The Universe... in Ruby

universe.rb

class Matter
  attr_accessor :position, :mass
  def initialize(position, mass)
    @position = position
    @mass = mass
  end
  
  def <=> other_matter
    @distance <=> other_matter.distance
  end
end

class Star < Matter
  def initialize(mass)
    super(0, mass)
  end
end

class Planet < Matter
  attr_accessor :moons
  def initialize(position, mass, moons = [])
    @moons = moons
    super(position, mass)
  end
  
  def mass
    @mass + @moons.inject(0) do |mass, moon|
      mass + moon.mass
    end
  end
end

class Moon < Matter
  attr_accessor :planet
  def initialize(position, mass, planet = nil)
    @planet = planet
    super(position, mass)
    @planet.moons << self if planet
  end
end

class PlanetarySystem
  def initialize(star)
    @star = star
    @planets = []
  end
  
  def <<(planet)
    @planets << planet
  end
  
  def each(&block)
    @planets.sort.each &block
  end
  
  def to_a
    self.map {|p| p }
  end
  
  def planetary_mass
    @planets.inject(0) do |mass, matter|
      mass + matter.mass
    end
  end
end

sun = Star.new(1.9891e+30)
solar_system = PlanetarySystem.new(sun)
earth = Planet.new(149_597_890, 5.9742e+23)
solar_system << earth
moon = Moon.new(384_399, 8.94e+22, earth)
venus = Planet.new(108_208_930, 4.8690e+24)
solar_system << venus
mars = Planet.new(227_936_640, 6.41913+23)

solar_system.planetary_mass.should == 486127859
(venus > earth).should == false
(mars > earth).should == true
solar_system.map do |planet|
  planet.distance
end.should == [227_936_640, 149_597_890, 108_208_930, 0]
solar_system.to_a.should == [mars, earth, venus, sun]