source: trunk/autoquest-core-tasktrees/src/main/java/de/ugoe/cs/autoquest/tasktrees/temporalrelation/utils/MostSimilarTaskDeterminer.java @ 1975

//   Copyright 2012 Georg-August-Universität Göttingen, Germany
//
//   Licensed 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
//
//   Unless required by applicable law or agreed to in writing, software
//   distributed under the License is distributed on an "AS IS" BASIS,
//   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
//   See the License for the specific language governing permissions and
//   limitations under the License.

package de.ugoe.cs.autoquest.tasktrees.temporalrelation.utils;

import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.logging.Level;

import de.ugoe.cs.autoquest.tasktrees.temporalrelation.TaskComparator;
import de.ugoe.cs.autoquest.tasktrees.treeifc.DefaultTaskInstanceTraversingVisitor;
import de.ugoe.cs.autoquest.tasktrees.treeifc.DefaultTaskTraversingVisitor;
import de.ugoe.cs.autoquest.tasktrees.treeifc.IEventTaskInstance;
import de.ugoe.cs.autoquest.tasktrees.treeifc.IStructuringTemporalRelationship;
import de.ugoe.cs.autoquest.tasktrees.treeifc.ITask;
import de.ugoe.cs.autoquest.tasktrees.treeifc.ITaskInfo;
import de.ugoe.cs.autoquest.tasktrees.treeifc.ITaskInstance;
import de.ugoe.cs.autoquest.tasktrees.treeifc.ITaskInstanceList;
import de.ugoe.cs.autoquest.tasktrees.treeifc.ITaskModel;
import de.ugoe.cs.autoquest.tasktrees.treeifc.TaskMetric;
import de.ugoe.cs.autoquest.tasktrees.treeifc.TaskTreeUtils;
import de.ugoe.cs.util.console.Console;

/**
 * <p>
 * This class is a utility class for detecting similar tasks. It compares all tasks in a given
 * list with each other and then chooses the pair with the highest level of similarity. For
 * comparison, the tasks are traversed and only the traversals are compared with each other.
 * </p>
 * <p>
 * Several task pairs may have the same similarity level. In this case, the class performs a
 * filtering of the pairs to result in a merging order for the tasks that is always the same.
 * </p>
 * <p>
 * If provided with many tasks, this class starts several threads to let the comparisons run in
 * parallel.
 * </p>
 * 
 * @author Patrick Harms
 */
public class MostSimilarTaskDeterminer {

    /**
     * If this similarity level is exceeded, two tasks are not considered similar anymore. 33 means
     * that at most 33% of the elements of both task traversals are not the same.
     */
    private static int MAX_DIFF_LEVEL = 33;
    
    /**
     * task comparator used internally
     */
    private TaskComparator comparator;
    
    /**
     * for performance reasons, some task comparisons can be excluded beforehand and the exclusions
     * are stored in this list
     */
    private Set<Long> comparisonsToSkip = new HashSet<>();
    
    /** TODO comment */
    private long comparisonCounter = 0;
    
    /** TODO comment */
    private long effectiveComparisonCounter = 0;

    /**
     * Initialize instances of this class with the task comparator to be used
     */
    public MostSimilarTaskDeterminer(TaskComparator comparator) {
        this.comparator = comparator;
    }

    /**
     * add a pair of comparisons that is not done to increase performance
     */
    public void addComparisonToSkip(ITask task1, ITask task2) {
        comparisonsToSkip.add(getMapId(task1, task2));
    }

    /**
     * returns a list of most similar tasks. Independent of the order of the provided tasks, the
     * returned list will always be the same for the same input elements.
     */
    public List<SimilarTasks> getMostSimilarTasks(List<ITask> tasks, ITaskModel taskModel) {
        Console.println("comparing " + tasks.size() + " sequences with each other");

        LinkedList<SimilarTasks> mostSimilarTasksList = performComparisons(taskModel, tasks);
        
        Console.println("initially found " + mostSimilarTasksList.size() + " similar sequences");
        
        applyFilterForSmallestDiffLevel(mostSimilarTasksList);
        
