16 bit checksum calculation

Question

I need to calculate checksum using 16 Bit ones complement addition. Here is the code:

while(byte>0)  //len = Total num of bytes
{
    Word = ((Buf[i]<<8) + Buf[i+1]) + Checksum; //get two bytes at a time and  add previous calculated checsum value

    Checksum = Word & 0x0FFFF; //Discard the carry if any

    Word = (Word>>16);     //Keep the carryout for value exceeding 16 Bit

    Checksum = Word + Checksum; //Add the carryout if any

    len -= 2; //decrease by 2 for 2 byte boundaries
    i += 2;
}

 Checksum = (unsigned int)~Checksum;

The above code works fine, if I understood the concept well ie to add 16 bits, add the carry if any and then take the compliment.

Any improvements or corrections in the program above?

Answer 1

If the packet size is less than 32k words, then you do not need to add the carry until the end:

while(byte>0)  //len = Total num of bytes
{
    Checksum = ((Buf[i]<<8) + Buf[i+1]) + Checksum; //get two bytes at a time and  add previous calculated checsum value

    len -= 2; //decrease by 2 for 2 byte boundaries
    i += 2;
}

 Checksum = (Checksum>>16) + Checksum; //Add the carryout

 Checksum = (unsigned int)~Checksum;