CNDB-16243: Improve hash quality for small numbers even further

Author: Piotr Kołaczkowski

src/java/org/apache/cassandra/cache/ChunkCache.java

@@ -314,7 +314,7 @@ public int hashCode()
         {
             // Mix readerId and position into a single long using a large prime multiplier
             // This constant is a mixing constant derived from the Golden Ratio
-            long mixed = (Long.rotateLeft(readerId, 16) + Long.rotateLeft(position, 16)) * 0x9E3779B97F4A7C15L;
+            long mixed = (Long.rotateLeft(readerId, 16) + position) * 0x9E3779B97F4A7C15L;
 
             // Spread the bits (XOR-shift) to ensure high bits affect low bits
             mixed ^= (mixed >>> 32);

test/unit/org/apache/cassandra/cache/ChunkCacheKeyTest.java

@@ -27,6 +27,8 @@
 
 import org.junit.Test;
 
+import org.apache.cassandra.utils.Pair;
+
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotEquals;
@@ -199,15 +201,36 @@ public void testHashCodeDistribution() throws Exception
     {
         // Test that hash codes are reasonably distributed
         Set<Integer> hashes = new HashSet<>();
-        for (long i = 0; i < 1000; i++)
+        Set<ChunkCache.Key> keys = new HashSet<>();
+
+        for (long i = 0; i < 10000; i++)
         {
-            ChunkCache.Key key = createKey(i, i * 2);
+            ChunkCache.Key key = createKey(i * 2, i);
+            keys.add(key);
             hashes.add(key.hashCode());
         }
 
-        // We should have a good distribution - at least 90% unique hashes
-        assertTrue("Hash distribution is poor: " + hashes.size() + " unique hashes out of 1000",
-                   hashes.size() > 900);
+        testHashCollisions(hashes, keys);
+
+        hashes.clear();
+        keys.clear();
+        for (long i = 0; i < 100; i++)
+        {
+            for (long j = 0; j < 100; j++)
+            {
+                ChunkCache.Key key = createKey(i, j);
+                keys.add(key);
+                hashes.add(key.hashCode());
+            }
+        }
+
+        testHashCollisions(hashes, keys);
+    }
+
+    private static void testHashCollisions(Set<Integer> hashes, Set<ChunkCache.Key> keys)
+    {
+        assertTrue("Hash distribution is poor: " + hashes.size() + " unique hashes out of " + keys.size(),
+                   hashes.size() >= keys.size() * 0.99);
     }
 
     @Test