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/*
 * (C) Copyright 2015-2021, by Joris Kinable and Contributors.
 *
 * JGraphT : a free Java graph-theory library
 *
 * See the CONTRIBUTORS.md file distributed with this work for additional
 * information regarding copyright ownership.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License 2.0 which is available at
 * http://www.eclipse.org/legal/epl-2.0, or the
 * GNU Lesser General Public License v2.1 or later
 * which is available at
 * http://www.gnu.org/licenses/old-licenses/lgpl-2.1-standalone.html.
 *
 * SPDX-License-Identifier: EPL-2.0 OR LGPL-2.1-or-later
 */
package org.jgrapht.graph.specifics;

import org.jgrapht.*;
import org.jgrapht.alg.util.*;
import org.jgrapht.graph.*;

import java.util.*;
import java.util.function.*;

/**
 * Fast implementation of UndirectedSpecifics. This class uses additional data structures to improve
 * the performance of methods which depend on edge retrievals, e.g. getEdge(V u, V v),
 * containsEdge(V u, V v), addEdge(V u, V v). A disadvantage is an increase in memory consumption.
 * If memory utilization is an issue, use a {@link UndirectedSpecifics} instead.
 *
 * @param  the graph vertex type
 * @param  the graph edge type
 *
 * @author Joris Kinable
 */
public class FastLookupUndirectedSpecifics
    extends
    UndirectedSpecifics
{
    private static final long serialVersionUID = 225772727571597846L;

    /**
     * Maps a pair of vertices <u,v> to a set of edges {(u,v)}. In case of a multigraph, all
     * edges which touch both u and v are included in the set.
     */
    protected Map, Set> touchingVerticesToEdgeMap;

    /**
     * Construct a new fast lookup undirected specifics.
     * 
     * @param graph the graph for which these specifics are for
     * @param vertexMap map for the storage of vertex edge sets. Needs to have a predictable
     *        iteration order.
     * @param touchingVerticesToEdgeMap Additional map for caching. No need for a predictable
     *        iteration order.
     * @param edgeSetFactory factory for the creation of vertex edge sets
     */
    public FastLookupUndirectedSpecifics(
        Graph graph, Map> vertexMap,
        Map, Set> touchingVerticesToEdgeMap, EdgeSetFactory edgeSetFactory)
    {
        super(graph, vertexMap, edgeSetFactory);
        this.touchingVerticesToEdgeMap = Objects.requireNonNull(touchingVerticesToEdgeMap);
    }

    @Override
    public Set getAllEdges(V sourceVertex, V targetVertex)
    {
        if (graph.containsVertex(sourceVertex) && graph.containsVertex(targetVertex)) {
            Set edges =
                touchingVerticesToEdgeMap.get(new UnorderedPair<>(sourceVertex, targetVertex));
            if (edges == null) {
                return Collections.emptySet();
            } else {
                Set edgeSet = edgeSetFactory.createEdgeSet(sourceVertex);
                edgeSet.addAll(edges);
                return edgeSet;
            }
        } else {
            return null;
        }
    }

    @Override
    public E getEdge(V sourceVertex, V targetVertex)
    {
        Set edges =
            touchingVerticesToEdgeMap.get(new UnorderedPair<>(sourceVertex, targetVertex));
        if (edges == null || edges.isEmpty())
            return null;
        else
            return edges.iterator().next();
    }

    @Override
    public boolean addEdgeToTouchingVertices(V sourceVertex, V targetVertex, E e)
    {
        if (!super.addEdgeToTouchingVertices(sourceVertex, targetVertex, e)) {
            return false;
        }
        addToIndex(sourceVertex, targetVertex, e);
        return true;
    }

    @Override
    public boolean addEdgeToTouchingVerticesIfAbsent(V sourceVertex, V targetVertex, E e)
    {
        // first lookup using our own index
        E edge = getEdge(sourceVertex, targetVertex);
        if (edge != null) {
            return false;
        }

        return addEdgeToTouchingVertices(sourceVertex, targetVertex, e);
    }

    @Override
    public E createEdgeToTouchingVerticesIfAbsent(
        V sourceVertex, V targetVertex, Supplier edgeSupplier)
    {
        // first lookup using our own index
        E edge = getEdge(sourceVertex, targetVertex);
        if (edge != null) {
            return null;
        }

        E e = edgeSupplier.get();
        addEdgeToTouchingVertices(sourceVertex, targetVertex, e);
        return e;
    }

    @Override
    public void removeEdgeFromTouchingVertices(V sourceVertex, V targetVertex, E e)
    {
        super.removeEdgeFromTouchingVertices(sourceVertex, targetVertex, e);

        removeFromIndex(sourceVertex, targetVertex, e);
    }

    /**
     * Add an edge to the index.
     * 
     * @param sourceVertex the source vertex
     * @param targetVertex the target vertex
     * @param e the edge
     */
    protected void addToIndex(V sourceVertex, V targetVertex, E e)
    {
        Pair vertexPair = new UnorderedPair<>(sourceVertex, targetVertex);
        Set edgeSet = touchingVerticesToEdgeMap.get(vertexPair);
        if (edgeSet != null)
            edgeSet.add(e);
        else {
            edgeSet = edgeSetFactory.createEdgeSet(sourceVertex);
            edgeSet.add(e);
            touchingVerticesToEdgeMap.put(vertexPair, edgeSet);
        }
    }

    /**
     * Remove an edge from the index.
     * 
     * @param sourceVertex the source vertex
     * @param targetVertex the target vertex
     * @param e the edge
     */
    protected void removeFromIndex(V sourceVertex, V targetVertex, E e)
    {
        Pair vertexPair = new UnorderedPair<>(sourceVertex, targetVertex);
        Set edgeSet = touchingVerticesToEdgeMap.get(vertexPair);
        if (edgeSet != null) {
            edgeSet.remove(e);
            if (edgeSet.isEmpty())
                touchingVerticesToEdgeMap.remove(vertexPair);
        }
    }

}