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Speedup DateLUT initialization #51347

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merged 2 commits into from Jun 24, 2023
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Speedup DateLUT initialization #51347

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8243b62
Speedup DateLUT initialization
davenger Jun 23, 2023
dfc1afa
Better naming and comments
davenger Jun 24, 2023
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86 changes: 50 additions & 36 deletions src/Common/DateLUTImpl.cpp
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Expand Up @@ -33,8 +33,24 @@ UInt8 getDayOfWeek(const cctz::civil_day & date)
UNREACHABLE();
}

inline cctz::time_point<cctz::seconds> lookupTz(const cctz::time_zone & cctz_time_zone, const cctz::civil_day & date)
{
cctz::time_zone::civil_lookup lookup = cctz_time_zone.lookup(date);

/// Ambiguity is possible if time was changed backwards at the midnight
/// or after midnight time has been changed back to midnight, for example one hour backwards at 01:00
/// or after midnight time has been changed to the previous day, for example two hours backwards at 01:00
/// Then midnight appears twice. Usually time change happens exactly at 00:00 or 01:00.

/// If transition did not involve previous day, we should use the first midnight as the start of the day,
/// otherwise it's better to use the second midnight.

return lookup.trans < lookup.post
? lookup.post /* Second midnight appears after transition, so there was a piece of previous day after transition */
: lookup.pre;
}

}

__attribute__((__weak__)) extern bool inside_main;

Expand Down Expand Up @@ -63,27 +79,49 @@ DateLUTImpl::DateLUTImpl(const std::string & time_zone_)
offset_is_whole_number_of_minutes_during_epoch = true;

cctz::civil_day date = lut_start;
cctz::time_point<cctz::seconds> start_of_day_time_point = lookupTz(cctz_time_zone, date);

auto next_transition_date = date;

UInt32 i = 0;
do
{
cctz::time_zone::civil_lookup lookup = cctz_time_zone.lookup(date);
Values & values = lut[i];

/// Ambiguity is possible if time was changed backwards at the midnight
/// or after midnight time has been changed back to midnight, for example one hour backwards at 01:00
/// or after midnight time has been changed to the previous day, for example two hours backwards at 01:00
/// Then midnight appears twice. Usually time change happens exactly at 00:00 or 01:00.
values.time_at_offset_change_value = 0;
values.amount_of_offset_change_value = 0;

/// If transition did not involve previous day, we should use the first midnight as the start of the day,
/// otherwise it's better to use the second midnight.
if (date >= next_transition_date)
{
start_of_day_time_point = lookupTz(cctz_time_zone, date);

/// If UTC offset was changed this day.
/// Change in time zone without transition is possible, e.g. Moscow 1991 Sun, 31 Mar, 02:00 MSK to EEST
cctz::time_zone::civil_transition transition{};
if (cctz_time_zone.next_transition(start_of_day_time_point - std::chrono::seconds(1), &transition)
&& (cctz::civil_day(transition.from) == date || cctz::civil_day(transition.to) == date)
&& transition.from != transition.to)
{
values.time_at_offset_change_value = (transition.from - cctz::civil_second(date)) / Values::OffsetChangeFactor;
values.amount_of_offset_change_value = (transition.to - transition.from) / Values::OffsetChangeFactor;

/// We don't support too large changes.
if (values.amount_of_offset_change_value > 24 * 4)
values.amount_of_offset_change_value = 24 * 4;
else if (values.amount_of_offset_change_value < -24 * 4)
values.amount_of_offset_change_value = -24 * 4;

/// We don't support cases when time change results in switching to previous day.
/// Shift the point of time change later.
if (values.time_at_offset_change_value + values.amount_of_offset_change_value < 0)
values.time_at_offset_change_value = -values.amount_of_offset_change_value;
}

std::chrono::time_point start_of_day_time_point = lookup.trans < lookup.post
? lookup.post /* Second midnight appears after transition, so there was a piece of previous day after transition */
: lookup.pre;
next_transition_date = std::min(cctz::civil_day(transition.to), cctz::civil_day(transition.from));
}

start_of_day = std::chrono::system_clock::to_time_t(start_of_day_time_point);

Values & values = lut[i];
values.year = date.year();
values.month = date.month();
values.day_of_month = date.day();
Expand All @@ -103,38 +141,14 @@ DateLUTImpl::DateLUTImpl(const std::string & time_zone_)
else
values.days_in_month = i != 0 ? lut[i - 1].days_in_month : 31;

values.time_at_offset_change_value = 0;
values.amount_of_offset_change_value = 0;

if (offset_is_whole_number_of_hours_during_epoch && start_of_day > 0 && start_of_day % 3600)
offset_is_whole_number_of_hours_during_epoch = false;

if (offset_is_whole_number_of_minutes_during_epoch && start_of_day > 0 && start_of_day % 60)
offset_is_whole_number_of_minutes_during_epoch = false;

/// If UTC offset was changed this day.
/// Change in time zone without transition is possible, e.g. Moscow 1991 Sun, 31 Mar, 02:00 MSK to EEST
cctz::time_zone::civil_transition transition{};
if (cctz_time_zone.next_transition(start_of_day_time_point - std::chrono::seconds(1), &transition)
&& (cctz::civil_day(transition.from) == date || cctz::civil_day(transition.to) == date)
&& transition.from != transition.to)
{
values.time_at_offset_change_value = (transition.from - cctz::civil_second(date)) / Values::OffsetChangeFactor;
values.amount_of_offset_change_value = (transition.to - transition.from) / Values::OffsetChangeFactor;

/// We don't support too large changes.
if (values.amount_of_offset_change_value > 24 * 4)
values.amount_of_offset_change_value = 24 * 4;
else if (values.amount_of_offset_change_value < -24 * 4)
values.amount_of_offset_change_value = -24 * 4;

/// We don't support cases when time change results in switching to previous day.
/// Shift the point of time change later.
if (values.time_at_offset_change_value + values.amount_of_offset_change_value < 0)
values.time_at_offset_change_value = -values.amount_of_offset_change_value;
}

/// Going to next day.
start_of_day_time_point += std::chrono::hours(24);
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++date;
++i;
}
Expand Down