module Italy
  class Holidays
    FIXED = [
      [1, 1],
      [1, 6],
      [4, 25],
      [5, 1],
      [6, 2],
      [8, 15],
      [11, 1],
      [12, 8],
      [12, 25],
      [12, 26]
    ].freeze

    def self.holiday?(date)
      date = date.to_date
      return true if FIXED.include?([date.month, date.day])

      easter = easter_date(date.year)
      date == easter || date == (easter + 1)
    end

    # Algoritmo gregoriano anonimo (Pasqua occidentale).
    def self.easter_date(year)
      a = year % 19
      b, c = year.divmod(100)
      d, e = b.divmod(4)
      f = (b + 8) / 25
      g = (b - f + 1) / 3
      h = (19 * a + b - d - g + 15) % 30
      i, k = c.divmod(4)
      l = (32 + 2 * e + 2 * i - h - k) % 7
      m = (a + 11 * h + 22 * l) / 451
      month, day = (h + l - 7 * m + 114).divmod(31)
      Date.new(year, month, day + 1)
    end
  end
end