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;
  
  import org.logicng.datastructures.Tristate;
  import org.logicng.formulas.Formula;
  import org.logicng.formulas.Literal;
  
  import java.util.BitSet;
  
  /**
   * Interface for a DNNF SAT Solver.
   * @version 2.0.0
   * @since 2.0.0
   */
  public interface DnnfSatSolver {
  
      /**
       * Adds a formula to the solver.  The formula is first converted to CNF.
       * @param formula the formula
       */
      void add(final Formula formula);
  
      /**
       * Initializes the solver by performing unit propagation
       * and returns {@code false} if UP finds a contradiction, otherwise {@code true}.
       * @return {@code false} if UP finds a contradiction, otherwise {@code true}
       */
      boolean start();
  
      /**
       * Performs a decision of the given variable to the given phase and performs UP.
       * @param var   the variable
       * @param phase the phase
       * @return {@code false} if UP finds a contradiction, otherwise {@code true}
       */
      boolean decide(final int var, boolean phase);
  
      /**
       * Reverts the decision of the given variable.
       * @param var the variable
       */
      void undoDecide(final int var);
  
      /**
       * Returns {@code true} if the current decision level was previously
       * selected as backtrack level.
       * @return {@code true} if the current decision level was previously
       * selected as backtrack level, otherwise {@code false}
       */
      boolean atAssertionLevel();
  
      /**
       * Enqueues the literal which was computed by the latest backtracking, performs UP
       * and return {@code false} if UP finds a contradiction, otherwise {@code true}.
       * @return {@code false} if UP finds a contradiction, otherwise {@code true}
       */
      boolean assertCdLiteral();
  
      /**
       * Returns a conjunction of all literals in the known variables which were
       * propagated by the last UP.
       * @param knownVariables the known variables
       * @return a conjunction of the relevant propagated literals
       */
      Formula newlyImplied(final BitSet knownVariables);
  
      /**
       * Returns the index of the variable of the given literal.
       * @param lit the literal
       * @return the index of the literal's variable
       */
      int variableIndex(final Literal lit);
  
      /**
       * Returns the literal corresponding to the given variable index.
       * @param var the variable index
       * @return the literal corresponding to the given variable index
       */
      Literal litForIdx(final int var);
  
      /**
       * Returns the current assignment of the given literal.
       * @param lit the literal
       * @return the current assignment of the given literal
       */
      Tristate valueOf(final int lit);
  }
  