class Calendar
  require 'date'

  attr_reader :year

  def initialize(year)
    if year.is_a?(Integer) && year > 0
      @year = year
    else
      @year = 0
    end
  end

  def is_leap_year?
    return false if @year <= 0
    (@year % 4).zero? && ((@year % 100 != 0) || (@year % 400).zero?)
  end

  def days_in_month(month)
    return nil unless month.is_a?(Integer) && month.between?(1, 12)
    month_days = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
    if month == 2 && is_leap_year?
      29
    else
      month_days[month - 1]
    end
  end

  def day_of_week(month, day)
    return nil unless is_valid_date?(month, day)
    Date.new(@year, month, day).wday
  rescue ArgumentError
    nil
  end

  def get_month_name(month)
    return nil unless month.is_a?(Integer) && month.between?(1, 12)
    months = %w[January February March April May June July August September October November December]
    months[month - 1]
  end

  def format_date(month, day)
    return nil unless is_valid_date?(month, day)
    "%04d-%02d-%02d" % [@year, month, day]
  end

  def get_days_until_date(month, day)
    return nil unless is_valid_date?(month, day)
    days = 0
    (1...month).each { |m| days += days_in_month(m) }
    days + (day - 1)
  end

  def is_valid_date?(month, day)
    return false unless month.is_a?(Integer) && day.is_a?(Integer)
    return false unless month.between?(1, 12)
    max_day = days_in_month(month)
    return false if max_day.nil? || day < 1 || day > max_day
    true
  end

  def get_weekdays_in_month(month)
    return [] unless month.is_a?(Integer) && month.between?(1, 12)
    weekdays = []
    (1..days_in_month(month)).each do |d|
      wday = day_of_week(month, d)
      weekdays << %w[Sunday Monday Tuesday Wednesday Thursday Friday Saturday][wday] if !wday.nil?
    end
    weekdays
  end

  def count_occurrences_of_day(month, target_day)
    return 0 unless month.is_a?(Integer) && month.between?(1, 12)
    return 0 unless target_day.is_a?(Integer) && target_day.between?(0, 6)
    count = 0
    (1..days_in_month(month)).each do |d|
      count += 1 if day_of_week(month, d) == target_day
    end
    count
  end
end