data=f.read(BLOCK_SIZE)

		while data:			

			yield hashlib.sha256(data).digest()[:HASH_LEN]

			data=f.read(BLOCK_SIZE)

def hashes():

	with sqlite3.connect(DB) as db:

		handle=db.cursor()

		handle.execute("""select `sector_hash` from `hashes` order by `sector_id` asc""")

		h=handle.fetchone()

		while h is not None:

			yield h

			h=handle.fetchone()

def findChanges(fileName):

	changelist=[]

	# build changelist

	# can use up to (HASH_LEN+size(int))*(filesize/BLOCK_SIZE) bytes of memory plus overhead

	# that's (8+4)*(8*2**30 / 4096) = 24MB for defaults

	for (i,(dataHash,savedHash)) in enumerate(itertools.zip_longest(chunks(fileName),hashes())):

		if dataHash!=savedHash:

			changelist.append((i,dataHash))

		if dataHash is None: break # shouldn't happen

	# write log

	with open(logFile, mode="w") as f:

		f.write("sector	hash\n")

		for (i,dataHash) in changelist:

			f.write("{0}\t{1}\n".format(i,dataHash))

	# update DB

	with sqlite3.connect(DB) as db:

		handle=db.cursor()

		handle.executemany(

			"""update `hashes` set `sector_hash`=:hash, `dirty`=1 where `sector_id`=:id""",

			({"id":i,"hash":dataHash} for (i,dataHash) in changelist)

		)

		db.commit()

def transferChanges(targetPath):

	# read changes

	with sqlite3.connect(DB) as db, open(fileName,mode="rb") as sf:

		handle=db.cursor()

		handle.execute("""select `sector_id` from `hashes` where `dirty`=1""")

		# transfer modified sectors and mark them as clean

		sectorIds=handle.fetchall()

			path=targetPath / getPath(sectorId)

			try: path.parent.mkdir(parents=True)

			except FileExistsError: pass

			df=path.open(mode="wb")

			sf.seek(sectorId)

			df.write(sf.read(BLOCK_SIZE))

			handle.execute("""update `hashes` set `dirty`=0 where `sector_id`=?""",(sectorId,))

def getPath(index):

	nodeIds=[]

	k=1

	while k<=FILE_COUNT:

		k*=FILES_PER_DIR

	nodeIds.reverse()

action=sys.argv[1]

fileName=sys.argv[2]

baseName=".".join(fileName.split(".")[:-1])

hashFile="{0}-hash.dat".format(baseName)

isoDate=datetime.datetime.now().strftime("%Y%m%dT%H%M%S")

logFile="{0}-{1}.log".format(baseName,isoDate)

if action=="init": initHashes(fileName)

elif action=="update": findChanges(fileName)

elif action=="transfer": transferChanges(sys.argv[3])

else: print("bad action")