        Console.println("only " + mostSimilarTasksList.size() + " have the smallest diff level");
        
        applyFilterForParents(mostSimilarTasksList);
        
        Console.println(mostSimilarTasksList.size() + " remain after filtering for parents");
        
        applyFilterForMostCoveredEvents(mostSimilarTasksList, taskModel);
        
        Console.println("calculated " + mostSimilarTasksList.size() + " most similar sequences");

        return mostSimilarTasksList;
    }

    /**
     * filters the given list of similar tasks for those having the smallest diff level, which is
     * in turn the highest similarity
     */
    private void applyFilterForSmallestDiffLevel(LinkedList<SimilarTasks> mostSimilarTasksList) {
        // determine the smallest diff level
        int smallestDiffLevel = Integer.MAX_VALUE;
        
        for (SimilarTasks candidate : mostSimilarTasksList) {
            if (candidate.getDiffLevel() < smallestDiffLevel) {
                smallestDiffLevel = candidate.getDiffLevel();
            }
        }
        
        if (smallestDiffLevel <= MAX_DIFF_LEVEL) {
            // remove all entries with a higher diff level
            Iterator<SimilarTasks> listIterator = mostSimilarTasksList.iterator();
        
            while (listIterator.hasNext()) {
                if (listIterator.next().getDiffLevel() > smallestDiffLevel) {
                    listIterator.remove();
                }
            }
            
            Console.println("smallest diff level is " + smallestDiffLevel);
        }
        else {
            mostSimilarTasksList.clear();
        }
    }
    
    /**
     * ensures that the given list of similar tasks does not contain two pairs, where one refers
     * to a child task of another pair.
     */
    private void applyFilterForParents(LinkedList<SimilarTasks> mostSimilarTasksList) {
        
        // remove all entries being parents of another entry or where both tasks are
        // generated through this rule
        Iterator<SimilarTasks> listIterator = mostSimilarTasksList.iterator();
        List<SimilarTasks> similarTasksToRemove = new LinkedList<SimilarTasks>();
    
        while (listIterator.hasNext()) {
            SimilarTasks candidate = listIterator.next();
            
            ITask task1 = candidate.getLeftHandSide();
            ITask task2 = candidate.getRightHandSide();
            for (SimilarTasks potentialChild : mostSimilarTasksList) {
                ITask task3 = potentialChild.getLeftHandSide();
                ITask task4 = potentialChild.getRightHandSide();
                if (TaskTreeUtils.isChild(task3, task1) ||
                    TaskTreeUtils.isChild(task3, task2) ||
                    TaskTreeUtils.isChild(task4, task1) ||
                    TaskTreeUtils.isChild(task4, task2))
                {
                    similarTasksToRemove.add(candidate);
                    break;
                }
            }
        }
        
        listIterator = mostSimilarTasksList.iterator();
        
        while (listIterator.hasNext()) {
            SimilarTasks candidate = listIterator.next();
            
            for (SimilarTasks toRemove : similarTasksToRemove) {
                if (candidate == toRemove) {
                    listIterator.remove();
                }
            }
        }
    }

    /**
     * performs a filtering of the detected similar tasks which ensures, that the list does not
     * contain two pairs referring to the same task and that in such cases always the same pair
     * will remain.
     */
    private void applyFilterForMostCoveredEvents(LinkedList<SimilarTasks> mostSimilarTasksList,
                                                 ITaskModel               taskModel)
    {
        for (SimilarTasks similarTasks : mostSimilarTasksList) {
            System.out.println(similarTasks.getLeftHandSide() + "  " + similarTasks.getRightHandSide());
        }
        
        // check, if several remaining similar tasks refer to the same task
        Map<ITask, LinkedList<SimilarTasks>> referredTasks =
            new HashMap<ITask, LinkedList<SimilarTasks>>();
        
        for (SimilarTasks similarTasks : mostSimilarTasksList) {
            ensureMapping(similarTasks.getLeftHandSide(), similarTasks, referredTasks);
            ensureMapping(similarTasks.getRightHandSide(), similarTasks, referredTasks);
        }
        
