Remove buckets from hashcode for performance and simplicity.

Author: JonasKunz

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

@@ -27,7 +27,6 @@
 import java.util.List;
 import java.util.OptionalLong;
 
-import static org.elasticsearch.exponentialhistogram.ExponentialHistogramUtils.bucketIteratorHash;
 import static org.elasticsearch.exponentialhistogram.ExponentialHistogramUtils.bucketIteratorsEqual;
 
 /**
@@ -152,8 +151,8 @@ public int hashCode() {
         int hash = scale();
         hash = 31 * hash + Double.hashCode(sum());
         hash = 31 * hash + zeroBucket().hashCode();
-        hash = 31 * hash + bucketIteratorHash(negativeBuckets().iterator());
-        hash = 31 * hash + bucketIteratorHash(positiveBuckets().iterator());
+        // 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
         return hash;
     }
 

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

@@ -73,14 +73,4 @@ static boolean bucketIteratorsEqual(BucketIterator a, BucketIterator b) {
         }
         return a.hasNext() == b.hasNext();
     }
-
-    static int bucketIteratorHash(BucketIterator it) {
-        int hash = 0;
-        while (it.hasNext()) {
-            hash = 31 * hash + Long.hashCode(it.peekIndex());
-            hash = 31 * hash + Long.hashCode(it.peekCount());
-            it.advance();
-        }
-        return hash;
-    }
 }

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

@@ -68,6 +68,12 @@ 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++) {