source: trunk/EventBenchCore/src/de/ugoe/cs/eventbench/models/HighOrderMarkovModel.java

package de.ugoe.cs.eventbench.models;

import java.security.InvalidParameterException;
import java.util.Collection;
import java.util.LinkedList;
import java.util.List;
import java.util.Random;

import de.ugoe.cs.eventbench.data.Event;

/**
 * <p>
 * Implements high-order Markov models.
 * </p>
 * 
 * @author Steffen Herbold
 * @version 1.0
 */
public class HighOrderMarkovModel extends TrieBasedModel {

        /**
         * <p>
         * Id for object serialization.
         * </p>
         */
        private static final long serialVersionUID = 1L;

        /**
         * <p>
         * Constructor. Creates a new HighOrderMarkovModel with a defined Markov
         * order.
         * </p>
         * 
         * @param maxOrder
         *            Markov order of the model
         * @param r
         *            random number generator used by probabilistic methods of the
         *            class
         */
        public HighOrderMarkovModel(int maxOrder, Random r) {
                super(maxOrder, r);
        }

        /**
         * <p>
         * Calculates the probability of the next Event being symbol based on the
         * order of the Markov model. The order is defined in the constructor
         * {@link #HighOrderMarkovModel(int, Random)}.
         * </p>
         * 
         * @see de.ugoe.cs.eventbench.models.IStochasticProcess#getProbability(java.util.List,
         *      de.ugoe.cs.eventbench.data.Event)
         */
        @Override
        public double getProbability(List<? extends Event<?>> context,
                        Event<?> symbol) {
                if (context == null) {
                        throw new InvalidParameterException("context must not be null");
                }
                if (symbol == null) {
                        throw new InvalidParameterException("symbol must not be null");
                }
                double result = 0.0d;

                List<Event<?>> contextCopy;
                if (context.size() >= trieOrder) {
                        contextCopy = new LinkedList<Event<?>>(context.subList(
                                        context.size() - trieOrder + 1, context.size()));
                } else {
                        contextCopy = new LinkedList<Event<?>>(context);
                }

                Collection<Event<?>> followers = trie.getFollowingSymbols(contextCopy);
                int sumCountFollowers = 0; // N(s\sigma')
                for (Event<?> follower : followers) {
                        sumCountFollowers += trie.getCount(contextCopy, follower);
                }

                int countSymbol = trie.getCount(contextCopy, symbol);
                if (sumCountFollowers != 0) {
                        result = ((double) countSymbol / sumCountFollowers);
                }

                return result;
        }

}