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.dnnf.datastructures.dtree;
  
  import org.logicng.formulas.Variable;
  import org.logicng.knowledgecompilation.dnnf.DnnfSatSolver;
  
  import java.util.BitSet;
  import java.util.List;
  import java.util.SortedSet;
  
  /**
   * Super class for a Decomposition Tree (DTree) for the DNNF Compiler
   * This is either a {@link DTreeNode} or a {@link DTreeLeaf}.
   * @version 2.0.0
   * @since 2.0.0
   */
  public abstract class DTree {
  
      protected int[] staticVariables;
      protected BitSet staticVarSet;
      protected int[] staticSeparator;
  
      /**
       * Initializes the DTree.
       * @param solver a specializes DNNF SAT solver
       */
      public abstract void initialize(final DnnfSatSolver solver);
  
      /**
       * Returns the size of the DTree.
       * @return the size of the DTree
       */
      public abstract int size();
  
      /**
       * Returns all variables of this DTree.
       * <p>
       * Since this set of variables can be cached, this is a constant time operation.
       * @return all variables of this DTree
       */
      int[] staticVarSetArray() {
          return this.staticVariables;
      }
  
      /**
       * Returns all variables of this DTree.
       * <p>
       * Since this set of variables can be cached, this is a constant time operation.
       * @return all variables of this DTree
       */
      public BitSet staticVarSet() {
          return this.staticVarSet;
      }
  
      /**
       * Returns all variables of this DTree.
       * <p>
       * Since this set of variables can be cached, this is a constant time operation.
       * @return all variables of this DTree
       */
      abstract SortedSet<Variable> staticVariableSet();
  
      /**
       * The dynamic separator of this DTree.  "Dynamic" means that subsumed clauses are ignored during the separator computation.
       * @return The dynamic separator of this DTree
       */
      public abstract BitSet dynamicSeparator();
  
      /**
       * The ids clauses in this DTree.
       * @return The clause ids
       */
      abstract int[] staticClauseIds();
  
      /**
       * Counts the number of unsubsumed occurrences for each variable in occurrences.
       * <p>
       * The parameter occurrences should be modified by the method accordingly.
       * @param occurrences The current number of occurrences for each variable which should be modified accordingly
       */
      public abstract void countUnsubsumedOccurrences(final int[] occurrences);
  
      /**
       * Returns the depth of this tree.
       * @return the depth of this tree
       */
      public abstract int depth();
  
      /**
       * Returns the widest separator of this tree.
       * @return the widest separator
       */
      public abstract int widestSeparator();
  
      /**
       * Returns all leafs of this tree.
       * @return all leafs of this tree
       */
      abstract List<DTreeLeaf> leafs();
  }
  