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+"""
+Graph summarization finds smaller representations of graphs resulting in faster
+runtime of algorithms, reduced storage needs, and noise reduction.
+Summarization has applications in areas such as visualization, pattern mining,
+clustering and community detection, and more.  Core graph summarization
+techniques are grouping/aggregation, bit-compression,
+simplification/sparsification, and influence based. Graph summarization
+algorithms often produce either summary graphs in the form of supergraphs or
+sparsified graphs, or a list of independent structures. Supergraphs are the
+most common product, which consist of supernodes and original nodes and are
+connected by edges and superedges, which represent aggregate edges between
+nodes and supernodes.
+
+Grouping/aggregation based techniques compress graphs by representing
+close/connected nodes and edges in a graph by a single node/edge in a
+supergraph. Nodes can be grouped together into supernodes based on their
+structural similarities or proximity within a graph to reduce the total number
+of nodes in a graph. Edge-grouping techniques group edges into lossy/lossless
+nodes called compressor or virtual nodes to reduce the total number of edges in
+a graph. Edge-grouping techniques can be lossless, meaning that they can be
+used to re-create the original graph, or techniques can be lossy, requiring
+less space to store the summary graph, but at the expense of lower
+reconstruction accuracy of the original graph.
+
+Bit-compression techniques minimize the amount of information needed to
+describe the original graph, while revealing structural patterns in the
+original graph.  The two-part minimum description length (MDL) is often used to
+represent the model and the original graph in terms of the model.  A key
+difference between graph compression and graph summarization is that graph
+summarization focuses on finding structural patterns within the original graph,
+whereas graph compression focuses on compressions the original graph to be as
+small as possible.  **NOTE**: Some bit-compression methods exist solely to
+compress a graph without creating a summary graph or finding comprehensible
+structural patterns.
+
+Simplification/Sparsification techniques attempt to create a sparse
+representation of a graph by removing unimportant nodes and edges from the
+graph.  Sparsified graphs differ from supergraphs created by
+grouping/aggregation by only containing a subset of the original nodes and
+edges of the original graph.
+
+Influence based techniques aim to find a high-level description of influence
+propagation in a large graph.  These methods are scarce and have been mostly
+applied to social graphs.
+
+*dedensification* is a grouping/aggregation based technique to compress the
+neighborhoods around high-degree nodes in unweighted graphs by adding
+compressor nodes that summarize multiple edges of the same type to
+high-degree nodes (nodes with a degree greater than a given threshold).
+Dedensification was developed for the purpose of increasing performance of
+query processing around high-degree nodes in graph databases and enables direct
+operations on the compressed graph.  The structural patterns surrounding
+high-degree nodes in the original is preserved while using fewer edges and
+adding a small number of compressor nodes.  The degree of nodes present in the
+original graph is also preserved. The current implementation of dedensification
+supports graphs with one edge type.
+
+For more information on graph summarization, see `Graph Summarization Methods
+and Applications: A Survey <https://dl.acm.org/doi/abs/10.1145/3186727>`_
+"""
+
+from collections import Counter, defaultdict
+
+import networkx as nx
+
+__all__ = ["dedensify", "snap_aggregation"]
+
+
+@nx._dispatchable(mutates_input={"not copy": 3}, returns_graph=True)
+def dedensify(G, threshold, prefix=None, copy=True):
+    """Compresses neighborhoods around high-degree nodes
+
+    Reduces the number of edges to high-degree nodes by adding compressor nodes
+    that summarize multiple edges of the same type to high-degree nodes (nodes
+    with a degree greater than a given threshold).  Dedensification also has
+    the added benefit of reducing the number of edges around high-degree nodes.
+    The implementation currently supports graphs with a single edge type.
+
+    Parameters
+    ----------
+    G: graph
+       A networkx graph
+    threshold: int
+       Minimum degree threshold of a node to be considered a high degree node.
+       The threshold must be greater than or equal to 2.
+    prefix: str or None, optional (default: None)
+       An optional prefix for denoting compressor nodes
+    copy: bool, optional (default: True)
+       Indicates if dedensification should be done inplace
+
+    Returns
+    -------
+    dedensified networkx graph : (graph, set)
+        2-tuple of the dedensified graph and set of compressor nodes
+
+    Notes
+    -----
+    According to the algorithm in [1]_, removes edges in a graph by
+    compressing/decompressing the neighborhoods around high degree nodes by
+    adding compressor nodes that summarize multiple edges of the same type
+    to high-degree nodes.  Dedensification will only add a compressor node when
+    doing so will reduce the total number of edges in the given graph. This
+    implementation currently supports graphs with a single edge type.
