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Checksum function

Posted on 2006-07-10
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Last Modified: 2010-04-15
I am trying to calculate the checksum (which is to obtain the sum of all bytes then obtain the 1's complement) - then store this as 1 byte to a file.  I am running on a Linux platform.

I received some feedback from the experts and was given this calculation -
unsigned char Data[12];
int i;

unsigned int CRC ()
{
  unsigned int sum;

  sum = 0;
  for (i=0; i < 12; ++i)
    sum += Data[i];

  sum ^= sum;
  return (sum);
}

However, i had some issues with it - it was populating some bizarre # -



When I use this approach  - I believe it works.  

#define       POLYNOMIAL      0x04C11DB7      
#define       WIDTH    (8 * sizeof(unsigned char))                                                                        
#define       TOPBIT   (1 << (WIDTH - 1))
      
  for (int byte = 0; byte < dataSize; ++byte)  {
       remainder ^= (data[byte] << (WIDTH - 8));
      for (unsigned char bit = 8; bit > 0; --bit) {    
                if (remainder & TOPBIT)
                              remainder = (remainder << 1) ^ POLYNOMIAL;
                         else
                         remainder = (remainder << 1);
                  }
                }
      }      
      return remainder;

I would like to  understand what the differences are between the 2 implementations.
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Question by:jewee
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LVL 45

Accepted Solution

by:
Kdo earned 300 total points
Comment Utility
Hi jewee,

They are quite different algorithms.  The first solution (which I provided) asked the question that you answered.  i.e. sum the bytes and take the 1's complement.

The equation that you've provided solves the checksum issue in quite a bit different fashion than what you'd originally asked.


Good Luck!
Kent
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by:jewee
Comment Utility
I was going to post this question int he previous one but I already closed it out.  

I was just curious - What are the differences in calculation???  do they both generate pretty much the same thing - just different implementation?
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LVL 8

Assisted Solution

by:manish_regmi
manish_regmi earned 200 total points
Comment Utility
As KDO said they are very different algorithms. The latter one is called CRC which is stronger then the first one.

please read the links
http://en.wikipedia.org/wiki/Checksum
http://en.wikipedia.org/wiki/Cyclic_redundancy_check

regards
Manish Regmi
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LVL 45

Expert Comment

by:Kdo
Comment Utility
Hi jewee,

> I was going to post this question int he previous one but I already
> closed it out.  

This is a legitimate follow up to the original post.  Don't be bashful about asking for clarification after closing a question.  Most of use that respond in a thread will revisit it after another post.


Kent
0
 

Author Comment

by:jewee
Comment Utility
Thank you Kent.  I wasn't aware of the fact that I could do that after it was closed.
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