[GitHub] [druid] gianm commented on a change in pull request #9308: Add MemoryOpenHashTable, a table similar to ByteBufferHashTable.

[GitBox <gi...@apache.org>]

gianm commented on a change in pull request #9308: Add MemoryOpenHashTable, a table similar to ByteBufferHashTable.
URL: https://github.com/apache/druid/pull/9308#discussion_r374972811
 
 

 ##########
 File path: processing/src/main/java/org/apache/druid/query/groupby/epinephelinae/collection/HashTableUtils.java
 ##########
 @@ -0,0 +1,162 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.druid.query.groupby.epinephelinae.collection;
+
+import org.apache.datasketches.memory.Memory;
+
+public class HashTableUtils
+{
+  private HashTableUtils()
+  {
+    // No instantiation.
+  }
+
+  /**
+   * Computes the previous power of two less than or equal to a given "n".
+   *
+   * The integer should be between 1 (inclusive) and {@link Integer#MAX_VALUE} for best results. Other parameters will
+   * return {@link Integer#MIN_VALUE}.
+   */
+  public static int previousPowerOfTwo(final int n)
+  {
+    if (n > 0) {
+      return Integer.highestOneBit(n);
+    } else {
+      return Integer.MIN_VALUE;
+    }
+  }
+
+  /**
+   * Compute a simple, fast hash code of some memory range.
+   *
+   * @param memory   a region of memory
+   * @param position position within the memory region
+   * @param length   length of memory to hash, starting at the position
+   */
+  public static int hashMemory(final Memory memory, final long position, final int length)
+  {
+    // Special cases for small, common key sizes to speed them up: e.g. one int key, two int keys, one long key, etc.
+    // The plus-one sizes (9, 13) are for nullable dimensions. The specific choices of special cases were chosen based
+    // on benchmarking (see MemoryBenchmark) on a Skylake-based cloud instance.
+
+    switch (length) {
+      case 4:
+        return 31 + memory.getInt(position);
+
+      case 8:
+        return 31 * (31 + memory.getInt(position)) + memory.getInt(position + Integer.BYTES);
 
 Review comment:
   I was doing the same kind of calculation that `Arrays.hashCode` would do:
   
   ```java
           int result = 1;
           for (int element : a)
               result = 31 * result + element;
   ```

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