@Override
public Expr optimize(final QueryContext qc, final VarScope scp) throws QueryException {
// number of predicates may change in loop
for (int p = 0; p < preds.length; p++) {
final Expr pred = preds[p];
if (pred instanceof CmpG || pred instanceof CmpV) {
final Cmp cmp = (Cmp) pred;
if (cmp.exprs[0].isFunction(Function.POSITION)) {
final Expr e2 = cmp.exprs[1];
final SeqType st2 = e2.seqType();
// position() = last() -> last()
// position() = $n (numeric) -> $n
if (e2.isFunction(Function.LAST) || st2.one() && st2.type.isNumber()) {
if (cmp instanceof CmpG && ((CmpG) cmp).op == OpG.EQ
|| cmp instanceof CmpV && ((CmpV) cmp).op == OpV.EQ) {
qc.compInfo(OPTWRITE, pred);
preds[p] = e2;
}
}
}
} else if (pred instanceof And) {
if (!pred.has(Flag.FCS)) {
// replace AND expression with predicates (don't swap position tests)
qc.compInfo(OPTPRED, pred);
final Expr[] and = ((Arr) pred).exprs;
final int m = and.length - 1;
final ExprList el = new ExprList(preds.length + m);
for (final Expr e : Arrays.asList(preds).subList(0, p)) el.add(e);
for (final Expr a : and) {
// wrap test with boolean() if the result is numeric
el.add(Function.BOOLEAN.get(null, info, a).optimizeEbv(qc, scp));
}
for (final Expr e : Arrays.asList(preds).subList(p + 1, preds.length)) el.add(e);
preds = el.finish();
}
} else if (pred instanceof ANum) {
final ANum it = (ANum) pred;
final long i = it.itr();
if (i == it.dbl()) {
preds[p] = Pos.get(i, info);
} else {
qc.compInfo(OPTREMOVE, this, pred);
return Empty.SEQ;
}
} else if (pred.isValue()) {
if (pred.ebv(qc, info).bool(info)) {
qc.compInfo(OPTREMOVE, this, pred);
preds = Array.delete(preds, p--);
} else {
// handle statically known predicates
qc.compInfo(OPTREMOVE, this, pred);
return Empty.SEQ;
}
}
}
return this;
}
/**
* Returns a new path instance. A path implementation is chosen that works fastest for the given
* steps.
*
* @param info input info
* @param root root expression; can be {@code null}
* @param steps steps
* @return path instance
*/
public static Path get(final InputInfo info, final Expr root, final Expr... steps) {
// new list with steps
final ExprList stps = new ExprList(steps.length);
// merge nested paths
Expr rt = root;
if (rt instanceof Path) {
final Path path = (Path) rt;
stps.add(path.steps);
rt = path.root;
}
// remove redundant context reference
if (rt instanceof Context) rt = null;
// add steps of input array
final int sl = steps.length;
for (int s = 0; s < sl; s++) {
Expr step = steps[s];
if (step instanceof Context) {
// remove redundant context references
if (sl > 1) continue;
// single step: rewrite to axis step (required to sort results of path)
step = Step.get(((Context) step).info, SELF, Test.NOD);
} else if (step instanceof Filter) {
// rewrite to axis step (can be evaluated faster than filter expression)
final Filter f = (Filter) step;
if (f.root instanceof Context) step = Step.get(f.info, SELF, Test.NOD, f.preds);
}
stps.add(step);
}
// check if all steps are axis steps
boolean axes = true;
final Expr[] st = stps.finish();
for (final Expr step : st) axes &= step instanceof Step;
// choose best implementation
return axes
? iterative(rt, st) ? new IterPath(info, rt, st) : new CachedPath(info, rt, st)
: new MixedPath(info, rt, st);
}
/**
* Merges expensive descendant-or-self::node() steps.
*
* @param qc query context
* @return original or new expression
*/
private Expr mergeSteps(final QueryContext qc) {
boolean opt = false;
final int sl = steps.length;
final ExprList stps = new ExprList(sl);
for (int s = 0; s < sl; s++) {
final Expr step = steps[s];
// check for simple descendants-or-self step with succeeding step
if (s < sl - 1 && step instanceof Step) {
final Step curr = (Step) step;
if (curr.simple(DESCORSELF, false)) {
// check succeeding step
final Expr next = steps[s + 1];
// descendant-or-self::node()/child::X -> descendant::X
if (simpleChild(next)) {
((Step) next).axis = DESC;
opt = true;
continue;
}
// descendant-or-self::node()/(X, Y) -> (descendant::X | descendant::Y)
Expr e = mergeList(next);
if (e != null) {
steps[s + 1] = e;
opt = true;
continue;
}
// //(X, Y)[text()] -> (/descendant::X | /descendant::Y)[text()]
if (next instanceof Filter && !next.has(Flag.FCS)) {
final Filter f = (Filter) next;
e = mergeList(f.root);
if (e != null) {
f.root = e;
opt = true;
continue;
}
}
}
}
stps.add(step);
}
if (opt) {
qc.compInfo(OPTDESC);
return get(info, root, stps.finish());
}
return this;
}
/**
* Returns an equivalent expression which accesses an index. If the expression cannot be
* rewritten, the original expression is returned.
