Getting "Null" as return value in ruby method

Following code is only for testing purposes

controller class

def show
  @game = Game.find(params[:id])
  matrix = [
     [':yes', ':No', ':none'],
     [':yes', ':No',' :Yes'],
     [':No', ':Yes', ':none'],
 ]
 @game.matrix = ::Gamer.new(matrix).next
 @game.save!()
 ...
end

Another class in lib folder

class Gamer
 @@current_value

 def initialize(args_value)
  @@current_value = Marshal.load(Marshal.dump(args_value))
 end

 def next
  prev_state = Marshal.load(Marshal.dump(@@current_value))
  for row in prev_state
   for item in row
    ...
    #making changes into `@@current_value` 
    ...
   end
  end
  return @@current_value
end 

Whenever I check the value of gamer.matrix it returns null everytime. If i replace return from return @@current_value with return @@prev_state I am getting the exact value which I passed to this function. That means my controller is able to call this class method.

What I am doing wrong here?