/*
* Kodkod -- Copyright (c) 2005-2007, 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.
* @effects 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}
* @effects {@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;
}
}