code: ///////////////////////////////////////////////////////////////////////////
  //                   __                _      _   ________               //
  //                  / /   ____  ____ _(_)____/ | / / ____/               //
  //                 / /   / __ \/ __ `/ / ___/  |/ / / __                 //
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  //               /_____/\____/\__, /_/\___/_/ |_/\____/                  //
  //                           /____/                                      //
  //                                                                       //
  //               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 static org.assertj.core.api.Assertions.assertThat;
  import static org.assertj.core.api.Assertions.assertThatThrownBy;
  
  import org.assertj.core.data.Offset;
  import org.junit.jupiter.api.Test;
  import org.logicng.formulas.FormulaFactory;
  import org.logicng.formulas.Variable;
  import org.logicng.graphs.generators.HypergraphGenerator;
  import org.logicng.io.parsers.ParserException;
  import org.logicng.io.parsers.PropositionalParser;
  
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.Map;
  
  /**
   * Unit tests for {@link HypergraphEdge}.
   * @version 2.0.0
   * @since 1.4.0
   */
  public class HypergraphEdgeTest {
  
      private final Offset<Double> offset = Offset.offset(0.000001);
  
      @Test
      public void testSimpleMethods() {
          final Hypergraph<String> hypergraph = new Hypergraph<>();
          final HypergraphNode<String> node1 = new HypergraphNode<>(hypergraph, "A");
          final HypergraphNode<String> node2 = new HypergraphNode<>(hypergraph, "B");
          final HypergraphEdge<String> edge1 = new HypergraphEdge<>(node1, node2);
          assertThat(edge1.nodes()).containsExactlyInAnyOrder(node1, node2);
      }
  
      @Test
      public void testCenterOfGravity() throws ParserException {
          final FormulaFactory f = new FormulaFactory();
          final PropositionalParser p = new PropositionalParser(f);
          final Hypergraph<Variable> hypergraph = HypergraphGenerator.fromCNF(Collections.singletonList(p.parse("A | B | ~C | D")));
          final HypergraphEdge<Variable> edge = hypergraph.edges().iterator().next();
          final Map<HypergraphNode<Variable>, Integer> ordering = new HashMap<>();
          ordering.put(new HypergraphNode<>(hypergraph, f.variable("A")), 1);
          ordering.put(new HypergraphNode<>(hypergraph, f.variable("B")), 2);
          ordering.put(new HypergraphNode<>(hypergraph, f.variable("C")), 3);
          ordering.put(new HypergraphNode<>(hypergraph, f.variable("D")), 4);
          assertThat(edge.centerOfGravity(ordering)).isCloseTo(2.5, this.offset);
          ordering.put(new HypergraphNode<>(hypergraph, f.variable("A")), 2);
          ordering.put(new HypergraphNode<>(hypergraph, f.variable("B")), 4);
          ordering.put(new HypergraphNode<>(hypergraph, f.variable("C")), 6);
          ordering.put(new HypergraphNode<>(hypergraph, f.variable("D")), 8);
          assertThat(edge.centerOfGravity(ordering)).isCloseTo(5, this.offset);
      }
  
      @Test
      public void testIllegalCenterOfGravity() throws ParserException {
          final FormulaFactory f = new FormulaFactory();
          final PropositionalParser p = new PropositionalParser(f);
          final Hypergraph<Variable> hypergraph = HypergraphGenerator.fromCNF(Collections.singletonList(p.parse("A | B | ~C | D")));
          final HypergraphEdge<Variable> edge = hypergraph.edges().iterator().next();
          assertThatThrownBy(() -> edge.centerOfGravity(new HashMap<>())).isInstanceOf(IllegalStateException.class);
      }
  
      @Test
      public void testEquals() {
          final Hypergraph<String> hypergraph = new Hypergraph<>();
          final HypergraphNode<String> node1 = new HypergraphNode<>(hypergraph, "A");
          final HypergraphNode<String> node2 = new HypergraphNode<>(hypergraph, "B");
          final HypergraphNode<String> node3 = new HypergraphNode<>(hypergraph, "C");
          final HypergraphEdge<String> edge1 = new HypergraphEdge<>(node1, node2);
          final HypergraphEdge<String> edge2 = new HypergraphEdge<>(node2, node3);
          final HypergraphEdge<String> edge3 = new HypergraphEdge<>(node1, node2);
  
          assertThat(edge1.equals(null)).isFalse();
          assertThat(edge1).isNotEqualTo(42);
  
          assertThat(edge1).isEqualTo(edge1);
          assertThat(edge1.equals(edge1)).isTrue();
          assertThat(edge1).isEqualTo(edge3);
          assertThat(edge3).isEqualTo(edge1);
          assertThat(edge1).isNotEqualTo(edge2);
          assertThat(edge2).isNotEqualTo(edge1);
      }
  }
  