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.transformations;
  
  import static org.logicng.formulas.cache.TransformationCacheEntry.ANONYMIZATION;
  
  import org.logicng.datastructures.Substitution;
  import org.logicng.formulas.Formula;
  import org.logicng.formulas.FormulaTransformation;
  import org.logicng.formulas.Variable;
  
  /**
   * An anonymizer replaces all variables in a formula with new variables generated from a given prefix and a counter.
   * <p>
   * An instance can be used to anonymize multiple formulas. In this case, variables with the same name will be replaced
   * with the same anonymized variable.
   * <p>
   * After anonymizing one or more formulas, the mapping from original variable to anonymized variable can be accessed
   * via {@link #getSubstitution()}.
   * @version 1.4.0
   * @since 1.4.0
   */
  public final class Anonymizer implements FormulaTransformation {
  
      private final Substitution substitution;
      private final String prefix;
      private int counter;
  
      /**
       * Constructs a new anonymizer with a given prefix for the newly introduced variables.
       * @param prefix       the prefix for the new variables
       * @param startCounter where should the counter start
       */
      public Anonymizer(final String prefix, final int startCounter) {
          this.prefix = prefix;
          this.substitution = new Substitution();
          this.counter = startCounter;
      }
  
      /**
       * Constructs a new anonymizer with a given prefix for the newly introduced variables.
       * @param prefix the prefix for the new variables
       */
      public Anonymizer(final String prefix) {
          this(prefix, 0);
      }
  
      /**
       * Constructs a new anonymizer with the standard variable prefix 'v'.
       */
      public Anonymizer() {
          this("v");
      }
  
      /**
       * Returns the substitution which was used to anonymize the formula(s).
       * <p>
       * Although a substitution maps from variables to formulas, it is guaranteed that
       * the substitution always maps to variables. So the following cast will always be
       * safe:
       * <p>
       * {@code (Variable) getSubstitution().getSubstitution(x)}
       * @return the substitution which was used to anonymize the formula(s)
       */
      public Substitution getSubstitution() {
          return new Substitution(this.substitution);
      }
  
      @Override
      public Formula apply(final Formula formula, final boolean cache) {
          if (formula.variables().isEmpty()) {
              return formula;
          }
          final Formula cached = formula.transformationCacheEntry(ANONYMIZATION);
          if (cache && cached != null) {
              return cached;
          }
          for (final Variable variable : formula.variables()) {
              if (this.substitution.getSubstitution(variable) == null) {
                  this.substitution.addMapping(variable, formula.factory().variable(this.prefix + this.counter++));
              }
          }
          final Formula transformed = formula.substitute(this.substitution);
          if (cache) {
              formula.setTransformationCacheEntry(ANONYMIZATION, transformed);
          }
          return transformed;
      }
  }
  