"""Unit tests for the sparsifier computation functions."""
import pytest
import networkx as nx
from networkx.utils import py_random_state


_seed = 2


def _test_spanner(G, spanner, stretch, weight=None):
    """Test whether a spanner is valid.

    This function tests whether the given spanner is a subgraph of the
    given graph G with the same node set. It also tests for all shortest
    paths whether they adhere to the given stretch.

    Parameters
    ----------
    G : NetworkX graph
        The original graph for which the spanner was constructed.

    spanner : NetworkX graph
        The spanner to be tested.

    stretch : float
        The proclaimed stretch of the spanner.

    weight : object
        The edge attribute to use as distance.
    """
    # check node set
    assert set(G.nodes()) == set(spanner.nodes())

    # check edge set and weights
    for u, v in spanner.edges():
        assert G.has_edge(u, v)
        if weight:
            assert spanner[u][v][weight] == G[u][v][weight]

    # check connectivity and stretch
    original_length = dict(nx.shortest_path_length(G, weight=weight))
    spanner_length = dict(nx.shortest_path_length(spanner, weight=weight))
    for u in G.nodes():
        for v in G.nodes():
            if u in original_length and v in original_length[u]:
                assert spanner_length[u][v] <= stretch * original_length[u][v]


@py_random_state(1)
def _assign_random_weights(G, seed=None):
    """Assigns random weights to the edges of a graph.

    Parameters
    ----------

    G : NetworkX graph
        The original graph for which the spanner was constructed.

    seed : integer, random_state, or None (default)
        Indicator of random number generation state.
        See :ref:`Randomness<randomness>`.
    """
    for u, v in G.edges():
        G[u][v]["weight"] = seed.random()


def test_spanner_trivial():
    """Test a trivial spanner with stretch 1."""
    G = nx.complete_graph(20)
    spanner = nx.spanner(G, 1, seed=_seed)

    for u, v in G.edges:
        assert spanner.has_edge(u, v)


def test_spanner_unweighted_complete_graph():
    """Test spanner construction on a complete unweighted graph."""
    G = nx.complete_graph(20)

    spanner = nx.spanner(G, 4, seed=_seed)
    _test_spanner(G, spanner, 4)

    spanner = nx.spanner(G, 10, seed=_seed)
    _test_spanner(G, spanner, 10)


def test_spanner_weighted_complete_graph():
    """Test spanner construction on a complete weighted graph."""
    G = nx.complete_graph(20)
    _assign_random_weights(G, seed=_seed)

    spanner = nx.spanner(G, 4, weight="weight", seed=_seed)
    _test_spanner(G, spanner, 4, weight="weight")

    spanner = nx.spanner(G, 10, weight="weight", seed=_seed)
    _test_spanner(G, spanner, 10, weight="weight")


def test_spanner_unweighted_gnp_graph():
    """Test spanner construction on an unweighted gnp graph."""
    G = nx.gnp_random_graph(20, 0.4, seed=_seed)

    spanner = nx.spanner(G, 4, seed=_seed)
    _test_spanner(G, spanner, 4)

    spanner = nx.spanner(G, 10, seed=_seed)
    _test_spanner(G, spanner, 10)


def test_spanner_weighted_gnp_graph():
    """Test spanner construction on an weighted gnp graph."""
    G = nx.gnp_random_graph(20, 0.4, seed=_seed)
    _assign_random_weights(G, seed=_seed)

    spanner = nx.spanner(G, 4, weight="weight", seed=_seed)
    _test_spanner(G, spanner, 4, weight="weight")

    spanner = nx.spanner(G, 10, weight="weight", seed=_seed)
    _test_spanner(G, spanner, 10, weight="weight")


def test_spanner_unweighted_disconnected_graph():
    """Test spanner construction on a disconnected graph."""
    G = nx.disjoint_union(nx.complete_graph(10), nx.complete_graph(10))

    spanner = nx.spanner(G, 4, seed=_seed)
    _test_spanner(G, spanner, 4)

    spanner = nx.spanner(G, 10, seed=_seed)
    _test_spanner(G, spanner, 10)


def test_spanner_invalid_stretch():
    """Check whether an invalid stretch is caught."""
    with pytest.raises(ValueError):
        G = nx.empty_graph()
        nx.spanner(G, 0)
