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Java Simplified. Extensions and Useful Constructs for the Java Platform. Codeprimate Elements (a.k.a. cp-elements) is a Java library and micro-framework used to simplify the development of software applications written in Java. Elements packages several APIs into one library in order to address various application concerns and aspects of software design and development collectively and conveniently. Elements is a highly simple, yet robust and proven library built on solid OO principles, software design patterns and best practices to effectively solve common and reoccurring problems in software development.

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/*
 * Copyright 2016 Author or Authors.
 *
 * 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.cp.elements.util;

import static org.cp.elements.util.CollectionUtils.asIterable;

import java.util.Deque;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.NoSuchElementException;
import java.util.concurrent.atomic.AtomicBoolean;

import org.cp.elements.lang.Assert;

/**
 * The BreadthFirstIterator class is an Iterator implementation and facade for an Iterator of Iterators
 * iterating elements in row order, left to right, top to bottom.
 *
 * @author John Blum
 * @see java.util.Iterator
 * @see org.cp.elements.util.DepthFirstIterator
 * @since 1.0.0
 */
@SuppressWarnings("unused")
public class BreadthFirstIterator implements Iterator {

  private AtomicBoolean nextCalled = new AtomicBoolean(false);

  private final Deque> iterators = new LinkedList<>();

  /**
   * Constructs an instance of the BreadthFirstIterator class wrapping the Iterator of Iterators to collectively
   * traverse the Iterators in order, left to right, top to bottom.
   *
   * @param iterators the Iterator of Iterators to encapsulate in a breadth-first traversal.
   * @throws java.lang.NullPointerException if the Iterator of Iterators reference is null.
   * @see java.util.Iterator
   */
  public BreadthFirstIterator(final Iterator> iterators) {
    Assert.notNull(iterators, "The Iterator of Iterators must not be null!");

    for (Iterator iterator : asIterable(iterators)) {
      if (iterator != null) {
        this.iterators.add(iterator);
      }
    }
  }

  /**
   * Determines whether another element is available in the iteration.
   *
   * @return a boolean value indicating whether another element is available in the iteration.
   * @see java.util.Iterator#hasNext()
   */
  @Override
  public boolean hasNext() {
    while (!(iterators.isEmpty() || iterators.peek().hasNext())) {
      Assert.isFalse(iterators.removeFirst().hasNext(), new IllegalStateException("removing a non-empty Iterator"));
    }

    return (!iterators.isEmpty() && iterators.peek().hasNext());
  }

  /**
   * Gets the next element in the iteration.
   *
   * @return the next element in the iteration.
   * @throws java.util.NoSuchElementException if the iteration has exhausted all elements.
   * @see java.util.Iterator#next()
   * @see #hasNext()
   */
  @Override
  public T next() {
    Assert.isTrue(hasNext(), new NoSuchElementException("The iteration has no more elements!"));
    T value = iterators.peek().next();
    nextCalled.set(true);
    return value;
  }

  /**
   * Removes the current element in the iteration.
   *
   * @throws java.lang.IllegalStateException if the next method has not yet been called, or the remove method
   * has already been called after the last call to the next method.
   * @see java.util.Iterator#remove()
   */
  @Override
  public void remove() {
    Assert.state(nextCalled.compareAndSet(true, false), "next was not called before remove");
    iterators.peek().remove();
  }
}