Fixed incorrect implementation of Bhattacharyya Distance and updated Bhattacharyya distance unit test

Author: DavidBerdik

src/com/jgaap/distances/BhattacharyyaDistance.java

@@ -10,10 +10,10 @@
 
 /**
  * Bhattacharyya Distance
- * d = abs( log(  sum( sqrt(  abs(xi - yi)  ) ) / sum(xi) ) )
+ * d = -log(sum(sqrt(xi * yi)))
  * 
- * @author Adam Sargent
- * @version 1.0
+ * @author David Berdik
+ * @version 2.0
  */
 
 public class BhattacharyyaDistance extends DistanceFunction {
@@ -36,16 +36,13 @@ public boolean showInGUI() {
 	@Override
 	public double distance(Histogram unknownHistogram, Histogram knownHistogram)
 			throws DistanceCalculationException {
-
 		Set<Event> events = Sets.union(unknownHistogram.uniqueEvents(), knownHistogram.uniqueEvents());
+		double distance = 0.0;
 
-		double distance = 0.0, sumNumer = 0.0, sumDenom = 0.0;
-
-		for(Event event: events){
-			sumNumer += Math.sqrt(Math.abs(unknownHistogram.relativeFrequency(event) - knownHistogram.relativeFrequency(event)));
-			sumDenom += unknownHistogram.relativeFrequency(event);
-		}
-		distance = Math.abs(Math.log(sumNumer / sumDenom));
+		for(Event event : events)
+			distance += Math.sqrt(unknownHistogram.relativeFrequency(event) * knownHistogram.relativeFrequency(event));
+		
+		distance = -Math.log(distance);
 		return distance;
 	}
 

unittests/com/jgaap/distances/BhattacharyyaDistanceTest.java

@@ -19,21 +19,17 @@ public void testDistance()
 		EventSet set1 = new EventSet();
 		EventSet set2 = new EventSet();
 		Vector<Event> test1 = new Vector<Event>();
-		test1.add(new Event("one", null));
-		test1.add(new Event("two", null));
-		test1.add(new Event("three", null));
-		test1.add(new Event("four", null));
-		test1.add(new Event("five", null));
-		test1.add(new Event("six", null));
-		test1.add(new Event("seven", null));
-		test1.add(new Event("eight", null));
-		test1.add(new Event("nine", null));
-		test1.add(new Event("ten", null));			
+		test1.add(new Event("Lorem", null));
+		test1.add(new Event("Lorem", null));
+		test1.add(new Event("ipsum", null));
+		test1.add(new Event("ipsum", null));
+		test1.add(new Event("ipsum", null));
+		test1.add(new Event("ipsum", null));
+		test1.add(new Event("ipsum", null));
 		set1.addEvents(test1);
 		set2.addEvents(test1);
 		double result = new BhattacharyyaDistance().distance(new EventMap(set1), new EventMap(set2));
-		assertTrue(Double.isInfinite(result));
-		
+		assertTrue(result == 0);
 
 		set2 = new EventSet();
 		Vector<Event> test2 = new Vector<Event>();
@@ -49,7 +45,7 @@ public void testDistance()
 		test2.add(new Event("10", null));
 		set2.addEvents(test2);
 		result = new BhattacharyyaDistance().distance(new EventMap(set1), new EventMap(set2));
-		assertTrue(DistanceTestHelper.inRange(result, Math.log(20 * Math.sqrt(.1)), 0.0000000001));
+		assertTrue(Double.isInfinite(result));
 	}
 
 }