/************************************************************************
 *         Copyright (C) 2006-2007 The University of Sheffield          *
 *      Developed by Mark A. Greenwood <m.greenwood@dcs.shef.ac.uk>     *
 *                                                                      *
 * This program is free software; you can redistribute it and/or modify *
 * it under the terms of the GNU General Public License as published by *
 * the Free Software Foundation; either version 2 of the License, or    *
 * (at your option) any later version.                                  *
 *                                                                      *
 * This program is distributed in the hope that it will be useful,      *
 * but WITHOUT ANY WARRANTY; without even the implied warranty of       *
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the        *
 * GNU General Public License for more details.                         *
 *                                                                      *
 * You should have received a copy of the GNU General Public License    *
 * along with this program; if not, write to the Free Software          *
 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.            *
 ************************************************************************/

package edu.isistan.uima.unified.algorithms.similarity;

import net.didion.jwnl.JWNLException;
import net.didion.jwnl.data.Synset;

/**
 * An implementation of the WordNet similarity measure developed by Lin. For
 * full details of the measure see:
 * <blockquote>Lin D. 1998. An information-theoretic definition of similarity. In
 * Proceedings of the 15th International Conference on Machine
 * Learning, Madison, WI.</blockquote>
 * @author Mark A. Greenwood
 */
public class Lin extends ICMeasure
{
	/**
	 * Instances of this similarity measure should be generated using the
	 * factory methods of {@link SimilarityMeasure}.
	 */
	protected Lin()
	{
		//A protected constructor to force the use of the newInstance method
	}
	
	@Override public double getSimilarity(Synset s1, Synset s2) throws JWNLException
	{
		//if the POS tags are not the same then return 0 as this measure
		//only works with 2 nouns or 2 verbs.
		if (!s1.getPOS().equals(s2.getPOS())) return 0;
		
		//see if the similarity is already cached and...
		Double cached = getFromCache(s1, s2);
		
		//if it is then simply return it
		if (cached != null) return cached.doubleValue();
		
		//Get the Information Content (IC) values for the two supplied synsets
		double ic1 = getIC(s1);
		double ic2 = getIC(s2);
		
		//if either IC value is zero then cache and return a sim of 0
		if (ic1 == 0 || ic2 == 0) return addToCache(s1,s2,0);
		
		//Get the Lowest Common Subsumer (LCS) of the two synsets
		Synset lcs = getLCSbyIC(s1,s2);
		
		//if there isn't an LCS then cache and return a sim of 0
		if (lcs == null) return addToCache(s1,s2,0);
		
		//get the IC valueof the LCS
		double icLCS = getIC(lcs);
		
		//caluclaue the similarity score
		double sim = (2*icLCS)/(ic1+ic2);
		
		//cache and return the calculated similarity
		return addToCache(s1,s2,sim);
	}
}