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 static org.assertj.core.api.Assertions.assertThat;
  import static org.assertj.core.api.Assertions.assertThatThrownBy;
  
  import org.junit.jupiter.api.Test;
  import org.logicng.formulas.Formula;
  import org.logicng.formulas.FormulaFactory;
  import org.logicng.formulas.Variable;
  
  import java.io.BufferedReader;
  import java.io.FileReader;
  import java.io.IOException;
  
  /**
   * Unit tests for the class {@link Graph} and the class {@link Node}.
   * @version 2.0.0
   * @since 1.2
   */
  public class GraphTest {
  
      public static Graph<Long> getLongGraph(final String id) throws IOException {
          final Graph<Long> g = new Graph<>(id + "-Long");
  
          final BufferedReader reader = new BufferedReader(new FileReader("src/test/resources/graphs/graph" + id + ".txt"));
  
          while (reader.ready()) {
              final String line = reader.readLine().trim();
              final String[] nodePair = line.split(":");
              g.connect(g.node(Long.valueOf(nodePair[0])), g.node(Long.valueOf(nodePair[1])));
          }
  
          return g;
      }
  
      @Test
      public void testLongGraph() {
          final Graph<Long> g = new Graph<>("Graph with Long nodes.");
  
          final Node<Long> a = g.node(1L);
          final Node<Long> b = g.node(2L);
          final Node<Long> c = g.node(3L);
          final Node<Long> d = g.node(4L);
          final Node<Long> d2 = g.node(4L);
  
          g.connect(d, d2);
          assertThat(d.neighbours().contains(d2)).isFalse();
  
          g.connect(a, b);
          g.connect(a, c);
          g.connect(a, d);
          g.connect(b, c);
  
          d.connectTo(d2);
          assertThat(d2.neighbours().contains(d)).isFalse();
  
          assertThat(a.neighbours().size()).isEqualTo(3);
          assertThat(c.neighbours().contains(a)).isTrue();
          assertThat(c.neighbours().contains(b)).isTrue();
  
          assertThat(a.toString()).isEqualTo("Node{content=1, neighbours:2,3,4}");
      }
  
      @Test
      public void testFormulaGraph() {
          final FormulaFactory f = new FormulaFactory();
          final Graph<Formula> g = new Graph<>("Graph with Formula nodes.");
  
          final Variable a = f.variable("A");
          final Node<Formula> an = g.node(a);
          final Variable b = f.variable("B");
          final Node<Formula> bn = g.node(b);
  
          final Formula aNb = f.and(a, b);
          final Node<Formula> aNbn = g.node(aNb);
          g.connect(aNbn, an);
          g.connect(aNbn, bn);
  
          assertThat(aNbn.neighbours().size()).isEqualTo(2);
          assertThat(an.neighbours().contains(aNbn)).isTrue();
          assertThat(bn.neighbours().contains(aNbn)).isTrue();
  
          assertThat(an.graph().name()).isEqualTo(g.name());
  
          g.disconnect(aNbn, an);
  
          assertThat(an.neighbours().isEmpty()).isTrue();
  
          assertThat(g.nodes().size()).isEqualTo(3);
      }
  
      @Test
      public void testTwoGraphs() {
          final Graph<String> g1 = new Graph<>("G1");
          final Graph<String> g2 = new Graph<>("G2");
  
          final Node<String> a = g1.node("A");
          final Node<String> b = g2.node("B");
  
          g1.disconnect(a, b);
          assertThat(a.neighbours().isEmpty()).isTrue();
          assertThatThrownBy(() -> g1.connect(a, b)).isInstanceOf(IllegalArgumentException.class);
      }
  
      @Test
      public void testNodes() {
          final Graph<String> g1 = new Graph<>("G1");
          final Node<String> node01 = new Node<>("nA", g1);
          final Node<String> node02 = new Node<>("nA", g1);
          final Node<String> node03 = new Node<>("nB", g1);
  
          assertThat(node01).isEqualTo(node01);
          assertThat(node01.equals(node01)).isTrue();
          assertThat(node01).isEqualTo(node02);
          assertThat(node02).isEqualTo(node01);
          assertThat(node01.equals(2)).isFalse();
          assertThat(node02).isNotEqualTo(node03);
          assertThat(node03).isNotEqualTo(node02);
      }
  }
  