/*
    * Copyright (c) 2002-2025 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.Collections;
  import java.util.List;
  
  import org.w3c.dom.Node;
  
  /**
   * A generic DomNodeList implementation of {@link org.w3c.dom.NodeList}.
   *
   * @param <E> The element type
   *
   * @author Daniel Gredler
   * @author Tom Anderson
   * @author Ronald Brill
   */
  public abstract class AbstractDomNodeList<E extends DomNode> extends AbstractList<E>
      implements DomNodeList<E>, Serializable {
  
      /** This node list's root node. */
      private final DomNode node_;
  
      /** Element cache, used to avoid XPath expression evaluation as much as possible. */
      private List<E> 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) {
          super();
          node_ = node;
  
          if (node == null) {
              cachedElements_ = Collections.EMPTY_LIST;
              return;
          }
  
          final DomHtmlAttributeChangeListenerImpl listener = new DomHtmlAttributeChangeListenerImpl(this);
          node_.addDomChangeListener(listener);
      }
  
      /**
       * Returns the DOM node.
       * @return the DOM node
       */
      protected DomNode getDomNode() {
          return node_;
      }
  
      /**
       * Returns the elements.
       * @return the elements
       */
      protected abstract List<E> provideElements();
  
      /**
       * Returns the nodes in this node list, caching as necessary.
       * @return the nodes in this node list
       */
      private List<E> getNodes() {
          // a bit of a hack, but I like to avoid synchronization
          // see https://github.com/HtmlUnit/htmlunit/issues/882
          //
          // there is a small chance that the cachedElements_ are
          // set to null after the assignment and before the return
          // but this is a race condition at all and depending on the
          // thread state the same overall result might happen also with sync
          List<E> shortLivedCache = cachedElements_;
          if (cachedElements_ == null) {
              shortLivedCache = provideElements();
              cachedElements_ = shortLivedCache;
          }
          return shortLivedCache;
      }
  
      /**
       * {@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 {
 
         private final transient WeakReference<AbstractDomNodeList<?>> 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();
         }
 
         private void clearCache() {
             if (nodeList_ != null) {
                 final AbstractDomNodeList<?> nodes = nodeList_.get();
                 if (nodes != null && nodes.node_ != null) {
                     nodes.cachedElements_ = null;
                 }
             }
         }
     }
 
 }