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
|
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 |
