Understanding Multiply and Accumulate Filter Code: Why Multiply by cof[0]?

I need a help with filter realization. I dont understand how this works. This is a multiply and accumulate filter, I dont understand why cof[0] constant is multiplied, here is the code cof is the filter coefficients and sample is the samples. Any help with this very appreciated. Thank you

long long mac(int len, int *sample, int *cof, int item)
{

   int i, io;

   static long long y;


   io = 0;

   y = 0;

   item /= 4;

   for ( i = 0; i < len; i++ )

      {

      y += (long long)sample[io] * (long long)cof[0];

      cof +=item;

      io++;

      if ( io >= len )

         io = 0;

      }

   return y;

}

Madu,
Let this be a lesson to you. This code needs comments.
It looks to me like cof is being incremented at each loop so it may be stepping through an array of coefficients. It's a bit puzzling why it is done  this way. And what relationship do len and item have?

To really understand how this works, I'm afraid that you'll have to step through the code with a set (or subset) of inputs. Afterward you should add useful comments for the next person (which could be you in a few months)

Hi Elizabeth,
Thank you very much for the reply. I dont know wheather you have account for this forum. I have posted a full code in another forum. Whether you could have a look at that post. Thanks

This appears to be a sum-of-products code, which is very common in digital signal processing. Look up Finite-Impulse-Response FIR filter. Look up Discrete Fourier Transform DFT. These are two examples of sum-of-products calculations. Compare these algorithms to your code, and add appropriate comments as Elizabeth suggests. Be careful with the details of how pointers are defined in your C compiler.

@madu Please copy and paste the full code here - Thanks

Hi everyone
Check this code 
Full code is here in original post