https://blog.jnito.com/entry/2019/05/03/121235 のカレンダー問題

calendar.rb

require 'date'

class Calendar
  using Module.new {
    refine(Date) do
      def travel_back =  Enumerator.produce(self, &:prev_day)
      def travel_forward =  Enumerator.produce(self, &:next_day)
      def beginning_of_month = Date.new(year, month, 1)
      def end_of_month = Date.new(year, month, -1)
    end
  }

  def initialize(date) = @date = date
  def to_s = [header, week_header, body].join("\n")

  private
  # delegation
  def month = @date.month
  def year = @date.year
  def beginning_of_month = @date.beginning_of_month
  def end_of_month = @date.end_of_month

  # print
  def body = weeks.map { week_format % _1.map {|d| d.day if d.month == month } }.join("\n")
  def header = "%6d  %4d" % [month, year]
  def week_header = week_format % weeks.first.map { _1.strftime('%A')[0, 2] }
  def week_format = "%-3s" * 7

  # dates
  def first_sunday = beginning_of_month.travel_back.find { _1.sunday? }
  def last_saturday = end_of_month.travel_forward.find { _1.saturday? }
  def weeks = dates.each_slice(7)
  def dates = first_sunday..last_saturday
end

today = Date.today
beginning_of_month = Date.new(today.year, today.month, 1)
Enumerator.produce(beginning_of_month, &:next_month).take(12).each do
  puts Calendar.new(_1)
end