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.orderings;
  
  import org.logicng.formulas.BinaryOperator;
  import org.logicng.formulas.Formula;
  import org.logicng.formulas.Literal;
  import org.logicng.formulas.Not;
  import org.logicng.formulas.PBConstraint;
  import org.logicng.formulas.Variable;
  
  import java.util.ArrayList;
  import java.util.LinkedHashSet;
  import java.util.List;
  
  /**
   * A depth-first-search BDD variable ordering.  Traverses the formula in a DFS manner
   * and gathers all variables in the occurrence.
   * @version 2.0.0
   * @since 1.4.0
   */
  public final class DFSOrdering implements VariableOrderingProvider {
  
      @Override
      public List<Variable> getOrder(final Formula formula) {
          final LinkedHashSet<Variable> order = new LinkedHashSet<>(formula.variables().size());
          dfs(formula, order);
          return new ArrayList<>(order);
      }
  
      private void dfs(final Formula formula, final LinkedHashSet<Variable> variables) {
          switch (formula.type()) {
              case LITERAL:
                  variables.add(((Literal) formula).variable());
                  break;
              case NOT:
                  dfs(((Not) formula).operand(), variables);
                  break;
              case IMPL:
              case EQUIV:
                  final BinaryOperator op = (BinaryOperator) formula;
                  dfs(op.left(), variables);
                  dfs(op.right(), variables);
                  break;
              case AND:
              case OR:
                  for (final Formula o : formula) {
                      dfs(o, variables);
                  }
                  break;
              case PBC:
                  final PBConstraint pbc = (PBConstraint) formula;
                  for (final Literal lit : pbc.operands()) {
                      variables.add(lit.variable());
                  }
                  break;
          }
      }
  }
  