simplepathinterpreter.java

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/*
 * Copyright 1999-2004 The Apache Software Foundation
 *
 * Licensed under the Apache License, Version 2.0 (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.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.jxpath.ri.axes;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

import org.apache.commons.jxpath.JXPathException;
import org.apache.commons.jxpath.ri.Compiler;
import org.apache.commons.jxpath.ri.EvalContext;
import org.apache.commons.jxpath.ri.InfoSetUtil;
import org.apache.commons.jxpath.ri.QName;
import org.apache.commons.jxpath.ri.compiler.Expression;
import org.apache.commons.jxpath.ri.compiler.NameAttributeTest;
import org.apache.commons.jxpath.ri.compiler.NodeNameTest;
import org.apache.commons.jxpath.ri.compiler.NodeTest;
import org.apache.commons.jxpath.ri.compiler.Step;
import org.apache.commons.jxpath.ri.model.NodeIterator;
import org.apache.commons.jxpath.ri.model.NodePointer;
import org.apache.commons.jxpath.ri.model.beans.LangAttributePointer;
import org.apache.commons.jxpath.ri.model.beans.NullElementPointer;
import org.apache.commons.jxpath.ri.model.beans.NullPropertyPointer;
import org.apache.commons.jxpath.ri.model.beans.PropertyOwnerPointer;
import org.apache.commons.jxpath.ri.model.beans.PropertyPointer;

/**
 * An evaluation mechanism for simple XPaths, which
 * is much faster than the usual process. It is only used for
 * xpaths which have no context-dependent parts, consist entirely of
 * <code>child::name</code> and <code>self::node()</code> steps with
 * predicates that either integer or have the form <code>[@name = ...]</code>.
 *
 * @author Dmitri Plotnikov
 * @version $Revision: 1.10 $ $Date: 2002/04/24 04:05:40 $
 */
public class SimplePathInterpreter {

    // Because of the complexity caused by the variety of situations
    // that need to be addressed by this class, we attempt to break up
    // the class into individual methods addressing those situations
    // individually.  The names of the methods are supposed to
    // give brief descriptions of those situations.

    private static final QName QNAME_NAME = new QName(null, "name");
    private static final int PERFECT_MATCH = 1000;

    // Uncomment this variable and the PATH = ... lines in
    // the two following methods in order to be able to print the
    // currently evaluated path for debugging of this class
//    private static String PATH;       // Debugging

    /**
     * Interpret a simple path that starts with the given root and
     * follows the given steps. All steps must have the axis "child::"
     * and a name test.  They can also optionally have predicates
     * of type [@name=expression] or simply [expression] interpreted
     * as an index.
     */
    public static NodePointer interpretSimpleLocationPath(
            EvalContext context, NodePointer root, Step steps[])
    {
//        PATH = createNullPointer(context, root, steps, 0).toString();  // Dbg
        NodePointer pointer = doStep(context, root, steps, 0);
//        return valuePointer(pointer);
        return pointer;
    }

    /**
     * Interpret the steps of a simple expression path that
     * starts with the given root, which is the result of evaluation
     * of the root expression of the expression path, applies the
     * given predicates to it and then follows the given steps.
     * All steps must have the axis "child::" or "attribute::"
     * and a name test.  They can also optionally have predicates
     * of type [@name=...] or simply [...] interpreted as an index.
     */
    public static NodePointer interpretSimpleExpressionPath(
                EvalContext context, NodePointer root,
                Expression predicates[], Step[] steps)
    {
//        PATH = createNullPointerForPredicates(context, root,
//                    steps, -1, predicates, 0).toString();  // Debugging
        NodePointer pointer =
            doPredicate(context, root, steps, -1, predicates, 0);
//        return valuePointer(pointer);
        return pointer;
    }

    /**
     * Recursive evaluation of a path. The general plan is:
     * Look at the current step,
     * find nodes that match it,
     * iterate over those nodes and
     * for each of them call doStep again for subsequent steps.
     */
    private static NodePointer doStep(
            EvalContext context, NodePointer parent,
            Step steps[], int currentStep)
    {
        if (parent == null) {
            return null;
        }

        if (currentStep == steps.length) {
            // We have reached the end of the list of steps
            return parent;
        }

        // Open all containers
        parent = valuePointer(parent);
        
        Step step = steps[currentStep];
        Expression predicates[] = step.getPredicates();

