class Date

  def format(string)
    delimiters = string.scan /\W+/
    formatters = string.scan /[a-z0-9_]+/i

    format_units = {
      'd'                     => 'd',
      'day'                   => 'd',
      'day_padded'            => 'd',
      'dd'                    => '-d',
      'Day'                   => '-d',
      'day_unpadded'          => '-d',
      'ddd'                   => '_d',
      'DAY'                   => '_d',
      'day_blank'             => '_d',
      'dddd'                  => 'j',
      'day_of_the_year'       => 'j',
      'm'                     => 'm',
      'month'                 => 'm',
      'month_padded'          => 'm',
      'mm'                    => '-m',
      'Month'                 => '-m',
      'month_unpadded'        => '-m',
      'mmm'                   => '_m',
      'MONTH'                 => '_m',
      'month_blank'           => '_m',
      'mmmm'                  => 'B',
      'month_name'            => 'B',
      'mmmmm'                 => 'b',
      'month_name_abbr'       => 'b',
      'w'                     => 'u',
      'weekday'               => 'u',
      'ww'                    => 'w',
      'weekday_offset'        => 'w',
      'www'                   => 'A',
      'weekday_name'          => 'A',
      'wwww'                  => 'a',
      'weekday_name_abbr'     => 'a',
      'wwwww'                 => 'W',
      'week'                  => 'W',
      'wwwwww'                => 'U',
      'week_offset'           => 'U',
      'yy'                    => 'y',
      'yr'                    => 'y',
      'yyyy'                  => 'Y',
      'year'                  => 'Y',
      'ampm'                  => 'P',
      'meridian'              => 'P',
      'AMPM'                  => 'p',
      'MERIDIAN'              => 'p'
    }.freeze

    strftime(formatters.map { |f| "%#{format_units.fetch(f)}#{delimiters.shift || ''}" }.join)
  end

  def to_format(key=:datetime_iso)
    format_units = {
      month:                       '%m',
      month_padded:                '%m',
      month_unpadded:              '%-m',
      month_blank:                 '%_m',
      month_name:                  '%B',
      month_name_abbr:             '%b',
      weekday:                     '%d',
      weekday_padded:              '%d',
      weekday_unpadded:            '%-d',
      weekday_blank:               '%_d',
      weekday_name:                '%A',
      weekday_name_abbr:           '%a',
      yr:                          '%y',
      year_abbr:                   '%y',
      year:                        '%Y',
      date:                        '%B %-d, %Y',
      date_abbr:                   '%b %-d, %Y',
      date_iso:                    '%Y-%m-%d',
      datetime:                    '%B %-d, %Y %H:%M',
      datetime_abbr:               '%b %-d, %Y %H:%M',
      datetime_iso:                '%Y-%m-%d %H:%M',
      datetime_imperical:          '%B %-d, %Y %I:%M %P',
      datetime_imperical_abbr:     '%b %-d, %Y %I:%M %P',
      datetime_imperical_iso:      '%Y-%m-%d %I:%M %P',
      datetime_tzn:                '%B %-d, %Y %H:%M %Z',
      datetime_abbr_tzn:           '%b %-d, %Y %H:%M %Z',
      datetime_iso_tzn:            '%Y-%m-%d %H:%M %z',
      datetime_imperical_tzn:      '%B %-d, %Y %I:%M %P %Z',
      datetime_imperical_abbr_tzn: '%b %-d, %Y %I:%M %P %Z',
      datetime_imperical_iso_tzn:  '%Y-%m-%d %I:%M %P %z',
      day:                         '%B %-d',
      day_abbr:                    '%b %-d',
      day_iso:                     '%m-%d',
      daytime:                     '%B %-d %H:%M',
      daytime_abbr:                '%b %-d %H:%M',
      daytime_iso:                 '%m-%d %H:%M',
      daytime_imperical:           '%B %-d %I:%M %P',
      daytime_imperical_abbr:      '%b %-d %I:%M %P',
      daytime_imperical_iso:       '%m-%d %I:%M %P'
    }.freeze

    strftime(format_units.fetch(key.to_sym))
  end

  alias_method :stamp, :to_format

end