#185

Author: cicirello

src/main/java/org/cicirello/permutations/distance/WeightedKendallTauDistance.java

@@ -100,7 +100,7 @@ public double distancef(Permutation p1, Permutation p2) {
 			w[arrayP2[i]] = weights[p2.get(i)];
 		}
 
-		return countWeightedInversions(arrayP2, w);
+		return countWeightedInversions(arrayP2, w, 0, arrayP2.length-1);
 	}
 
 	/**
@@ -115,15 +115,21 @@ public double maxf(int length) {
 		return maxDistance;
 	}
 
-	private double countWeightedInversions(int[] array, double[] w) {
-		if (array.length <= 1) return 0;
-		int m = array.length >> 1;
-		int[] left = Arrays.copyOfRange(array, 0, m);
-		int[] right = Arrays.copyOfRange(array, m, array.length);
-		double weightedCount = countWeightedInversions(left, w) + countWeightedInversions(right, w);
+	private double countWeightedInversions(int[] array, double[] w, int first, int last) {
+		if (last <= first) {
+			return 0;
+		}
+		int m = (first + last) >> 1;
+		return countWeightedInversions(array, w, first, m) + countWeightedInversions(array, w, m+1, last) + merge(array, w, first, m+1, last+1);
+	}
+	
+	private double merge(int[] array, double[] w, int first, int midPlus, int lastPlus) {
+		int[] left = Arrays.copyOfRange(array, first, midPlus);
+		int[] right = Arrays.copyOfRange(array, midPlus, lastPlus);
 		int i = 0;
 		int j = 0;
-		int k = 0;
+		int k = first;
+		double weightedCount = 0;
 		while (i < left.length && j < right.length) {
 			if (left[i] < right[j]) {
 				array[k] = left[i];