Jskad/source/org/apache/commons/jrcs/diff/SimpleDiff.java

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package org.apache.commons.jrcs.diff;

import java.util.*;

/**
 * Implements a simple differencing algortithm.<p>
 *
 * @date $Date: 2003/07/14 12:22:29 $
 * @version $Revision: 1.1 $
 * @author <a href="mailto:juanco@suigeneris.org">Juanco Anez</a>
 *
 * <p><b>Overview of Algorithm</b></p>
 *
 * <p><i>by <a
 * href='http://www.topmeadow.net/bwm'> bwm</a>
 * </p>
 *
 * <p>The algorithm is optimised for situations where the input sequences
 * have few repeated objects.  If it is given input with many repeated
 * objects it will report sub-optimal changes.  However, given appropriate
 * input, it is fast, and linear in memory usage.</p>
 *
 * <p>The algorithm consists of the following steps:</p>
 * <ul>
 *   <li>compute an equivalence set for the input data</li>
 *   <li>translate each element of the orginal
 *       and revised input sequences to a member of the equivalence set
 *   </li>
 *   <li>match the the input sequences to determine the deltas, i.e.
 *       the differences between the original and revised sequences.</li>
 * </ul>
 *
 * <p>The first step is to compute a an equivalence set for the input data.
 *  The equivalence set is computed from objects that are in the original
 *  input sequence</p>
 * <pre>
 *   eq(x) = the index of the first occurence of x in the original sequence.
 * </pre>
 *
 * <p>With this equivalence function, the algorithm can compare integers rather
 * than strings, which is considerably more efficient.</p>
 *
 * <p>The second step is to compute the datastructure on which the
 * algorithm will operate.  Having computed the equivalence function
 * in the previous step, we can compute two arrays where
 * indx[i] = eqs(orig[i]) and jndx[i] = eqs(rev[i]).  The algorithm can
 * now operate on indx and jndx instead of orig and rev. Thus, comparisons
 * are then on O(int == int) instead of O(Object.equals(Object)).
 * </p>
 *
 * <p>The algorithm now matches indx and jndx.  Whilst indx[i] == jndx[i]
 * it skips matching objects in the sequence.  In seeking to match objects
 * in the input sequence it assumes that each object is likely to be unique.
 * It uses the known characteristics of the unique equivalence function.  It can
 * tell from the eq value if this object appeared in the other sequence
 * at all.  If it did not, there is no point in searching for a match.</p>
 *
 * <p>Recall that the eq function value is the index earliest occurrence in
 * the orig sequence.  This information is used to search efficiently for
 * the next match.  The algorithm is perfect when all input objects are
 * unique, but degrades when input objects are not unique.  When input
 * objects are not unique an optimal match may not be found, but a
 * correct match will be.</p>
 *
 * <p>Having identified common matching objects in the orig and revised
 * sequences, the differences between them are easily computed.
 * </p>
 *
 * @see Delta
 * @see Revision
 * Modifications:
 *
 * 27/Apr/2003 bwm
 * Added some comments whilst trying to figure out the algorithm
 *
 * 03 May 2003 bwm
 * Created this implementation class by refactoring it out of the Diff
 * class to enable plug in difference algorithms
 *
 */
public class SimpleDiff
    implements DiffAlgorithm
{

    static final int NOT_FOUND_i = -2;
    static final int NOT_FOUND_j = -1;
    static final int EOS = Integer.MAX_VALUE;

    public SimpleDiff()
    {
    }

    protected int scan(int[] ndx, int i, int target)
    {
        while (ndx[i] < target)
        {
            i++;
        }
        return i;
    }

    /**
     * Compute the difference between original and revised sequences.
     *
     * @param orig The original sequence.
     * @param rev The revised sequence to be compared with the original.
     * @return A Revision object describing the differences.
     * @throws DifferenciationFailedException if the diff could not be computed.
     */
    public Revision diff(Object[] orig, Object[] rev)
        throws DifferentiationFailedException
    {
        // create map eqs, such that for each item in both orig and rev
        // eqs(item) = firstOccurrence(item, orig);
        Map eqs = buildEqSet(orig, rev);

