static void
autocorr(real X[], real R[], int K, int M, int N)
{
  int  ii,jj;
  float tmp;

  for(ii=0; ii<=K; ii++) {
    R[ii] = 0;
    for(jj=M; jj<N; jj++) {
      tmp = X[jj] * X[jj-ii];
      R[ii] += tmp;
    }
  }
}