Calculates the b(u) lookup table.

def __calculate_bu_dfs(dfs_data):
    """Calculates the b(u) lookup table."""
    u = dfs_data['ordering'][0]
    b = {}
    b[u] = D(u, dfs_data)
    __calculate_bu_dfs_recursively(u, b, dfs_data)
    return b