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.generators;
  
  import static org.assertj.core.api.Assertions.assertThat;
  
  import org.junit.jupiter.api.Test;
  import org.logicng.formulas.FormulaFactory;
  import org.logicng.formulas.Variable;
  import org.logicng.graphs.datastructures.Hypergraph;
  import org.logicng.graphs.datastructures.HypergraphEdge;
  import org.logicng.graphs.datastructures.HypergraphNode;
  import org.logicng.io.parsers.ParserException;
  import org.logicng.io.parsers.PropositionalParser;
  
  import java.util.Arrays;
  import java.util.Collections;
  
  /**
   * Unit tests for {@link HypergraphGenerator}.
   * @version 2.0.0
   * @since 1.4.0
   */
  public class HypergraphGeneratorTest {
  
      @Test
      public void testCNF() throws ParserException {
          final FormulaFactory f = new FormulaFactory();
          final PropositionalParser p = new PropositionalParser(f);
          assertThat(HypergraphGenerator.fromCNF(p.parse("$false")).nodes()).isEmpty();
          assertThat(HypergraphGenerator.fromCNF(p.parse("$false")).edges()).isEmpty();
          assertThat(HypergraphGenerator.fromCNF(p.parse("$true")).nodes()).isEmpty();
          assertThat(HypergraphGenerator.fromCNF(p.parse("$true")).edges()).isEmpty();
  
          Hypergraph<Variable> hypergraph = HypergraphGenerator.fromCNF(p.parse("A"));
          HypergraphNode<Variable> nodeA = new HypergraphNode<>(hypergraph, f.variable("A"));
          assertThat(hypergraph.nodes()).containsExactly(nodeA);
          assertThat(hypergraph.edges()).containsExactly(new HypergraphEdge<>(Collections.singletonList(nodeA)));
  
          hypergraph = HypergraphGenerator.fromCNF(p.parse("A | B | ~C"));
          nodeA = new HypergraphNode<>(hypergraph, f.variable("A"));
          HypergraphNode<Variable> nodeB = new HypergraphNode<>(hypergraph, f.variable("B"));
          HypergraphNode<Variable> nodeC = new HypergraphNode<>(hypergraph, f.variable("C"));
          assertThat(hypergraph.nodes()).containsExactlyInAnyOrder(nodeA, nodeB, nodeC);
          assertThat(hypergraph.edges()).containsExactlyInAnyOrder(new HypergraphEdge<>(Arrays.asList(nodeA, nodeB, nodeC)));
  
          hypergraph = HypergraphGenerator.fromCNF(p.parse("(A | B | ~C) & (B | ~D) & (C | ~E) & (~B | ~D | E) & X & ~Y"));
          nodeA = new HypergraphNode<>(hypergraph, f.variable("A"));
          nodeB = new HypergraphNode<>(hypergraph, f.variable("B"));
          nodeC = new HypergraphNode<>(hypergraph, f.variable("C"));
          final HypergraphNode<Variable> nodeD = new HypergraphNode<>(hypergraph, f.variable("D"));
          final HypergraphNode<Variable> nodeE = new HypergraphNode<>(hypergraph, f.variable("E"));
          final HypergraphNode<Variable> nodeX = new HypergraphNode<>(hypergraph, f.variable("X"));
          final HypergraphNode<Variable> nodeY = new HypergraphNode<>(hypergraph, f.variable("Y"));
          assertThat(hypergraph.nodes()).containsExactlyInAnyOrder(nodeA, nodeB, nodeC, nodeD, nodeE, nodeX, nodeY);
          assertThat(hypergraph.edges()).containsExactlyInAnyOrder(
                  new HypergraphEdge<>(Arrays.asList(nodeA, nodeB, nodeC)),
                  new HypergraphEdge<>(Arrays.asList(nodeB, nodeD)),
                  new HypergraphEdge<>(Arrays.asList(nodeC, nodeE)),
                  new HypergraphEdge<>(Arrays.asList(nodeB, nodeD, nodeE)),
                  new HypergraphEdge<>(Collections.singletonList(nodeX)),
                  new HypergraphEdge<>(Collections.singletonList(nodeY))
          );
      }
  
      @Test
      public void testCNFFromList() throws ParserException {
          final FormulaFactory f = new FormulaFactory();
          final PropositionalParser p = new PropositionalParser(f);
          assertThat(HypergraphGenerator.fromCNF(Collections.singletonList(p.parse("$false"))).nodes()).isEmpty();
          assertThat(HypergraphGenerator.fromCNF(Collections.singletonList(p.parse("$false"))).edges()).isEmpty();
          assertThat(HypergraphGenerator.fromCNF(Collections.singletonList(p.parse("$true"))).nodes()).isEmpty();
          assertThat(HypergraphGenerator.fromCNF(Collections.singletonList(p.parse("$true"))).edges()).isEmpty();
  
