/**
* add the conjunctive expressions to specified list, has recursive step.
*
* @param expr the expr, in the best case in CNF
* @param conjExpr the list to which expressions will be added
*/
protected void getConjunctiveExpressions(ValueExpr expr, List<ValueExpr> conjExpr) {
if (expr instanceof And) {
And and = (And) expr;
getConjunctiveExpressions(and.getLeftArg(), conjExpr);
getConjunctiveExpressions(and.getRightArg(), conjExpr);
} else conjExpr.add(expr);
}
@Override
public void meet(And node) throws RuntimeException {
builder.append("(");
node.getLeftArg().visit(this);
builder.append(" AND ");
node.getRightArg().visit(this);
builder.append(")");
}
@Override
public void meet(Filter filter) {
if (filter.getArg() instanceof EmptyResult) {
log.debug(
"Argument of filter expression does not yield results at the provided sources, replacing Filter node.");
filter.replaceWith(filter.getArg());
return;
}
/*
* TODO idea:
* if we have a FILTER such as ?s='a' OR ?s='b' OR ?s='c' handle this appropriately
*/
ValueExpr valueExpr = filter.getCondition();
/*
* TODO transform condition into some normal form, e.g. CNF
*/
// determine conjunctive expressions
List<ValueExpr> conjunctiveExpressions = new ArrayList<ValueExpr>();
getConjunctiveExpressions(valueExpr, conjunctiveExpressions);
FilterExprInsertVisitor filterExprVst = new FilterExprInsertVisitor();
List<ValueExpr> remainingExpr = new ArrayList<ValueExpr>(conjunctiveExpressions.size());
for (ValueExpr cond : conjunctiveExpressions) {
/*
* Determine if this filter is applicable for optimization.
* Currently only leaf expressions are applicable, i.e.
* not combined expressions.
*/
if (isCompatibleExpr(cond)) {
HashSet<String> exprVars = new VarFinder().findVars(cond);
FilterExpr filterExpr = new FilterExpr(cond, exprVars);
filterExprVst.initialize(filterExpr);
filter.getArg().visit(filterExprVst);
// if the filter expr. is handled in the stmt we do not have to keep it
if (filterExprVst.canRemove()) continue;
remainingExpr.add(filterExpr.getExpression());
} else {
remainingExpr.add(cond);
}
}
if (remainingExpr.size() == 0) {
filter.replaceWith(filter.getArg()); // remove the filter
} else if (remainingExpr.size() == 1) {
filter.setCondition(remainingExpr.get(0)); // just apply the remaining condition
} else {
// construct conjunctive value expr
And root = new And();
root.setLeftArg(remainingExpr.get(0));
And tmp = root;
for (int i = 1; i < remainingExpr.size() - 1; i++) {
And _a = new And();
_a.setLeftArg(remainingExpr.get(i));
tmp.setRightArg(_a);
tmp = _a;
}
tmp.setRightArg(remainingExpr.get(remainingExpr.size() - 1));
filter.setCondition(root);
}
}