The program checksum.py is very simple: it can be used to calculate checksums
of different flavors (addition, xor-based, or fletcher) on either random
values (as generated by the program) or on values you pass in as a series of
bytes.
When run in the default mode:
prompt> ./checksum.py
OPTIONS seed 0
OPTIONS data_size 4
OPTIONS data
Decimal: 216 194 107 66
Hex: 0xd8 0xc2 0x6b 0x42
Bin: 0b11011000 0b11000010 0b01101011 0b01000010
Add: ?
Xor: ?
Fletcher: ?
prompt>In this example, the program produces a random set of four numbers (the "data") of 216 194 107 66 (decimal). The numbers are also shown in hex and binary.
The program then asks you to compute the additive, xor-based, and fletcher checksums. The addition is just the result of adding each of the bytes together, with the result modulo 256 (it's just a single byte checksum). The xor-based checksum is the result of xor'ing each byte together (it is also a single byte). Finally, fletcher is the result of computing the two parts of the fletcher checksum (as described in the chapter), which is two bytes in total.
You can change the seed to get a different problem:
prompt> ./checksum.py -s 1
OPTIONS seed 1
OPTIONS data_size 4
OPTIONS data
Decimal: Hex: Bin:
34 0x22 0b00100010
216 0xd8 0b11011000
195 0xc3 0b11000011
65 0x41 0b01000001
Add: ?
Xor: ?
Fletcher: ?
prompt> You can specify a different length for the random data:
prompt> ./checksum.py -d 2
OPTIONS seed 0
OPTIONS data_size 2
OPTIONS data
Decimal: Hex: Bin:
216 0xd8 0b11011000
194 0xc2 0b11000010
Add: ?
Xor: ?
Fletcher: ?
prompt> You can also specify your own data string:
prompt> ./checksum.py -D 1,2,3,4
OPTIONS seed 0
OPTIONS data_size 4
OPTIONS data 1,2,3,4
Decimal: Hex: Bin:
1 0x01 0b00000001
2 0x02 0b00000010
3 0x03 0b00000011
4 0x04 0b00000100
Add: ?
Xor: ?
Fletcher: ?
prompt> Finally, you can use -c to have the program compute the checksums for you.
prompt> ./checksum.py -D 1,2,3,4 -c
OPTIONS seed 0
OPTIONS data_size 4
OPTIONS data 1,2,3,4
Decimal: Hex: Bin:
1 0x01 0b00000001
2 0x02 0b00000010
3 0x03 0b00000011
4 0x04 0b00000100
Add: 10 (0b00001010)
Xor: 4 (0b00000100)
Fletcher(a,b): 10, 20 (0b00001010,0b00010100)
prompt> Thus ends the worst README in this collection of READMEs.