About the wavelet transform

I'm just starting out with wavelet transform, and I want to do pitch detection.So I downloaded gnu's GSL computing library.But I don't understand a few lines of code in the wavelet transform function, can someone help me explain it?I posted the function related code.thank you

#define ELEMENT(a,stride,i) ((a)[(stride)*(i)])

int
gsl_wavelet_transform (const gsl_wavelet * w, 
                       double *data, size_t stride, size_t n,
                       gsl_wavelet_direction dir, 
                       gsl_wavelet_workspace * work)
{
  size_t i;

  if (work->n < n)
    {
      //GSL_ERROR ("not enough workspace provided", GSL_EINVAL);
    }

  if (binary_logn (n) == -1)
    {
      //GSL_ERROR ("n is not a power of 2", GSL_EINVAL);
    }

  if (n < 2)
    {
      return GSL_SUCCESS;
    }

  if (dir == gsl_wavelet_forward)
    {
      for (i = n; i >= 2; i >>= 1)
        {
          dwt_step (w, data, stride, i, dir, work);
        }
    }
  else
    {
      for (i = 2; i <= n; i <<= 1)
        {
          dwt_step (w, data, stride, i, dir, work);
        }
    }

  return GSL_SUCCESS;
}

static void
dwt_step (const gsl_wavelet * w, double *a, size_t stride, size_t n,
          gsl_wavelet_direction dir, gsl_wavelet_workspace * work)
{
  double ai, ai1;
  size_t i, ii;
  size_t jf;
  size_t k;
  size_t n1, ni, nh, nmod;

  for (i = 0; i < work->n; i++)
    {
      work->scratch[i] = 0.0;
    }


  **nmod = w->nc * n;** //Can not understand the meaning of this code
  **nmod -= w->offset;**    //Why does it need to be offset        /* center support */

  **n1 = n - 1;**
  **nh = n >> 1;**

  if (dir == gsl_wavelet_forward)
    {
      for (ii = 0, i = 0; i < n; i += 2, ii++)
        {
          double h = 0, g = 0;

          **ni = i + nmod;** //What meaning

          for (k = 0; k < w->nc; k++)
            {
              **jf = n1 & (ni + k);**  //What meaning
              h += w->h1[k] * ELEMENT (a, stride, jf);
              g += w->g1[k] * ELEMENT (a, stride, jf);
            }

          work->scratch[ii] += h;
          work->scratch[ii + nh] += g;
        }
    }

/* else { for (ii = 0, i = 0; i < n; i += 2, ii++) { ai = ELEMENT (a, stride, ii); ai1 = ELEMENT (a, stride, ii + nh); ni = i + nmod; for (k = 0; k < w->nc; k++) { jf = (n1 & (ni + k)); work->scratch[jf] += (w->h2[k] * ai + w->g2[k] * ai1); } } } */ for (i = 0; i < n; i++) { ELEMENT (a, stride, i) = work->scratch[i]; } }