internal override LocalInstant AddMonths(LocalInstant localInstant, int months)
{
// Note: this method gives the same result regardless of the month numbering used
// by the instance. The method works in terms of civil month numbers for most of
// the time in order to simplify the logic.
if (months == 0)
{
return(localInstant);
}
long tickOfDay = TimeOfDayCalculator.GetTickOfDay(localInstant);
var startDate = HebrewScripturalCalculator.HebrewFromAbsolute(AbsoluteDayFromLocalInstant(localInstant));
int year = startDate.Year;
int month = HebrewMonthConverter.ScripturalToCivil(year, startDate.Month);
// This arithmetic works the same both backwards and forwards.
year += (months / MonthsPerLeapCycle) * YearsPerLeapCycle;
months = months % MonthsPerLeapCycle;
if (months > 0)
{
// Add as many months as we need to in order to act as if we'd begun at the start
// of the year, for simplicity.
months += month - 1;
// Add a year at a time
while (months >= GetMaxMonth(year))
{
months -= GetMaxMonth(year);
year++;
}
// However many months we've got left to add tells us the final month.
month = months + 1;
}
else
{
// Pretend we were given the month at the end of the years.
months -= GetMaxMonth(year) - month;
// Subtract a year at a time
while (months + GetMaxMonth(year) <= 0)
{
months += GetMaxMonth(year);
year--;
}
// However many months we've got left to add (which will still be negative...)
// tells us the final month.
month = GetMaxMonth(year) + months;
}
// Convert back to scriptural for the last bit
month = HebrewMonthConverter.CivilToScriptural(year, month);
int day = Math.Min(HebrewScripturalCalculator.DaysInMonth(year, month), startDate.Day);
int absoluteDay = HebrewScripturalCalculator.AbsoluteFromHebrew(year, month, day);
return(LocalInstantFromAbsoluteDay(absoluteDay, tickOfDay));
}
internal override YearMonthDay GetYearMonthDay(int year, int dayOfYear)
{
YearMonthDay scriptural = HebrewScripturalCalculator.GetYearMonthDay(year, dayOfYear);
return(monthNumbering == HebrewMonthNumbering.Scriptural ? scriptural : new YearMonthDay(year, HebrewMonthConverter.ScripturalToCivil(year, scriptural.Month), scriptural.Day));
}
private int ScripturalToCalendarMonth(int year, int month) => monthNumbering == HebrewMonthNumbering.Scriptural ? month : HebrewMonthConverter.ScripturalToCivil(year, month);
// Note to self: this is (minuendInstant - subtrahendInstant) in months. So if minuendInstant
// is later than subtrahendInstant, the result should be positive.
internal override int MonthsBetween(LocalInstant minuendInstant, LocalInstant subtrahendInstant)
{
var minuendDate = HebrewScripturalCalculator.HebrewFromAbsolute(AbsoluteDayFromLocalInstant(minuendInstant));
var subtrahendDate = HebrewScripturalCalculator.HebrewFromAbsolute(AbsoluteDayFromLocalInstant(subtrahendInstant));
// First (quite rough) guess... we could probably be more efficient than this, but it's unlikely to be very far off.
double minuendMonths = (minuendDate.Year * MonthsPerLeapCycle) / (double)YearsPerLeapCycle + HebrewMonthConverter.ScripturalToCivil(minuendDate.Year, minuendDate.Month);
double subtrahendMonths = (subtrahendDate.Year * MonthsPerLeapCycle) / (double)YearsPerLeapCycle + HebrewMonthConverter.ScripturalToCivil(subtrahendDate.Year, subtrahendDate.Month);
int diff = (int)(minuendMonths - subtrahendMonths);
if (subtrahendInstant <= minuendInstant)
{
// Go backwards until we've got a tight upper bound...
while (AddMonths(subtrahendInstant, diff) > minuendInstant)
{
diff--;
}
// Go forwards until we've overshot
while (AddMonths(subtrahendInstant, diff) <= minuendInstant)
{
diff++;
}
// Take account of the overshoot
return(diff - 1);
}
else
{
// Moving backwards, so we need to end up with a result greater than or equal to
// minuendInstant...
// Go forwards until we've got a tight upper bound...
while (AddMonths(subtrahendInstant, diff) < minuendInstant)
{
diff++;
}
// Go backwards until we've overshot
while (AddMonths(subtrahendInstant, diff) >= minuendInstant)
{
diff--;
}
// Take account of the overshoot
return(diff + 1);
}
}
