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.satlab.ReductionStrategy;
  import kodkod.engine.satlab.ResolutionTrace;
  import kodkod.util.ints.IntSet;
  import kodkod.util.ints.Ints;
  
  /**
   * A non-optimal minimization strategy based on the Empty Clause Cone algorithm.
   * @author Emina Torlak
   * @see <a href="http://research.microsoft.com/users/lintaoz/papers/SAT_2003_core.pdf">L. Zhang and S. Malik. <i>Extracting small unsatisfiable cores from unsatisfiable
   * Boolean formula.</i>  In Proceedings of Sixth International Conference on Theory and Applications of 
   * Satisfiability Testing (SAT '03). 2003.</a>
   */
  public final class ECFPStrategy implements ReductionStrategy {
  	private int lastCore;
  	/**
  	 * Constructs a new instance of the empty clause cone strategy for 
  	 * minimizing unsatisfiable cores.
  	 */
  	public ECFPStrategy() {
  		lastCore = Integer.MAX_VALUE;
  	}
  
  	/**
  	 * {@inheritDoc}
  	 * @see kodkod.engine.satlab.ReductionStrategy#next(kodkod.engine.satlab.ResolutionTrace)
  	 */
  	public IntSet next(final ResolutionTrace trace) {
  		final IntSet core = trace.core();
  		if (lastCore > core.size()) {
  			lastCore = core.size();
  			return core;
  		} else {
  			lastCore = Integer.MIN_VALUE;                  
  			return Ints.EMPTY_SET;
  		}
  	}
  
  }
  