listfile="files.txt" #list of files to be mreged
num_ev=15 # number of events per merged file, needs to be more than events in each file
outfname="Events_merged" # appended with "_i.hepmc

first=True
counter_ev=0
counter_evtot=0
counter_file=0

header=[]

outfile=None 

with open(listfile) as inputlist:
    for infile in inputlist:
        with open(infile.rstrip()) as hepmc:
            for line in hepmc:
                #ignore empty lines
                if not line.strip():
                    continue
                #extract the header from the first file
                if line.startswith("HepMC::Version"):
                    if first:
                        header.append(line)
                    continue
                if line.startswith("HepMC::IO_GenEvent-START_EVENT_LISTING"):
                    if first:
                        header.append(line)
                        first = False
                    continue
                #end-of-file doesn't matter
                if line.startswith("HepMC::IO_GenEvent-END_EVENT_LISTING"):
                    continue
                
                if line.startswith("E"):
                    counter_ev=counter_ev+1
                    counter_evtot=counter_evtot+1
                    
                    #is this the first event for a new file?
                    if counter_ev > num_ev:
                        outfile.write("HepMC::IO_GenEvent-END_EVENT_LISTING")
                        outfile.close()
                        outfile=None
                        counter_ev=1

                    if outfile==None:
                        print "Opening new file "+outfname+"_"+str(counter_file)+".hepmc for event "+str(counter_evtot)
                        outfile=open(outfname+"_"+str(counter_file)+".hepmc","w")
                        for entry in header:
                            outfile.write(entry)
                            #print entry
                        counter_file=counter_file+1
                    adjustedline = line.split()
                    #print line
                    #print adjustedline
                    adjustedline[1]=str(counter_ev)
                    outfile.write(' '.join(adjustedline)+"\n")
                    continue

                outfile.write(line)



outfile.write("HepMC::IO_GenEvent-END_EVENT_LISTING")
outfile.close()
print "merged "+ str(counter_evtot) + " evnets into "+ str(counter_file) + " Files"