// array_associations will have shape: (6N-12, 2)
array_associations = np.dstack((point_indices, tri_indices))[0]
array_associations = array_associations[np.lexsort((
array_associations[...,1],
array_associations[...,0]))]
array_associations = array_associations.astype(np.intp)
// group by generator indices to produce
After Change
// indices will have shape: (6N-12,)
indices = np.argsort(point_indices, kind="mergesort")
// flattened_groups will have shape: (6N-12,)
flattened_groups = tri_indices[indices].astype(np.intp)
// intervals will have shape: (N+1,)
intervals = np.cumsum(np.bincount(point_indices + 1))
