func makedag(ctx *cli.Context) {
	utils.CheckLegalese(ctx.GlobalString(utils.DataDirFlag.Name))

	args := ctx.Args()
	wrongArgs := func() {
		utils.Fatalf(`Usage: geth makedag <block number> <outputdir>`)
	}
	switch {
	case len(args) == 2:
		blockNum, err := strconv.ParseUint(args[0], 0, 64)
		dir := args[1]
		if err != nil {
			wrongArgs()
		} else {
			dir = filepath.Clean(dir)
			// seems to require a trailing slash
			if !strings.HasSuffix(dir, "/") {
				dir = dir + "/"
			}
			_, err = ioutil.ReadDir(dir)
			if err != nil {
				utils.Fatalf("Can't find dir")
			}
			fmt.Println("making DAG, this could take awhile...")
			ethash.MakeDAG(blockNum, dir)
		}
	default:
		wrongArgs()
	}
}

func (self *minerApi) MakeDAG(req *shared.Request) (interface{}, error) {
	args := new(MakeDAGArgs)
	if err := self.codec.Decode(req.Params, &args); err != nil {
		return nil, err
	}

	if args.BlockNumber < 0 {
		return false, shared.NewValidationError("BlockNumber", "BlockNumber must be positive")
	}

	err := ethash.MakeDAG(uint64(args.BlockNumber), "")
	if err == nil {
		return true, nil
	}
	return false, err
}

// StartAutoDAG() spawns a go routine that checks the DAG every autoDAGcheckInterval
// by default that is 10 times per epoch
// in epoch n, if we past autoDAGepochHeight within-epoch blocks,
// it calls ethash.MakeDAG  to pregenerate the DAG for the next epoch n+1
// if it does not exist yet as well as remove the DAG for epoch n-1
// the loop quits if autodagquit channel is closed, it can safely restart and
// stop any number of times.
// For any more sophisticated pattern of DAG generation, use CLI subcommand
// makedag
func (self *Ethereum) StartAutoDAG() {
	if self.autodagquit != nil {
		return // already started
	}
	go func() {
		glog.V(logger.Info).Infof("Automatic pregeneration of ethash DAG ON (ethash dir: %s)", ethash.DefaultDir)
		var nextEpoch uint64
		timer := time.After(0)
		self.autodagquit = make(chan bool)
		for {
			select {
			case <-timer:
				glog.V(logger.Info).Infof("checking DAG (ethash dir: %s)", ethash.DefaultDir)
				currentBlock := self.ChainManager().CurrentBlock().NumberU64()
				thisEpoch := currentBlock / epochLength
				if nextEpoch <= thisEpoch {
					if currentBlock%epochLength > autoDAGepochHeight {
						if thisEpoch > 0 {
							previousDag, previousDagFull := dagFiles(thisEpoch - 1)
							os.Remove(filepath.Join(ethash.DefaultDir, previousDag))
							os.Remove(filepath.Join(ethash.DefaultDir, previousDagFull))
							glog.V(logger.Info).Infof("removed DAG for epoch %d (%s)", thisEpoch-1, previousDag)
						}
						nextEpoch = thisEpoch + 1
						dag, _ := dagFiles(nextEpoch)
						if _, err := os.Stat(dag); os.IsNotExist(err) {
							glog.V(logger.Info).Infof("Pregenerating DAG for epoch %d (%s)", nextEpoch, dag)
							err := ethash.MakeDAG(nextEpoch*epochLength, "") // "" -> ethash.DefaultDir
							if err != nil {
								glog.V(logger.Error).Infof("Error generating DAG for epoch %d (%s)", nextEpoch, dag)
								return
							}
						} else {
							glog.V(logger.Error).Infof("DAG for epoch %d (%s)", nextEpoch, dag)
						}
					}
				}
				timer = time.After(autoDAGcheckInterval)
			case <-self.autodagquit:
				return
			}
		}
	}()
}