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"""Implements the _Crc64Calculator class.
"""
from __future__ import absolute_import
from .crc64result import Crc64Result
class _Crc64Calculator(object):
"""Implements a class that calculates a 64-bit Cyclic Redundancy Check checksum.
Class initialiser requires a polynomial to seed the construction of a lookup table and an
optional initial XOR value.
def __init__(self, polynomial, initial_xor=0xffffffffffffffff):
self._construct_lookup_table(polynomial)
self._crc64 = initial_xor
@property
def crc64(self):
"""Returns the current CRC-64.
return Crc64Result(self._crc64)
def update(self, content):
"""Enumerates the bytes of the supplied bytearray and updates the CRC-64.
No return value.
for byte in content:
self._crc64 = (self._crc64 >> 8) ^ self._lookup_table[(self._crc64 & 0xff) ^ byte]
def _construct_lookup_table(self, polynomial):
"""Precomputes a CRC-64 lookup table seeded from the supplied polynomial.
self._lookup_table = []
for i in range(0, 256):
lookup_value = i
for _ in range(0, 8):
if lookup_value & 0x1 == 0x1:
lookup_value = (lookup_value >> 1) ^ polynomial
else:
lookup_value = lookup_value >> 1
self._lookup_table.append(lookup_value)
sjwood /
pydvdid
