ranges.rb

def ranges(io, max_step = 1)
  in_range = false
  first = nil
  last = nil
  
  io.each_line do |line|
    id = line.to_i
    if first.nil?
      first = id
      last = id
      next
    end
    
    if in_range
      if id - last > max_step
        yield first, last if last - first > 1
        in_range = false
      end
    elsif id - last <= max_step
      in_range = true
      first = last
    end
    
    last = id
  end
  
  if in_range
    yield first, last
  end
end

io = StringIO.new(<<-EOS
             1
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             8
             11
             12
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             15
             16
             EOS
            )

p ranges(io) {|x, y| puts [x,y].inspect }

p ranges(io, 2) {|x, y| puts [x,y].inspect }