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package org.apache.wicket.util.diff.myers;

import org.apache.wicket.util.diff.Chunk;
import org.apache.wicket.util.diff.Delta;
import org.apache.wicket.util.diff.Diff;
import org.apache.wicket.util.diff.DiffAlgorithm;
import org.apache.wicket.util.diff.DifferentiationFailedException;
import org.apache.wicket.util.diff.Revision;


/**
 * A clean-room implementation of <a href="http://www.cs.arizona.edu/people/gene/"> Eugene Myers</a>
 * differencing algorithm.
 * <p>
 * See the paper at <a href="http://www.cs.arizona.edu/people/gene/PAPERS/diff.ps">
 * http://www.cs.arizona.edu/people/gene/PAPERS/diff.ps</a>
 * 
 * @version $Revision: 1.1 $ $Date: 2006/03/12 00:24:21 $
 * @author <a href="mailto:juanco@suigeneris.org">Juanco Anez</a>
 * @see Delta
 * @see Revision
 * @see Diff
 */
public class MyersDiff implements DiffAlgorithm
{
	/**
	 * Constructs an instance of the Myers differencing algorithm.
	 */
	public MyersDiff()
	{
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public Revision diff(final Object[] orig, final Object[] rev)
		throws DifferentiationFailedException
	{
		PathNode path = buildPath(orig, rev);
		return buildRevision(path, orig, rev);
	}

	/**
	 * Computes the minimum diffpath that expresses de differences between the original and revised
	 * sequences, according to Gene Myers differencing algorithm.
	 * 
	 * @param orig
	 *            The original sequence.
	 * @param rev
	 *            The revised sequence.
	 * @return A minimum {@link PathNode Path} across the differences graph.
	 * @throws DifferentiationFailedException
	 *             if a diff path could not be found.
	 */
	public static PathNode buildPath(final Object[] orig, final Object[] rev)
		throws DifferentiationFailedException
	{
		if (orig == null)
		{
			throw new IllegalArgumentException("original sequence is null");
		}
		if (rev == null)
		{
			throw new IllegalArgumentException("revised sequence is null");
		}

		// these are local constants
		final int N = orig.length;
		final int M = rev.length;

		final int MAX = N + M + 1;
		final int size = 1 + 2 * MAX;
		final int middle = (size + 1) / 2;
		final PathNode diagonal[] = new PathNode[size];

		diagonal[middle + 1] = new Snake(0, -1, null);
		for (int d = 0; d < MAX; d++)
		{
			for (int k = -d; k <= d; k += 2)
			{
				final int kmiddle = middle + k;
				final int kplus = kmiddle + 1;
				final int kminus = kmiddle - 1;
				PathNode prev = null;

				int i;
				if ((k == -d) || ((k != d) && (diagonal[kminus].i < diagonal[kplus].i)))
				{
					i = diagonal[kplus].i;
					prev = diagonal[kplus];
				}
				else
				{
					i = diagonal[kminus].i + 1;
					prev = diagonal[kminus];
				}

				diagonal[kminus] = null; // no longer used

				int j = i - k;

				PathNode node = new DiffNode(i, j, prev);

				// orig and rev are zero-based
				// but the algorithm is one-based
				// that's why there's no +1 when indexing the sequences
				while ((i < N) && (j < M) && orig[i].equals(rev[j]))
				{
					i++;
					j++;
				}
				if (i > node.i)
				{
					node = new Snake(i, j, node);
				}

				diagonal[kmiddle] = node;

				if ((i >= N) && (j >= M))
				{
					return diagonal[kmiddle];
				}
			}
			diagonal[middle + d - 1] = null;

		}
		// According to Myers, this cannot happen
		throw new DifferentiationFailedException("could not find a diff path");
	}

	/**
	 * Constructs a {@link Revision} from a difference path.
	 * 
	 * @param path
	 *            The path.
	 * @param orig
	 *            The original sequence.
	 * @param rev
	 *            The revised sequence.
	 * @return A {@link Revision} script corresponding to the path.
	 */
	public static Revision buildRevision(PathNode path, final Object[] orig, final Object[] rev)
	{
		if (path == null)
		{
			throw new IllegalArgumentException("path is null");
		}
		if (orig == null)
		{
			throw new IllegalArgumentException("original sequence is null");
		}
		if (rev == null)
		{
			throw new IllegalArgumentException("revised sequence is null");
		}

		Revision revision = new Revision();
		if (path.isSnake())
		{
			path = path.prev;
		}
		while ((path != null) && (path.prev != null) && (path.prev.j >= 0))
		{
			if (path.isSnake())
			{
				throw new IllegalStateException("bad diffpath: found snake when looking for diff");
			}
			int i = path.i;
			int j = path.j;

			path = path.prev;
			int ianchor = path.i;
			int janchor = path.j;

			Delta delta = Delta.newDelta(new Chunk(orig, ianchor, i - ianchor), new Chunk(rev,
				janchor, j - janchor));
			revision.insertDelta(delta);
			if (path.isSnake())
			{
				path = path.prev;
			}
		}
		return revision;
	}

}
