"""Functions for computing dominating sets in a graph."""
from itertools import chain

import networkx as nx
from networkx.utils import arbitrary_element

__all__ = ["dominating_set", "is_dominating_set"]


def dominating_set(G, start_with=None):
    r"""Finds a dominating set for the graph G.

    A *dominating set* for a graph with node set *V* is a subset *D* of
    *V* such that every node not in *D* is adjacent to at least one
    member of *D* [1]_.

    Parameters
    ----------
    G : NetworkX graph

    start_with : node (default=None)
        Node to use as a starting point for the algorithm.

    Returns
    -------
    D : set
        A dominating set for G.

    Notes
    -----
    This function is an implementation of algorithm 7 in [2]_ which
    finds some dominating set, not necessarily the smallest one.

    See also
    --------
    is_dominating_set

    References
    ----------
    .. [1] https://en.wikipedia.org/wiki/Dominating_set

    .. [2] Abdol-Hossein Esfahanian. Connectivity Algorithms.
        http://www.cse.msu.edu/~cse835/Papers/Graph_connectivity_revised.pdf

    """
    all_nodes = set(G)
    if start_with is None:
        start_with = arbitrary_element(all_nodes)
    if start_with not in G:
        raise nx.NetworkXError(f"node {start_with} is not in G")
    dominating_set = {start_with}
    dominated_nodes = set(G[start_with])
    remaining_nodes = all_nodes - dominated_nodes - dominating_set
    while remaining_nodes:
        # Choose an arbitrary node and determine its undominated neighbors.
        v = remaining_nodes.pop()
        undominated_neighbors = set(G[v]) - dominating_set
        # Add the node to the dominating set and the neighbors to the
        # dominated set. Finally, remove all of those nodes from the set
        # of remaining nodes.
        dominating_set.add(v)
        dominated_nodes |= undominated_neighbors
        remaining_nodes -= undominated_neighbors
    return dominating_set


def is_dominating_set(G, nbunch):
    """Checks if `nbunch` is a dominating set for `G`.

    A *dominating set* for a graph with node set *V* is a subset *D* of
    *V* such that every node not in *D* is adjacent to at least one
    member of *D* [1]_.

    Parameters
    ----------
    G : NetworkX graph

    nbunch : iterable
        An iterable of nodes in the graph `G`.

    See also
    --------
    dominating_set

    References
    ----------
    .. [1] https://en.wikipedia.org/wiki/Dominating_set

    """
    testset = {n for n in nbunch if n in G}
    nbrs = set(chain.from_iterable(G[n] for n in testset))
    return len(set(G) - testset - nbrs) == 0
