ranges.rb

def group_continuous(io)
  result = []
  range_start = nil
  offset = 0

  io.each_line do |line|
    id = line.to_i
    puts "#{id}\t#{range_start}\t#{offset}"
    if !range_start.nil? && id == range_start + offset + 1
      offset = offset + 1
    else
      if !range_start.nil?
        result << (range_start..(range_start + offset))
        range_start = id
        offset = 0
      else
        range_start = id
        offset = 0
      end
    end
  end
  result << (range_start..(range_start + offset))
  result
end

io = StringIO.new(<<-EOS
             1
             2
             3
             5
             6
             8
             11
             12
             13
             15
             16
             EOS
            )

p group_continuous(io)