def METHOD_NAME(self, particle_idx):
    """Add particles to the hashtable"""
    # See _deactivate_particles.
    add_split = hash_split(self._particle_hashes[particle_idx])
    for cur_hash, add_idx in add_split.items():
        if cur_hash not in self._hashtable:
            self._hashtable[cur_hash] = particle_idx[add_idx]
            self._hash_idx[particle_idx[add_idx]] = np.arange(len(add_idx))
        else:
            self._hash_idx[particle_idx[add_idx]] = np.arange(
                len(self._hashtable[cur_hash]),
                len(self._hashtable[cur_hash]) + len(add_idx))
            self._hashtable[cur_hash] = np.append(
                self._hashtable[cur_hash], particle_idx[add_idx])