package net.sf.saxon.functions;
import net.sf.saxon.expr.*;
import net.sf.saxon.om.GroundedIterator;
import net.sf.saxon.om.GroundedValue;
import net.sf.saxon.om.Item;
import net.sf.saxon.om.SequenceIterator;
import net.sf.saxon.trans.XPathException;
import net.sf.saxon.value.NumericValue;
import net.sf.saxon.value.EmptySequence;
import net.sf.saxon.type.ItemType;
/**
* Implements the saxon:item-at() function. This is handled specially because it is generated
* by the optimizer.
*
* <p>The function takes two arguments: the first is an arbitrary sequence, the second is optional numeric.
* The function returns the same result as let $N := NUMBER return SEQUENCE[$N], including cases where the
* numeric argument is not a whole number.</p>
*/
public class ItemAt extends SystemFunction {
/**
* Perform optimisation of an expression and its subexpressions.
* <p/>
* <p>This method is called after all references to functions and variables have been resolved
* to the declaration of the function or variable, and after all type checking has been done.</p>
*
* @param visitor an expression visitor
* @param contextItemType the static type of "." at the point where this expression is invoked.
* The parameter is set to null if it is known statically that the context item will be undefined.
* If the type of the context item is not known statically, the argument is set to
* {@link net.sf.saxon.type.Type#ITEM_TYPE}
* @return the original expression, rewritten if appropriate to optimize execution
* @throws net.sf.saxon.trans.XPathException
* if an error is discovered during this phase
* (typically a type error)
*/
public Expression optimize(ExpressionVisitor visitor, ItemType contextItemType) throws XPathException {
Expression e = super.optimize(visitor, contextItemType);
if (e != this) {
return e;
}
if (argument[1] instanceof Literal) {
NumericValue val = (NumericValue)((Literal)argument[1]).getValue();
if (val.compareTo(1) < 0 || val.compareTo(Integer.MAX_VALUE) > 0 || !val.isWholeNumber()) {
return new Literal(EmptySequence.getInstance());
}
}
return this;
}
/**
* Evaluate the function to return the selected item.
*/
public Item evaluateItem(XPathContext context) throws XPathException {
NumericValue index = (NumericValue)argument[1].evaluateItem(context);
if (index == null) {
return null;
}
if (index.compareTo(Integer.MAX_VALUE) <= 0 && index.isWholeNumber()) {
int intindex = (int)index.longValue();
if (intindex < 1) {
return null;
}
SequenceIterator base = argument[0].iterate(context);
if (intindex == 1) {
return base.next();
} else if (base instanceof GroundedIterator) {
GroundedValue value = ((GroundedIterator)base).materialize();
return value.itemAt(intindex-1);
} else {
SequenceIterator tail = TailIterator.make(base, intindex);
return tail.next();
}
} else {
// there is no item at the required position
return null;
}
}
}
//
// 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.
//
// Contributor(s):
//