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   *  to you under the Apache License, Version 2.0 (the
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   *
   *    https://www.apache.org/licenses/LICENSE-2.0
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  package org.apache.cayenne.graph;
  
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Collections;
  import java.util.List;
  import java.util.ListIterator;
  
  /**
   * A GraphDiff that is a list of other GraphDiffs.
   * 
   * @since 1.2
   */
  public class CompoundDiff implements GraphDiff {
  
  	private static final long serialVersionUID = 5930690302335603082L;
  
  	protected List<GraphDiff> diffs;
  
  	/**
  	 * Creates an empty CompoundDiff instance.
  	 */
  	public CompoundDiff() {
  	}
  
  	/**
  	 * Creates CompoundDiff instance. Note that a List is not cloned in this
  	 * constructor, so subsequent calls to add and addAll would modify the
  	 * original list.
  	 */
  	public CompoundDiff(List<GraphDiff> diffs) {
  		this.diffs = diffs;
  	}
  
  	/**
  	 * Returns true if this diff has no other diffs or if all of its diffs are
  	 * noops.
  	 */
  	public boolean isNoop() {
  		if (diffs == null || diffs.isEmpty()) {
  			return true;
  		}
  
  		for (GraphDiff diff : diffs) {
  			if (!diff.isNoop()) {
  				return false;
  			}
  		}
  		return true;
  	}
  
  	public List<GraphDiff> getDiffs() {
  		return (diffs != null) ? Collections.unmodifiableList(diffs) : Collections.<GraphDiff> emptyList();
  	}
  
  	public void add(GraphDiff diff) {
  		nonNullDiffs().add(diff);
  	}
  
  	public void addAll(Collection<? extends GraphDiff> diffs) {
  		nonNullDiffs().addAll(diffs);
  	}
  
  	/**
  	 * Iterates over diffs list, calling "apply" on each individual diff.
  	 */
  	public void apply(GraphChangeHandler tracker) {
  		if (diffs == null) {
  			return;
  		}
  
  		// implements a naive linear commit - simply replay stored operations
  		for (GraphDiff change : diffs) {
  			change.apply(tracker);
  		}
  	}
  
  	/**
  	 * Iterates over diffs list in reverse order, calling "apply" on each
  	 * individual diff.
  	 */
  	public void undo(GraphChangeHandler tracker) {
  		if (diffs == null) {
  			return;
  		}
  
  		ListIterator<GraphDiff> it = diffs.listIterator(diffs.size());
  		while (it.hasPrevious()) {
  			GraphDiff change = it.previous();
  			change.undo(tracker);
  		}
  	}
  
  	List<GraphDiff> nonNullDiffs() {
  		if (diffs == null) {
  			synchronized (this) {
  				if (diffs == null) {
  					diffs = new ArrayList<>();
  				}
  			}
  		}
  
  		return diffs;
  	}
  }
  