def compaction(array)
  return array if array.size == 0
  result = [[array.shift]]
  array.each{|n|
    result.last.last == n - 1 ? result.last << n : result << [n] 
  }
  result.map!{|ary|
    ary.size >= 3 ? "#{ary.first}-#{ary.last}" : ary.map{|n| n.to_s }.join(", ")
  }
  result.join(", ")
end

p compaction([1, 2, 3, 10, 11, 20, 100, 101, 102, 200])

"1-3, 10, 11, 20, 100-102, 200"