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

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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.
This commit is contained in:
Timothy Flynn 2022-09-27 13:50:41 -04:00 committed by Linus Groh
parent a194303948
commit 9a1b24d5be
2 changed files with 14 additions and 4 deletions
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14
Meta/Lagom/Tools/CodeGenerators/LibTimeZone/GenerateTimeZoneData.cpp
View file

@ -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 };

4
Tests/LibTimeZone/TestTimeZone.cpp
View file

@ -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