code: ///////////////////////////////////////////////////////////////////////////
  //                   __                _      _   ________               //
  //                  / /   ____  ____ _(_)____/ | / / ____/               //
  //                 / /   / __ \/ __ `/ / ___/  |/ / / __                 //
  //                / /___/ /_/ / /_/ / / /__/ /|  / /_/ /                 //
  //               /_____/\____/\__, /_/\___/_/ |_/\____/                  //
  //                           /____/                                      //
  //                                                                       //
  //               The Next Generation Logic Library                       //
  //                                                                       //
  ///////////////////////////////////////////////////////////////////////////
  //                                                                       //
  //  Copyright 2015-20xx Christoph Zengler                                //
  //                                                                       //
  //  Licensed under the Apache License, Version 2.0 (the "License");      //
  //  you may not use this file except in compliance with the License.     //
  //  You may obtain a copy of the License at                              //
  //                                                                       //
  //  http://www.apache.org/licenses/LICENSE-2.0                           //
  //                                                                       //
  //  Unless required by applicable law or agreed to in writing, software  //
  //  distributed under the License is distributed on an "AS IS" BASIS,    //
  //  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or      //
  //  implied.  See the License for the specific language governing        //
  //  permissions and limitations under the License.                       //
  //                                                                       //
  ///////////////////////////////////////////////////////////////////////////
  
  package org.logicng.graphs.datastructures;
  
  import java.util.Arrays;
  import java.util.Collection;
  import java.util.LinkedHashSet;
  import java.util.Map;
  import java.util.Objects;
  import java.util.Set;
  
  /**
   * An edge in a hypergraph.
   * @param <T> the content type of the graph's nodes
   * @version 2.0.0
   * @since 1.4.0
   */
  public final class HypergraphEdge<T> {
  
      private final LinkedHashSet<HypergraphNode<T>> nodes;
  
      /**
       * Constructs a new edge for a given set of nodes.
       * @param nodes the nodes connected by this edge
       */
      public HypergraphEdge(final Collection<HypergraphNode<T>> nodes) {
          this.nodes = new LinkedHashSet<>(nodes);
          for (final HypergraphNode<T> node : nodes) {
              node.addEdge(this);
          }
      }
  
      /**
       * Constructs a new edge for a given set of nodes.
       * @param nodes the nodes connected by this edge
       */
      @SafeVarargs
      public HypergraphEdge(final HypergraphNode<T>... nodes) {
          this(Arrays.asList(nodes));
      }
  
      /**
       * Returns the nodes connected by this edge.
       * @return the nodes connected by this edge
       */
      public Set<HypergraphNode<T>> nodes() {
          return this.nodes;
      }
  
      /**
       * Computes the center of gravity for this edge (see Aloul, Markov, and Sakallah).
       * @param nodeOrdering the node ordering for which the COG is computed
       * @return the center of gravity for this edge
       */
      public double centerOfGravity(final Map<HypergraphNode<T>, Integer> nodeOrdering) {
          int cog = 0;
          for (final HypergraphNode<T> node : this.nodes) {
              final Integer level = nodeOrdering.get(node);
              if (level == null) {
                  throw new IllegalStateException("Could not find node " + node + " in the node ordering.");
              }
              cog += level;
          }
          return (double) cog / this.nodes.size();
      }
  
      @Override
      public boolean equals(final Object o) {
          if (this == o) {
              return true;
          }
          if (o == null || getClass() != o.getClass()) {
              return false;
          }
          final HypergraphEdge<?> that = (HypergraphEdge<?>) o;
          return Objects.equals(this.nodes, that.nodes);
      }
  
      @Override
      public int hashCode() {
          return Objects.hash(this.nodes);
      }
  
      @Override
      public String toString() {
          return "HypergraphEdge{" +
                  "nodes=" + this.nodes +
                  '}';
      }
  }
  