        // remove all entries of tasks occurring only once
        List<ITask> tasksOccuringOnce = new LinkedList<ITask>();

        for (Map.Entry<ITask, LinkedList<SimilarTasks>> entry : referredTasks.entrySet()) {
            if (entry.getValue().size() <= 1) {
                tasksOccuringOnce.add(entry.getKey());
            }
        }

        for (ITask taskToRemove : tasksOccuringOnce) {
            referredTasks.remove(taskToRemove);
        }

        // if there are remaining tasks occurring several times, try to extract one similar tasks
        // object, that should be merged first

        if (referredTasks.size() > 0) {
            mostSimilarTasksList.clear();

            Console.traceln(Level.FINEST, "several comparisons for the same task exist with " +
                            "same diff level --> filtering for pair to be merged first");

            SimilarTasks firstToMerge = null;

            for (LinkedList<SimilarTasks> pairs : referredTasks.values()) {
                for (SimilarTasks current : pairs) {
                    if (firstToMerge == null) {
                        firstToMerge = current;
                    }
                    else if (firstToMerge != current) {
                        firstToMerge = getFirstToMerge(firstToMerge, current, taskModel);
                    }
                }
            }
            
            if (firstToMerge != null) {
                mostSimilarTasksList.add(firstToMerge);
            }
        }
        
    }

    /**
     * <p>
     * compares two similar tasks and decides which of them is to be merged first.
     * </p>
     */
    private SimilarTasks getFirstToMerge(SimilarTasks first,
                                         SimilarTasks second,
                                         ITaskModel   taskModel)
    {
        
        int valFirst = getTaskMetric(first, TaskMetric.EVENT_COVERAGE, taskModel);
        int valSecond = getTaskMetric(second, TaskMetric.EVENT_COVERAGE, taskModel);

        if (valSecond > valFirst) {
            return second;
        }
        else if (valSecond < valFirst) {
            return first;
        }

        // no of covered events is equal, try to distinguish by count

        valFirst = getTaskMetric(first, TaskMetric.COUNT, taskModel);
        valSecond = getTaskMetric(second, TaskMetric.COUNT, taskModel);

        if (valSecond > valFirst) {
            return second;
        }
        else if (valSecond < valFirst) {
            return first;
        }

        // count is equal, try to distinguish by depth

        valFirst = getTaskMetric(first, TaskMetric.DEPTH, taskModel);
        valSecond = getTaskMetric(second, TaskMetric.DEPTH, taskModel);

        if (valSecond < valFirst) {
            return second;
        }
        else if (valSecond > valFirst) {
            return first;
        }

        // no of covered events is equal, try to distinguish by count

        valFirst = cumulateTaskMetric(first, TaskMetric.COUNT, taskModel);
        valSecond = cumulateTaskMetric(second, TaskMetric.COUNT, taskModel);

        if (valSecond > valFirst) {
            return second;
        }
        else if (valSecond < valFirst) {
            return first;
        }

        // depth is equal. Calculate for both the similarity
        // based on which the merging will take place 
        SimilarTasks tmp = SimilarTasks.getMergableLevelOfSimilarity(first, comparator);
        valFirst = tmp != null ? tmp.getDiffLevel() : Integer.MAX_VALUE;
            
        tmp = SimilarTasks.getMergableLevelOfSimilarity(second, comparator);
        valSecond = tmp != null ? tmp.getDiffLevel() : Integer.MAX_VALUE;

        if (valSecond < valFirst) {
            return second;
        }
        else if (valSecond > valFirst) {
            return first;
        }

        first.dump(System.out);
        second.dump(System.out);

        throw new RuntimeException
            ("several tasks are similar so that it is undecidable which to merge first");
    }

    /**
     *
     */
    private void ensureMapping(ITask                                task,
                               SimilarTasks                         similarTasks,
                               Map<ITask, LinkedList<SimilarTasks>> map)
    {
        LinkedList<SimilarTasks> value = map.get(task);
        
        if (value == null) {
            value = new LinkedList<SimilarTasks>();
            map.put(task, value);
        }
        
        value.add(similarTasks);
    }


    /**
     * starts several threads performing the task comparisons
     */
    private LinkedList<SimilarTasks> performComparisons(ITaskModel taskModel, List<ITask> tasks) {
        LinkedList<SimilarTasks> mostSimilarTasks = new LinkedList<SimilarTasks>();
        List<Runnable> startedRunnables = new LinkedList<Runnable>();

        comparisonCounter = 0;
        
