arma::vec counts(classes);
counts.zeros();

for (size_t i = 0; i < labels.n_elem; ++i)
  counts(labels[i])++;

double entropy = 0.0;

for (size_t i = 0; i < classes; ++i)
{
  const double p1 = counts[i] / labels.n_elem;
  entropy += (p1 == 0) ? 0 : p1 * log2(p1);
}