class Holidays::DateCalculator::Easter::Gregorian

def calculate_easter_for(year)
  y = year
  a = y % 19
  b = y / 100
  c = y % 100
  d = b / 4
  e = b % 4
  f = (b + 8) / 25
  g = (b - f + 1) / 3
  h = (19 * a + b - d - g + 15) % 30
  i = c / 4
  k = c % 4
  l = (32 + 2 * e + 2 * i - h - k) % 7
  m = (a + 11 * h + 22 * l) / 451
  month = (h + l - 7 * m + 114) / 31
  day = ((h + l - 7 * m + 114) % 31) + 1
  Date.civil(year, month, day)
end

def calculate_orthodox_easter_for(year)
  j_date = Julian.new.calculate_orthodox_easter_for(year)
  case
  # up until 1582, julian and gregorian easter dates were identical
  when year <= 1582
    offset = 0
  # between the years 1583 and 1699 10 days are added to the julian day count
  when (year >= 1583 and year <= 1699)
    offset = 10
  # after 1700, 1 day is added for each century, except if the century year is exactly divisible by 400 (in which case no days are added).
  # Safe until 4100 AD, when one leap day will be removed.
  when year >= 1700
    offset = (year - 1700).divmod(100)[0] + ((year - year.divmod(100)[1]).divmod(400)[1] == 0 ? 0 : 1) - (year - year.divmod(100)[1] - 1700).divmod(400)[0] + 10
  end
  Date.jd(j_date.jd + offset)
end