        // groups size is minimal 100 or maximal 1000
        int groupSize = Math.max
            (100, Math.min(3000, 1 + (tasks.size() / Runtime.getRuntime().availableProcessors())));
        
        synchronized (startedRunnables) {
            int start1 = 0;
            int end1;
            int start2;
            int end2;
            
            do {
                end1 = Math.min(start1 + groupSize, tasks.size());
                TaskTraversal[] leftHandTraversals = new TaskTraversal[end1 - start1];
                
                Console.traceln(Level.FINE, "calculating left hand traversals from " + start1 +
                                " to " + end1);
                
                int index = 0;
                for (int j = start1; j < end1; j++) {
                    leftHandTraversals[index++] =
                        TaskTraversal.getTraversal(tasks.get(j), null);
                }

                start2 = 0;
                do {
                    end2 = Math.min(start2 + groupSize, end1 - 1);
                    TaskTraversal[] rightHandTraversals = new TaskTraversal[end2 - start2];

                    if (end2 <= start1) {
                        Console.traceln(Level.FINE, "calculating right hand traversals from " +
                                        start2 + " to " + end2);
                        
                        // traversals need to be created
                        index = 0;
                        for (int j = start2; j < end2; j++) {
                            rightHandTraversals[index++] =
                                TaskTraversal.getTraversal(tasks.get(j), null);
                        }
                        
                    }
                    else {
                        // traversals can be reused
                        Console.traceln(Level.FINE, "reusing traversals for right hand from " +
                                        start2 + " to " + end2);
                        
                        rightHandTraversals = leftHandTraversals;
                    }
                    
                    Runnable runnable = new CompareRunnable(taskModel, leftHandTraversals,
                                                            rightHandTraversals,
                                                            mostSimilarTasks, startedRunnables);
                    
                    while (startedRunnables.size() >= 
                               Math.max(1, Runtime.getRuntime().availableProcessors()))
                    {
                        try {
                            Console.traceln(Level.FINER, "waiting for next thread to finish");
                            startedRunnables.wait();
                            Console.traceln(Level.FINER, "thread finished");
                        }
                        catch (InterruptedException e) {
                            // should not happen
                            Console.logException(e);
                        }
                    }
                
                    Console.traceln(Level.FINER, "starting next thread");
                    startedRunnables.add(runnable);
                    new Thread(runnable).start();
                    Console.traceln(Level.FINER, "started next thread " + runnable);
                    
                    start2 = end2;
                }
                while (end2 < (end1 - 1));
                
                start1 = end1;
            }
            while (end1 < tasks.size());
            
            
            while (startedRunnables.size() > 0) {
                try {
                    Console.traceln(Level.FINER, "waiting for next thread to finish");
                    startedRunnables.wait();
                    Console.traceln(Level.FINER, "thread finished");
                }
                catch (InterruptedException e) {
                    // should not happen
                    Console.logException(e);
                }
            }
        }
        
        Console.traceln
            (Level.FINER, "all threads finished, " + comparisonCounter + " comparisons done (" +
             effectiveComparisonCounter + " effectively)");
        
        if (comparisonCounter != (((tasks.size() - 1) * tasks.size()) / 2)) {
            throw new RuntimeException(comparisonCounter + "  " +
                                       (((tasks.size() - 1) * tasks.size()) / 2));
        }
        
        return mostSimilarTasks;
    }

    /**
     * convenience method to get the value of a task metric
     */
    private int getTaskMetric(SimilarTasks similarTasks, TaskMetric metric, ITaskModel taskModel) {
        if (similarTasks == null) {
            return 0;
        }
        
        ITaskInfo info1 = taskModel.getTaskInfo(similarTasks.getLeftHandSide());
        ITaskInfo info2 = taskModel.getTaskInfo(similarTasks.getRightHandSide());
        
        return info1.getMeasureValue(metric) + info2.getMeasureValue(metric);
    }
    
    /**
     * convenience method to get the cumulative value of a task metric
     */
    private int cumulateTaskMetric(SimilarTasks     similarTasks,
                                   final TaskMetric metric,
                                   final ITaskModel taskModel)
    {
        final int[] value = new int[1];
        value[0] = 0;
        
