code: ///////////////////////////////////////////////////////////////////////////
  //                   __                _      _   ________               //
  //                  / /   ____  ____ _(_)____/ | / / ____/               //
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  //                           /____/                                      //
  //                                                                       //
  //               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.dnnf.functions;
  
  import org.logicng.formulas.Formula;
  import org.logicng.formulas.Variable;
  import org.logicng.formulas.cache.FunctionCacheEntry;
  
  import java.math.BigInteger;
  import java.util.HashMap;
  import java.util.Map;
  import java.util.SortedSet;
  import java.util.TreeSet;
  
  /**
   * A DNNF function which counts models.
   * @version 2.0.0
   * @since 2.0.0
   */
  public final class DnnfModelCountFunction implements DnnfFunction<BigInteger> {
  
      private static final DnnfModelCountFunction INSTANCE = new DnnfModelCountFunction();
  
      private DnnfModelCountFunction() {
          // intentionally left empty
      }
  
      /**
       * Returns the singleton instance of this function.
       * @return an instance of this function
       */
      public static DnnfModelCountFunction get() {
          return INSTANCE;
      }
  
      @Override
      public BigInteger apply(final SortedSet<Variable> originalVariables, final Formula formula) {
          final Object cached = formula.functionCacheEntry(FunctionCacheEntry.DNNF_MODELCOUNT);
          final BigInteger result;
          if (cached != null) {
              result = (BigInteger) cached;
          } else {
              result = count(formula, new HashMap<>());
          }
          formula.setFunctionCacheEntry(FunctionCacheEntry.DNNF_MODELCOUNT, result);
          final SortedSet<Variable> dontCareVariables = new TreeSet<>();
          final SortedSet<Variable> dnnfVariables = formula.variables();
          for (final Variable originalVariable : originalVariables) {
              if (!dnnfVariables.contains(originalVariable)) {
                  dontCareVariables.add(originalVariable);
              }
          }
          final BigInteger factor = BigInteger.valueOf(2).pow(dontCareVariables.size());
          return result.multiply(factor);
      }
  
      private BigInteger count(final Formula dnnf, final Map<Formula, BigInteger> internalCache) {
          BigInteger c = internalCache.get(dnnf);
          if (c == null) {
              switch (dnnf.type()) {
                  case LITERAL:
                  case TRUE:
                      c = BigInteger.ONE;
                      break;
                  case AND:
                      c = BigInteger.ONE;
                      for (final Formula op : dnnf) {
                          c = c.multiply(count(op, internalCache));
                      }
                      break;
                  case OR:
                      final int allVariables = dnnf.variables().size();
                      c = BigInteger.ZERO;
                      for (final Formula op : dnnf) {
                          final BigInteger opCount = count(op, internalCache);
                          final BigInteger factor = BigInteger.valueOf(2L).pow(allVariables - op.variables().size());
                          c = c.add(opCount.multiply(factor));
                      }
                      break;
                  case FALSE:
                      c = BigInteger.ZERO;
                      break;
              }
              internalCache.put(dnnf, c);
          }
          return c;
      }
  }
  