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The XSLT and XQuery Processor
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// Copyright (c) 2015 Saxonica Limited.
// This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0.
// If a copy of the MPL was not distributed with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
// This Source Code Form is "Incompatible With Secondary Licenses", as defined by the Mozilla Public License, v. 2.0.
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
package net.sf.saxon.value;
import net.sf.saxon.Configuration;
import net.sf.saxon.event.SequenceReceiver;
import net.sf.saxon.expr.*;
import net.sf.saxon.expr.instruct.SlotManager;
import net.sf.saxon.expr.parser.ExplicitLocation;
import net.sf.saxon.lib.FeatureKeys;
import net.sf.saxon.om.*;
import net.sf.saxon.trans.XPathException;
import net.sf.saxon.tree.iter.ManualIterator;
/**
* A Closure represents a value that has not yet been evaluated: the value is represented
* by an expression, together with saved values of all the context variables that the
* expression depends on.
*
* This Closure is designed for use when the value is only read once. If the value
* is read more than once, a new iterator over the underlying expression is obtained
* each time: this may (for example in the case of a filter expression) involve
* significant re-calculation.
*
* The expression may depend on local variables and on the context item; these values
* are held in the saved XPathContext object that is kept as part of the Closure, and they
* will always be read from that object. The expression may also depend on global variables;
* these are unchanging, so they can be read from the Bindery in the normal way. Expressions
* that depend on other contextual information, for example the values of position(), last(),
* current(), current-group(), should not be evaluated using this mechanism: they should
* always be evaluated eagerly. This means that the Closure does not need to keep a copy
* of these context variables.
*/
public class Closure implements Sequence, ContextOriginator {
protected Expression expression;
/*@Nullable*/ protected XPathContextMajor savedXPathContext;
protected int depth = 0;
// The base iterator is used to copy items on demand from the underlying value
// to the reservoir. It only ever has one instance (for each Closure) and each
// item is read only once.
/*@Nullable*/ protected SequenceIterator inputIterator;
// private static int countClosures = 0;
// private static int countMemoClosures = 0;
/**
* Constructor should not be called directly, instances should be made using the make() method.
*/
//private static int closureCount = 0;
public Closure() {
// this.expression = exp;
// this.savedXPathContext = context.newContext();
// this.savedXPathContext.setOriginatingConstructType(Location.LAZY_EVALUATION);
// saveContext(expression, context);
}
/**
* Construct a Closure by supplying the expression and the set of context variables.
*
* @param expression the expression to be lazily evaluated
* @param context the dynamic context of the expression including for example the variables
* on which it depends
* @param ref the number of references to the value being lazily evaluated; this affects
* the kind of Closure that is created
* @return the Closure, a virtual value that can later be materialized when its content is required
* @throws net.sf.saxon.trans.XPathException
* if a dynamic error occurs
*/
/*@NotNull*/
public static Sequence make(/*@NotNull*/ Expression expression, /*@NotNull*/ XPathContext context, int ref) throws XPathException {
// special cases such as TailExpressions and shared append expressions are now picked up before
// this method is called (where possible, at compile time)
Configuration config = context.getConfiguration();
if (config.getBooleanProperty(FeatureKeys.EAGER_EVALUATION)) {
// Using eager evaluation can make for easier debugging
SequenceIterator iter = expression.iterate(context);
return SequenceExtent.makeSequenceExtent(iter);
}
Sequence v = config.makeClosure(expression, ref, context);
if (v instanceof Closure) {
Closure c = (Closure) v;
c.expression = expression;
c.savedXPathContext = context.newContext();
c.savedXPathContext.setOrigin(c);
c.saveContext(expression, context);
return c;
} else {
return v;
}
}
public void saveContext(/*@NotNull*/ Expression expression, /*@NotNull*/ XPathContext context) throws XPathException {
// Make a copy of all local variables. If the value of any local variable is a closure
// whose depth exceeds a certain threshold, we evaluate the closure eagerly to avoid
// creating deeply nested lists of Closures, which consume memory unnecessarily
// We only copy the local variables if the expression has dependencies on local variables.
