flattner.rb

class Flattner

  def initialize
    @res_arr = []
  end

  def flatten_ary ary
    unwind(ary)
    @res_arr
  end

  def unwind input_ary
    input_ary.each do  |el|
      if el.class.eql?(Array)
        unwind(el)
      else
        @res_arr << el
      end
    end
  end

end

test_flattner.rb

require 'test/unit'
require '[change_to_proper_path]/flattner'

class TestFlattner < Test::Unit::TestCase
  def test_flattner_empty_ary
    flattner = Flattner.new
    ary = []
    assert_equal flattner.flatten_ary(ary), []
  end

  def test_flattner_flat_ary
    flattner = Flattner.new
    ary = [1,2,3,4,5,6,7,8,9]
    assert_equal flattner.flatten_ary(ary), [1,2,3,4,5,6,7,8,9]
  end

  def test_flattner_full_ary
    flattner = Flattner.new
    ary = [1,2,[3],4,[5,[6,[7,8]]],9]
    assert_equal flattner.flatten_ary(ary), [1,2,3,4,5,6,7,8,9]
  end
end