Number of children per parent, anywhere in the tree sequence

[hyanwong]

Here's a neat method that I think should work, and does not require any sorting:

def arg_num_children(ts):
    # Count the number of unique child IDs for each node in the ARG
    # Relies on the fact that edges in a tree seq are sorted by parent ID then by child ID
    same_parent = np.concatenate((ts.edges_parent[1:] == ts.edges_parent[:-1], [False]))
    same_child  = np.concatenate((ts.edges_child[1:] == ts.edges_child[:-1], [False]))
    is_last_unique = ~same_parent | ~same_child # last occurrence of each unique child per parent
    return np.bincount(ts.edges_parent[is_last_unique], minlength=ts.num_nodes)