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.knowledgecompilation.bdds.functions;
  
  import org.logicng.formulas.Variable;
  import org.logicng.knowledgecompilation.bdds.BDD;
  import org.logicng.knowledgecompilation.bdds.datastructures.BDDConstant;
  import org.logicng.knowledgecompilation.bdds.datastructures.BDDInnerNode;
  import org.logicng.knowledgecompilation.bdds.datastructures.BDDNode;
  import org.logicng.knowledgecompilation.bdds.jbuddy.BDDKernel;
  import org.logicng.knowledgecompilation.bdds.jbuddy.BDDOperations;
  
  import java.util.HashMap;
  import java.util.Map;
  import java.util.stream.Collectors;
  
  /**
   * Creates a LogicNG internal BDD data structure of a given BDD.
   * @version 2.3.0
   * @since 2.0.0
   */
  public final class LngBDDFunction implements BDDFunction<BDDNode> {
  
      private static final LngBDDFunction INSTANCE = new LngBDDFunction();
  
      /**
       * @deprecated In the next version, the standard constructor will be replaced by a private constructor.
       * In order to instantiate an object of this class, use the {@link #get()} method.
       */
      @Deprecated
      public LngBDDFunction() {
          // intentionally left empty
      }
  
      /**
       * Returns the singleton instance of this function.
       * @return an instance of this function
       */
      public static LngBDDFunction get() {
          return INSTANCE;
      }
  
      @Override
      public BDDNode apply(final BDD bdd) {
          final BDDKernel kernel = bdd.underlyingKernel();
          final int index = bdd.index();
          final Map<Integer, int[]> kernelNodeMap = new BDDOperations(kernel).allNodes(index).stream()
                  .collect(Collectors.toMap(node -> node[0], node -> node));
          return buildBDDNode(index, kernel, kernelNodeMap, new HashMap<>());
      }
  
      private BDDNode buildBDDNode(final int index, final BDDKernel kernel, final Map<Integer, int[]> kernelNodeMap,
                                   final Map<Integer, BDDNode> nodeMap) {
          BDDNode node = nodeMap.get(index);
          if (node != null) {
              return node;
          }
          if (index == BDDKernel.BDD_FALSE) {
              node = BDDConstant.getFalsumNode(kernel.factory());
          } else if (index == BDDKernel.BDD_TRUE) {
              node = BDDConstant.getVerumNode(kernel.factory());
          } else {
              final int[] kernelNode = kernelNodeMap.get(index);
              final Variable variable = kernel.getVariableForIndex(kernelNode[1]);
              final BDDNode lowNode = buildBDDNode(kernelNode[2], kernel, kernelNodeMap, nodeMap);
              final BDDNode highNode = buildBDDNode(kernelNode[3], kernel, kernelNodeMap, nodeMap);
              node = new BDDInnerNode(variable, lowNode, highNode);
          }
          nodeMap.put(index, node);
          return node;
      }
  }
  