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.solvers.functions;
  
  import static org.logicng.handlers.Handler.start;
  
  import org.logicng.backbones.Backbone;
  import org.logicng.backbones.BackboneType;
  import org.logicng.datastructures.Tristate;
  import org.logicng.formulas.Variable;
  import org.logicng.handlers.SATHandler;
  import org.logicng.solvers.MiniSat;
  import org.logicng.solvers.SolverState;
  
  import java.util.Arrays;
  import java.util.Collection;
  import java.util.function.Consumer;
  
  /**
   * A solver function which computes a backbone for the formula on the solver.
   * <p>
   * Backbone functions are instantiated via their builder {@link #builder()}.
   * @version 2.1.0
   * @since 2.0.0
   */
  public final class BackboneFunction implements SolverFunction<Backbone> {
  
      private final SATHandler handler;
      private final Collection<Variable> variables;
      private final BackboneType type;
  
      private BackboneFunction(final SATHandler handler, final Collection<Variable> variables, final BackboneType type) {
          this.handler = handler;
          this.variables = variables;
          this.type = type;
      }
  
      /**
       * Returns the builder for this function.
       * @return the builder
       */
      public static Builder builder() {
          return new Builder();
      }
  
      @Override
      public Backbone apply(final MiniSat solver, final Consumer<Tristate> resultSetter) {
          start(this.handler);
          SolverState stateBeforeBackbone = null;
          if (solver.canSaveLoadState()) {
              stateBeforeBackbone = solver.saveState();
          }
          final Backbone backbone = solver.underlyingSolver().computeBackbone(this.variables, this.type, this.handler);
          if (solver.canSaveLoadState()) {
              solver.loadState(stateBeforeBackbone);
          }
          return backbone;
      }
  
      /**
       * The builder for a backbone function.
       */
      public static class Builder {
  
          private SATHandler handler;
          private Collection<Variable> variables;
          private BackboneType type = BackboneType.POSITIVE_AND_NEGATIVE;
  
          private Builder() {
              // Initialize only via factory
          }
  
          /**
           * Sets the SAT handler for this function.
           * @param handler the handler
           * @return the current builder
           */
          public Builder handler(final SATHandler handler) {
              this.handler = handler;
              return this;
          }
  
          /**
           * Sets the variables which are relevant for the backbone computation.
           * @param variables the variables
           * @return the current builder
           */
          public Builder variables(final Collection<Variable> variables) {
              this.variables = variables;
              return this;
          }
  
          /**
           * Sets the variables which are relevant for the backbone computation.
           * @param variables the variables
           * @return the current builder
           */
          public Builder variables(final Variable... variables) {
              this.variables = Arrays.asList(variables);
              return this;
          }
  
          /**
           * Sets the type of backbone which should be computed (default: POSITIVE_AND_NEGATIVE).
           * @param type the backbone type
           * @return the current builder
           */
          public Builder type(final BackboneType type) {
              this.type = type;
              return this;
          }
  
          /**
           * Builds the backbone function with the current builder's configuration.
           * @return the backbone function
           */
          public BackboneFunction build() {
              return new BackboneFunction(this.handler, this.variables, this.type);
          }
      }
  }
  