+
+    Examples
+    --------
+    Dedensification will only add compressor nodes when doing so would result
+    in fewer edges::
+
+        >>> original_graph = nx.DiGraph()
+        >>> original_graph.add_nodes_from(
+        ...     ["1", "2", "3", "4", "5", "6", "A", "B", "C"]
+        ... )
+        >>> original_graph.add_edges_from(
+        ...     [
+        ...         ("1", "C"), ("1", "B"),
+        ...         ("2", "C"), ("2", "B"), ("2", "A"),
+        ...         ("3", "B"), ("3", "A"), ("3", "6"),
+        ...         ("4", "C"), ("4", "B"), ("4", "A"),
+        ...         ("5", "B"), ("5", "A"),
+        ...         ("6", "5"),
+        ...         ("A", "6")
+        ...     ]
+        ... )
+        >>> c_graph, c_nodes = nx.dedensify(original_graph, threshold=2)
+        >>> original_graph.number_of_edges()
+        15
+        >>> c_graph.number_of_edges()
+        14
+
+    A dedensified, directed graph can be "densified" to reconstruct the
+    original graph::
+
+        >>> original_graph = nx.DiGraph()
+        >>> original_graph.add_nodes_from(
+        ...     ["1", "2", "3", "4", "5", "6", "A", "B", "C"]
+        ... )
+        >>> original_graph.add_edges_from(
+        ...     [
+        ...         ("1", "C"), ("1", "B"),
+        ...         ("2", "C"), ("2", "B"), ("2", "A"),
+        ...         ("3", "B"), ("3", "A"), ("3", "6"),
+        ...         ("4", "C"), ("4", "B"), ("4", "A"),
+        ...         ("5", "B"), ("5", "A"),
+        ...         ("6", "5"),
+        ...         ("A", "6")
+        ...     ]
+        ... )
+        >>> c_graph, c_nodes = nx.dedensify(original_graph, threshold=2)
+        >>> # re-densifies the compressed graph into the original graph
+        >>> for c_node in c_nodes:
+        ...     all_neighbors = set(nx.all_neighbors(c_graph, c_node))
+        ...     out_neighbors = set(c_graph.neighbors(c_node))
+        ...     for out_neighbor in out_neighbors:
+        ...         c_graph.remove_edge(c_node, out_neighbor)
+        ...     in_neighbors = all_neighbors - out_neighbors
+        ...     for in_neighbor in in_neighbors:
+        ...         c_graph.remove_edge(in_neighbor, c_node)
+        ...         for out_neighbor in out_neighbors:
+        ...             c_graph.add_edge(in_neighbor, out_neighbor)
+        ...     c_graph.remove_node(c_node)
+        ...
+        >>> nx.is_isomorphic(original_graph, c_graph)
+        True
+
+    References
+    ----------
+    .. [1] Maccioni, A., & Abadi, D. J. (2016, August).
+       Scalable pattern matching over compressed graphs via dedensification.
+       In Proceedings of the 22nd ACM SIGKDD International Conference on
+       Knowledge Discovery and Data Mining (pp. 1755-1764).
