cal.rb

# Pointless implementation of cal(1).
#
# Author :: Jon-Michael Deldin <[email protected]>
# Date   :: 2011-06-27
#
class Calendar

  # width of calendar in spaces
  WIDTH = 20

  # month lookup
  MONTHS = {
      1  =>  'January',
      2  =>  'February',
      3  =>  'March',
      4  =>  'April',
      5  =>  'May',
      6  =>  'June',
      7  =>  'July',
      8  =>  'August',
      9  =>  'September',
      10 => 'October',
      11 => 'November',
      12 => 'December',
  }

  # day lookup, 0-based for Sakamoto's algorithm
  DAYS = {
    0 => 'Su',
    1 => 'Mo',
    2 => 'Tu',
    3 => 'We',
    4 => 'Th',
    5 => 'Fr',
    6 => 'Sa',
  }

  # @param [Fixnum] year
  # @param [Fixnum] month
  #
  def initialize(year, month)
    @year  = year
    @month = month
  end

  # Returns the month's calendar as a string.
  #
  # @return [String]
  #
  def to_s
    s = header(@month) + "\n"

    day_tbl = {}
    DAYS.values.each { |d| day_tbl[d] = [] }

    # associate days to names
    (1 .. days_in_month(@month)).each do |i|
      day_tbl[DAYS[day_of_week(@year, @month, i)]] << i
    end

    # pad as many days necessary till the start
    day_tbl.each do |name, days|
      break if days.first == 1
      day_tbl[name].unshift(nil)
    end

    i = 0
    last = 0

    while day_tbl.any?
      # check sunday
      if day_tbl[DAYS[i]].first == last + 1
        s << "%2d" % day_tbl[DAYS[i]].first
        if (i+1) % 7 == 0
          s << "\n"
        else
          s << " "
        end
        day_tbl[DAYS[i]].shift
        last += 1
      elsif day_tbl[DAYS[i]].empty?
        day_tbl.delete(DAYS[i])
      else
        s << "   "
        day_tbl[DAYS[i]].shift
      end

      i = (i + 1) % 7
    end
    s.rstrip + "\n\n"
     end
  
  # Uses Sakamoto's algorithm <http://c-faq.com/misc/zeller.html>
  def day_of_week(y, m, d)
    t = [0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4]
    y -= m < 3 ? 1 : 0
    (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7
  end

 
  # Returns the number of days in a month.
  #
  #  L = 30 + { [ M + floor(M/8) ] MOD 2 }
  #
  # @param [Fixnum] month
  # @return [Array]
  #
  def days_in_month(month)
    days = 30 + ( month + (month / 8.0).floor ) % 2

    if month == 2
      days - 2
    else
      days
    end
  end

  # Leap years occur either every four years when not divisible by 100
  # or every year that is divisible by 400.
  #
  # @return [Boolean]
  def leap_year?
    (@year % 4 == 0 && @year % 100 != 0) || @year % 400 == 0
  end

  # Prints a header compatible with cal(1).
  #
  # cal prints more space on the right side if the header cannot be
  # evenly divided into two parts.
  #
  # @param [Fixnum] month
  #
  # @return [String]
  def header(month)
    lbl  = MONTHS[month] + ' ' + @year.to_s
    padl = (WIDTH - lbl.length) / 2
    padr = WIDTH - lbl.length - padl

    hdr = ' ' * padl + lbl
    hdr += "\n"
    hdr += "Su Mo Tu We Th Fr Sa"
  end

end

if $0 == __FILE__
  cal = Calendar.new(2011, 06)
  puts cal
end