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.formulas;
  
  import org.logicng.collections.LNGIntVector;
  
  import java.util.HashMap;
  import java.util.Iterator;
  import java.util.LinkedHashMap;
  import java.util.LinkedHashSet;
  import java.util.Map;
  import java.util.Set;
  
  /**
   * An extended version of the formula factory that allows saving and loading states to reduce memory usage.
   * <p>
   * New formulas can only be generated by a formula factory.  It is implemented s.t. it is guaranteed that equivalent
   * formulas (in terms of associativity and commutativity) are hold exactly once in memory.
   * <p>
   * A formula factory is NOT thread-safe.  If you generate formulas from more than one thread you either need to
   * synchronize the formula factory yourself or you use a formula factory for each single thread.
   * @version 2.3.2
   * @since 1.2
   */
  public class ExtendedFormulaFactory extends FormulaFactory {
  
      private final LNGIntVector validStates = new LNGIntVector();
      private int nextStateId;
  
      /**
       * Shrinks a given map to a given size
       * @param map     the map to be shrunk
       * @param newSize the new size, the map shall be shrunk to
       */
      static <T, U> void shrinkMap(final Map<T, U> map, final int newSize) {
          if (!(map instanceof LinkedHashMap)) {
              throw new IllegalStateException("Cannot shrink a map which is not of type LinkedHashMap");
          }
          if (newSize > map.size()) {
              throw new IllegalStateException("Cannot shrink a map of size " + map.size() + " to new size " + newSize);
          }
          final Iterator<Map.Entry<T, U>> entryIterator = map.entrySet().iterator();
          int count = 0;
          while (count < newSize) {
              entryIterator.next();
              count++;
          }
          while (entryIterator.hasNext()) {
              entryIterator.next();
              entryIterator.remove();
          }
      }
  
      /**
       * Shrinks a given set to a given size
       * @param set     the set to be shrunk
       * @param newSize the new size, the set shall be shrunk to
       */
      static <T> void shrinkSet(final Set<T> set, final int newSize) {
          if (!(set instanceof LinkedHashSet)) {
              throw new IllegalStateException("Cannot shrink a set which is not of type LinkedHashSet");
          }
          if (newSize > set.size()) {
              throw new IllegalStateException("Cannot shrink a set of size " + set.size() + " to new size " + newSize);
          }
          final Iterator<T> entryIterator = set.iterator();
          int count = 0;
          while (count < newSize) {
              entryIterator.next();
              count++;
          }
          while (entryIterator.hasNext()) {
              entryIterator.next();
              entryIterator.remove();
          }
      }
  
      @Override
      public void clear() {
          this.posLiterals = new LinkedHashMap<>();
          this.negLiterals = new LinkedHashMap<>();
          this.generatedVariables = new LinkedHashSet<>();
          this.nots = new LinkedHashMap<>();
          this.implications = new LinkedHashMap<>();
          this.equivalences = new LinkedHashMap<>();
          this.ands2 = new LinkedHashMap<>();
          this.ands3 = new LinkedHashMap<>();
          this.ands4 = new LinkedHashMap<>();
          this.andsN = new LinkedHashMap<>();
          this.ors2 = new LinkedHashMap<>();
          this.ors3 = new LinkedHashMap<>();
          this.ors4 = new LinkedHashMap<>();
          this.orsN = new LinkedHashMap<>();
          this.pbConstraints = new LinkedHashMap<>();
          this.cardinalityConstraints = new LinkedHashMap<>();
          this.ccCounter = 0;
          this.pbCounter = 0;
          this.cnfCounter = 0;
          this.transformationCache = new HashMap<>();
          this.predicateCache = new HashMap<>();
          this.functionCache = new HashMap<>();
          this.pbEncodingCache = new HashMap<>();
      }
  
      /**
       * Saves the FormulaFactoryState to be loaded later.
       * @return the FormulaFactoryState.
       */
      public FormulaFactoryState save() {
          final int[] state = new int[19];
          state[0] = this.posLiterals.size();
          state[1] = this.negLiterals.size();
          state[2] = this.generatedVariables.size();
          state[3] = this.nots.size();
          state[4] = this.implications.size();
          state[5] = this.equivalences.size();
          state[6] = this.ands2.size();
          state[7] = this.ands3.size();
          state[8] = this.ands4.size();
          state[9] = this.andsN.size();
          state[10] = this.ors2.size();
          state[11] = this.ors3.size();
          state[12] = this.ors4.size();
          state[13] = this.orsN.size();
          state[14] = this.pbConstraints.size();
          state[15] = this.cardinalityConstraints.size();
          state[16] = this.ccCounter;
          state[17] = this.pbCounter;
          state[18] = this.cnfCounter;
          final int id = this.nextStateId++;
          this.validStates.push(id);
          return new FormulaFactoryState(id, state);
      }
  
      /**
       * Loads a previously saved FormulaFactoryState. All formula factory states saved after the loaded one become invalid.
       * @param state the FormulaFactoryState to be loaded
       * @throws IllegalArgumentException if the FormulaFactoryState to be loaded is invalid
       */
      public void load(final FormulaFactoryState state) {
          int index = -1;
          for (int i = this.validStates.size() - 1; i >= 0 && index == -1; i--) {
              if (this.validStates.get(i) == state.id()) {
                  index = i;
              }
          }
          if (index == -1) {
              throw new IllegalArgumentException("The given formula factory state is not valid anymore.");
          }
          this.validStates.shrinkTo(index + 1);
  
          shrinkMap(this.posLiterals, state.state()[0]);
          shrinkMap(this.negLiterals, state.state()[1]);
          shrinkSet(this.generatedVariables, state.state()[2]);
          shrinkMap(this.nots, state.state()[3]);
          shrinkMap(this.implications, state.state()[4]);
          shrinkMap(this.equivalences, state.state()[5]);
          shrinkMap(this.ands2, state.state()[6]);
          shrinkMap(this.ands3, state.state()[7]);
          shrinkMap(this.ands4, state.state()[8]);
          shrinkMap(this.andsN, state.state()[9]);
          shrinkMap(this.ors2, state.state()[10]);
          shrinkMap(this.ors3, state.state()[11]);
          shrinkMap(this.ors4, state.state()[12]);
          shrinkMap(this.orsN, state.state()[13]);
          shrinkMap(this.pbConstraints, state.state()[14]);
          shrinkMap(this.cardinalityConstraints, state.state()[15]);
          this.ccCounter = state.state()[16];
          this.pbCounter = state.state()[17];
          this.cnfCounter = state.state()[18];
  
          clearCaches();
      }
  
      /**
       * Clears the transformation, function, and PB encoding caches.
       */
      public void clearCaches() {
          this.transformationCache.clear();
          this.functionCache.clear();
          this.pbEncodingCache.clear();
      }
  }
  