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.AIG;
  
  import org.logicng.formulas.And;
  import org.logicng.formulas.Equivalence;
  import org.logicng.formulas.Formula;
  import org.logicng.formulas.FormulaFactory;
  import org.logicng.formulas.FormulaTransformation;
  import org.logicng.formulas.Implication;
  import org.logicng.formulas.Not;
  import org.logicng.formulas.Or;
  import org.logicng.formulas.cache.PredicateCacheEntry;
  
  import java.util.LinkedHashSet;
  
  /**
   * And-inverter-graph (AIG) transformation.  Returns the AIG of the given formula.
   * @version 2.3.0
   * @since 1.0
   */
  public final class AIGTransformation implements FormulaTransformation {
  
      private FormulaFactory f;
      private boolean cache;
      private static final AIGTransformation INSTANCE = new AIGTransformation();
  
      /**
       * @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 AIGTransformation() {
          // Intentionally left empty
      }
  
      /**
       * Returns the singleton instance of this function.
       * @return an instance of this function
       */
      public static AIGTransformation get() {
          return INSTANCE;
      }
  
      @Override
      public Formula apply(final Formula formula, final boolean cache) {
          this.f = formula.factory();
          this.cache = cache;
          switch (formula.type()) {
              case FALSE:
              case TRUE:
              case LITERAL:
                  return formula;
              case NOT:
                  return transformNot((Not) formula);
              case IMPL:
                  return transformImplication((Implication) formula);
              case EQUIV:
                  return transformEquivalence((Equivalence) formula);
              case AND:
                  return transformAnd((And) formula);
              case OR:
                  return transformOr((Or) formula);
              case PBC:
                  return apply(formula.cnf(), cache);
              default:
                  throw new IllegalArgumentException("Could not process the formula type " + formula.type());
          }
      }
  
      private Formula transformNot(final Not not) {
          Formula aig = not.transformationCacheEntry(AIG);
          if (aig == null) {
              aig = this.f.not(apply(not.operand(), this.cache));
              if (this.cache) {
                  not.setTransformationCacheEntry(AIG, aig);
                  aig.setPredicateCacheEntry(PredicateCacheEntry.IS_AIG, true);
              }
          }
          return aig;
      }
  
      private Formula transformImplication(final Implication impl) {
          Formula aig = impl.transformationCacheEntry(AIG);
          if (aig == null) {
              aig = this.f.not(this.f.and(apply(impl.left(), this.cache), this.f.not(apply(impl.right(), this.cache))));
              if (this.cache) {
                  impl.setTransformationCacheEntry(AIG, aig);
                  aig.setPredicateCacheEntry(PredicateCacheEntry.IS_AIG, true);
              }
          }
          return aig;
      }
  
      private Formula transformEquivalence(final Equivalence equiv) {
          Formula aig = equiv.transformationCacheEntry(AIG);
          if (aig == null) {
              aig = this.f.and(this.f.not(this.f.and(apply(equiv.left(), this.cache), this.f.not(apply(equiv.right(), this.cache)))),
                      this.f.not(this.f.and(this.f.not(equiv.left()), equiv.right())));
              if (this.cache) {
                  equiv.setTransformationCacheEntry(AIG, aig);
                  aig.setPredicateCacheEntry(PredicateCacheEntry.IS_AIG, true);
              }
          }
          return aig;
      }
  
      private Formula transformAnd(final And and) {
          Formula aig = and.transformationCacheEntry(AIG);
          if (aig == null) {
              final LinkedHashSet<Formula> nops = new LinkedHashSet<>(and.numberOfOperands());
              for (final Formula op : and) {
                  nops.add(apply(op, this.cache));
              }
              aig = this.f.and(nops);
              if (this.cache) {
                  and.setTransformationCacheEntry(AIG, aig);
                  aig.setPredicateCacheEntry(PredicateCacheEntry.IS_AIG, true);
              }
          }
          return aig;
      }
  
      private Formula transformOr(final Or or) {
          Formula aig = or.transformationCacheEntry(AIG);
          if (aig == null) {
              final LinkedHashSet<Formula> nops = new LinkedHashSet<>(or.numberOfOperands());
              for (final Formula op : or) {
                  nops.add(this.f.not(apply(op, this.cache)));
              }
              aig = this.f.not(this.f.and(nops));
              if (this.cache) {
                  or.setTransformationCacheEntry(AIG, aig);
                  aig.setPredicateCacheEntry(PredicateCacheEntry.IS_AIG, true);
              }
          }
          return aig;
      }
  
      @Override
      public String toString() {
          return this.getClass().getSimpleName();
      }
  }
  