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

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emkornfield commented on a change in pull request #3966:
URL: https://github.com/apache/iceberg/pull/3966#discussion_r800308967



##########
File path: core/src/test/java/org/apache/iceberg/util/TestZOrderByteUtil.java
##########
@@ -0,0 +1,260 @@
+/*
+ * 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.util.Arrays;
+import java.util.Random;
+import org.apache.iceberg.relocated.com.google.common.primitives.UnsignedBytes;
+import org.apache.iceberg.types.Types;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class TestZOrderByteUtil {
+  private static final byte IIIIIIII = (byte) 255;
+  private static final byte IOIOIOIO = (byte) 170;
+  private static final byte OIOIOIOI = (byte) 85;
+  private static final byte OOOOIIII = (byte) 15;
+  private static final byte OOOOOOOI = (byte) 1;
+  private static final byte OOOOOOOO = (byte) 0;
+
+  private static final int NUM_TESTS = 100000;
+
+  private final Random random = new Random(42);
+
+  private String bytesToString(byte[] bytes) {
+    StringBuilder result = new StringBuilder();
+    for (byte b : bytes) {
+      result.append(String.format("%8s", Integer.toBinaryString(b & 0xFF)).replace(' ', '0'));
+    }
+    return result.toString();
+  }
+
+  /**
+   * Returns a non-0 length byte array
+   */
+  private byte[]  generateRandomBytes() {
+    int length = Math.abs(random.nextInt(100) + 1);
+    byte[] result = new byte[length];
+    random.nextBytes(result);
+    return result;
+  }
+
+  /**
+   * Test method to ensure correctness of byte interleaving code
+   */
+  private String interleaveStrings(String[] strings) {
+    StringBuilder result = new StringBuilder();
+    int totalLength = Arrays.stream(strings).mapToInt(String::length).sum();
+    int substringIndex = 0;
+    int characterIndex = 0;
+    while (characterIndex < totalLength) {
+      for (String str : strings) {
+        if (substringIndex < str.length()) {
+          result.append(str.charAt(substringIndex));
+          characterIndex++;
+        }
+      }
+      substringIndex++;
+    }
+    return result.toString();
+  }
+
+  /**
+   * Compares the result of a string based interleaving algorithm implemented above
+   * versus the binary bit-shifting algorithm used in ZOrderByteUtils. Either both
+   * algorithms are identically wrong or are both identically correct.
+   */
+  @Test
+  public void testInterleaveRandomExamples() {
+    for (int test = 0; test < NUM_TESTS; test++) {
+      int numByteArrays = Math.abs(random.nextInt(6)) + 1;
+      byte[][] testBytes =  new byte[numByteArrays][];
+      String[] testStrings = new String[numByteArrays];
+      for (int byteIndex = 0;  byteIndex < numByteArrays; byteIndex++) {
+        testBytes[byteIndex] = generateRandomBytes();
+        testStrings[byteIndex] = bytesToString(testBytes[byteIndex]);
+      }
+      byte[] byteResult = ZOrderByteUtils.interleaveBits(testBytes);
+      String byteResultAsString = bytesToString(byteResult);
+
+      String stringResult = interleaveStrings(testStrings);
+
+      Assert.assertEquals("String interleave didn't match byte interleave", stringResult, byteResultAsString);
+    }
+  }
+
+  @Test
+  public void testInterleaveEmptyBits() {
+    byte[][] test = new byte[4][10];
+    byte[] expected = new byte[40];
+
+    Assert.assertArrayEquals("Should combine empty arrays",
+        expected, ZOrderByteUtils.interleaveBits(test));
+  }
+
+  @Test
+  public void testInterleaveFullBits() {
+    byte[][] test = new byte[4][];
+    test[0] = new byte[]{IIIIIIII, IIIIIIII};
+    test[1] = new byte[]{IIIIIIII};
+    test[2] = new byte[0];
+    test[3] = new byte[]{IIIIIIII, IIIIIIII, IIIIIIII};
+    byte[] expected = new byte[]{IIIIIIII, IIIIIIII, IIIIIIII, IIIIIIII, IIIIIIII, IIIIIIII};
+
+    Assert.assertArrayEquals("Should combine full arrays",
+        expected, ZOrderByteUtils.interleaveBits(test));
+  }
+
+  @Test
+  public void testInterleaveMixedBits() {

Review comment:
       > Result is going to be: 0b1010101010101010101010101010101011111111. All values of v1 interleave with just the upper bits of v2. Is that the intended behavior?
   
   nit: I think the result fails to account for `*ToBytes` calls which flip the first leading bits.  Which should make the byte ordering similar to strings
   
   > This may be handy when trying to handle magnitude problems, assuming that the magnitude is always reflected in the type. But you could easily have a case where v1 and v2 are both in the range of 0x0000 to 0xFFFF and then this behavior would not actually produce a useful z-order.
   
   Not sure if the nit above addresses your concerns and what exactly you mean by not a useful z-order, could you expand on them?  My understanding is that as long as the input bytes that get interleaved have an equivalent lexicographic order to the original inputs it is a z-order and should maintain the clustering properties z-orders provide, if you then compare the output bytes lexicographically.   The only other principled option I could see here would be to zero pad all numeric values to the same number of bytes, but without thinking too deeply about it I think this just takes extra space.




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