1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
|
# pycrc -- parameterisable CRC calculation utility and C source code generator
#
# Copyright (c) 2006-2013 Thomas Pircher <tehpeh@gmx.net>
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to
# deal in the Software without restriction, including without limitation the
# rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
# sell copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
# IN THE SOFTWARE.
"""
CRC algorithms implemented in Python.
If you want to study the Python implementation of the CRC routines, then this
is a good place to start from.
The algorithms Bit by Bit, Bit by Bit Fast and Table-Driven are implemented.
This module can also be used as a library from within Python.
Examples
========
This is an example use of the different algorithms:
>>> from crc_algorithms import Crc
>>>
>>> crc = Crc(width = 16, poly = 0x8005,
... reflect_in = True, xor_in = 0x0000,
... reflect_out = True, xor_out = 0x0000)
>>> print("0x%x" % crc.bit_by_bit("123456789"))
>>> print("0x%x" % crc.bit_by_bit_fast("123456789"))
>>> print("0x%x" % crc.table_driven("123456789"))
"""
# Class Crc
###############################################################################
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
|
