insert_dummy.rb

require "minitest/autorun"

module InsertDummy
  def self.insert_dummy(a)
    dummy = "d"
    if a.length < 2
      a + [dummy].cycle(3).to_a
    else
      case a.length % 4
        when 0 then a
        when 1 then (a << dummy).insert(a.length - 4, dummy).insert(a.length - 8, dummy)
        when 2 then (a << dummy).insert(a.length - 4, dummy)
        when 3 then (a << dummy)
      end
    end
  end
end

class TestInsertDummy < Minitest::Test
  def test_length_after_inserting_dummy
    assert_equal InsertDummy.insert_dummy(%w(a)).length, 4
    assert_equal InsertDummy.insert_dummy(%w(a a)).length, 4
    assert_equal InsertDummy.insert_dummy(%w(a a a)).length, 4
    assert_equal InsertDummy.insert_dummy(%w(a a a a)).length, 4
    assert_equal InsertDummy.insert_dummy(%w(a a a a a)).length, 8
    
    1000.times do
      length = rand(100)
      if length != 0
        expected_length = (length / 4.0).ceil * 4
        assert_equal InsertDummy.insert_dummy(["a"].cycle(length).to_a).length, expected_length
      end
    end
  end
end