package net.sf.saxon.expr;
import net.sf.saxon.instruct.UserFunction;
import net.sf.saxon.om.Item;
import net.sf.saxon.om.SequenceIterator;
import net.sf.saxon.om.ValueRepresentation;
import net.sf.saxon.trans.XPathException;
import net.sf.saxon.type.ItemType;
import net.sf.saxon.type.TypeHierarchy;
import net.sf.saxon.value.SequenceExtent;
import net.sf.saxon.value.Value;
/**
* A TailCallLoop wraps the body of a function that contains tail-recursive function calls. On completion
* of the "real" body of the function it tests whether the function has executed a tail call, and if so,
* iterates to evaluate the tail call.
*/
public final class TailCallLoop extends UnaryExpression {
UserFunction containingFunction;
/**
* Constructor - create a TailCallLoop
* @param function the function in which this tail call loop appears
*/
public TailCallLoop(UserFunction function) {
super(function.getBody());
containingFunction = function;
}
/**
* Get the containing function
* @return the containing function
*/
public UserFunction getContainingFunction() {
return containingFunction;
}
/**
* Type-check the expression
*/
public Expression typeCheck(ExpressionVisitor visitor, ItemType contextItemType) throws XPathException {
operand = visitor.typeCheck(operand, contextItemType);
return this;
}
/**
* An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process().
* This method indicates which of these methods is provided. This implementation provides both iterate() and
* process() methods natively.
*/
public int getImplementationMethod() {
return operand.getImplementationMethod();
}
/**
* Copy an expression. This makes a deep copy.
*
* @return the copy of the original expression
*/
public Expression copy() {
throw new UnsupportedOperationException("copy");
}
/**
* Iterate over the sequence of values
*/
public SequenceIterator iterate(XPathContext context) throws XPathException {
final XPathContextMajor cm = (XPathContextMajor)context;
while (true) {
SequenceIterator iter = operand.iterate(cm);
ValueRepresentation extent = SequenceExtent.makeSequenceExtent(iter);
UserFunction fn = cm.getTailCallFunction();
if (fn == null) {
return Value.asIterator(extent);
}
if (fn != containingFunction) {
return Value.asIterator(tailCallDifferentFunction(fn, cm));
}
// otherwise, loop round to execute the tail call
}
}
/**
* Evaluate as an Item.
*/
public Item evaluateItem(XPathContext context) throws XPathException {
final XPathContextMajor cm = (XPathContextMajor)context;
while (true) {
Item item = operand.evaluateItem(context);
UserFunction fn = cm.getTailCallFunction();
if (fn == null) {
return item;
}
if (fn != containingFunction) {
return Value.asItem(tailCallDifferentFunction(fn, cm));
}
// otherwise, loop round to execute the tail call
}
}
/**
* Process the function body
* @param context The dynamic context, giving access to the current node,
* the current variables, etc.
*/
public void process(XPathContext context) throws XPathException {
final XPathContextMajor cm = (XPathContextMajor)context;
while (true) {
operand.process(context);
UserFunction fn = cm.getTailCallFunction();
if (fn == null) {
return;
}
if (fn != containingFunction) {
Value.asValue(tailCallDifferentFunction(fn, cm)).process(cm);
return;
}
// otherwise, loop round to execute the tail call
}
}
/**
* Make a tail call on a different function. This reuses the context object and the stack frame array
* where possible, but it does consume some Java stack space. It's still worth it, because we don't use
* as much stack as we would if we didn't return down to the TailCallLoop level.
* @param fn the function to be called
* @param cm the dynamic context
* @return the result of calling the other function
* @throws XPathException if the called function fails
*/
private ValueRepresentation tailCallDifferentFunction(UserFunction fn, XPathContextMajor cm) throws XPathException {
cm.resetStackFrameMap(fn.getStackFrameMap(), fn.getNumberOfArguments());
try {
return ExpressionTool.evaluate(fn.getBody(), fn.getEvaluationMode(), cm, 1);
} catch (XPathException err) {
err.maybeSetLocation(this);
err.maybeSetContext(cm);
throw err;
}
}
/**
* Determine the data type of the items returned by the expression
* @param th The type hierarchy cache
*/
public ItemType getItemType(TypeHierarchy th) {
return operand.getItemType(th);
}
/**
* Give a string representation of the expression name for use in diagnostics
* @return the expression name, as a string
*/
protected String displayExpressionName() {
return "tailCallLoop";
}
}
//
// The contents of this file are subject to the Mozilla Public License Version 1.0 (the "License");
// you may not use this file except in compliance with the License. You may obtain a copy of the
// License at http://www.mozilla.org/MPL/
//
// Software distributed under the License is distributed on an "AS IS" basis,
// WITHOUT WARRANTY OF ANY KIND, either express or implied.
// See the License for the specific language governing rights and limitations under the License.
//
// The Original Code is: all this file.
//
// The Initial Developer of the Original Code is Michael H. Kay
//
// Portions created by (your name) are Copyright (C) (your legal entity). All Rights Reserved.
//
// Contributor(s): none.
//