*
* <p>The following types of queries can be rewritten (in the examples, the equality comparison is
* used, which will be rewritten to {@link ValueAccess} instances):
*
* <pre>
* 1. A[text() = '...'] -> IA('...')
* 2. A[. = '...'] -> IA('...', A)
* 3. text()[. = '...'] -> IA('...')
* 4. A[B = '...'] -> IA('...', B)/parent::A
* 1. A[B/text() = '...'] -> IA('...')/parent::B/parent::A
* 2. A[B/C = '...'] -> IA('...', C)/parent::B/parent::A
* 7. A[@a = '...'] -> IA('...', @a)/parent::A
* 8. @a[. = '...'] -> IA('...', @a)</pre>
*
* Queries of type 1, 3, 5 will not yield any results if the string to be compared is empty.
*
* @param qc query context
* @param rt root value
* @return original or new expression
* @throws QueryException query exception
*/
private Expr index(final QueryContext qc, final Value rt) throws QueryException {
// only rewrite paths with data reference
final Data data = rt.data();
if (data == null) return this;
// cache index access costs
IndexInfo index = null;
// cheapest predicate and step
int iPred = 0, iStep = 0;
// check if path can be converted to an index access
final int sl = steps.length;
for (int s = 0; s < sl; s++) {
// only accept descendant steps without positional predicates
final Step step = axisStep(s);
if (step == null || !step.axis.down || step.has(Flag.FCS)) break;
// check if path is iterable (i.e., will be duplicate-free)
final boolean iter = pathNodes(data, s) != null;
final IndexContext ictx = new IndexContext(data, iter);
// choose cheapest index access
final int pl = step.preds.length;
for (int p = 0; p < pl; p++) {
final IndexInfo ii = new IndexInfo(ictx, qc, step);
if (!step.preds[p].indexAccessible(ii)) continue;
if (ii.costs == 0) {
// no results...
qc.compInfo(OPTNOINDEX, this);
return Empty.SEQ;
}
if (index == null || index.costs > ii.costs) {
index = ii;
iPred = p;
iStep = s;
}
}
}
// skip rewriting if no index access is possible, or if it is too expensive
if (index == null || index.costs > data.meta.size) return this;
// rewrite for index access
qc.compInfo(index.info);
// replace expressions for index access
final Step indexStep = index.step;
// collect remaining predicates
final int pl = indexStep.preds.length;
final ExprList newPreds = new ExprList(pl - 1);
for (int p = 0; p < pl; p++) {
if (p != iPred) newPreds.add(indexStep.preds[p]);
}
// invert steps that occur before index step and add them as predicate
final Test test = InvDocTest.get(rt);
final ExprList invSteps = new ExprList();
if (test != Test.DOC || !data.meta.uptodate || predSteps(data, iStep)) {
for (int s = iStep; s >= 0; s--) {
final Axis ax = axisStep(s).axis.invert();
if (s == 0) {
// add document test for collections and axes other than ancestors
if (test != Test.DOC || ax != Axis.ANC && ax != Axis.ANCORSELF)
invSteps.add(Step.get(info, ax, test));
} else {
final Step prev = axisStep(s - 1);
invSteps.add(Step.get(info, ax, prev.test, prev.preds));
}
}
}
if (!invSteps.isEmpty()) newPreds.add(get(info, null, invSteps.finish()));
// create resulting expression
final ExprList resultSteps = new ExprList();
final Expr resultRoot;
if (index.expr instanceof Path) {
final Path p = (Path) index.expr;
resultRoot = p.root;
resultSteps.add(p.steps);
} else {
resultRoot = index.expr;
}
if (!newPreds.isEmpty()) {
int ls = resultSteps.size() - 1;
Step step;
if (ls < 0 || !(resultSteps.get(ls) instanceof Step)) {
// add at least one self axis step
step = Step.get(info, Axis.SELF, Test.NOD);
ls++;
} else {
step = (Step) resultSteps.get(ls);
}
// add remaining predicates to last step
resultSteps.set(ls, step.addPreds(newPreds.finish()));
}
// add remaining steps
for (int s = iStep + 1; s < sl; s++) resultSteps.add(steps[s]);
return get(info, resultRoot, resultSteps.finish());
}