1,11,21,1211 ... sequence

algo.rb

def generate(cardinality=1, result=[1])
  if cardinality == 0
    return result
  else
    elem = result.last
    sub_ary = elem.to_s.split('').map(&:to_i)
    result << group(sub_ary)
    generate(cardinality - 1, result)
  end
end

def group(ary)
  len = ary.length
  counter = 0
  res = []
  while counter < len
    tmp_count = 0
    first_element_in_seq = ary[counter]

    100.times do |i|
      if first_element_in_seq == ary[counter + i]
        tmp_count += 1
      else
        break
      end
    end
    res << "#{tmp_count}#{first_element_in_seq}".to_i
    counter += tmp_count
  end

  res.join.to_i
end