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package org.exoplatform.services.document.impl.diff;

import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

import org.exoplatform.services.document.diff.DiffAlgorithm;
import org.exoplatform.services.document.diff.Revision;

/**
 * Implements a simple differencing algortithm.
 * <p>
 * 
 * @date $Date: 2003/10/13 08:00:24 $
 * @version $Revision: 1.7 $
 * @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 DeltaImpl
 * @see RevisionImpl 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 Exception if the diff could not be computed.
   */
  public Revision diff(Object[] orig, Object[] rev) throws Exception {
    // 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 RevisionImpl(); // !!! 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(DeltaImpl.newDelta(new ChunkImpl(orig, ia, i - ia), new ChunkImpl(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;
  }

}