LibTimeZone: Add an API to retrieve both daylight and standard offsets

This API will also include the formatted name of the time zone, with
respect for DST (e.g. EST vs EDT for America/New_York).

Meta/Lagom/Tools/CodeGenerators/LibTimeZone/GenerateTimeZoneData.cpp

@@ -582,6 +582,38 @@ Optional<Offset> get_time_zone_offset(TimeZone time_zone, AK::Time time)
     return dst_offset;
 }
 
+Optional<Array<NamedOffset, 2>> get_named_time_zone_offsets(TimeZone time_zone, AK::Time time)
+{
+    auto const& time_zone_offset = find_time_zone_offset(time_zone, time);
+    Array<NamedOffset, 2> named_offsets;
+
+    auto format_name = [](auto format, auto offset) -> String {
+        if (offset == 0)
+            return s_string_list[format].replace("{}"sv, ""sv);
+        return String::formatted(s_string_list[format], s_string_list[offset]);
+    };
+
+    auto set_named_offset = [&](auto& named_offset, auto dst_offset, auto in_dst, auto format, auto offset) {
+        named_offset.seconds = time_zone_offset.offset + dst_offset;
+        named_offset.in_dst = in_dst;
+        named_offset.name = format_name(format, offset);
+    };
+
+    if (time_zone_offset.dst_rule != -1) {
+        auto offsets = find_dst_offsets(time_zone_offset, time);
+        auto in_dst = offsets[1]->offset == 0 ? InDST::No : InDST::Yes;
+
+        set_named_offset(named_offsets[0], offsets[0]->offset, InDST::No, time_zone_offset.standard_format, offsets[0]->format);
+        set_named_offset(named_offsets[1], offsets[1]->offset, in_dst, time_zone_offset.daylight_format, offsets[1]->format);
+    } else {
+        auto in_dst = time_zone_offset.dst_offset == 0 ? InDST::No : InDST::Yes;
+        set_named_offset(named_offsets[0], time_zone_offset.dst_offset, in_dst, time_zone_offset.standard_format, 0);
+        set_named_offset(named_offsets[1], time_zone_offset.dst_offset, in_dst, time_zone_offset.daylight_format, 0);
+    }
+
+    return named_offsets;
+}
+
 Span<StringView const> all_time_zones()
 {
     static constexpr auto all_time_zones = Array {

Tests/LibTimeZone/TestTimeZone.cpp

@@ -149,6 +149,31 @@ TEST_CASE(get_time_zone_offset_with_dst)
     test_offset("Europe/Moscow"sv, -1589068800, offset(+1, 3, 00, 00), No);  // Monday, August 25, 1919 12:00:00 AM
 }
 
