LibTimeZone: Use the last DST rule in the TZDB if a match isn't found

Some time zones, like "Asia/Shanghai", use a set of DST rules that end before present day. In these cases, we should fall back to last possible RULE entry from the TZDB. The time zone compiler published by IANA (zic) performs the same fallback starting with version 2 of the time zone file format.
lanmonster · Sep 28, 2022 · 9a1b24d · 9a1b24d
1 parent a194303
commit 9a1b24d
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Showing 2 changed files with 14 additions and 4 deletions.
diff --git a/Meta/Lagom/Tools/CodeGenerators/LibTimeZone/GenerateTimeZoneData.cpp b/Meta/Lagom/Tools/CodeGenerators/LibTimeZone/GenerateTimeZoneData.cpp
@@ -641,6 +641,7 @@ static Array<DaylightSavingsOffset const*, 2> find_dst_offsets(TimeZoneOffset co
 
  DaylightSavingsOffset const* standard_offset = nullptr;
  DaylightSavingsOffset const* daylight_offset = nullptr;
+ DaylightSavingsOffset const* last_offset = nullptr;
 
  auto preferred_rule = [&](auto* current_offset, auto& new_offset) {
  if (!current_offset)
@@ -652,6 +653,12 @@ static Array<DaylightSavingsOffset const*, 2> find_dst_offsets(TimeZoneOffset co
 
  for (size_t index = 0; (index < dst_rules.size()) && (!standard_offset || !daylight_offset); ++index) {
  auto const& dst_rule = dst_rules[index];
+
+ if (last_offset == nullptr)
+ last_offset = &dst_rule;
+ else if (dst_rule.time_in_effect(dst_rule.year_to) > last_offset->time_in_effect(last_offset->year_to))
+ last_offset = &dst_rule;
+
  if ((time < dst_rule.year_from) || (time >= dst_rule.year_to))
  continue;
 
@@ -661,11 +668,10 @@ static Array<DaylightSavingsOffset const*, 2> find_dst_offsets(TimeZoneOffset co
  daylight_offset = preferred_rule(daylight_offset, dst_rule);
  }
 
- // In modern times, there will always be a standard rule in the TZDB, but that isn't true in
- // all time zones in or before the early 1900s. For example, the "US" rules begin in 1918.
+ // If there isn't a standard or daylight rule in effect, fall back to the last rule given in the TZDB.
  if (!standard_offset) {
- static DaylightSavingsOffset const empty_offset {};
- return { &empty_offset, &empty_offset };
+ VERIFY(last_offset != nullptr);
+ standard_offset = last_offset;
  }
 
  return { standard_offset, daylight_offset ? daylight_offset : standard_offset };

diff --git a/Tests/LibTimeZone/TestTimeZone.cpp b/Tests/LibTimeZone/TestTimeZone.cpp
@@ -177,6 +177,10 @@ TEST_CASE(get_named_time_zone_offsets)
  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
+
+ // Shanghai's DST rules end in 1991.
+ test_named_offsets("Asia/Shanghai"sv, 694223999, offset(+1, 8, 00, 00), offset(+1, 9, 00, 00), "CST"sv, "CDT"sv); // Tuesday, December 31, 1991 11:59:59 PM
+ test_named_offsets("Asia/Shanghai"sv, 694224000, offset(+1, 8, 00, 00), offset(+1, 8, 00, 00), "CST"sv, "CST"sv); // Wednesday, January 1, 1992 12:00:00 AM
 }
 
 #else