Include value count in hashcode

Author: JonasKunz

libs/exponential-histogram/src/main/java/org/elasticsearch/exponentialhistogram/ExponentialHistogram.java

@@ -102,6 +102,15 @@ public abstract class ExponentialHistogram implements Accountable {
      */
     public abstract double sum();
 
+    /**
+     * Returns the sum of the counts of all values represented by this histogram.
+     *
+     * @return the value count, guaranteed to be zero for empty histograms
+     */
+    public long valueCount() {
+        return zeroBucket().count() + positiveBuckets().valueCount() + negativeBuckets().valueCount();
+    }
+
     /**
      * Represents a bucket range of an {@link ExponentialHistogram}, either the positive or the negative range.
      */
@@ -163,8 +172,10 @@ public int hashCode() {
         int hash = scale();
         hash = 31 * hash + Double.hashCode(sum());
         hash = 31 * hash + zeroBucket().hashCode();
+        hash = 31 * hash + Long.hashCode(valueCount());
         // we intentionally don't include the hash of the buckets here, because that is likely expensive to compute
-        // we assume that the sum is a good enough differentiator for most use cases
+        // instead, we assume that the value count and sum are a good enough approximation in most cases to minimize collisions
+        // the value count is typically available as a cached value and doesn't involve iterating over all buckets
         return hash;
     }
 

libs/exponential-histogram/src/test/java/org/elasticsearch/exponentialhistogram/ExponentialHistogramEqualityTests.java

@@ -68,12 +68,6 @@ public void testEquality() {
     }
 
     public void testHashQuality() {
-        switch (modification) {
-            // we intentionally don't include the buckets in the hash, so skip them in tests
-            case POSITIVE_BUCKETS, NEGATIVE_BUCKETS:
-                return;
-        }
-
         ExponentialHistogram histo = randomHistogram();
         // of 10 tries, at least one should produce a different hash code
         for (int i = 0; i < 10; i++) {

x-pack/plugin/mapper-exponential-histogram/src/main/java/org/elasticsearch/xpack/exponentialhistogram/CompressedExponentialHistogram.java

@@ -59,6 +59,11 @@ public double sum() {
         return sum;
     }
 
+    @Override
+    public long valueCount() {
+        return valueCount;
+    }
+
     @Override
     public ExponentialHistogram.Buckets positiveBuckets() {
         return positiveBuckets;