composite_pattern.rb

module Leaf
   def land_size 
     @land_size || 0
   end
end

module Composite
  def append_child(child)
    @children << child
  end

  def remove_child(child)
    @children.delete child
  end

  def land_size
    @children.inject(0){ |sum, child| sum + child.land_size }
  end
end
   
class Building
  include Leaf
  def initialize( land_size )
    @land_size = land_size
  end
end

class Neighbourhood
  include Leaf
  include Composite

  def initialize 
    @children = []
  end
end

class City
  include Composite
  
  def initialize
    @children = []
  end
end

#example
cervia = City.new
terme = Neighbourhood.new
pinarella = Neighbourhood.new
pinarella.append_child Building.new( 400 )
pinarella.append_child Building.new( 600 )
terme.append_child Building.new( 200 )
terme.append_child Building.new( 1000 )
cervia.append_child pinarella
cervia.append_child terme
cervia.append_child Building.new( 500 )

puts "cervia size: #{cervia.land_size}" #=>cervia size: 2700
puts "pinarella size: #{pinarella.land_size}" #=>pinarella size: 1000
puts "terme size: #{terme.land_size}" #=> terme size: 1200