GGraziadei commented on code in PR #16827: URL: https://github.com/apache/iceberg/pull/16827#discussion_r3692181620
########## core/src/main/java/org/apache/iceberg/util/HilbertByteUtils.java: ########## @@ -0,0 +1,140 @@ +/* + * 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 org.apache.iceberg.relocated.com.google.common.base.Preconditions; + +/** + * Maps a set of columns (each already converted to fixed-width, lexicographically-ordered unsigned + * bytes by {@link ZOrderByteUtils}) onto a single byte array whose unsigned big-endian + * lexicographic ordering follows the multi-dimensional Hilbert space-filling curve. + * + * <p>Unlike Z-ordering, the Hilbert transform requires every dimension to contribute the same + * number of bits, so each column is read to a fixed {@code bitsPerColumn} precision. + * + * <p>The transform is the standard "axes to transposed Hilbert index" algorithm from J. Skilling, + * "Programming the Hilbert curve" (2004); the transposed index is then serialized to a scalar with + * {@link ZOrderByteUtils#interleaveBits}. + */ +public class HilbertByteUtils { + + private HilbertByteUtils() {} + + /** See {@link #hilbertIndex(byte[][], int, ByteBuffer)}. */ + public static byte[] hilbertIndex(byte[][] columnsBinary, int bitsPerColumn) { + int outputBytes = columnsBinary.length * (bitsPerColumn / 8); + return hilbertIndex(columnsBinary, bitsPerColumn, ByteBuffer.allocate(outputBytes)); + } + + /** + * Compute the Hilbert index for the given columns. + * + * @param columnsBinary one ordered-byte array per column; each must be at least {@code + * bitsPerColumn / 8} bytes long (only the leading bytes are used) + * @param bitsPerColumn bits taken from each column; a positive multiple of 8, no greater than 64 + * @param reuse a buffer with capacity at least {@code numColumns * bitsPerColumn / 8} + * @return the Hilbert index, of length {@code numColumns * bitsPerColumn / 8} + */ + public static byte[] hilbertIndex(byte[][] columnsBinary, int bitsPerColumn, ByteBuffer reuse) { + Preconditions.checkArgument( + bitsPerColumn > 0 && bitsPerColumn % 8 == 0, + "Hilbert bitsPerColumn must be a positive multiple of 8, was %s", + bitsPerColumn); + Preconditions.checkArgument( + bitsPerColumn <= 64, + "Hilbert bitsPerColumn must be no greater than 64, was %s", + bitsPerColumn); + Preconditions.checkArgument( + columnsBinary.length > 0, "Cannot compute a Hilbert index for zero columns"); + + int numColumns = columnsBinary.length; + int bytesPerColumn = bitsPerColumn / 8; + + long[] transpose = new long[numColumns]; + for (int i = 0; i < numColumns; i++) { + Preconditions.checkArgument( + columnsBinary[i].length >= bytesPerColumn, + "Column %s contributes %s bytes but %s are required", + i, + columnsBinary[i].length, + bytesPerColumn); + transpose[i] = readBigEndian(columnsBinary[i], bytesPerColumn); + } + + axesToTranspose(transpose, bitsPerColumn); + + byte[][] transposedBytes = new byte[numColumns][bytesPerColumn]; + for (int i = 0; i < numColumns; i++) { + writeBigEndian(transpose[i], transposedBytes[i], bytesPerColumn); + } + + return ZOrderByteUtils.interleaveBits(transposedBytes, numColumns * bytesPerColumn, reuse); + } + + // please refer to the paper Review Comment: Hi Russell, thanks for checking the code. I am adding the reference to J. Skilling, "Programming the Hilbert curve" (AIP Conf. Proc. 707, 381, 2004; https://doi.org/10.1063/1.1751381) -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
