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XLT (Xceptance LoadTest) is an extensive load and performance test tool developed and maintained by Xceptance.
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/*
 * Copyright (c) 2002-2023 Gargoyle Software Inc.
 *
 * 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
 * https://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.htmlunit.html;

import java.io.Serializable;
import java.lang.ref.WeakReference;
import java.util.AbstractList;
import java.util.ArrayList;
import java.util.List;

import org.w3c.dom.Node;

/**
 * A generic DomNodeList implementation of {@link org.w3c.dom.NodeList}.
 *
 * @param  The element type
 *
 * @author Daniel Gredler
 * @author Tom Anderson
 * @author Ronald Brill
 */
public abstract class AbstractDomNodeList extends AbstractList
    implements DomNodeList, Serializable {

    /** This node list's root node. */
    private DomNode node_;

    /** Element cache, used to avoid XPath expression evaluation as much as possible. */
    private List cachedElements_;

    /**
     * Creates a new node list. The elements will be "calculated" using the specified XPath
     * expression applied on the specified node.
     * @param node the node to serve as root for the XPath expression
     */
    public AbstractDomNodeList(final DomNode node) {
        if (node != null) {
            node_ = node;
            final DomHtmlAttributeChangeListenerImpl listener = new DomHtmlAttributeChangeListenerImpl(this);
            node_.addDomChangeListener(listener);
            if (node_ instanceof HtmlElement) {
                ((HtmlElement) node_).addHtmlAttributeChangeListener(listener);
                cachedElements_ = null;
            }
        }
    }

    /**
     * Returns the DOM node.
     * @return the DOM node
     */
    protected DomNode getDomNode() {
        return node_;
    }

    /**
     * Returns the elements.
     * @return the elements
     */
    protected abstract List provideElements();

    /**
     * Returns the nodes in this node list, caching as necessary.
     * @return the nodes in this node list
     */
    private List getNodes() {
        if (cachedElements_ == null) {
            if (node_ == null) {
                cachedElements_ = new ArrayList<>();
            }
            else {
                cachedElements_ = provideElements();
            }
        }
        return cachedElements_;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public int size() {
        return getLength();
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public int getLength() {
        return getNodes().size();
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public Node item(final int index) {
        return getNodes().get(index);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public E get(final int index) {
        return getNodes().get(index);
    }

    /**
     * DOM change listener which clears the node cache when necessary.
     */
    private static final class DomHtmlAttributeChangeListenerImpl
                                    implements DomChangeListener, HtmlAttributeChangeListener {

        private final transient WeakReference> nodeList_;

        DomHtmlAttributeChangeListenerImpl(final AbstractDomNodeList nodeList) {
            super();

            nodeList_ = new WeakReference<>(nodeList);
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void nodeAdded(final DomChangeEvent event) {
            clearCache();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void nodeDeleted(final DomChangeEvent event) {
            clearCache();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void attributeAdded(final HtmlAttributeChangeEvent event) {
            clearCache();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void attributeRemoved(final HtmlAttributeChangeEvent event) {
            clearCache();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void attributeReplaced(final HtmlAttributeChangeEvent event) {
            clearCache();
        }

        private void clearCache() {
            if (nodeList_ != null) {
                final AbstractDomNodeList nodes = nodeList_.get();
                if (nodes != null) {
                    nodes.cachedElements_ = null;
                }
            }
        }
    }

}