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.knowledgecompilation.bdds;
  
  import static org.assertj.core.api.Assertions.assertThat;
  
  import org.junit.jupiter.api.Test;
  import org.logicng.datastructures.Assignment;
  import org.logicng.formulas.CType;
  import org.logicng.formulas.Formula;
  import org.logicng.formulas.FormulaFactory;
  import org.logicng.formulas.Variable;
  import org.logicng.knowledgecompilation.bdds.jbuddy.BDDKernel;
  import org.logicng.testutils.NQueensGenerator;
  
  import java.math.BigInteger;
  import java.util.ArrayList;
  import java.util.HashSet;
  import java.util.List;
  import java.util.Set;
  import java.util.SortedSet;
  
  /**
   * Unit tests for the BDDs.
   * @version 2.0.0
   * @since 1.4.0
   */
  public class BDDModelEnumerationTest {
  
      private final FormulaFactory f;
  
      private final List<Formula> formulas;
      private final List<SortedSet<Variable>> variables;
      private final BigInteger[] expected;
  
      public BDDModelEnumerationTest() {
          final int[] problems = new int[]{3, 4, 5, 6, 7, 8, 9};
          this.expected = new BigInteger[]{
                  BigInteger.valueOf(0),
                  BigInteger.valueOf(2),
                  BigInteger.valueOf(10),
                  BigInteger.valueOf(4),
                  BigInteger.valueOf(40),
                  BigInteger.valueOf(92),
                  BigInteger.valueOf(352)
          };
  
          this.f = new FormulaFactory();
          final NQueensGenerator generator = new NQueensGenerator(this.f);
          this.formulas = new ArrayList<>(problems.length);
          this.variables = new ArrayList<>(problems.length);
  
          for (final int problem : problems) {
              final Formula p = generator.generate(problem);
              this.formulas.add(p);
              this.variables.add(p.variables());
          }
      }
  
      @Test
      public void testModelCount() {
          for (int i = 0; i < this.formulas.size(); i++) {
              final BDDKernel kernel = new BDDKernel(this.f, this.variables.get(i).size(), 10000, 10000);
              final BDD bdd = BDDFactory.build(this.formulas.get(i), kernel);
              assertThat(bdd.modelCount()).isEqualTo(this.expected[i]);
          }
      }
  
      @Test
      public void testModelEnumeration() {
          for (int i = 0; i < this.formulas.size(); i++) {
              final BDDKernel kernel = new BDDKernel(this.f, this.variables.get(i).size(), 10000, 10000);
              final BDD bdd = BDDFactory.build(this.formulas.get(i), kernel);
              final Set<Assignment> models = new HashSet<>(bdd.enumerateAllModels());
              assertThat(models.size()).isEqualTo(this.expected[i].intValue());
              for (final Assignment model : models) {
                  assertThat(this.formulas.get(i).evaluate(model)).isTrue();
              }
          }
      }
  
      @Test
      public void testExo() {
          final FormulaFactory f = new FormulaFactory();
          final Formula constraint = f.exo(generateVariables(100, f)).cnf();
          final BDDKernel kernel = new BDDKernel(f, constraint.variables().size(), 100000, 1000000);
          final BDD bdd = BDDFactory.build(constraint, kernel);
          assertThat(bdd.modelCount()).isEqualTo(BigInteger.valueOf(100));
          assertThat(bdd.enumerateAllModels()).hasSize(100);
      }
  
      @Test
      public void testExk() {
          final FormulaFactory f = new FormulaFactory();
          final Formula constraint = f.cc(CType.EQ, 8, generateVariables(15, f)).cnf();
          final BDDKernel kernel = new BDDKernel(f, constraint.variables().size(), 100000, 1000000);
          final BDD bdd = BDDFactory.build(constraint, kernel);
          assertThat(bdd.modelCount()).isEqualTo(BigInteger.valueOf(6435));
          assertThat(bdd.enumerateAllModels()).hasSize(6435);
      }
  
      @Test
      public void testAmo() {
          final FormulaFactory f = new FormulaFactory();
          final Formula constraint = f.amo(generateVariables(100, f)).cnf();
          final BDDKernel kernel = new BDDKernel(f, constraint.variables().size(), 100000, 1000000);
          final BDD bdd = BDDFactory.build(constraint, kernel);
          assertThat(bdd.modelCount()).isEqualTo(BigInteger.valueOf(221));
          assertThat(bdd.enumerateAllModels(generateVariables(100, f))).hasSize(101);
      }
  
      private List<Variable> generateVariables(final int n, final FormulaFactory f) {
          final List<Variable> result = new ArrayList<>(n);
          for (int i = 0; i < n; i++) {
              result.add(f.variable("v" + i));
          }
          return result;
      }
  }
  