Close tdunning#169 Re-order terms in division to avoid periodic decim…

…al and rounding issues in Centroid#add, fixing out-of-order centroids

core/src/main/java/com/tdunning/math/stats/Centroid.java

@@ -81,7 +81,15 @@ public void add(double x, int w) {
             actualData.add(x);
         }
         count += w;
-        centroid += w * (x - centroid) / count;
+        // NOTE: The previous calculation was computed with `w * (x - centroid) / count;`.
+        //       Now we calculate a factor here for cases where `count` might be a
+        //       number like 3 that could lead to a repeating decimal and possible
+        //       rounding issues. e.g. `count` starts as `0` (zero) and `w` as `3`.
+        //       That could cause a calculation like `3 * (0.8046947099707735 - 0.0) / 3`.
+        //       Calculating this we would get `0.8046947099707736` (same error for `w`
+        //       equals to `6`, `11`, etc.)
+        double factor = (double) w / count;
+        centroid += factor * (x - centroid);
     }
 
     public double mean() {

core/src/test/java/com/tdunning/math/stats/TDigestTest.java

@@ -548,6 +548,15 @@ public void testSorted() {
         }
     }
 
+    /**
+     * Issue: https://github.com/tdunning/t-digest/issues/169
+     */
+    public void testCentroidAdd() {
+        double x = 0.8046947099707735;
+        Centroid centroid = new Centroid(x, 3);
+        assertEquals(x, centroid.mean(), 0.0);
+    }
+
     @Test
     public void testNaN() {
         final TDigest digest = factory().create();