@Override
public final Expr compile(final QueryContext qc, final VarScope scp) throws QueryException {
if (root != null) root = root.compile(qc, scp);
// no steps
if (steps.length == 0) return root == null ? new Context(info) : root;
final Value init = qc.value, cv = initial(qc);
final boolean doc = cv != null && cv.type == NodeType.DOC;
qc.value = cv;
try {
final int sl = steps.length;
for (int s = 0; s < sl; s++) {
Expr e = steps[s];
// axis step: if input is a document, its type is temporarily generalized
final boolean as = e instanceof Step;
if (as && s == 0 && doc) cv.type = NodeType.NOD;
e = e.compile(qc, scp);
if (e.isEmpty()) return optPre(qc);
steps[s] = e;
// no axis step: invalidate context value
if (!as) qc.value = null;
}
} finally {
if (doc) cv.type = NodeType.DOC;
qc.value = init;
}
// optimize path
return optimize(qc, scp);
}
@Override
public final Expr optimize(final QueryContext qc, final VarScope scp) throws QueryException {
final Value v = initial(qc);
if (v != null && v.isEmpty() || emptyPath(v)) return optPre(qc);
// merge descendant steps
Expr e = mergeSteps(qc);
if (e == this && v != null && v.type == NodeType.DOC) {
// check index access
e = index(qc, v);
// rewrite descendant to child steps
if (e == this) e = children(qc, v);
}
// recompile path if it has changed
if (e != this) return e.compile(qc, scp);
// set atomic type for single attribute steps to speed up predicate tests
if (root == null && steps.length == 1 && steps[0] instanceof Step) {
final Step curr = (Step) steps[0];
if (curr.axis == ATTR && curr.test.kind == Kind.URI_NAME) curr.seqType(SeqType.NOD_ZO);
}
// choose best path implementation and set type information
final Path path = get(info, root, steps);
path.size = path.size(qc);
path.seqType = SeqType.get(steps[steps.length - 1].seqType().type, size);
return path;
}
@Override
public final boolean has(final Flag flag) {
// first step or root expression will be used as context
if (flag == Flag.CTX) return root == null || root.has(flag);
for (final Expr s : steps) if (s.has(flag)) return true;
return root != null && root.has(flag);
}
@Override
public Expr optimize(final QueryContext ctx, final VarScope scp) throws QueryException {
if (root instanceof Context) {
ctx.compInfo(OPTREMCTX);
root = null;
}
for (final Expr e : steps) {
// check for empty steps
if (e.isEmpty()) return optPre(null, ctx);
}
return this;
}
/**
* Returns the initial context value of a path or {@code null}.
*
* @param qc query context (may be @code null)
* @return root
*/
private Value initial(final QueryContext qc) {
// current context value
final Value v = qc != null ? qc.value : null;
// no root or context expression: return context
if (root == null || root instanceof Context) return v;
// root reference
if (root instanceof Root) return v != null && v instanceof Item ? Root.root(v) : v;
// root is value: return root
if (root.isValue()) return (Value) root;
// data reference
final Data d = root.data();
if (d != null) return new DBNode(d, 0, Data.ELEM);
// otherwise, return null
return null;
}
@Override
public final Expr inline(final QueryContext ctx, final VarScope scp, final Var v, final Expr e)
throws QueryException {
final Value oldVal = ctx.value;
try {
ctx.value = root(ctx);
final Expr rt = root == null ? null : root.inline(ctx, scp, v, e);
if (rt != null) {
setRoot(ctx, rt);
ctx.value = oldVal;
ctx.value = root(ctx);
}
boolean change = rt != null;
for (int i = 0; i < steps.length; i++) {
final Expr nw = steps[i].inline(ctx, scp, v, e);
if (nw != null) {
steps[i] = nw;
change = true;
}
}
return change ? optimize(ctx, scp) : null;
} finally {
ctx.value = oldVal;
}
}
/**
* Checks if the path can be rewritten for iterative evaluation.
*
* @param root root expression; can be a {@code null} reference
* @param steps path steps
* @return result of check
*/
private static boolean iterative(final Expr root, final Expr... steps) {
if (root == null || !root.iterable()) return false;
final int sl = steps.length;
for (int s = 0; s < sl; ++s) {
switch (((Step) steps[s]).axis) {
// reverse axes - don't iterate
case ANC:
case ANCORSELF:
case PREC:
case PRECSIBL:
return false;
// multiple, unsorted results - only iterate at last step,
// or if last step uses attribute axis
case DESC:
case DESCORSELF:
case FOLL:
case FOLLSIBL:
return s + 1 == sl || s + 2 == sl && ((Step) steps[s + 1]).axis == Axis.ATTR;
// allow iteration for CHILD, ATTR, PARENT and SELF axes
default:
}
}
return true;
}
/**
* 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;
}
@Override
public final Expr compile(final QueryContext ctx, final VarScope scp) throws QueryException {
if (root != null) setRoot(ctx, root.compile(ctx, scp));
final Value v = ctx.value;
try {
ctx.value = root(ctx);
return compilePath(ctx, scp);
} finally {
ctx.value = v;
}
}
/**
* Returns the root of the current context or {@code null}.
*
* @param ctx query context
* @return root
*/
final Value root(final QueryContext ctx) {
final Value v = ctx != null ? ctx.value : null;
// no root specified: return context, if it does not reference a document
// as e.g. happens in //a(b|c)
if (root == null) return v == null || v.type != NodeType.DOC ? v : null;
// root is value: return root
if (root.isValue()) return (Value) root;
// no root reference, no context: return null
if (!(root instanceof Root) || v == null) return null;
// return context sequence or root of current context
return v.size() == 1 ? Root.root(v) : v;
}
@Override
public boolean accept(final ASTVisitor visitor) {
if (root == null) {
if (!visitor.lock(DBLocking.CTX)) return false;
} else if (!root.accept(visitor)) {
return false;
}
visitor.enterFocus();
if (!visitAll(visitor, steps)) return false;
visitor.exitFocus();
return true;
}
@Override
public Data data() {
if (root != null) {
// data reference
final Data data = root.data();
if (data != null) {
final int sl = steps.length;
for (int s = 0; s < sl; s++) {
if (axisStep(s) == null) return null;
}
return data;
}
}
return null;
}
@Override
public final Expr inline(final QueryContext qc, final VarScope scp, final Var var, final Expr ex)
throws QueryException {
boolean changed = false;
if (root != null) {
final Expr rt = root.inline(qc, scp, var, ex);
if (rt != null) {
root = rt;
changed = true;
}
}
final int sl = steps.length;
for (int s = 0; s < sl; s++) {
final Expr nw = steps[s].inline(qc, scp, var, ex);
if (nw != null) {
steps[s] = nw;
changed = true;
}
}
return changed ? optimize(qc, scp) : null;
}
@Override
public final int exprSize() {
int sz = 1;
for (final Expr e : steps) sz += e.exprSize();
return root == null ? sz : sz + root.exprSize();
}
@Override
public VarUsage count(final Var var) {
final VarUsage inRoot = root == null ? VarUsage.NEVER : root.count(var);
return VarUsage.sum(var, steps) == VarUsage.NEVER ? inRoot : VarUsage.MORE_THAN_ONCE;
}
@Override
public final boolean removable(final Var var) {
for (final Expr step : steps) if (step.uses(var)) return false;
return root == null || root.removable(var);
}
@Override
public boolean removable(final Var v) {
return root == null || root.removable(v);
}