 12:00:00 AM
+
     for (auto const& test : test_data) {
-        auto time_zone = Unicode::get_time_zone_name(test.locale, test.time_zone, test.style);
-        VERIFY(time_zone.has_value());
-        EXPECT_EQ(*time_zone, test.expected_result);
+        auto time_zone = Unicode::format_time_zone(test.locale, test.time_zone, test.style, jan_1_2022);
+        EXPECT_EQ(time_zone, test.expected_result);
     }
 }
 

Userland/Libraries/LibUnicode/DateTimeFormat.cpp

@@ -262,7 +262,9 @@ String format_time_zone(StringView locale, StringView time_zone, CalendarPattern
     case CalendarPatternStyle::Long:
     case CalendarPatternStyle::ShortGeneric:
     case CalendarPatternStyle::LongGeneric:
-        return get_time_zone_name(locale, time_zone, style).value_or(time_zone);
+        if (auto name = get_time_zone_name(locale, time_zone, style); name.has_value())
+            return *name;
+        break;
 
     case CalendarPatternStyle::ShortOffset:
     case CalendarPatternStyle::LongOffset:
@@ -271,6 +273,21 @@ String format_time_zone(StringView locale, StringView time_zone, CalendarPattern
     default:
         VERIFY_NOT_REACHED();
     }
+
+    // If more styles are added, consult the following table to ensure always falling back to GMT offset is still correct:
+    // https://unicode.org/reports/tr35/tr35-dates.html#dfst-zone
+    switch (style) {
+    case CalendarPatternStyle::Short:
+    case CalendarPatternStyle::ShortGeneric:
+        return format_time_zone(locale, time_zone, CalendarPatternStyle::ShortOffset, time);
+
+    case CalendarPatternStyle::Long:
+    case CalendarPatternStyle::LongGeneric:
+        return format_time_zone(locale, time_zone, CalendarPatternStyle::LongOffset, time);
+
+    default:
+        VERIFY_NOT_REACHED();
+    }
 }
 
 }