+TEST_CASE(get_named_time_zone_offsets)
+{
+    auto test_named_offsets = [](auto time_zone, i64 time, i64 expected_standard_offset, i64 expected_daylight_offset, auto expected_standard_name, auto expected_daylight_name) {
+        auto actual_offsets = TimeZone::get_named_time_zone_offsets(time_zone, AK::Time::from_seconds(time));
+        VERIFY(actual_offsets.has_value());
+
+        EXPECT_EQ(actual_offsets->at(0).seconds, expected_standard_offset);
+        EXPECT_EQ(actual_offsets->at(1).seconds, expected_daylight_offset);
+        EXPECT_EQ(actual_offsets->at(0).name, expected_standard_name);
+        EXPECT_EQ(actual_offsets->at(1).name, expected_daylight_name);
+    };
+
+    test_named_offsets("America/New_York"sv, 1642558528, offset(-1, 5, 00, 00), offset(-1, 4, 00, 00), "EST"sv, "EDT"sv); // Wednesday, January 19, 2022 2:15:28 AM
+    test_named_offsets("UTC"sv, 1642558528, offset(+1, 0, 00, 00), offset(+1, 0, 00, 00), "UTC"sv, "UTC"sv);              // Wednesday, January 19, 2022 2:15:28 AM
+    test_named_offsets("GMT"sv, 1642558528, offset(+1, 0, 00, 00), offset(+1, 0, 00, 00), "GMT"sv, "GMT"sv);              // Wednesday, January 19, 2022 2:15:28 AM
+
+    // Phoenix does not observe DST.
+    test_named_offsets("America/Phoenix"sv, 1642558528, offset(-1, 7, 00, 00), offset(-1, 7, 00, 00), "MST"sv, "MST"sv); // Wednesday, January 19, 2022 2:15:28 AM
+
+    // Moscow's observed DST changed several times in 1919.
+    test_named_offsets("Europe/Moscow"sv, -1609459200, offset(+1, 2, 31, 19), offset(+1, 3, 31, 19), "MSK"sv, "MSD"sv);  // Wednesday, January 1, 1919 12:00:00 AM
+    test_named_offsets("Europe/Moscow"sv, -1596412800, offset(+1, 2, 31, 19), offset(+1, 4, 31, 19), "MSK"sv, "MDST"sv); // Sunday, June 1, 1919 12:00:00 AM
+    test_named_offsets("Europe/Moscow"sv, -1589068800, offset(+1, 3, 00, 00), offset(+1, 4, 00, 00), "MSK"sv, "MSD"sv);  // Monday, August 25, 1919 12:00:00 AM
+}
+
 #else
 
 TEST_CASE(time_zone_from_string)

Userland/Libraries/LibTimeZone/TimeZone.cpp

@@ -4,7 +4,6 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include <AK/Array.h>
 #include <AK/String.h>
 #include <LibTimeZone/TimeZone.h>
 #include <stdio.h>
@@ -157,4 +156,25 @@ Optional<Offset> get_time_zone_offset(StringView time_zone, AK::Time time)
     return {};
 }
 
+Optional<Array<NamedOffset, 2>> __attribute__((weak)) get_named_time_zone_offsets([[maybe_unused]] TimeZone time_zone, AK::Time)
+{
+#if !ENABLE_TIME_ZONE_DATA
+    VERIFY(time_zone == TimeZone::UTC);
+
+    NamedOffset utc_offset {};
+    utc_offset.name = "UTC"sv;
+
+    return Array { utc_offset, utc_offset };
+#else
+    return {};
+#endif
+}
+
+Optional<Array<NamedOffset, 2>> get_named_time_zone_offsets(StringView time_zone, AK::Time time)
+{
+    if (auto maybe_time_zone = time_zone_from_string(time_zone); maybe_time_zone.has_value())
+        return get_named_time_zone_offsets(*maybe_time_zone, time);
+    return {};
+}
+
 }

Userland/Libraries/LibTimeZone/TimeZone.h

@@ -6,8 +6,10 @@
 
 #pragma once
 
+#include <AK/Array.h>
 #include <AK/Error.h>
 #include <AK/Optional.h>
+#include <AK/String.h>
 #include <AK/StringView.h>
 #include <AK/Time.h>
 #include <AK/Types.h>
@@ -25,6 +27,10 @@ struct Offset {
     InDST in_dst { InDST::No };
 };
 
+struct NamedOffset : public Offset {
+    String name;
+};
+
 StringView current_time_zone();
 ErrorOr<void> change_time_zone(StringView time_zone);
 Span<StringView const> all_time_zones();
@@ -39,4 +45,7 @@ StringView daylight_savings_rule_to_string(DaylightSavingsRule daylight_savings_
 Optional<Offset> get_time_zone_offset(TimeZone time_zone, AK::Time time);
 Optional<Offset> get_time_zone_offset(StringView time_zone, AK::Time time);
 
+Optional<Array<NamedOffset, 2>> get_named_time_zone_offsets(TimeZone time_zone, AK::Time time);
+Optional<Array<NamedOffset, 2>> get_named_time_zone_offsets(StringView time_zone, AK::Time time);
+
 }