/*
 * This file is part of the LibreOffice project.
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * This file incorporates work covered by the following license notice:
 *
 *   Licensed to the Apache Software Foundation (ASF) under one or more
 *   contributor license agreements. See the NOTICE file distributed
 *   with this work for additional information regarding copyright
 *   ownership. The ASF licenses this file to you under the Apache
 *   License, Version 2.0 (the "License"); you may not use this file
 *   except in compliance with the License. You may obtain a copy of
 *   the License at http://www.apache.org/licenses/LICENSE-2.0 .
 */

package org.openoffice.xmerge.merger.diff;

import java.util.ArrayList;
import org.openoffice.xmerge.merger.DiffAlgorithm;
import org.openoffice.xmerge.merger.Difference;
import org.openoffice.xmerge.merger.Iterator;
import org.openoffice.xmerge.util.Debug;

/**
 * This is one of the implementations of {@code DiffAlgorithm} interface.
 *
 * <p>Using Longest Common Subsequence (LCS). The algorithm here is based on the
 * book "Introduction to Algorithms" by Thomas H.Cormen, Charles E.Leiserson and
 * Ronald L.Riverst (MIT Press 1990) page 314.</p>
 */
public class IteratorLCSAlgorithm implements DiffAlgorithm {

    public Difference[] computeDiffs(Iterator orgSeq, Iterator modSeq) {

        int orgSeqlen = orgSeq.elementCount();
        int modSeqlen = modSeq.elementCount();

        // Diff table is used to keep track which element is the same or not
        // in those 2 sequences
        int[][] diffTable = createDiffTable(orgSeq, modSeq);

        // debug purpose...
        if (Debug.isFlagSet(Debug.INFO)) {
            printDiffTable(diffTable);
        }

        ArrayList<Difference> diffResult = new ArrayList<Difference>();

        generateResult(diffTable, orgSeqlen, modSeqlen, diffResult);

        // convert the vector to array, it has to do in here as
        // generateResult is called recursively
        Difference[] diffArray = new Difference[diffResult.size()];
        diffResult.toArray(diffArray);

        return diffArray;
    }

    /**
     * Debug function used to print out the nicely formatted difference table.
     *
     * @param  diffTable  The difference table to display.
     */
    private void printDiffTable(int[][] diffTable) {

        for (int i = 0; i < diffTable.length; i++) {
            StringBuilder sb = new StringBuilder();
            for (int j = 0; j < diffTable[i].length; j++) {
               sb.append(" ").append(diffTable[i][j]).append(" ");
            }
            Debug.log(Debug.INFO, sb.toString());
        }
    }

    /**
     * Create the difference table.
     *
     * <p>The difference table is used internal to keep track what elements are
     * common or different in the two sequences.</p>
     *
     * @param   orgSeq  The original sequence to be used as a base.
     * @param   modSeq  The modified sequence to compare.
     *
     * @return  A difference table as a two-dimensional array of integers.
     */
    private int[][] createDiffTable(Iterator orgSeq, Iterator modSeq) {
        int orgSeqlen = orgSeq.elementCount() + 1;
        int modSeqlen = modSeq.elementCount() + 1;
        int[][] diffTable;

        // initialize the diffTable
        diffTable = new int[orgSeqlen][];
        for (int i = 0; i < orgSeqlen; i++) {
            diffTable[i] = new int[modSeqlen];
        }

        // compute the diff Table using LCS algorithm, refer to the book
        // mentioned at the top of the program

        int i, j;

        Object orgSeqObject, modSeqObject;

        for (orgSeqObject = orgSeq.start(), i = 1;
             orgSeqObject != null;
             orgSeqObject = orgSeq.next(), i++) {

            for (modSeqObject = modSeq.start(), j = 1;
                 modSeqObject != null;
                 modSeqObject = modSeq.next(), j++) {

                if (orgSeq.equivalent(orgSeqObject, modSeqObject)) {
                    diffTable[i][j] = diffTable[i-1][j-1]+1;
                } else {
                    if (diffTable[i-1][j] >= diffTable[i][j-1]) {
                        diffTable[i][j] = diffTable[i-1][j];
                    } else {
                        diffTable[i][j] = diffTable[i][j-1];
                    }
                }
            }
        }

        return diffTable;
    }

    /**
     * Generate the {@code Difference} object result vector.
     *
     * <p>This method will be called recursively to backtrack the difference
     * table to get the difference result (and also the LCS).</p>
     *
     * @param   diffTable   The difference table containing the {@code Difference}
     *                      result.
     * @param   i           The nth element in original sequence to compare. This
     *                      method is called recursively with {@code i} and
     *                      {@code j} decreased until {@code 0}.
     * @param   j           The nth element in modified sequence to compare.
     * @param   diffVector  A vector to output the {@code Difference} result.
     *                      Can not use a return variable as it is a recursive
     *                      method.  The vector will contain {@code Difference}
     *                      objects with operation and positions fill in.
     */
    private void generateResult(int[][] diffTable,
                                int i, int j, ArrayList<Difference> diffVector) {

        // handle the first element
        if (i == 0 && j == 0) {
            return;

        } else if (j == 0) {
            for (int cnt = 0; cnt < i; cnt++) {
                Difference diff =
                    new Difference(Difference.DELETE, cnt, j);
                diffVector.add(diff);
            }
            return;

        } else if (i == 0) {
            for (int cnt = 0; cnt < j; cnt++) {
                Difference diff =
                    new Difference(Difference.ADD, i, cnt);
                diffVector.add(diff);
            }
            return;
        }

        // for the detail of this algorithm, refer to the book mentioned on
        // the top and page 317 and 318.
        if ((diffTable[i-1][j-1] == diffTable[i][j] -1) &&
            (diffTable[i-1][j-1] == diffTable[i-1][j]) &&
            (diffTable[i-1][j-1] == diffTable[i][j-1])) {

            // the element of ith and jth in org and mod sequence is the same
            generateResult(diffTable, i-1, j-1, diffVector);
        } else {
            if (diffTable[i-1][j] > diffTable[i][j-1]) {

                // recursively call first, then add the result so that
                // the beginning of the diffs will be stored first
                generateResult(diffTable, i-1, j, diffVector);

                Difference diff =
                    new Difference(Difference.DELETE, i-1, j);
                diffVector.add(diff);
            } else if (diffTable[i-1][j] < diffTable[i][j-1]) {

                // recursively call first, then add the result so that
                // the beginning of the diffs will be stored first
                generateResult(diffTable, i, j-1, diffVector);

                Difference diff =
                    new Difference(Difference.ADD, i, j-1);
                diffVector.add(diff);
            } else { // diffTable[i-1][j] == diffTable[i][j-1]
                // recursively call first, then add the result so that
                // the beginning of the diffs will be stored first
                generateResult(diffTable, i-1, j-1, diffVector);

                Difference diff =
                    new Difference(Difference.CHANGE, i-1, j-1);
                diffVector.add(diff);

            }
        }
    }
}
