/*
* 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.Iterator;
import kodkod.engine.fol2sat.TranslationLog;
import kodkod.engine.fol2sat.Translator;
import kodkod.engine.satlab.Clause;
import kodkod.engine.satlab.ReductionStrategy;
import kodkod.engine.satlab.ResolutionTrace;
import kodkod.util.ints.IntBitSet;
import kodkod.util.ints.IntIterator;
import kodkod.util.ints.IntSet;
import kodkod.util.ints.Ints;
/**
* A hybrid strategy for generating unsat cores that are minimal when mapped
* back onto 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). Note that this does
* not guarantee that F(C) itself is minimal. In other words, there could be
* an f in F(C) such that F(C) - f is unsatisfiable. To get a minimal logic core,
* use {@linkplain RCEStrategy}.
*
* <p>This strategy will work properly only on CNFs generated by the kodkod {@linkplain Translator}. </p>
* @author Emina Torlak
* @see RCEStrategy
*/
public final class HybridStrategy implements ReductionStrategy {
private final IntSet topVars;
/**
* Constructs a hybrid strategy that will use the given translation
* log to relate the cnf clauses back to the logic constraints from
* which they were generated.
*/
public HybridStrategy(TranslationLog log) {
topVars = StrategyUtils.rootVars(log);
}
/**
* {@inheritDoc}
* @see kodkod.engine.satlab.ReductionStrategy#next(kodkod.engine.satlab.ResolutionTrace)
*/
public IntSet next(ResolutionTrace trace) {
if (topVars.isEmpty()) return Ints.EMPTY_SET; // tried everything
final IntSet core = trace.core();
for(Iterator<Clause> iter = trace.iterator(core); iter.hasNext();) {
Clause clause = iter.next();
int maxVar = clause.maxVariable();
if (topVars.remove(maxVar)) {
// get all core clauses with the given maximum variable
IntSet exclude = coreClausesWithMaxVar(trace, maxVar);
assert !exclude.isEmpty();
// get all clauses reachable from the conflict clause
IntSet next = trace.reachable(Ints.singleton(trace.size()-1));
// remove all clauses backward reachable from the clauses with the given maxVar
next.removeAll(trace.backwardReachable(exclude));
if (!next.isEmpty()) {
return next;
}
}
}
topVars.clear();
return Ints.EMPTY_SET;
}
/**
* Returns the indices of the clauses in the unsatisfiable core of the
* given trace that have the specified maximum variable.
* @return { i: trace.core() | trace[i].maxVariable() = maxVariable }
*/
private static IntSet coreClausesWithMaxVar(ResolutionTrace trace, int maxVariable) {
final IntSet core = trace.core();
final IntSet restricted = new IntBitSet(core.max()+1);
final Iterator<Clause> clauses = trace.iterator(core);
final IntIterator indices = core.iterator();
while(clauses.hasNext()) {
Clause clause = clauses.next();
int index = indices.next();
if (clause.maxVariable()==maxVariable)
restricted.add(index);
}
return restricted;
}
}