module DateExtensions
  def wday_from_monday
    wday == 0 ? 6 : wday - 1
  end
  
  def rest_of_days_in_week
    (0...(7 - wday_from_monday)).map { |i| self + i }
  end
  
  def start_of_next_week(interval = 1)
    (self + (7 * interval)).start_of_week
  end
  
  def start_of_week
    self - wday_from_monday
  end
  
  def start_of_next_month(interval = 1)
    new_month, new_year = month + interval, year
    
    until new_month <= 12 do
      new_month -= 12
      new_year += 1
    end
    
    Date.new(new_year, new_month, 1)
  end
  
  def start_of_next_year(interval = 1)
    Date.new(year + interval, 1, 1)
  end
  
  def day_occurrence_range(type)
    case type.to_s
      when 'first' : 1..7
      when 'second': 8..14
      when 'third' : 15..21
      when 'fourth': 22..28
      when 'last'  : ((days_in_month - 6)..days_in_month)
    end
  end
  
  def day_occurrences(type)
    day_occurrence_range(type).map do |i|
      renew(:day => i)
    end
  end
  
  def weekday?
    (1..5).include?(wday)
  end
  
  def days_in_month
    self.class.days_in_month(year, month)
  end
  
  def renew(options = {})
    self.class.new(options[:year] || year, options[:month] || month, options[:day] || day)
  end
end

class Date
  NORMAL_MONTH_LENGTHS = {
    1 => 31, 2 => 28, 3 => 31, 4 => 30, 5 => 31, 6 => 30,
    7 => 31, 8 => 31, 9 => 30, 10 => 31, 11 => 30, 12 => 31
  } unless const_defined? :NORMAL_MONTH_LENGTHS
  
  DAY_OCCURRENCES = %w{first second third fourth last}
  
  def self.days_in_month(year, month)
    days = NORMAL_MONTH_LENGTHS[month]
    days += 1 if month == 2 && new(year, month, 1).leap?
    days
  end
  
  include DateExtensions
end