        // create an array such that
        //   indx[i] = NOT_FOUND_i if orig[i] is not in rev
        //   indx[i] = firstOccurrence(orig[i], orig)
        int[] indx = buildIndex(eqs, orig, NOT_FOUND_i);

        // create an array such that
        //   jndx[j] = NOT_FOUND_j if orig[j] is not in rev
        //   jndx[j] = firstOccurrence(rev[j], orig)
        int[] jndx = buildIndex(eqs, rev, NOT_FOUND_j);

        // what in effect has been done is to build a unique hash
        // for each item that is in both orig and rev
        // and to label each item in orig and new with that hash value
        // or a marker that the item is not common to both.

        eqs = null; // let gc know we're done with this

        Revision deltas = new Revision(); //!!! new Revision()
        int i = 0;
        int j = 0;

        // skip matching
        // skip leading items that are equal
        // could be written
        // for (i=0; indx[i] != EOS && indx[i] == jndx[i]; i++);
        // j = i;
        for (; indx[i] != EOS && indx[i] == jndx[j]; i++, j++)
        {
            /* void */
        }

        while (indx[i] != jndx[j])
        { // only equal if both == EOS
            // they are different
            int ia = i;
            int ja = j;

            // size of this delta
            do
            {
                // look down rev for a match
                // stop at a match
                // or if the FO(rev[j]) > FO(orig[i])
                // or at the end
                while (jndx[j] < 0 || jndx[j] < indx[i])
                {
                    j++;
                }
                // look down orig for a match
                // stop at a match
                // or if the FO(orig[i]) > FO(rev[j])
                // or at the end
                while (indx[i] < 0 || indx[i] < jndx[j])
                {
                    i++;
                }

                // this doesn't do a compare each line with each other line
                // so it won't find all matching lines
            }
            while (indx[i] != jndx[j]);

            // on exit we have a match

            // they are equal, reverse any exedent matches
            // it is possible to overshoot, so count back matching items
            while (i > ia && j > ja && indx[i - 1] == jndx[j - 1])
            {
                --i;
                --j;
            }

            deltas.addDelta(Delta.newDelta(new Chunk(orig, ia, i - ia),
                                           new Chunk(rev, ja, j - ja)));
            // skip matching
            for (; indx[i] != EOS && indx[i] == jndx[j]; i++, j++)
            {
                /* void */
            }
        }
        return deltas;
    }

    /**
     * create a <code>Map</code> from each common item in orig and rev
     * to the index of its first occurrence in orig
     *
     * @param orig the original sequence of items
     * @param rev  the revised sequence of items
     */
    protected Map buildEqSet(Object[] orig, Object[] rev)
    {
        // construct a set of the objects that orig and rev have in common

        // first construct a set containing all the elements in orig
        Set items = new HashSet(Arrays.asList(orig));

        // then remove all those not in rev
        items.retainAll(Arrays.asList(rev));

        Map eqs = new HashMap();
        for (int i = 0; i < orig.length; i++)
        {
            // if its a common item and hasn't been found before
            if (items.contains(orig[i]))
            {
                // add it to the map
                eqs.put(orig[i], new Integer(i));
                // and make sure its not considered again
                items.remove(orig[i]);
            }
        }
        return eqs;
    }

    /**
     * build a an array such each a[i] = eqs([i]) or NF if eqs([i]) undefined
     *
     * @param eqs a mapping from Object to Integer
     * @param seq a sequence of objects
     * @param NF  the not found marker
     */
    protected int[] buildIndex(Map eqs, Object[] seq, int NF)
    {
        int[] result = new int[seq.length + 1];
        for (int i = 0; i < seq.length; i++)
        {
            Integer value = (Integer) eqs.get(seq[i]);
            if (value == null || value.intValue() < 0)
            {
                result[i] = NF;
            }
            else
            {
                result[i] = value.intValue();
            }
        }
        result[seq.length] = EOS;
        return result;
    }

}