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class Crc(object):
"""
A base class for CRC routines.
"""
# Class constructor
###############################################################################
def __init__(self, width, poly, reflect_in, xor_in, reflect_out, xor_out, table_idx_width = None):
"""The Crc constructor.
The parameters are as follows:
width
poly
reflect_in
xor_in
reflect_out
xor_out
"""
self.Width = width
self.Poly = poly
self.ReflectIn = reflect_in
self.XorIn = xor_in
self.ReflectOut = reflect_out
self.XorOut = xor_out
self.TableIdxWidth = table_idx_width
self.MSB_Mask = 0x1 << (self.Width - 1)
self.Mask = ((self.MSB_Mask - 1) << 1) | 1
if self.TableIdxWidth != None:
self.TableWidth = 1 << self.TableIdxWidth
else:
self.TableIdxWidth = 8
self.TableWidth = 1 << self.TableIdxWidth
self.DirectInit = self.XorIn
self.NonDirectInit = self.__get_nondirect_init(self.XorIn)
if self.Width < 8:
self.CrcShift = 8 - self.Width
else:
self.CrcShift = 0
# function __get_nondirect_init
###############################################################################
def __get_nondirect_init(self, init):
"""
return the non-direct init if the direct algorithm has been selected.
"""
crc = init
for i in range(self.Width):
bit = crc & 0x01
if bit:
crc^= self.Poly
crc >>= 1
if bit:
crc |= self.MSB_Mask
return crc & self.Mask
# function reflect
###############################################################################
def reflect(self, data, width):
"""
reflect a data word, i.e. reverts the bit order.
"""
x = data & 0x01
for i in range(width - 1):
data >>= 1
x = (x << 1) | (data & 0x01)
return x
# function bit_by_bit
###############################################################################
def bit_by_bit(self, in_data):
"""
Classic simple and slow CRC implementation. This function iterates bit
by bit over the augmented input message and returns the calculated CRC
value at the end.
"""
# If the input data is a string, convert to bytes.
if isinstance(in_data, str):
in_data = [ord(c) for c in in_data]
register = self.NonDirectInit
for octet in in_data:
if self.ReflectIn:
octet = self.reflect(octet, 8)
for i in range(8):
topbit = register & self.MSB_Mask
register = ((register << 1) & self.Mask) | ((octet >> (7 - i)) & 0x01)
if topbit:
register ^= self.Poly
for i in range(self.Width):
topbit = register & self.MSB_Mask
register = ((register << 1) & self.Mask)
if topbit:
register ^= self.Poly
if self.ReflectOut:
register = self.reflect(register, self.Width)
return register ^ self.XorOut
# function bit_by_bit_fast
###############################################################################
def bit_by_bit_fast(self, in_data):
"""
This is a slightly modified version of the bit-by-bit algorithm: it
does not need to loop over the augmented bits, i.e. the Width 0-bits
wich are appended to the input message in the bit-by-bit algorithm.
"""
# If the input data is a string, convert to bytes.
if isinstance(in_data, str):
in_data = [ord(c) for c in in_data]
register = self.DirectInit
for octet in in_data:
if self.ReflectIn:
octet = self.reflect(octet, 8)
for i in range(8):
topbit = register & self.MSB_Mask
if octet & (0x80 >> i):
topbit ^= self.MSB_Mask
register <<= 1
if topbit:
register ^= self.Poly
register &= self.Mask
if self.ReflectOut:
register = self.reflect(register, self.Width)
return register ^ self.XorOut
# function gen_table
###############################################################################
def gen_table(self):
"""
This function generates the CRC table used for the table_driven CRC
algorithm. The Python version cannot handle tables of an index width
other than 8. See the generated C code for tables with different sizes
instead.
"""
table_length = 1 << self.TableIdxWidth
tbl = [0] * table_length
for i in range(table_length):
register = i
if self.ReflectIn:
register = self.reflect(register, self.TableIdxWidth)
register = register << (self.Width - self.TableIdxWidth + self.CrcShift)
for j in range(self.TableIdxWidth):
if register & (self.MSB_Mask << self.CrcShift) != 0:
register = (register << 1) ^ (self.Poly << self.CrcShift)
else:
register = (register << 1)
if self.ReflectIn:
register = self.reflect(register >> self.CrcShift, self.Width) << self.CrcShift
tbl[i] = register & (self.Mask << self.CrcShift)
return tbl
# function table_driven
###############################################################################
def table_driven(self, in_data):
"""
The Standard table_driven CRC algorithm.
"""
# If the input data is a string, convert to bytes.
if isinstance(in_data, str):
in_data = [ord(c) for c in in_data]
tbl = self.gen_table()
register = self.DirectInit << self.CrcShift
if not self.ReflectIn:
for octet in in_data:
tblidx = ((register >> (self.Width - self.TableIdxWidth + self.CrcShift)) ^ octet) & 0xff
register = ((register << (self.TableIdxWidth - self.CrcShift)) ^ tbl[tblidx]) & (self.Mask << self.CrcShift)
register = register >> self.CrcShift
else:
register = self.reflect(register, self.Width + self.CrcShift) << self.CrcShift
for octet in in_data:
tblidx = ((register >> self.CrcShift) ^ octet) & 0xff
register = ((register >> self.TableIdxWidth) ^ tbl[tblidx]) & (self.Mask << self.CrcShift)
register = self.reflect(register, self.Width + self.CrcShift) & self.Mask
if self.ReflectOut:
register = self.reflect(register, self.Width)
return register ^ self.XorOut |
Calcul de crc
Envoyé par fdesl
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