Ruby 1.9と2.0で同じ結果を返す Array#shuffle と同等のメソッド http://sue445.hatenablog.com/entry/2013/06/23/222144

shuffle.rb

class Array
  # ruby 1.9系と2.0系で Array#shuffle の結果が異なるため独自実装
  # ref.
  # * https://github.com/ruby/ruby/blob/e3efce6df1aa691e17c59f442b35b4fd129d3a13/array.c#L4012
  # * https://github.com/ruby/ruby/blob/ruby_2_0_0/NEWS#L35-L36
  # * https://github.com/ruby/ruby/commit/e3efce6df1aa691e17c59f442b35b4fd129d3a13
  def my_shuffle(random)
    i = self.length
    dst_array = self.clone

    while i > 0 do
      j = random.rand(i)
      i -= 1
      dst_array[i], dst_array[j] = dst_array[j], dst_array[i]
    end

    dst_array
  end
end