/*
* 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;
}
}