Change calculation for baseline divergence. (#598)

Change the calculation for baseline divergence: 0.0 means no difference, 1.0 means all are different.

Author: huizhilu

helix-core/src/main/java/org/apache/helix/controller/rebalancer/util/ResourceUsageCalculator.java

@@ -81,7 +81,9 @@ public static Map<String, Integer> getResourceUsage(ResourcesStateMap resourceAs
    *       }
    *    }
    * }
-   * baseline divergence = (matched: 1) / (total(matched + doesn't match): 3) = 1/3 ~= 0.333
+   * baseline divergence = (doesn't match: 2) / (total(matched + doesn't match): 3) = 2/3 ~= 0.66667
+   * If divergence == 1.0, all are different(no match); divergence == 0.0, no difference.
+   *
    * @param baseline baseline assignment
    * @param bestPossibleAssignment best possible assignment
    * @return double value range at [0.0, 1.0]
@@ -137,7 +139,7 @@ public static double measureBaselineDivergence(Map<String, ResourceAssignment> b
       }
     }
 
-    return numTotalBestPossibleReplicas == 0 ? 0.0d
-        : (double) numMatchedReplicas / (double) numTotalBestPossibleReplicas;
+    return numTotalBestPossibleReplicas == 0 ? 1.0d
+        : (1.0d - (double) numMatchedReplicas / (double) numTotalBestPossibleReplicas);
   }
 }

helix-core/src/test/java/org/apache/helix/controller/rebalancer/util/TestResourceUsageCalculator.java

@@ -45,19 +45,19 @@ public void testMeasureBaselineDivergence(Map<String, Map<String, Map<String, St
 
     // Empty best possible assignment.
     Assert.assertEquals(ResourceUsageCalculator
-        .measureBaselineDivergence(baselineAssignment, Collections.emptyMap()), 0.0d);
+        .measureBaselineDivergence(baselineAssignment, Collections.emptyMap()), 1.0d);
     // Empty baseline assignment.
     Assert.assertEquals(ResourceUsageCalculator
-        .measureBaselineDivergence(Collections.emptyMap(), noMatchBestPossibleAssignment), 0.0d);
+        .measureBaselineDivergence(Collections.emptyMap(), noMatchBestPossibleAssignment), 1.0d);
 
     Assert.assertEquals(ResourceUsageCalculator
-        .measureBaselineDivergence(baselineAssignment, noMatchBestPossibleAssignment), 0.0d);
+        .measureBaselineDivergence(baselineAssignment, noMatchBestPossibleAssignment), 1.0d);
     Assert.assertEquals(ResourceUsageCalculator
             .measureBaselineDivergence(baselineAssignment, someMatchBestPossibleAssignment),
-        (double) 1 / (double) 3);
+        (1.0d - (double) 1 / (double) 3));
     Assert.assertEquals(
         ResourceUsageCalculator.measureBaselineDivergence(baselineAssignment, baselineAssignment),
-        1.0d);
+        0.0d);
   }
 
   private Map<String, ResourceAssignment> buildResourceAssignment(

helix-core/src/test/java/org/apache/helix/controller/rebalancer/waged/TestWagedRebalancerMetrics.java

@@ -138,7 +138,7 @@ public void testWagedRebalanceMetrics()
     // Wait for asyncReportBaselineDivergenceGauge to complete and verify.
     Assert.assertTrue(TestHelper.verify(() -> (double) metricCollector.getMetric(
         WagedRebalancerMetricCollector.WagedRebalancerMetricNames.BaselineDivergenceGauge.name(),
-        RatioMetric.class).getLastEmittedMetricValue() == 1.0d, TestHelper.WAIT_DURATION));
+        RatioMetric.class).getLastEmittedMetricValue() == 0.0d, TestHelper.WAIT_DURATION));
   }
 
   @Override