Launch School challenge: Meetup

meetup.rb

class Meetup
  def initialize(month, year)
    @month, @year = month, year
  end

  def day(weekday, schedule)
    @date = Date.new(@year, @month, 1)
    wday = Date::DAYNAMES.index(weekday.to_s.capitalize)

    case schedule
    when :first
      @date.next_weekday(wday)

    when :second
      @date.next_weekday(wday) + 7

    when :third
      @date.next_weekday(wday) + 14

    when :fourth
      @date.next_weekday(wday) + 21

    when :last
      @end_of_month = Date.new(@year, @month, 1).next_month - 1
      @end_of_month.previous_weekday(wday)

    when :teenth
      @date = Date.new(@year, @month, 13)
      @date.next_weekday(wday)
    end
  end
end

class Date
  def next_weekday(wday)
    self + ((wday - self.wday) % 7)
  end

  def previous_weekday(wday)
    self - ((self.wday - wday) % 7)
  end
end