package org.pdf4j.saxon.expr;
import org.pdf4j.saxon.om.Item;
import org.pdf4j.saxon.om.SequenceIterator;
import org.pdf4j.saxon.trans.XPathException;
import org.pdf4j.saxon.type.ItemType;
/**
* A LastItemExpression returns the last item in the sequence returned by a given
* base expression. The evaluation strategy is to read the input sequence with a one-item lookahead.
*/
public final class LastItemExpression extends UnaryExpression {
/**
* Constructor
* @param base A sequence expression denoting sequence whose first item is to be returned
*/
public LastItemExpression(Expression base) {
super(base);
computeStaticProperties();
}
/**
* 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 expresion 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 org.pdf4j.saxon.type.Type#ITEM_TYPE}
* @return the original expression, rewritten if appropriate to optimize execution
* @throws org.pdf4j.saxon.trans.XPathException if an error is discovered during this phase
* (typically a type error)
*/
public Expression optimize(ExpressionVisitor visitor, ItemType contextItemType) throws XPathException {
operand = visitor.optimize(operand, contextItemType);
if (operand instanceof LastItemExpression || operand instanceof FirstItemExpression) {
return operand;
}
return super.optimize(visitor, contextItemType);
}
/**
* Promote this expression if possible
*/
public Expression promote(PromotionOffer offer) throws XPathException {
Expression exp = offer.accept(this);
if (exp != null) {
return exp;
} else {
if (offer.action != PromotionOffer.UNORDERED) {
// we can't push the UNORDERED property down to the operand, because order is significant
operand = doPromotion(operand, offer);
}
return this;
}
}
/**
* Get the static cardinality
*/
public int computeCardinality() {
return operand.getCardinality() & ~StaticProperty.ALLOWS_MANY;
}
/**
* Copy an expression. This makes a deep copy.
*
* @return the copy of the original expression
*/
public Expression copy() {
return new LastItemExpression(getBaseExpression().copy());
}
/**
* Evaluate the expression
*/
public Item evaluateItem(XPathContext context) throws XPathException {
SequenceIterator forwards = operand.iterate(context);
if (forwards instanceof ReversibleIterator) {
return ((ReversibleIterator)forwards).getReverseIterator().next();
} else {
Item current = null;
while (true) {
Item item = forwards.next();
if (item == null) {
return current;
}
current = item;
}
}
}
public String displayExpressionName() {
return "lastItem";
}
}
//
// 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):
//