/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You 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.apache.commons.collections4.iterators;
  
  import java.util.List;
  import java.util.ListIterator;
  import java.util.Objects;
  
  import org.apache.commons.collections4.ResettableListIterator;
  
  /**
   * Iterates backwards through a List, starting with the last element
   * and continuing to the first. This is useful for looping around
   * a list in reverse order without needing to actually reverse the list.
   * <p>
   * The first call to {@code next()} will return the last element
   * from the list, and so on. The {@code hasNext()} method works
   * in concert with the {@code next()} method as expected.
   * However, the {@code nextIndex()} method returns the correct
   * index in the list, thus it starts high and reduces as the iteration
   * continues. The previous methods work similarly.
   * </p>
   *
   * @param <E> The type of elements returned by this iterator.
   * @since 3.2
   */
  public class ReverseListIterator<E> implements ResettableListIterator<E> {
  
      /** The list being wrapped. */
      private final List<E> list;
  
      /** The list iterator being wrapped. */
      private ListIterator<E> iterator;
  
      /** Flag to indicate if updating is possible at the moment. */
      private boolean validForUpdate = true;
  
      /**
       * Constructor that wraps a list.
       *
       * @param list  The list to create a reversed iterator for
       * @throws NullPointerException if the list is null
       */
      public ReverseListIterator(final List<E> list) {
          this.list = Objects.requireNonNull(list, "list");
          iterator = list.listIterator(list.size());
      }
  
      /**
       * Adds a new element to the list between the next and previous elements.
       *
       * @param obj  The object to add
       * @throws UnsupportedOperationException if the list is unmodifiable
       * @throws IllegalStateException if the iterator is not in a valid state for set
       */
      @Override
      public void add(final E obj) {
          // the validForUpdate flag is needed as the necessary previous()
          // method call re-enables remove and add
          if (!validForUpdate) {
              throw new IllegalStateException("Cannot add to list until next() or previous() called");
          }
          validForUpdate = false;
          iterator.add(obj);
          iterator.previous();
      }
  
      /**
       * Checks whether there is another element.
       *
       * @return true if there is another element
       */
      @Override
      public boolean hasNext() {
          return iterator.hasPrevious();
      }
  
      /**
       * Checks whether there is a previous element.
       *
       * @return true if there is a previous element
       */
      @Override
      public boolean hasPrevious() {
          return iterator.hasNext();
     }
 
     /**
      * Gets the next element.
      * The next element is the previous in the list.
      *
      * @return The next element in the iterator
      */
     @Override
     public E next() {
         final E obj = iterator.previous();
         validForUpdate = true;
         return obj;
     }
 
     /**
      * Gets the index of the next element.
      *
      * @return The index of the next element in the iterator
      */
     @Override
     public int nextIndex() {
         return iterator.previousIndex();
     }
 
     /**
      * Gets the previous element.
      * The next element is the previous in the list.
      *
      * @return The previous element in the iterator
      */
     @Override
     public E previous() {
         final E obj = iterator.next();
         validForUpdate = true;
         return obj;
     }
 
     /**
      * Gets the index of the previous element.
      *
      * @return The index of the previous element in the iterator
      */
     @Override
     public int previousIndex() {
         return iterator.nextIndex();
     }
 
     /**
      * Removes the last returned element.
      *
      * @throws UnsupportedOperationException if the list is unmodifiable
      * @throws IllegalStateException if there is no element to remove
      */
     @Override
     public void remove() {
         if (!validForUpdate) {
             throw new IllegalStateException("Cannot remove from list until next() or previous() called");
         }
         iterator.remove();
     }
 
     /**
      * Resets the iterator back to the start (which is the
      * end of the list as this is a reversed iterator)
      */
     @Override
     public void reset() {
         iterator = list.listIterator(list.size());
     }
 
     /**
      * Replaces the last returned element.
      *
      * @param obj  The object to set
      * @throws UnsupportedOperationException if the list is unmodifiable
      * @throws IllegalStateException if the iterator is not in a valid state for set
      */
     @Override
     public void set(final E obj) {
         if (!validForUpdate) {
             throw new IllegalStateException("Cannot set to list until next() or previous() called");
         }
         iterator.set(obj);
     }
 
 }