        DefaultTaskTraversingVisitor visitor = new DefaultTaskTraversingVisitor() {
            @Override
            public void visit(IStructuringTemporalRelationship relationship) {
                value[0] += taskModel.getTaskInfo(relationship).getMeasureValue(metric);
                super.visit(relationship);
            }
        };
        
        similarTasks.getLeftHandSide().accept(visitor);
        similarTasks.getRightHandSide().accept(visitor);
        
        return value[0];
    }

    /**
     * convenience method to check if a specific comparison shall be skipped
     */
    private boolean isComparisonToSkip(ITask task1, ITask task2) {
        return comparisonsToSkip.contains(getMapId(task1, task2));
    }

    /**
     * convenience method to get a unique id for representing a pair of two tasks
     */
    private long getMapId(ITask task1, ITask task2) {
        if (task1.getId() < task2.getId()) {
            return (((long) task1.getId()) << 32) + task2.getId();
        }
        else {
            return (((long) task2.getId()) << 32) + task1.getId();
        }
    }

    /**
     * Runnable performing a subset of task comparisons in a dedicated thread
     */
    public class CompareRunnable implements Runnable {

        /** */
        private ITaskModel taskModel;
        
        /** */
        private TaskTraversal[] leftHandTraversals;
        
        /** */
        private TaskTraversal[] rightHandTraversals;
        
        /** */
        private List<SimilarTasks> mostSimilarTasksList;

        /** */
        private List<Runnable> unfinishedRunnables;
        
        /**
         *
         */
        public CompareRunnable(ITaskModel         taskModel,
                               TaskTraversal[]    leftHandTraversals,
                               TaskTraversal[]    rightHandTraversals,
                               List<SimilarTasks> mostSimilarTasksList,
                               List<Runnable>     unfinishedRunnables)
        {
            this.taskModel = taskModel;
            this.leftHandTraversals = leftHandTraversals;
            this.rightHandTraversals = rightHandTraversals;
            this.mostSimilarTasksList = mostSimilarTasksList;
            this.unfinishedRunnables = unfinishedRunnables;
        }

        /* (non-Javadoc)
         * @see java.lang.Runnable#run()
         */
        @Override
        public void run() {
            SimilarTasks mostSimilarTasks = SimilarTasks.UNEQUAL_TASKS;
            int mostSimilarDiffLevel = MAX_DIFF_LEVEL;
            List<SimilarTasks> allMostSimilarTasks = new LinkedList<SimilarTasks>();
            int counter = 0;
            int effectiveComparisons = 0;
            
            LEFT_HAND_TRAVERSAL:
            for (int i = 0; i < leftHandTraversals.length; i++) {
                
                ITask leftHandTask = leftHandTraversals[i].getTask();
                
                RIGHT_HAND_TRAVERSAL:
                for (int j = 0; j < rightHandTraversals.length; j++) {

                    ITask rightHandTask = rightHandTraversals[j].getTask();
                    
                    try {
                        if (leftHandTask == rightHandTask) {
                            continue LEFT_HAND_TRAVERSAL;
                        }

                        counter++;

                        if (isComparisonToSkip(leftHandTask, rightHandTask)) {
                            continue RIGHT_HAND_TRAVERSAL;
                        }
                        
                        if (!isBasicallySimilar(leftHandTraversals[i], rightHandTraversals[j])) {
                            continue RIGHT_HAND_TRAVERSAL;
                        }
                        
                        effectiveComparisons++;

                        SimilarTasks similarTasks1 = SimilarTasks.compareTraversals
                            (leftHandTraversals[i], rightHandTraversals[j], comparator);

                        SimilarTasks similarTasks2 = SimilarTasks.compareTraversals
                            (rightHandTraversals[j], leftHandTraversals[i], comparator);