// What's more, we only copy those variables that the expression actually depends on.
if ((expression.getDependencies() & StaticProperty.DEPENDS_ON_LOCAL_VARIABLES) != 0) {
StackFrame localStackFrame = context.getStackFrame();
Sequence[] local = localStackFrame.getStackFrameValues();
int[] slotsUsed = expression.getSlotsUsed(); // computed on first call
if (local != null) {
final SlotManager stackFrameMap = localStackFrame.getStackFrameMap();
final Sequence[] savedStackFrame =
new Sequence[stackFrameMap.getNumberOfVariables()];
for (int i : slotsUsed) {
if (local[i] instanceof Closure) {
int cdepth = ((Closure) local[i]).depth;
if (cdepth >= 10) {
local[i] = SequenceExtent.makeSequenceExtent(local[i].iterate());
} else if (cdepth + 1 > depth) {
depth = cdepth + 1;
}
}
savedStackFrame[i] = local[i];
}
savedXPathContext.setStackFrame(stackFrameMap, savedStackFrame);
}
}
// Make a copy of the context item
FocusIterator currentIterator = context.getCurrentIterator();
if (currentIterator != null) {
Item contextItem = currentIterator.current();
ManualIterator single = new ManualIterator(contextItem);
savedXPathContext.setCurrentIterator(single);
// we don't save position() and last() because we have no way
// of restoring them. So the caller must ensure that a Closure is not
// created if the expression depends on position() or last()
}
savedXPathContext.setReceiver(null);
}
/**
* Get the first item in the sequence.
*
* @return the first item in the sequence if there is one, or null if the sequence
* is empty
* @throws net.sf.saxon.trans.XPathException
* in the situation where the sequence is evaluated lazily, and
* evaluation of the first item causes a dynamic error.
*/
public Item head() throws XPathException {
return iterate().next();
}
public Expression getExpression() {
return expression;
}
/*@Nullable*/
public XPathContextMajor getSavedXPathContext() {
return savedXPathContext;
}
public void setExpression(Expression expression) {
this.expression = expression;
}
public void setSavedXPathContext(XPathContextMajor savedXPathContext) {
this.savedXPathContext = savedXPathContext;
}
/**
* Evaluate the expression in a given context to return an iterator over a sequence
*/
/*@NotNull*/
public SequenceIterator iterate() throws XPathException {
if (inputIterator == null) {
SequenceIterator in = expression.iterate(savedXPathContext);
return inputIterator = in;
} else {
// In an ideal world this shouldn't happen: if the value is needed more than once, we should
// have chosen a MemoClosure. In fact, this path is never taken when executing the standard
// test suite (April 2005). However, it provides robustness in case the compile-time analysis
// is flawed. I believe it's also possible that this path can be taken if a Closure needs to be
// evaluated when the chain of dependencies gets too long: this was happening routinely when
// all local variables were saved, rather than only those that the expression depends on.
// Changed Feb 2015 to throw an exception. Processing the expression more than once is not only
// inefficient, it is wrong in the case where node-identity affects the outcome. See for example
// test case fn-fold-left-018
//return inputIterator.getAnother();
throw new IllegalStateException("A Closure can only be read once");
}
}
/**
* Process the instruction, without returning any tail calls
*
* @param context The dynamic context, giving access to the current node,
* the current variables, etc.
* @throws net.sf.saxon.trans.XPathException if an error occurs evaluating the input expression
*/
public void process(/*@NotNull*/ XPathContext context) throws XPathException {
if (expression == null) {
// This is a Closure that simply wraps a SequenceIterator supplied from the Java level
SequenceReceiver out = context.getReceiver();
Item item;
while ((item = inputIterator.next()) != null) {
out.append(item, ExplicitLocation.UNKNOWN_LOCATION, NodeInfo.ALL_NAMESPACES);
}
inputIterator = inputIterator.getAnother();
} else {
// To evaluate the closure in push mode, we need to use the original context of the
// expression for everything except the current output destination, which is newly created
XPathContextMajor c2 = savedXPathContext.newContext();
SequenceReceiver out = context.getReceiver();
c2.setReceiver(out);
c2.setTemporaryOutputState(StandardNames.XSL_VARIABLE);
expression.process(c2);
}
}
/**
* Reduce a value to its simplest form. If the value is a closure or some other form of deferred value
* such as a FunctionCallPackage, then it is reduced to a SequenceExtent. If it is a SequenceExtent containing
* a single item, then it is reduced to that item. One consequence that is exploited by class FilterExpression
* is that if the value is a singleton numeric value, then the result will be an instance of NumericValue
* @return the simplified value
* @throws XPathException if an error occurs doing the lazy evaluation
*/
public GroundedValue reduce() throws XPathException {
return SequenceExtent.makeSequenceExtent(iterate());
}
}
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