/*
    * 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
    *
    *      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.collections4.iterators;
  
  import java.util.ArrayList;
  import java.util.Iterator;
  import java.util.List;
  import java.util.NoSuchElementException;
  import java.util.Objects;
  
  import org.apache.commons.collections4.FluentIterable;
  
  /**
   * Provides an interleaved iteration over the elements contained in a
   * collection of Iterators.
   * <p>
   * Given two {@link Iterator} instances {@code A} and {@code B}, the
   * {@link #next} method on this iterator will switch between {@code A.next()}
   * and {@code B.next()} until both iterators are exhausted.
   * </p>
   *
   * @param <E> the type of elements returned by this iterator.
   * @since 4.1
   */
  public class ZippingIterator<E> implements Iterator<E> {
  
      /** The {@link Iterator}s to evaluate. */
      private final Iterator<Iterator<? extends E>> iterators;
  
      /** The next iterator to use for next(). */
      private Iterator<? extends E> nextIterator;
  
      /** The last iterator which was used for next(). */
      private Iterator<? extends E> lastReturned;
  
      /**
       * Constructs a new {@code ZippingIterator} that will provide
       * interleaved iteration of the specified iterators.
       *
       * @param iterators  the array of iterators
       * @throws NullPointerException if any iterator is null
       */
      public ZippingIterator(final Iterator<? extends E>... iterators) {
          // create a mutable list to be able to remove exhausted iterators
          final List<Iterator<? extends E>> list = new ArrayList<>();
          for (final Iterator<? extends E> iterator : iterators) {
              Objects.requireNonNull(iterator, "iterator");
              list.add(iterator);
          }
          this.iterators = FluentIterable.of(list).loop().iterator();
      }
  
      /**
       * Constructs a new {@code ZippingIterator} that will provide
       * interleaved iteration over the two given iterators.
       *
       * @param a  the first child iterator
       * @param b  the second child iterator
       * @throws NullPointerException if either iterator is null
       */
      @SuppressWarnings("unchecked")
      public ZippingIterator(final Iterator<? extends E> a, final Iterator<? extends E> b) {
          this(new Iterator[] {a, b});
      }
  
      /**
       * Constructs a new {@code ZippingIterator} that will provide
       * interleaved iteration over the three given iterators.
       *
       * @param a  the first child iterator
       * @param b  the second child iterator
       * @param c  the third child iterator
       * @throws NullPointerException if either iterator is null
       */
      @SuppressWarnings("unchecked")
      public ZippingIterator(final Iterator<? extends E> a,
                             final Iterator<? extends E> b,
                             final Iterator<? extends E> c) {
          this(new Iterator[] {a, b, c});
      }
  
      /**
       * Returns {@code true} if any child iterator has remaining elements.
       *
       * @return true if this iterator has remaining elements
      */
     @Override
     public boolean hasNext() {
         // the next iterator has already been determined
         // this might happen if hasNext() is called multiple
         if (nextIterator != null) {
             return true;
         }
 
         while (iterators.hasNext()) {
             final Iterator<? extends E> childIterator = iterators.next();
             if (childIterator.hasNext()) {
                 nextIterator = childIterator;
                 return true;
             }
             // iterator is exhausted, remove it
             iterators.remove();
         }
         return false;
     }
 
     /**
      * Returns the next element from a child iterator.
      *
      * @return the next interleaved element
      * @throws NoSuchElementException if no child iterator has any more elements
      */
     @Override
     public E next() throws NoSuchElementException {
         if (!hasNext()) {
             throw new NoSuchElementException();
         }
 
         final E val = nextIterator.next();
         lastReturned = nextIterator;
         nextIterator = null;
         return val;
     }
 
     /**
      * Removes the last returned element from the child iterator that produced it.
      *
      * @throws IllegalStateException if there is no last returned element, or if
      *   the last returned element has already been removed
      */
     @Override
     public void remove() {
         if (lastReturned == null) {
             throw new IllegalStateException("No value can be removed at present");
         }
         lastReturned.remove();
         lastReturned = null;
     }
 
 }