                        if ((similarTasks1.getDiffLevel() <= mostSimilarDiffLevel) ||
                            (similarTasks2.getDiffLevel() <= mostSimilarDiffLevel))
                        {
                            SimilarTasks similarTasks =
                                getSimilarTasksToPrefer(similarTasks1, similarTasks2);

                            if (similarTasks.isInBetweenDifference() ||
                                (similarTasks.getPatch().getDeltas().size() == 0))
                            {
                                if (similarTasks.getDiffLevel() < mostSimilarDiffLevel) {
                                    mostSimilarTasks = similarTasks;
                                    mostSimilarDiffLevel = mostSimilarTasks.getDiffLevel();
                                    allMostSimilarTasks.clear();
                                    allMostSimilarTasks.add(similarTasks);
                                }
                                else if (similarTasks.getDiffLevel() == mostSimilarDiffLevel) {
                                    allMostSimilarTasks.add(similarTasks);
                                }
                            }
                        }
                    }
                    catch (Exception e) {
                        e.printStackTrace();

                        SimilarTasks similarTasks1 = SimilarTasks.compareTraversals
                            (leftHandTraversals[i], rightHandTraversals[j], comparator);

                        SimilarTasks similarTasks2 = SimilarTasks.compareTraversals
                            (rightHandTraversals[j], leftHandTraversals[i], comparator);

                        similarTasks1.dump(System.err);

                        similarTasks2.dump(System.err);
                    }
                }
            }
            
            synchronized (unfinishedRunnables) {
                mostSimilarTasksList.addAll(allMostSimilarTasks);
                comparisonCounter += counter;
                effectiveComparisonCounter += effectiveComparisons;
                
                for (int i = 0; i < unfinishedRunnables.size(); i++) {
                    if (unfinishedRunnables.get(i) == this) {
                        unfinishedRunnables.remove(i);
                        unfinishedRunnables.notify();
                    }
                }
            }
        }

        /**
         *
         */
        private boolean isBasicallySimilar(TaskTraversal traversal1, TaskTraversal traversal2) {
            int length1 = traversal1.size();
            int length2 = traversal2.size();
            int maxLength = Math.max(length1, length2);
            int lengthDiff = 100 * Math.abs(length1 - length2) / maxLength;
            
            if (lengthDiff > MAX_DIFF_LEVEL) {
                return false;
            }
            else {
                ITask firstTask1 = traversal1.get(0);
                ITask firstTask2 = traversal2.get(0);
                
                if (!comparator.equals(firstTask1, firstTask2)) {
                    return false;
                }
                
                ITask lastTask1 = traversal1.get(traversal1.size() - 1);
                ITask lastTask2 = traversal2.get(traversal2.size() - 1);
                
                if (!comparator.equals(lastTask1, lastTask2)) {
                    return false;
                }
                
                if (TaskTreeUtils.isChild(traversal1.getTask(), traversal2.getTask()) ||
                    TaskTreeUtils.isChild(traversal2.getTask(), traversal1.getTask()))
                {
                    return false;
                }
                
                return true;
            }
        }

        /**
         *
         */
        private SimilarTasks getSimilarTasksToPrefer(SimilarTasks similarTasks1,
                                                     SimilarTasks similarTasks2)
        {
            if (similarTasks2.getDiffLevel() > similarTasks1.getDiffLevel()) {
                return similarTasks2;
            }
            else if (similarTasks1.getDiffLevel() > similarTasks2.getDiffLevel()) {
                return similarTasks1;
            }
            else if (similarTasks1.getDiffLevel() == similarTasks2.getDiffLevel()) {
                ITask first = similarTasks1.getLeftHandSide();
                ITask second = similarTasks2.getLeftHandSide();
                
                long valFirst = getTaskMetric(first, TaskMetric.EVENT_COVERAGE);
                long valSecond = getTaskMetric(second, TaskMetric.EVENT_COVERAGE);

                if (valSecond > valFirst) {
                    return similarTasks2;
                }
                else if (valSecond < valFirst) {
                    return similarTasks1;
                }

                // no of covered events is equal, try to distinguish by count

                valFirst = getTaskMetric(first, TaskMetric.COUNT);
                valSecond = getTaskMetric(second, TaskMetric.COUNT);

                if (valSecond > valFirst) {
                    return similarTasks2;
                }
                else if (valSecond < valFirst) {
                    return similarTasks1;
                }

