From Newsgroup: comp.lang.c++

This is the current code to convert a time_point with 100ns-ticks
since 1.1.1601 00:00:00.0 to a date in Gregorian or Julian calendar.

template<bool Gregorian>
void date_1601<Gregorian>::from_time_point( time_point timePoint ) noexcept
{
uint64_t tsCalc = timePoint.time_since_epoch().count();
bool leapYear = false;
constexpr int64_t Y4S = FOUR_YEARS_W_LY;
int y400 = 0, y100 = 0, y4, yMod4, patch = 0;
bool leapQuad = true;
if constexpr( Gregorian )
{
constexpr int64_t Y400S = FOUR_HUNDRED_YEARS;
if( (int64_t)tsCalc >= -(int64_t)LEAP_YEAR )
{
if( (int64_t)(tsCalc + LEAP_YEAR) < (int64_t)tsCalc )
{
tsCalc -= FOUR_HUNDRED_YEARS;
patch = 400;
}
tsCalc += LEAP_YEAR;
y400 += (int)(tsCalc / FOUR_HUNDRED_YEARS);
}
else
{
tsCalc += LEAP_YEAR;
if( (int64_t)tsCalc <= -Y400S )
{
tsCalc += FOUR_HUNDRED_YEARS;
patch = -400;
}
y400 += (int)((int64_t)(tsCalc - (Y400S - 1)) / Y400S);
}
tsCalc -= y400 * Y400S;
if( tsCalc >= FIRST_CENTURY )
{
y100 = (int)((tsCalc - FIRST_CENTURY) / REMAINING_CENUTRIES); // 0 ... 2
tsCalc -= FIRST_CENTURY + y100++ * REMAINING_CENUTRIES; // -> 1 ... 3
if( tsCalc >= FOUR_YEARS_WO_LY )
{
y4 = (int)((tsCalc - FOUR_YEARS_WO_LY) / FOUR_YEARS_W_LY); // 0 ... 23
tsCalc -= FOUR_YEARS_WO_LY + y4++ * FOUR_YEARS_W_LY; // -> 1 ... 24
}
else
{
y4 = 0;
yMod4 = (int)(tsCalc / NON_LEAP_YEAR);
tsCalc -= yMod4 * NON_LEAP_YEAR;
leapQuad = false;
}
}
else
{
y100 = 0;
y4 = (int)(tsCalc / FOUR_YEARS_W_LY);
tsCalc -= y4 * FOUR_YEARS_W_LY;
}
}
else
{
if( (int64_t)tsCalc >= -(int64_t)JULIAN_ZERO_1600 )
{
if( (int64_t)(tsCalc + JULIAN_ZERO_1600) < (int64_t)tsCalc )
{
tsCalc -= FOUR_YEARS_W_LY;
patch = 4;
}
tsCalc += JULIAN_ZERO_1600;
y4 = (int)(tsCalc / FOUR_YEARS_W_LY);
}
else
{
tsCalc += JULIAN_ZERO_1600;
if( (int64_t)tsCalc <= -Y4S )
{
tsCalc += FOUR_YEARS_W_LY;
patch = -4;
}
y4 = (int)((int64_t)(tsCalc - (Y4S - 1)) / Y4S);
}
tsCalc -= y4 * Y4S;
}
if( leapQuad )
if( leapYear = tsCalc < LEAP_YEAR; !leapYear )
{
tsCalc -= LEAP_YEAR;
yMod4 = (int)(tsCalc / NON_LEAP_YEAR); // 0 ... 2
tsCalc -= yMod4++ * NON_LEAP_YEAR; // -> 1 ... 3
}
else
yMod4 = 0;
year = (int16_t)(1600 + 400 * y400 + 100 * y100 + 4 * y4 + yMod4 + patch);
{
uint16_t yDay = (uint16_t)(tsCalc / DAY);
tsCalc %= DAY;
span dualDaysAfterYear( g_daysAfterYear );
span daysAfterYear( dualDaysAfterYear[leapYear] );
month = 0;
for( ; yDay >= daysAfterYear[month + 1]; ++month );
assert(month < 12);
day = yDay - daysAfterYear[month] + 1;
++month;
}
hour = (uint8_t)(tsCalc / HOUR);
tsCalc %= HOUR;
minute = (uint8_t)(tsCalc / MINUTE);
tsCalc %= MINUTE;
second = (uint8_t)((uint32_t)tsCalc / (uint32_t)SECOND);
ticks = (uint32_t)tsCalc % (uint32_t)SECOND;
weekday = get_weekday( timePoint );
}
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