+       http://www.cs.umd.edu/~abadi/papers/graph-dedense.pdf
+    """
+    if threshold < 2:
+        raise nx.NetworkXError("The degree threshold must be >= 2")
+
+    degrees = G.in_degree if G.is_directed() else G.degree
+    # Group nodes based on degree threshold
+    high_degree_nodes = {n for n, d in degrees if d > threshold}
+    low_degree_nodes = G.nodes() - high_degree_nodes
+
+    auxiliary = {}
+    for node in G:
+        high_degree_nbrs = frozenset(high_degree_nodes & set(G[node]))
+        if high_degree_nbrs:
+            if high_degree_nbrs in auxiliary:
+                auxiliary[high_degree_nbrs].add(node)
+            else:
+                auxiliary[high_degree_nbrs] = {node}
+
+    if copy:
+        G = G.copy()
+
+    compressor_nodes = set()
+    for index, (high_degree_nodes, low_degree_nodes) in enumerate(auxiliary.items()):
+        low_degree_node_count = len(low_degree_nodes)
+        high_degree_node_count = len(high_degree_nodes)
+        old_edges = high_degree_node_count * low_degree_node_count
+        new_edges = high_degree_node_count + low_degree_node_count
+        if old_edges <= new_edges:
+            continue
+        compression_node = "".join(str(node) for node in high_degree_nodes)
+        if prefix:
+            compression_node = str(prefix) + compression_node
+        for node in low_degree_nodes:
+            for high_node in high_degree_nodes:
+                if G.has_edge(node, high_node):
+                    G.remove_edge(node, high_node)
+
+            G.add_edge(node, compression_node)
+        for node in high_degree_nodes:
+            G.add_edge(compression_node, node)
+        compressor_nodes.add(compression_node)
+    return G, compressor_nodes
+
+
+def _snap_build_graph(
+    G,
+    groups,
+    node_attributes,
+    edge_attributes,
+    neighbor_info,
+    edge_types,
+    prefix,
+    supernode_attribute,
+    superedge_attribute,
+):
+    """
+    Build the summary graph from the data structures produced in the SNAP aggregation algorithm
+
+    Used in the SNAP aggregation algorithm to build the output summary graph and supernode
+    lookup dictionary.  This process uses the original graph and the data structures to
+    create the supernodes with the correct node attributes, and the superedges with the correct
+    edge attributes
+
+    Parameters
+    ----------
+    G: networkx.Graph
+        the original graph to be summarized
+    groups: dict
+        A dictionary of unique group IDs and their corresponding node groups
+    node_attributes: iterable
+        An iterable of the node attributes considered in the summarization process
+    edge_attributes: iterable
+        An iterable of the edge attributes considered in the summarization process
+    neighbor_info: dict
+        A data structure indicating the number of edges a node has with the
+        groups in the current summarization of each edge type
+    edge_types: dict
+        dictionary of edges in the graph and their corresponding attributes recognized
+        in the summarization
+    prefix: string
+        The prefix to be added to all supernodes
+    supernode_attribute: str
+        The node attribute for recording the supernode groupings of nodes
+    superedge_attribute: str
+        The edge attribute for recording the edge types represented by superedges
+
+    Returns
+    -------
+    summary graph: Networkx graph
+    """
+    output = G.__class__()
+    node_label_lookup = {}
+    for index, group_id in enumerate(groups):
+        group_set = groups[group_id]
+        supernode = f"{prefix}{index}"
+        node_label_lookup[group_id] = supernode
+        supernode_attributes = {
+            attr: G.nodes[next(iter(group_set))][attr] for attr in node_attributes
+        }
+        supernode_attributes[supernode_attribute] = group_set
+        output.add_node(supernode, **supernode_attributes)
+
+    for group_id in groups:
+        group_set = groups[group_id]
+        source_supernode = node_label_lookup[group_id]
+        for other_group, group_edge_types in neighbor_info[
+            next(iter(group_set))
+        ].items():
+            if group_edge_types:
+                target_supernode = node_label_lookup[other_group]
+                summary_graph_edge = (source_supernode, target_supernode)
+
+                edge_types = [
+                    dict(zip(edge_attributes, edge_type))
+                    for edge_type in group_edge_types
+                ]
+
+                has_edge = output.has_edge(*summary_graph_edge)
+                if output.is_multigraph():
+                    if not has_edge:
+                        for edge_type in edge_types:
+                            output.add_edge(*summary_graph_edge, **edge_type)
+                    elif not output.is_directed():
+                        existing_edge_data = output.get_edge_data(*summary_graph_edge)
+                        for edge_type in edge_types:
+                            if edge_type not in existing_edge_data.values():
+                                output.add_edge(*summary_graph_edge, **edge_type)
+                else:
+                    superedge_attributes = {superedge_attribute: edge_types}
+                    output.add_edge(*summary_graph_edge, **superedge_attributes)
+
+    return output
+
+
+def _snap_eligible_group(G, groups, group_lookup, edge_types):
+    """
+    Determines if a group is eligible to be split.
+
+    A group is eligible to be split if all nodes in the group have edges of the same type(s)
+    with the same other groups.