                // count is equal, try to distinguish by depth

                valFirst = getTaskMetric(first, TaskMetric.DEPTH);
                valSecond = getTaskMetric(second, TaskMetric.DEPTH);

                if (valSecond < valFirst) {
                    return similarTasks2;
                }
                else if (valSecond > valFirst) {
                    return similarTasks1;
                }

                // no of covered events is equal, try to distinguish by count

                valFirst = cumulateTaskMetric(first, TaskMetric.COUNT);
                valSecond = cumulateTaskMetric(second, TaskMetric.COUNT);

                if (valSecond > valFirst) {
                    return similarTasks2;
                }
                else if (valSecond < valFirst) {
                    return similarTasks1;
                }

                // depth is equal. Calculate for both the similarity
                // based on which the merging will take place
                SimilarTasks tmp =
                    SimilarTasks.getMergableLevelOfSimilarity(similarTasks1, comparator);
                
                valFirst = tmp != null ? tmp.getDiffLevel() : Integer.MAX_VALUE;
                
                tmp = SimilarTasks.getMergableLevelOfSimilarity(similarTasks2, comparator);
                valSecond = tmp != null ? tmp.getDiffLevel() : Integer.MAX_VALUE;

                if (valSecond < valFirst) {
                    return similarTasks2;
                }
                else if (valSecond > valFirst) {
                    return similarTasks1;
                }

                valFirst = getFirstTimestamp(first);
                valSecond = getFirstTimestamp(second);
                
                if (valSecond > valFirst) {
                    return similarTasks1;
                }
                else if (valSecond < valFirst) {
                    return similarTasks2;
                }

                similarTasks1.dump(System.out);
                similarTasks2.dump(System.out);

                throw new RuntimeException
                    ("several tasks are similar so that it is undecidable which to merge first");
            }
            
            return null;
        }

        /**
         * 
         */
        private int getTaskMetric(ITask task, TaskMetric metric) {
            ITaskInfo info = taskModel.getTaskInfo(task);
            return info.getMeasureValue(metric);
        }
        
        /**
         * convenience method to get the cumulative value of a task metric
         */
        private int cumulateTaskMetric(ITask            task,
                                       final TaskMetric metric)
        {
            final int[] value = new int[1];
            value[0] = 0;
            
            DefaultTaskTraversingVisitor visitor = new DefaultTaskTraversingVisitor() {
                @Override
                public void visit(IStructuringTemporalRelationship relationship) {
                    value[0] += taskModel.getTaskInfo(relationship).getMeasureValue(metric);
                    super.visit(relationship);
                }
            };
            
            task.accept(visitor);
            
            return value[0];
        }

        /**
         * 
         */
        private long getFirstTimestamp(ITask task) {
            long timestamp = Long.MAX_VALUE;
            
            final List<IEventTaskInstance> eventTaskInstances = new LinkedList<>();
            for (ITaskInstance instance : task.getInstances()) {
                eventTaskInstances.clear();
                
                instance.accept(new DefaultTaskInstanceTraversingVisitor() {

                    @Override
                    public void visit(IEventTaskInstance eventTaskInstance) {
                        eventTaskInstances.add(eventTaskInstance);
                    }

                    @Override
                    public void visit(ITaskInstanceList taskInstanceList) {
                        for (ITaskInstance child : taskInstanceList) {
                            if (eventTaskInstances.size() > 0) {
                                break;
                            }
                            
                            child.accept(this);
                        }
                    }
                    
                });
                
                if (eventTaskInstances.size() > 0) {
                    long newTimestamp =
                        ((IEventTaskInstance) eventTaskInstances.get(0)).getEvent().getTimestamp();
                    
                    if (timestamp > newTimestamp) {
                        timestamp = newTimestamp;
                    }
                }
            }
            
            return timestamp;
        }
        
        /* (non-Javadoc)
         * @see java.lang.Object#toString()
         */
        @Override
        public String toString() {
            return "CompareThread(" + leftHandTraversals.length + ", " +
                rightHandTraversals.length + ")";
        }

    }
}