Rubyでカレンダー

require 'date'

# Returns array of weeks of the year/month.
#   wday_start: Start day of week (0=Sun, 1=Mon, ..., 6=Sat)
#
# Example:
#   cal(2013, 1)
#     => [[nil,nil,  1,  2,  3,  4,  5],
#         [  6,  7,  8,  9, 10, 11, 12],
#         [ 13, 14, 15, 16, 17, 18, 19],
#         [ 20, 21, 22, 23, 24, 25, 26],
#         [ 27, 28, 29, 30, 31]]
def cal(year, month, wday_start = 0)
  last_day = Date.new(year, month, -1).day
  wday = (Date.new(year, month, 1).wday - wday_start) % 7
  weeks = (wday + last_day + 6) / 7
 
  (0 ... weeks).map do |i|
    day = i * 7 - wday + 1
    if day <= 0  # First week of the month.
      [nil] * wday + (1 .. 7 - wday).to_a
    elsif day + 7 > last_day  # Last week of the month.                         
      (day .. last_day).to_a
    else
      (day .. day + 6).to_a
    end
  end
end


if $0 == __FILE__
=begin
2013/1
 .  .  1  2  3  4  5
 6  7  8  9 10 11 12
13 14 15 16 17 18 19
20 21 22 23 24 25 26
27 28 29 30 31
=end
  now = Time.now
  month = (ARGV.shift || now.month).to_i
  year = (ARGV.shift || now.year).to_i

  weeks = cal(year, month)

  puts "#{year}/#{month}"
  weeks.each do |week|
    puts week.map {|d| d ? sprintf('%2d', d) : ' .'}.join(' ')
  end
end