+
+    Parameters
+    ----------
+    G: graph
+        graph to be summarized
+    groups: dict
+        A dictionary of unique group IDs and their corresponding node groups
+    group_lookup: dict
+        dictionary of nodes and their current corresponding group ID
+    edge_types: dict
+        dictionary of edges in the graph and their corresponding attributes recognized
+        in the summarization
+
+    Returns
+    -------
+    tuple: group ID to split, and neighbor-groups participation_counts data structure
+    """
+    nbr_info = {node: {gid: Counter() for gid in groups} for node in group_lookup}
+    for group_id in groups:
+        current_group = groups[group_id]
+
+        # build nbr_info for nodes in group
+        for node in current_group:
+            nbr_info[node] = {group_id: Counter() for group_id in groups}
+            edges = G.edges(node, keys=True) if G.is_multigraph() else G.edges(node)
+            for edge in edges:
+                neighbor = edge[1]
+                edge_type = edge_types[edge]
+                neighbor_group_id = group_lookup[neighbor]
+                nbr_info[node][neighbor_group_id][edge_type] += 1
+
+        # check if group_id is eligible to be split
+        group_size = len(current_group)
+        for other_group_id in groups:
+            edge_counts = Counter()
+            for node in current_group:
+                edge_counts.update(nbr_info[node][other_group_id].keys())
+
+            if not all(count == group_size for count in edge_counts.values()):
+                # only the nbr_info of the returned group_id is required for handling group splits
+                return group_id, nbr_info
+
+    # if no eligible groups, complete nbr_info is calculated
+    return None, nbr_info
+
+
+def _snap_split(groups, neighbor_info, group_lookup, group_id):
+    """
+    Splits a group based on edge types and updates the groups accordingly
+
+    Splits the group with the given group_id based on the edge types
+    of the nodes so that each new grouping will all have the same
+    edges with other nodes.
+
+    Parameters
+    ----------
+    groups: dict
+        A dictionary of unique group IDs and their corresponding node groups
+    neighbor_info: dict
+        A data structure indicating the number of edges a node has with the
+        groups in the current summarization of each edge type
+    edge_types: dict
+        dictionary of edges in the graph and their corresponding attributes recognized
+        in the summarization
+    group_lookup: dict
+        dictionary of nodes and their current corresponding group ID
+    group_id: object
+        ID of group to be split
+
+    Returns
+    -------
+    dict
+        The updated groups based on the split
+    """
+    new_group_mappings = defaultdict(set)
+    for node in groups[group_id]:
+        signature = tuple(
+            frozenset(edge_types) for edge_types in neighbor_info[node].values()
+        )
+        new_group_mappings[signature].add(node)
+
+    # leave the biggest new_group as the original group
+    new_groups = sorted(new_group_mappings.values(), key=len)
+    for new_group in new_groups[:-1]:
+        # Assign unused integer as the new_group_id
+        # ids are tuples, so will not interact with the original group_ids
+        new_group_id = len(groups)
+        groups[new_group_id] = new_group
+        groups[group_id] -= new_group
+        for node in new_group:
+            group_lookup[node] = new_group_id
+
+    return groups
+
+
+@nx._dispatchable(
+    node_attrs="[node_attributes]", edge_attrs="[edge_attributes]", returns_graph=True
+)
+def snap_aggregation(
+    G,
+    node_attributes,
+    edge_attributes=(),
+    prefix="Supernode-",
+    supernode_attribute="group",
+    superedge_attribute="types",
+):
+    """Creates a summary graph based on attributes and connectivity.
+
+    This function uses the Summarization by Grouping Nodes on Attributes
+    and Pairwise edges (SNAP) algorithm for summarizing a given
+    graph by grouping nodes by node attributes and their edge attributes
+    into supernodes in a summary graph.  This name SNAP should not be
+    confused with the Stanford Network Analysis Project (SNAP).
+
+    Here is a high-level view of how this algorithm works:
+
+    1) Group nodes by node attribute values.
+
+    2) Iteratively split groups until all nodes in each group have edges
+    to nodes in the same groups. That is, until all the groups are homogeneous
+    in their member nodes' edges to other groups.  For example,
+    if all the nodes in group A only have edge to nodes in group B, then the
+    group is homogeneous and does not need to be split. If all nodes in group B
+    have edges with nodes in groups {A, C}, but some also have edges with other
+    nodes in B, then group B is not homogeneous and needs to be split into
+    groups have edges with {A, C} and a group of nodes having
+    edges with {A, B, C}.  This way, viewers of the summary graph can
+    assume that all nodes in the group have the exact same node attributes and
+    the exact same edges.
+
+    3) Build the output summary graph, where the groups are represented by
+    super-nodes. Edges represent the edges shared between all the nodes in each
+    respective groups.
+
+    A SNAP summary graph can be used to visualize graphs that are too large to display
+    or visually analyze, or to efficiently identify sets of similar nodes with similar connectivity
+    patterns to other sets of similar nodes based on specified node and/or edge attributes in a graph.
