[GitHub] [iceberg] rdblue commented on a change in pull request #3966: Core: Adds Utility Class for Implementing ZOrdering

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rdblue commented on a change in pull request #3966:
URL: https://github.com/apache/iceberg/pull/3966#discussion_r800250355



##########
File path: core/src/main/java/org/apache/iceberg/util/ZOrderByteUtils.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.iceberg.util;
+
+import java.nio.ByteBuffer;
+import java.util.Arrays;
+
+/**
+ * Within Z-Ordering the byte representations of objects being compared must be ordered,
+ * this requires several types to be transformed when converted to bytes. The goal is to
+ * map object's whose byte representation are not lexicographically ordered into representations
+ * that are lexicographically ordered.
+ * Most of these techniques are derived from
+ * https://aws.amazon.com/blogs/database/z-order-indexing-for-multifaceted-queries-in-amazon-dynamodb-part-2/
+ *
+ * Some implementation is taken from
+ * https://github.com/apache/hbase/blob/master/hbase-common/src/main/java/org/apache/hadoop/hbase/util/OrderedBytes.java
+ */
+public class ZOrderByteUtils {
+
+  private ZOrderByteUtils() {
+
+  }
+
+  /**
+   * Signed ints do not have their bytes in magnitude order because of the sign bit.
+   * To fix this, flip the sign bit so that all negatives are ordered before positives. This essentially
+   * shifts the 0 value so that we don't break our ordering when we cross the new 0 value.
+   */
+  public static byte[] intToOrderedBytes(int val) {
+    ByteBuffer bytes = ByteBuffer.allocate(Integer.BYTES);

Review comment:
       I'm okay with this if you want to get this through quickly, but in the long term I think we should avoid the allocation cost. What I'd recommend since this is a util method is to add a `reuse` argument:
   
   ```java
     // this should probably live in the ByteBuffers util class
     public static ByteBuffer reuseOrAllocate(ByteBuffer reuse, int length) {
       if (reuse.hasArray() && reuse.arrayOffset() == 0 && reuse.capacity() == length) {
         reuse.position(0);
         reuse.limit(length);
         return reuse;
       } else {
         return ByteBuffer.allocate(length);
       }
     }
   
     public static ByteBuffer intToOrderedBytes(int val, ByteBuffer reused) {
       ByteBuffer bytes = reuseOrAllocate(reused, 4);
       bytes.putInt(val ^ 0x80000000);
       return bytes;
     }
   ```
   
   Then the caller can always call `ByteBuffer.array()` to get the underlying bytes, but it can keep reusing the space:
   ```java
     v1Bytes = intToOrderedBytes(v1, v1Bytes);
     v2Bytes = intToOrderedBytes(v2, v2Bytes);
     zorderBytes = zorder(v1Bytes, v2Bytes, zorderBytes);
   ```




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