from hashlib import new as newhash

from os import stat

from os.path import isfile

import sys

def verifyFile(filePath, fileLength, checksums):

	actualLength = stat(filePath).st_size

	if actualLength != fileLength:

		raise OSError(

			'Expected length %d, actual length %d' % (fileLength, actualLength)

			)

	hashers = {}

	for algo in checksums.keys():

		try:

			hashers[algo] = newhash(algo)

		except ValueError as ex:

			raise OSError('Failed to create "%s" hasher: %s' % (algo, ex))

	bufSize = 16384

	with open(filePath, 'rb') as inp:

		while True:

			buf = inp.read(bufSize)

			if not buf:

				break

			for hasher in hashers.values():

				hasher.update(buf)

	for algo, hasher in sorted(hashers.items()):

		if checksums[algo] != hasher.hexdigest():

			raise OSError('%s checksum mismatch' % algo)

def main(filePath, fileLengthStr, checksumStrs):

	if not isfile(filePath):

		print('No such file: %s' % filePath, file=sys.stderr)

		sys.exit(2)

	try:

		fileLength = int(fileLengthStr)

	except ValueError:

		print('Length should be an integer', file=sys.stderr)

		sys.exit(2)

	checksums = {}

	for checksumStr in checksumStrs:

		try:

			algo, hashval = checksumStr.split('=')

		except ValueError:

			print('Invalid checksum format: %s' % checksumStr, file=sys.stderr)

			sys.exit(2)

		else:

			checksums[algo] = hashval

	print('Validating: %s' % filePath)

	try:

		verifyFile(filePath, fileLength, checksums)

	except OSError as ex:

		print('Validation FAILED: %s' % ex, file=sys.stderr)

		sys.exit(1)

	else:

		print('Validation passed')

		sys.exit(0)

if __name__ == '__main__':

	if len(sys.argv) >= 3:

		main(

			sys.argv[1],

			sys.argv[2],

			sys.argv[3 : ]

			)

	else:

		print(

			'Usage: python3 checksum.py FILE LENGTH (ALGO=HASH)*',

			file=sys.stderr

			)

		sys.exit(2)