def est_entropy(self):
        r"""
        Estimates the entropy of the current particle distribution
        as :math:`-\sum_i w_i \log w_i` where :math:`\{w_i\}`
        is the set of particles with nonzero weight.
        """
        nz_weights = self.particle_weights[self.particle_weights > 0]
        return -np.sum(np.log(nz_weights) * nz_weights)