Incorrect LRC value calculated from checksum

Question

I'm trying to calculate an LRC (Longitudinal Redundancy Check) value with Python. My Python code is pulled from other posts on StackOverflow. It looks like this:

lrc = 0
for b in message:
    lrc ^= b
print lrc

If I plug in the value '\x02\x47\x30\x30\x03', I get an LRC value of 70 or 0x46 (F) However, I am expecting a value of 68 - 0x44 (D) instead.

I have calculated the correct LRC value via C# code:

byte LRC = 0;
for (int i = 1; i < bytes.Length; i++)
 {
     LRC ^= bytes[i];
 }
 return LRC;

If I plug in the same byte array values, I get the expected result of 0x44.

Functionally, the code looks very similar. So I'm wondering what the difference is between the code. Is it my input value? Should I format my string differently?

score 2 · Accepted Answer · edited May 23 '17 at 10:28

2

Arrays are 0-ordered in C#, so by starting iteration from int i = 1; you are skipping 1st byte.

Python result is correct one.

Fixed reference code:

byte LRC = 0;
for (int i = 0; i < bytes.Length; i++)
{
     LRC ^= bytes[i];
}
return LRC;

To avoid such mistake you should consider using foreach syntactic sugar (although I'm not familiar with C# practices).

/edit

To skip first byte in Python simply use slice syntax:

lrc = 0
for b in message[1:]:
    lrc ^= b
print lrc

edited May 23 '17 at 10:28

Community

1
1

answered Apr 10 '15 at 20:07

Łukasz Rogalski

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Appreciate the input but the C# result is the one I am looking for. The Python implementation is the one that I am unhappy with. Good point with noting that it starts at 1 though. I will explore that further – metalcharms Apr 10 '15 at 20:29

score -1 · Answer 2 · answered Apr 10 '15 at 20:46

So I figured out the answer to my question. Thanks to Nsh for his insight. I found a way to make the algorithm work. I just had to skip the first byte in the for-loop. There's probably a better way to do this but it was quick and it's readable.

def calcLRC(input):
    input=input.decode('hex')
    lrc = 0
    i = 0
    message = bytearray(input)
    for b in message:
        if(i == 0):
            pass
        else:
            lrc ^= b
        i+=1;
    return lrc

It now returns the expected 0x44 in my use case.

Incorrect LRC value calculated from checksum

2 Answers2