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 java.util.HashSet;
  import java.util.Set;
  
  import kodkod.engine.satlab.Clause;
  import kodkod.engine.satlab.ReductionStrategy;
  import kodkod.engine.satlab.ResolutionTrace;
  import kodkod.util.ints.IntIterator;
  import kodkod.util.ints.IntSet;
  import kodkod.util.ints.Ints;
  
  
  /**
   * A basic implementation of the Complete ResolutionTrace Refutation algorithm
   * for for producing locally minimal cores.
   * An unsatisfiable core is locally minimal iff removing any single clause from
   * the core will make the resulting formula satisfiable.  No heuristic is used
   * to pick the clauses to be excluded from the core.
   * @specfield traces: [0..)->ResolutionTrace
   * @specfield nexts: [0..)->Set<Clause>
   * @invariant traces.ResolutionTrace = nexts.Set<Clause>
   * @invariant all i: [1..) | some traces[i] => some traces[i-1]
   * @invariant all i: [0..#nexts) | nexts[i] in traces[i].conflict.^antecedents
   * @invariant no disj i,j: [0..#nexts) | traces[i] = traces[j] && nexts[i] = nexts[j]
   * @author Emina Torlak
   * @see <a href="http://www.cs.tau.ac.il/~ale1/muc_sat06_short_8.pdf">N. Dershowitz, Z. Hanna, and A. Nadel.  <i>A scalable algorithm for minimal unsatisfiable core
   * extraction.</i>  In Proceedings of Ninth International Conference on Theory and Applications of 
   * Satisfiability Testing (SAT '06). 2006.</a>
   */
  public final class CRRStrategy implements ReductionStrategy {
  	private Set<Clause> excluded;
  	
  	/** 
  	 * Constructs a new instance of CRRStrategy. 
  	 * @ensures no this.traces' and no this.nexts'
  	 **/
  	public CRRStrategy() {
  		excluded = null;
  	}
  	
  	/**
  	 * Returns the next subset of clauses in the given trace to be analyzed.  
  	 * @requires {@inheritDoc} 
  	 * @ensures {@inheritDoc}
  	 * @return  last(this.nexts')
  	 */
  	public final IntSet next(final ResolutionTrace trace) {
  		final IntSet core = trace.core();
  		if (excluded==null) { // the first time this method is called
  			excluded = new HashSet<Clause>((int)(StrictMath.round(core.size()*.75)));
  		}
  		
  		for(IntIterator iter = core.iterator(Integer.MAX_VALUE, Integer.MIN_VALUE); iter.hasNext();) {
  			int index = iter.next();
  			if (excluded.add(trace.get(index))) { // haven't tried excluding this one
  				// get all clauses reachable from the conflict clause
  				IntSet next = trace.reachable(Ints.singleton(trace.size()-1)); 
  				// remove all clauses backward reachable from the excluded clause
  				next.removeAll(trace.backwardReachable(Ints.singleton(index)));
  				return next;
  			}
  		}
  		
  		return Ints.EMPTY_SET;
  	}	
  }
  