+
+    Parameters
+    ----------
+    G: graph
+        Networkx Graph to be summarized
+    node_attributes: iterable, required
+        An iterable of the node attributes used to group nodes in the summarization process. Nodes
+        with the same values for these attributes will be grouped together in the summary graph.
+    edge_attributes: iterable, optional
+        An iterable of the edge attributes considered in the summarization process.  If provided, unique
+        combinations of the attribute values found in the graph are used to
+        determine the edge types in the graph.  If not provided, all edges
+        are considered to be of the same type.
+    prefix: str
+        The prefix used to denote supernodes in the summary graph. Defaults to 'Supernode-'.
+    supernode_attribute: str
+        The node attribute for recording the supernode groupings of nodes. Defaults to 'group'.
+    superedge_attribute: str
+        The edge attribute for recording the edge types of multiple edges. Defaults to 'types'.
+
+    Returns
+    -------
+    networkx.Graph: summary graph
+
+    Examples
+    --------
+    SNAP aggregation takes a graph and summarizes it in the context of user-provided
+    node and edge attributes such that a viewer can more easily extract and
+    analyze the information represented by the graph
+
+    >>> nodes = {
+    ...     "A": dict(color="Red"),
+    ...     "B": dict(color="Red"),
+    ...     "C": dict(color="Red"),
+    ...     "D": dict(color="Red"),
+    ...     "E": dict(color="Blue"),
+    ...     "F": dict(color="Blue"),
+    ... }
+    >>> edges = [
+    ...     ("A", "E", "Strong"),
+    ...     ("B", "F", "Strong"),
+    ...     ("C", "E", "Weak"),
+    ...     ("D", "F", "Weak"),
+    ... ]
+    >>> G = nx.Graph()
+    >>> for node in nodes:
+    ...     attributes = nodes[node]
+    ...     G.add_node(node, **attributes)
+    >>> for source, target, type in edges:
+    ...     G.add_edge(source, target, type=type)
+    >>> node_attributes = ("color",)
+    >>> edge_attributes = ("type",)
+    >>> summary_graph = nx.snap_aggregation(
+    ...     G, node_attributes=node_attributes, edge_attributes=edge_attributes
+    ... )
+
+    Notes
+    -----
+    The summary graph produced is called a maximum Attribute-edge
+    compatible (AR-compatible) grouping.  According to [1]_, an
+    AR-compatible grouping means that all nodes in each group have the same
+    exact node attribute values and the same exact edges and
+    edge types to one or more nodes in the same groups.  The maximal
+    AR-compatible grouping is the grouping with the minimal cardinality.
+
+    The AR-compatible grouping is the most detailed grouping provided by
+    any of the SNAP algorithms.
+
+    References
+    ----------
+    .. [1] Y. Tian, R. A. Hankins, and J. M. Patel. Efficient aggregation
+       for graph summarization. In Proc. 2008 ACM-SIGMOD Int. Conf.
+       Management of Data (SIGMOD’08), pages 567–580, Vancouver, Canada,
+       June 2008.
+    """
+    edge_types = {
+        edge: tuple(attrs.get(attr) for attr in edge_attributes)
+        for edge, attrs in G.edges.items()
+    }
+    if not G.is_directed():
+        if G.is_multigraph():
+            # list is needed to avoid mutating while iterating
+            edges = [((v, u, k), etype) for (u, v, k), etype in edge_types.items()]
+        else:
+            # list is needed to avoid mutating while iterating
+            edges = [((v, u), etype) for (u, v), etype in edge_types.items()]
+        edge_types.update(edges)
+
+    group_lookup = {
+        node: tuple(attrs[attr] for attr in node_attributes)
+        for node, attrs in G.nodes.items()
+    }
+    groups = defaultdict(set)
+    for node, node_type in group_lookup.items():
+        groups[node_type].add(node)
+
+    eligible_group_id, nbr_info = _snap_eligible_group(
+        G, groups, group_lookup, edge_types
+    )
+    while eligible_group_id:
+        groups = _snap_split(groups, nbr_info, group_lookup, eligible_group_id)
+        eligible_group_id, nbr_info = _snap_eligible_group(
+            G, groups, group_lookup, edge_types
+        )
+    return _snap_build_graph(
+        G,
+        groups,
+        node_attributes,
+        edge_attributes,
+        nbr_info,
+        edge_types,
+        prefix,
+        supernode_attribute,
+        superedge_attribute,
+    )