content: /* 
   * Kodkod -- Copyright (c) 2005-present, Emina Torlak
   *
   * Permission is hereby granted, free of charge, to any person obtaining a copy
   * of this software and associated documentation files (the "Software"), to deal
   * in the Software without restriction, including without limitation the rights
   * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
   * copies of the Software, and to permit persons to whom the Software is
   * furnished to do so, subject to the following conditions:
   *
   * The above copyright notice and this permission notice shall be included in
   * all copies or substantial portions of the Software.
   *
   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
   * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
   * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
   * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
   * THE SOFTWARE.
   */
  package kodkod.engine.ucore;
  
  import kodkod.engine.fol2sat.TranslationLog;
  import kodkod.engine.fol2sat.Translator;
  import kodkod.engine.satlab.ReductionStrategy;
  import kodkod.engine.satlab.ResolutionTrace;
  import kodkod.util.ints.IntCollection;
  import kodkod.util.ints.IntSet;
  import kodkod.util.ints.IntTreeSet;
  import kodkod.util.ints.Ints;
  
  /**
  * Naive Core Extraction is a strategy for generating unsat cores that are minimal at the logic level.  
   * Specifically, let C be a core that is minimal according to this strategy, 
   * and let F(C) be the top-level logic constraints
   * corresponding to C.  Then, this strategy guarantees that there is no clause
   * c in C such that F(C - c) is a strict subset of F(C). Furthermore, it also
   * guarantees that for all f in F(C), F(C) - f is satisfiable.  This is a stronger
   * guarantee than that of {@linkplain HybridStrategy}.  In general, using this strategy
   * is more expensive, timewise, than using {@linkplain HybridStrategy}.
   * 
   * <p>This implementation of NCE will work properly only on CNFs generated by the kodkod {@linkplain Translator}. </p>
  * @author Emina Torlak
  * @see HybridStrategy
  */
  public final class NCEStrategy implements ReductionStrategy {
  	private final IntCollection varsToTry;
  	private final IntSet coreVars;
  	
  	/**
  	 * Constructs an NCE strategy that will use the given translation
  	 * log to relate the cnf clauses back to the logic constraints from 
  	 * which they were generated. 
  	 */
  	public NCEStrategy(final TranslationLog log) {
  		varsToTry = StrategyUtils.rootVars(log);
  		coreVars = new IntTreeSet();//new IntTreeSet(varsToTry);
  		coreVars.addAll(varsToTry);
  	}
  	
  	/**
  	 * {@inheritDoc}
  	 * @see kodkod.engine.satlab.ReductionStrategy#next(kodkod.engine.satlab.ResolutionTrace)
  	 */
  	public IntSet next(ResolutionTrace trace) {
  		if (varsToTry.isEmpty()) return Ints.EMPTY_SET;
  		// if the last attempt at reduction was unsuccessful,
  		// add the unit clauses that we tried to discard back to coreVars
  		coreVars.addAll(StrategyUtils.coreTailUnits(trace));
  		final int first = varsToTry.iterator().next();//varsToTry.min();
  		varsToTry.remove(first);
  		coreVars.remove(first);
  		// get all axioms corresponding to the clauses that
  		// form the translations of formulas identified by coreVars
  		final IntSet relevantClauses = StrategyUtils.clausesFor(trace, coreVars); 
  		assert !relevantClauses.isEmpty() && !relevantClauses.contains(trace.size()-1);
  		return relevantClauses;
  	}
  
  }
  