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.knowledgecompilation.bdds.datastructures;
  
  import static org.assertj.core.api.Assertions.assertThat;
  
  import org.junit.jupiter.api.Test;
  import org.logicng.formulas.FormulaFactory;
  
  /**
   * Unit tests for {@link BDDNode}, {@link BDDConstant}, and {@link BDDInnerNode}.
   * @version 2.0.0
   * @since 1.4.0
   */
  public class BDDNodeTest {
  
      @Test
      public void testSimpleMethods() {
          final FormulaFactory f = new FormulaFactory();
          final BDDNode verumNode = BDDConstant.getVerumNode(f);
          final BDDNode falsumNode = BDDConstant.getFalsumNode(f);
          final BDDNode innerNode = new BDDInnerNode(f.variable("A"), verumNode, falsumNode);
          assertThat(verumNode.isInnerNode()).isFalse();
          assertThat(falsumNode.isInnerNode()).isFalse();
          assertThat(innerNode.isInnerNode()).isTrue();
          assertThat(verumNode.label()).isEqualTo(f.verum());
          assertThat(falsumNode.label()).isEqualTo(f.falsum());
          assertThat(innerNode.label()).isEqualTo(f.variable("A"));
          assertThat(verumNode.low()).isNull();
          assertThat(verumNode.high()).isNull();
          assertThat(falsumNode.low()).isNull();
          assertThat(falsumNode.high()).isNull();
          assertThat(innerNode.low()).isEqualTo(verumNode);
          assertThat(innerNode.high()).isEqualTo(falsumNode);
      }
  
      @Test
      public void testNodes() {
          final FormulaFactory f = new FormulaFactory();
          final BDDNode verumNode = BDDConstant.getVerumNode(f);
          final BDDNode falsumNode = BDDConstant.getFalsumNode(f);
          final BDDNode innerNode = new BDDInnerNode(f.variable("A"), verumNode, falsumNode);
          assertThat(verumNode.nodes()).containsExactly(verumNode);
          assertThat(falsumNode.nodes()).containsExactly(falsumNode);
          assertThat(innerNode.nodes()).containsExactlyInAnyOrder(verumNode, falsumNode, innerNode);
      }
  
      @Test
      public void testHashCode() {
          final FormulaFactory f = new FormulaFactory();
          final BDDNode verumNode = BDDConstant.getVerumNode(f);
          final BDDNode falsumNode = BDDConstant.getFalsumNode(f);
          final BDDNode innerNode1 = new BDDInnerNode(f.variable("A"), verumNode, falsumNode);
          final BDDNode innerNode2 = new BDDInnerNode(f.variable("A"), verumNode, falsumNode);
          assertThat(verumNode.hashCode()).isEqualTo(verumNode.hashCode());
          assertThat(falsumNode.hashCode()).isEqualTo(falsumNode.hashCode());
          assertThat(innerNode1.hashCode()).isEqualTo(innerNode2.hashCode());
      }
  
      @Test
      public void testEquals() {
          final FormulaFactory f = new FormulaFactory();
          final BDDNode verumNode = BDDConstant.getVerumNode(f);
          final BDDNode falsumNode = BDDConstant.getFalsumNode(f);
          final BDDNode innerNode1 = new BDDInnerNode(f.variable("A"), verumNode, falsumNode);
          final BDDNode innerNode1a = new BDDInnerNode(f.variable("A"), verumNode, falsumNode);
          final BDDNode innerNode2 = new BDDInnerNode(f.variable("A"), falsumNode, verumNode);
          final BDDNode innerNode3 = new BDDInnerNode(f.variable("B"), verumNode, falsumNode);
          assertThat(verumNode).isEqualTo(verumNode);
          assertThat(falsumNode).isEqualTo(falsumNode);
          assertThat(innerNode1).isEqualTo(innerNode1);
          assertThat(innerNode1.equals(innerNode1)).isTrue();
          assertThat(innerNode1).isEqualTo(innerNode1a);
          assertThat(innerNode1).isNotEqualTo(innerNode2);
          assertThat(innerNode1).isNotEqualTo(innerNode3);
          assertThat(innerNode1).isNotEqualTo(null);
          assertThat(innerNode1).isNotEqualTo("String");
          assertThat(verumNode).isNotEqualTo(falsumNode);
          assertThat(verumNode).isNotEqualTo(innerNode3);
          assertThat(verumNode).isNotEqualTo(null);
          assertThat(verumNode).isNotEqualTo("String");
      }
  }
  