        // Divide and conquer: the process is broken out into
        // four major use cases.
        // 1. Current step has no predicates and
        //    the root is a property owner (e.g. bean or map)
        // 2. Current step has predicates and
        //    the root is a property owner (e.g. bean or map)
        // 3. Current step has no predicates and
        //    the root is an InfoSet standard node (e.g. DOM Node)
        // 4. Current step has predicates and
        //    the root is an InfoSet standard node (e.g. DOM Node)

        if (parent instanceof PropertyOwnerPointer) {
            if (predicates == null || predicates.length == 0) {
                return doStepNoPredicatesPropertyOwner(
                    context,
                    (PropertyOwnerPointer) parent,
                    steps,
                    currentStep);
            }
            else {
                return doStepPredicatesPropertyOwner(
                    context,
                    (PropertyOwnerPointer) parent,
                    steps,
                    currentStep);
            }
        }
        else {
            if (predicates == null || predicates.length == 0) {
                return doStepNoPredicatesStandard(
                    context,
                    parent,
                    steps,
                    currentStep);
            }
            else {
                return doStepPredicatesStandard(
                    context,
                    parent,
                    steps,
                    currentStep);
            }
        }
    }

    /**
     * We have a step that starts with a property owner (bean, map, etc) and has
     * no predicates.  The name test of the step may map to a scalar property
     * or to a collection.  If it is a collection, we should apply the tail of
     * the path to each element until we find a match. If we don't find
     * a perfect match, we should return the "best quality" pointer, which
     * has the longest chain of steps mapping to existing nodes and the shortes
     * tail of Null* pointers.
     */
    private static NodePointer doStepNoPredicatesPropertyOwner(
                EvalContext context, PropertyOwnerPointer parentPointer,
                Step[] steps, int currentStep)
    {
        Step step = steps[currentStep];
        NodePointer childPointer =
            createChildPointerForStep(parentPointer, step);

        if (!childPointer.isActual()) {
            // The property does not exist - create a null pointer.
            return createNullPointer(
                context,
                parentPointer,
                steps,
                currentStep);
        }
        else if (currentStep == steps.length - 1) {
            // If this is the last step - we are done, we found it
            return childPointer;
        }
        else if (childPointer.isCollection()) {
            // Iterate over all values and
            // execute remaining steps for each node,
            // looking for the best quality match
            int bestQuality = 0;
            childPointer = (NodePointer) childPointer.clone();
            NodePointer bestMatch = null;
            int count = childPointer.getLength();
            for (int i = 0; i < count; i++) {
                childPointer.setIndex(i);
                NodePointer pointer =
                    doStep(context, childPointer, steps, currentStep + 1);
                int quality = computeQuality(pointer);
                if (quality == PERFECT_MATCH) {
                    return pointer;
                }
                else if (quality > bestQuality) {
                    bestQuality = quality;
                    bestMatch = (NodePointer) pointer.clone();
                }
            }
            if (bestMatch != null) {
                return bestMatch;
            }
            // This step did not find anything - return a null pointer
            return createNullPointer(context, childPointer, steps, currentStep);
        }
        else {
            // Evaluate subsequent steps
            return doStep(context, childPointer, steps, currentStep + 1);
        }
    }

    /**
     * A path that starts with a standard InfoSet node (e.g. DOM Node) and
     * has no predicates.  Get a child iterator and apply the tail of
     * the path to each element until we find a match. If we don't find
     * a perfect match, we should return the "best quality" pointer, which
     * has the longest chain of steps mapping to existing nodes and the shortes
     * tail of Null* pointers.
     */
    private static NodePointer doStepNoPredicatesStandard(
                EvalContext context, NodePointer parentPointer,
                Step[] steps, int currentStep)
    {
        Step step = steps[currentStep];

        if (step.getAxis() == Compiler.AXIS_SELF) {
            return doStep(context, parentPointer, steps, currentStep + 1);
        }

        int bestQuality = 0;
        NodePointer bestMatch = null;
        NodeIterator it = getNodeIterator(context, parentPointer, step);
        if (it != null) {
            for (int i = 1; it.setPosition(i); i++) {
                NodePointer childPointer = it.getNodePointer();
                if (steps.length == currentStep + 1) {
                    // If this is the last step - we are done, we found it
                    return childPointer;
                }
                NodePointer pointer = doStep(
                        context, childPointer, steps, currentStep + 1);
                int quality = computeQuality(pointer);
                if (quality == PERFECT_MATCH) {
                    return pointer;
                }
                else if (quality > bestQuality) {
                    bestQuality = quality;
                    bestMatch = (NodePointer) pointer.clone();
                }
            }
        }

        if (bestMatch != null) {
            return bestMatch;
        }

        return createNullPointer(