private Item intern(Item item) {
if (item.sameSymbol(ANY.getSymbol())) return ANY;
if (item.sameSymbol(VAR.getSymbol())) return VAR;
if (item.sameSymbol(TERM.getSymbol())) return TERM;
if (item.sameSymbol(URI.getSymbol())) return URI;
if (item.sameSymbol(LITERAL.getSymbol())) return LITERAL;
if (item.sameSymbol(BNODE.getSymbol())) return BNODE;
return item;
}
@Override
public Formula apply(final Formula formula, boolean cache) {
if (!this.proceed) return null;
if (formula.type().precedence() >= LITERAL.precedence()) return formula;
Formula cached = formula.transformationCacheEntry(FACTORIZED_CNF);
if (cached != null) return cached;
switch (formula.type()) {
case NOT:
case IMPL:
case EQUIV:
cached = this.apply(formula.nnf(), cache);
break;
case OR:
LinkedHashSet<Formula> nops = new LinkedHashSet<>();
for (final Formula op : formula) {
if (!this.proceed) return null;
nops.add(this.apply(op, cache));
}
final Iterator<Formula> it = nops.iterator();
cached = it.next();
while (it.hasNext()) {
if (!this.proceed) return null;
cached = this.distribute(cached, it.next());
}
break;
case AND:
nops = new LinkedHashSet<>();
for (final Formula op : formula) {
final Formula apply = this.apply(op, cache);
if (!this.proceed) return null;
nops.add(apply);
}
cached = formula.factory().and(nops);
break;
case PBC:
cached = formula.nnf();
break;
default:
throw new IllegalArgumentException("Could not process the formula type " + formula.type());
}
if (this.proceed) {
if (cache) formula.setTransformationCacheEntry(FACTORIZED_CNF, cached);
return cached;
}
return null;
}