stater.rb

class Stater
  
  def initialize(*roles)
    @state = 0
    @roles = roles
  end
  
  
  def current
    @roles[@state % @roles.size]
  end
  
  
  def next
    @roles[(@state + 1) % @roles.size]
  end
  
  
  def next!
    @state += 1
    current
  end
  
end

stater_spec.rb

require 'spec_helper'

describe Stater do
  
  before :all do
    @stater = Stater.new(:killers, :doctor, :detective)
  end
  
  it "should start with killers round" do
    @stater.current.should == :killers
  end
  
  it "should be followed by doctor round" do
    @stater.next.should    == :doctor
    @stater.next!.should   == :doctor
    @stater.current.should == :doctor
  end
  
  it "should be followed by detective round" do
    @stater.next.should    == :detective
    @stater.next!.should   == :detective
    @stater.current.should == :detective
  end
  
  it "should go back to killers round" do
    @stater.next.should    == :killers
    @stater.next!.should   == :killers
    @stater.current.should == :killers
    
  end
  
  it "should be followed by doctor round" do
    @stater.next.should    == :doctor
    @stater.next!.should   == :doctor
    @stater.current.should == :doctor
  end
  
end