          Hypergraph<Variable> hypergraph = HypergraphGenerator.fromCNF(Collections.singletonList(p.parse("A")));
          HypergraphNode<Variable> nodeA = new HypergraphNode<>(hypergraph, f.variable("A"));
          assertThat(hypergraph.nodes()).containsExactly(nodeA);
          assertThat(hypergraph.edges()).containsExactly(new HypergraphEdge<>(Collections.singletonList(nodeA)));
  
          hypergraph = HypergraphGenerator.fromCNF(Collections.singletonList(p.parse("A | B | ~C")));
          nodeA = new HypergraphNode<>(hypergraph, f.variable("A"));
          HypergraphNode<Variable> nodeB = new HypergraphNode<>(hypergraph, f.variable("B"));
          HypergraphNode<Variable> nodeC = new HypergraphNode<>(hypergraph, f.variable("C"));
          assertThat(hypergraph.nodes()).containsExactlyInAnyOrder(nodeA, nodeB, nodeC);
          assertThat(hypergraph.edges()).containsExactlyInAnyOrder(new HypergraphEdge<>(Arrays.asList(nodeA, nodeB, nodeC)));
  
          hypergraph = HypergraphGenerator.fromCNF(Arrays.asList(
                  p.parse("(A | B | ~C)"),
                  p.parse("(B | ~D)"),
                  p.parse("(C | ~E)"),
                  p.parse("(~B | ~D | E)"),
                  p.parse("X"),
                  p.parse("~Y")
          ));
          nodeA = new HypergraphNode<>(hypergraph, f.variable("A"));
          nodeB = new HypergraphNode<>(hypergraph, f.variable("B"));
          nodeC = new HypergraphNode<>(hypergraph, f.variable("C"));
          HypergraphNode<Variable> nodeD = new HypergraphNode<>(hypergraph, f.variable("D"));
          HypergraphNode<Variable> nodeE = new HypergraphNode<>(hypergraph, f.variable("E"));
          HypergraphNode<Variable> nodeX = new HypergraphNode<>(hypergraph, f.variable("X"));
          HypergraphNode<Variable> nodeY = new HypergraphNode<>(hypergraph, f.variable("Y"));
          assertThat(hypergraph.nodes()).containsExactlyInAnyOrder(nodeA, nodeB, nodeC, nodeD, nodeE, nodeX, nodeY);
          assertThat(hypergraph.edges()).containsExactlyInAnyOrder(
                  new HypergraphEdge<>(Arrays.asList(nodeA, nodeB, nodeC)),
                  new HypergraphEdge<>(Arrays.asList(nodeB, nodeD)),
                  new HypergraphEdge<>(Arrays.asList(nodeC, nodeE)),
                  new HypergraphEdge<>(Arrays.asList(nodeB, nodeD, nodeE)),
                  new HypergraphEdge<>(Collections.singletonList(nodeX)),
                  new HypergraphEdge<>(Collections.singletonList(nodeY))
          );
  
          hypergraph = HypergraphGenerator.fromCNF(
                  p.parse("(A | B | ~C)"),
                  p.parse("(B | ~D)"),
                  p.parse("(C | ~E)"),
                  p.parse("(~B | ~D | E)"),
                  p.parse("X"),
                  p.parse("~Y")
          );
          nodeA = new HypergraphNode<>(hypergraph, f.variable("A"));
          nodeB = new HypergraphNode<>(hypergraph, f.variable("B"));
          nodeC = new HypergraphNode<>(hypergraph, f.variable("C"));
          nodeD = new HypergraphNode<>(hypergraph, f.variable("D"));
          nodeE = new HypergraphNode<>(hypergraph, f.variable("E"));
          nodeX = new HypergraphNode<>(hypergraph, f.variable("X"));
          nodeY = new HypergraphNode<>(hypergraph, f.variable("Y"));
          assertThat(hypergraph.nodes()).containsExactlyInAnyOrder(nodeA, nodeB, nodeC, nodeD, nodeE, nodeX, nodeY);
          assertThat(hypergraph.edges()).containsExactlyInAnyOrder(
                  new HypergraphEdge<>(Arrays.asList(nodeA, nodeB, nodeC)),
                  new HypergraphEdge<>(Arrays.asList(nodeB, nodeD)),
                  new HypergraphEdge<>(Arrays.asList(nodeC, nodeE)),
                  new HypergraphEdge<>(Arrays.asList(nodeB, nodeD, nodeE)),
                  new HypergraphEdge<>(Collections.singletonList(nodeX)),
                  new HypergraphEdge<>(Collections.singletonList(nodeY))
          );
      }
  
      @Test
      public void testNonCNF() throws ParserException {
          final FormulaFactory f = new FormulaFactory();
          final PropositionalParser p = new PropositionalParser(f);
          try {
              HypergraphGenerator.fromCNF(p.parse("A => B"));
          } catch (final IllegalArgumentException e) {
              assertThat(e).hasMessage("Cannot generate a hypergraph from a non-cnf formula");
          }
      }
  }
  