1 | package de.ugoe.cs.quest.usageprofiles;
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2 |
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3 | import java.security.InvalidParameterException;
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4 | import java.util.Collection;
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5 | import java.util.LinkedList;
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6 | import java.util.List;
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7 | import java.util.Random;
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8 |
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9 | import de.ugoe.cs.quest.eventcore.Event;
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10 |
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11 | /**
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12 | * <p>
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13 | * Implements high-order Markov models.
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14 | * </p>
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15 | *
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16 | * @author Steffen Herbold
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17 | * @version 1.0
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18 | */
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19 | public class HighOrderMarkovModel extends TrieBasedModel {
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20 |
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21 | /**
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22 | * <p>
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23 | * Id for object serialization.
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24 | * </p>
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25 | */
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26 | private static final long serialVersionUID = 1L;
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27 |
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28 | /**
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29 | * <p>
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30 | * Constructor. Creates a new HighOrderMarkovModel with a defined Markov
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31 | * order.
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32 | * </p>
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33 | *
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34 | * @param maxOrder
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35 | * Markov order of the model
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36 | * @param r
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37 | * random number generator used by probabilistic methods of the
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38 | * class
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39 | */
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40 | public HighOrderMarkovModel(int maxOrder, Random r) {
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41 | super(maxOrder, r);
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42 | }
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43 |
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44 | /**
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45 | * <p>
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46 | * Calculates the probability of the next Event being symbol based on the
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47 | * order of the Markov model. The order is defined in the constructor
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48 | * {@link #HighOrderMarkovModel(int, Random)}.
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49 | * </p>
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50 | *
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51 | * @see de.ugoe.cs.quest.usageprofiles.IStochasticProcess#getProbability(java.util.List,
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52 | * de.ugoe.cs.quest.eventcore.Event)
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53 | */
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54 | @Override
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55 | public double getProbability(List<Event> context,
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56 | Event symbol) {
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57 | if (context == null) {
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58 | throw new InvalidParameterException("context must not be null");
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59 | }
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60 | if (symbol == null) {
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61 | throw new InvalidParameterException("symbol must not be null");
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62 | }
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63 | double result = 0.0d;
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64 |
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65 | List<Event> contextCopy;
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66 | if (context.size() >= trieOrder) {
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67 | contextCopy = new LinkedList<Event>(context.subList(
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68 | context.size() - trieOrder + 1, context.size()));
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69 | } else {
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70 | contextCopy = new LinkedList<Event>(context);
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71 | }
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72 |
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73 | Collection<Event> followers = trie.getFollowingSymbols(contextCopy);
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74 | int sumCountFollowers = 0; // N(s\sigma')
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75 | for (Event follower : followers) {
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76 | sumCountFollowers += trie.getCount(contextCopy, follower);
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77 | }
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78 |
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79 | int countSymbol = trie.getCount(contextCopy, symbol);
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80 | if (sumCountFollowers != 0) {
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81 | result = ((double) countSymbol / sumCountFollowers);
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82 | }
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83 |
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84 | return result;
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85 | }
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86 |
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87 | }
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