231 lines
8.5 KiB
Python
231 lines
8.5 KiB
Python
# -*- coding: Latin-1 -*-
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# pycrc -- parametrisable CRC calculation utility and C source code generator
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#
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# Copyright (c) 2006-2012 Thomas Pircher <tehpeh@gmx.net>
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#
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# Permission is hereby granted, free of charge, to any person obtaining a copy
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# of this software and associated documentation files (the "Software"), to
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# deal in the Software without restriction, including without limitation the
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# rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
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# sell copies of the Software, and to permit persons to whom the Software is
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# furnished to do so, subject to the following conditions:
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#
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# The above copyright notice and this permission notice shall be included in
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# all copies or substantial portions of the Software.
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#
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# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
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# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
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# IN THE SOFTWARE.
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"""
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CRC algorithms implemented in Python.
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If you want to study the Python implementation of the CRC routines, then this
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is a good place to start from.
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The algorithms Bit by Bit, Bit by Bit Fast and Table-Driven are implemented.
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This module can also be used as a library from within Python.
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Examples
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========
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This is an example use of the different algorithms:
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>>> from crc_algorithms import Crc
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>>>
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>>> crc = Crc(width = 16, poly = 0x8005,
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... reflect_in = True, xor_in = 0x0000,
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... reflect_out = True, xor_out = 0x0000)
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>>> print("0x%x" % crc.bit_by_bit("123456789"))
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>>> print("0x%x" % crc.bit_by_bit_fast("123456789"))
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>>> print("0x%x" % crc.table_driven("123456789"))
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"""
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# Class Crc
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###############################################################################
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class Crc(object):
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"""
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A base class for CRC routines.
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"""
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# Class constructor
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###############################################################################
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def __init__(self, width, poly, reflect_in, xor_in, reflect_out, xor_out, table_idx_width = None):
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"""The Crc constructor.
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The parameters are as follows:
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width
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poly
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reflect_in
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xor_in
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reflect_out
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xor_out
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"""
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self.Width = width
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self.Poly = poly
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self.ReflectIn = reflect_in
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self.XorIn = xor_in
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self.ReflectOut = reflect_out
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self.XorOut = xor_out
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self.TableIdxWidth = table_idx_width
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self.MSB_Mask = 0x1 << (self.Width - 1)
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self.Mask = ((self.MSB_Mask - 1) << 1) | 1
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if self.TableIdxWidth != None:
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self.TableWidth = 1 << self.TableIdxWidth
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else:
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self.TableIdxWidth = 8
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self.TableWidth = 1 << self.TableIdxWidth
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self.DirectInit = self.XorIn
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self.NonDirectInit = self.__get_nondirect_init(self.XorIn)
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if self.Width < 8:
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self.CrcShift = 8 - self.Width
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else:
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self.CrcShift = 0
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# function __get_nondirect_init
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###############################################################################
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def __get_nondirect_init(self, init):
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"""
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return the non-direct init if the direct algorithm has been selected.
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"""
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crc = init
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for i in range(self.Width):
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bit = crc & 0x01
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if bit:
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crc^= self.Poly
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crc >>= 1
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if bit:
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crc |= self.MSB_Mask
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return crc & self.Mask
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# function reflect
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###############################################################################
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def reflect(self, data, width):
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"""
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reflect a data word, i.e. reverts the bit order.
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"""
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x = data & 0x01
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for i in range(width - 1):
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data >>= 1
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x = (x << 1) | (data & 0x01)
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return x
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# function bit_by_bit
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###############################################################################
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def bit_by_bit(self, in_str):
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"""
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Classic simple and slow CRC implementation. This function iterates bit
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by bit over the augmented input message and returns the calculated CRC
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value at the end.
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"""
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register = self.NonDirectInit
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for c in in_str:
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octet = ord(c)
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if self.ReflectIn:
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octet = self.reflect(octet, 8)
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for i in range(8):
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topbit = register & self.MSB_Mask
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register = ((register << 1) & self.Mask) | ((octet >> (7 - i)) & 0x01)
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if topbit:
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register ^= self.Poly
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for i in range(self.Width):
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topbit = register & self.MSB_Mask
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register = ((register << 1) & self.Mask)
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if topbit:
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register ^= self.Poly
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if self.ReflectOut:
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register = self.reflect(register, self.Width)
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return register ^ self.XorOut
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# function bit_by_bit_fast
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###############################################################################
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def bit_by_bit_fast(self, in_str):
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"""
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This is a slightly modified version of the bit-by-bit algorithm: it
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does not need to loop over the augmented bits, i.e. the Width 0-bits
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wich are appended to the input message in the bit-by-bit algorithm.
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"""
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register = self.DirectInit
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for c in in_str:
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octet = ord(c)
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if self.ReflectIn:
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octet = self.reflect(octet, 8)
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for i in range(8):
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topbit = register & self.MSB_Mask
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if octet & (0x80 >> i):
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topbit ^= self.MSB_Mask
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register <<= 1
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if topbit:
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register ^= self.Poly
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register &= self.Mask
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if self.ReflectOut:
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register = self.reflect(register, self.Width)
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return register ^ self.XorOut
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# function gen_table
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###############################################################################
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def gen_table(self):
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"""
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This function generates the CRC table used for the table_driven CRC
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algorithm. The Python version cannot handle tables of an index width
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other than 8. See the generated C code for tables with different sizes
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instead.
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"""
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table_length = 1 << self.TableIdxWidth
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tbl = [0] * table_length
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for i in range(table_length):
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register = i
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if self.ReflectIn:
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register = self.reflect(register, self.TableIdxWidth)
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register = register << (self.Width - self.TableIdxWidth + self.CrcShift)
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for j in range(self.TableIdxWidth):
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if register & (self.MSB_Mask << self.CrcShift) != 0:
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register = (register << 1) ^ (self.Poly << self.CrcShift)
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else:
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register = (register << 1)
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if self.ReflectIn:
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register = self.reflect(register >> self.CrcShift, self.Width) << self.CrcShift
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tbl[i] = register & (self.Mask << self.CrcShift)
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return tbl
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# function table_driven
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###############################################################################
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def table_driven(self, in_str):
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"""
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The Standard table_driven CRC algorithm.
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"""
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tbl = self.gen_table()
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register = self.DirectInit << self.CrcShift
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if not self.ReflectIn:
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for c in in_str:
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tblidx = ((register >> (self.Width - self.TableIdxWidth + self.CrcShift)) ^ ord(c)) & 0xff
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register = ((register << (self.TableIdxWidth - self.CrcShift)) ^ tbl[tblidx]) & (self.Mask << self.CrcShift)
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register = register >> self.CrcShift
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else:
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register = self.reflect(register, self.Width + self.CrcShift) << self.CrcShift
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for c in in_str:
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tblidx = ((register >> self.CrcShift) ^ ord(c)) & 0xff
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register = ((register >> self.TableIdxWidth) ^ tbl[tblidx]) & (self.Mask << self.CrcShift)
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register = self.reflect(register, self.Width + self.CrcShift) & self.Mask
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if self.ReflectOut:
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register = self.reflect(register, self.Width)
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return register ^ self.XorOut
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