sequence_parser.rb

require "minitest/autorun"

class SequenceParser
  def self.calc_sequence(input)
    words = []
    input.each_char do |char|
      last_elem = words.last
      if last_elem.nil?
        words << { elem_count: 1, elem: char }
      else
        if (last_elem[:elem] == char)
          last_elem[:elem_count] += 1
        else
          words << { elem_count: 1, elem: char }
        end
      end
    end

    words.map{|o| o.values.join }.join
  end
end

class TestSequence < Minitest::Test
  def test_known_sequences
    assert_equal '11', SequenceParser.calc_sequence('1')
    assert_equal '21', SequenceParser.calc_sequence('11')
    assert_equal '1211', SequenceParser.calc_sequence('21')
    assert_equal '111221', SequenceParser.calc_sequence('1211')
    assert_equal '312211', SequenceParser.calc_sequence('111221')
  end

  def test_smaple_sequences
    start = nil
    10.times do
      start = SequenceParser.calc_sequence(start || '1')
    end
    assert_equal '11131221133112132113212221', start
  end
end