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.predicates;
  
  import static org.logicng.formulas.cache.PredicateCacheEntry.IS_DNF;
  import static org.logicng.predicates.TermPredicate.minterm;
  
  import org.logicng.datastructures.Tristate;
  import org.logicng.formulas.Formula;
  import org.logicng.formulas.FormulaPredicate;
  
  /**
   * DNF predicate.  Indicates whether a formula is in DNF or not.
   * @version 2.3.0
   * @since 1.0
   */
  public final class DNFPredicate implements FormulaPredicate {
  
      private final static DNFPredicate INSTANCE = new DNFPredicate();
  
      /**
       * Private empty constructor.  Singleton class.
       */
      private DNFPredicate() {
          // Intentionally left empty
      }
  
      /**
       * Returns the singleton of the predicate.
       * @return the predicate instance
       */
      public static DNFPredicate get() {
          return INSTANCE;
      }
  
      @Override
      public boolean test(final Formula formula, final boolean cache) {
          final Tristate cached = formula.predicateCacheEntry(IS_DNF);
          if (cached != Tristate.UNDEF) {
              return cached == Tristate.TRUE;
          }
          boolean result;
          switch (formula.type()) {
              case FALSE:
              case TRUE:
              case LITERAL:
                  return true;
              case NOT:
              case IMPL:
              case EQUIV:
              case PBC:
                  return false;
              case OR:
                  result = true;
                  for (final Formula op : formula) {
                      if (!minterm().test(op, false)) {
                          result = false;
                      }
                  }
                  break;
              case AND:
                  result = minterm().test(formula, false);
                  break;
              default:
                  throw new IllegalArgumentException("Cannot compute DNF predicate on " + formula.type());
          }
          if (cache) {
              formula.setPredicateCacheEntry(IS_DNF, result);
          }
          return result;
      }
  
      @Override
      public String toString() {
          return this.getClass().getSimpleName();
      }
  }
  