package cz.cuni.amis.utils;
   
   import java.util.Iterator;
   import java.util.NoSuchElementException;
   
   
   /**
    * This class allows you to combine several iterators in the single one allowing you to seamlessly iterate over several
    * collections at once.
   * <p><p>
   * This class behaves as defined by {@link Iterator} contract.
   * 
   * @author Jimmy
   *
   * @param <NODE>
   */
  public class Iterators<NODE> implements Iterator<NODE> {
  	
  	/**
  	 * Array of iterators that are going to be iterated over. 
  	 */
  	private Iterator<NODE>[] iterators;
  	
  	/**
  	 * Current index of {@link Iterators#iterator} inside {@link Iterators#iterators}. If this index is >= iterators.length then
  	 * there are no more iterators and current iterator is not valid.
  	 */
  	private int currentIteratorIndex;
  	
  	/**
  	 * Previous iterator from {@link Iterators#iterators}, this is stored because of {@link Iterators#remove()} operation that
  	 * is a bit complicated by the fact that one may call {@link Iterators#hasNext()} which advances {@link Iterators#currentIteratorIndex}
  	 * but remove() must still remove previously next()ed node.
  	 */
  	private Iterator<NODE> previousIterator;
  	
  	/**
  	 * Current iterator to be used for {@link Iterators#hasNext()} and {@link Iterators#next()} operations.
  	 */
  	private Iterator<NODE> iterator;
  	
  	/**
  	 * Whether {@link Iterators#currentIteratorIndex} has been recently advanced and {@link Iterators#next()} has not yet been called.
  	 * This means if this is true {@link Iterators#remove()} must use {@link Iterators#previousIterator} instead of {@link Iterators#iterator}.
  	 */
  	private boolean switched;
  	
  	/**
  	 * Whether the last NODE returned by {@link Iterators#next()} was already removed or not.
  	 */
  	private boolean removed;
  	
  	/**
  	 * Whether the {@link Iterators#next()} has ever been called.
  	 */
  	private boolean next;
  	
  	/**
  	 * Initialize this class to use "iterators" in the order as they are passed into the constructor.
  	 * @param iterators may contain nulls
  	 */
  	public Iterators(Iterator<NODE>... iterators) {
  		this.iterators = iterators;
  		if (this.iterators == null || this.iterators.length == 0) {
  			currentIteratorIndex = 1;
  			iterator = null;
  		} else {
  			currentIteratorIndex = 0;
  			previousIterator = null;
  			iterator = iterators[0];
  			switched = false;
  			removed = false;
  			next = false;
  		}
  	}
  	
  	/**
  	 * Advances {@link Iterators#currentIteratorIndex} and {@link Iterators#iterator}.
  	 * @return
  	 */
  	private boolean nextIterator() {		
  		if (iterators == null || currentIteratorIndex >= iterators.length) return false;
  		previousIterator = iterator;
  		switched = true;
  		while (true) {
  			++currentIteratorIndex;
  			if (currentIteratorIndex >= iterators.length) {
  				return false;
  			}			
  			iterator = iterators[currentIteratorIndex];
  			if (iterator == null) continue;
  			if (iterator.hasNext()) {
  				return true;
  			}
  		}
  	}
  	
  	@Override
  	public boolean hasNext() {
 		return (iterator != null && iterator.hasNext()) || nextIterator();
 	}
 
 	@Override
 	public NODE next() {
 		if (iterator == null) throw new NoSuchElementException("Last iterator fully used.");
 		if (iterator.hasNext() || nextIterator()) {
 			next = true;
 			switched = false;
 			removed = false;
 			return iterator.next();
 		}
 		throw new NoSuchElementException("Last iterator fully used.");
 	}
 
 	@Override
 	public void remove() {
 		if (!next) throw new IllegalStateException("next() method has never been successfully called, no element to remove!");
 		if (removed) throw new IllegalStateException("remove() was called twice for the same element, unsupported!");
 		if (switched) {
 			previousIterator.remove();
 		} else {
 			iterator.remove();
 		}
 		removed = true;
 	}
 
 }