Class TDayNoToYearLookupTable

util/datetime/systime.cpp:136–173

    }

    class TDayNoToYearLookupTable {
        static constexpr int TableSize = 128;
        // lookup table for years in [StartYear, StartYear + TableSize] range
        ui16 DaysSinceEpoch[TableSize] = {};

    public:
        static constexpr int StartYear = 1970;
        static constexpr int StartDays = (StartYear - UNIX_TIME_BASE_YEAR) * DAYS_IN_YEAR + LeapYearCount(StartYear - 1) - LeapYearCount(UNIX_TIME_BASE_YEAR - 1);
        static constexpr i64 MinTimestamp = StartDays * static_cast<i64>(SECONDS_PER_DAY);
        static constexpr i64 MaxTimestamp = MinTimestamp + static_cast<i64>(TableSize) * DAYS_IN_LEAP_YEAR * SECONDS_PER_DAY - 1;
        constexpr TDayNoToYearLookupTable() {
            ui16 daysAccumulated = 0;

            for (int year = StartYear; year < StartYear + TableSize; ++year) {
                daysAccumulated += YearSize(year);
                DaysSinceEpoch[year - StartYear] = daysAccumulated;
            }
        }

        // lookup year by days since epoch, decrement day counter to the corresponding amount of days.
        // The method returns the last year in the table, if year is too big
        int FindYear(ui32& days) const {
            const ui32 yearIndex = days / DAYS_IN_LEAP_YEAR;

            // we can miss by at most 1 year
            Y_ASSERT(yearIndex < TableSize);
            if (const auto diff = DaysSinceEpoch[yearIndex]; diff <= days) {
                days -= diff;
                return static_cast<int>(yearIndex + StartYear + 1);
            }

            if (yearIndex > 0) {
                days -= DaysSinceEpoch[yearIndex - 1];
            }

            return static_cast<int>(yearIndex + StartYear);
        }
    };

    constexpr TDayNoToYearLookupTable DAYS_TO_YEAR_LOOKUP;
} // namespace