roee88 commented on a change in pull request #11067: URL: https://github.com/apache/arrow/pull/11067#discussion_r726479595
########## File path: java/ffi/src/main/java/org/apache/arrow/ffi/Metadata.java ########## @@ -0,0 +1,102 @@ +/* + * 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.arrow.ffi; + +import static org.apache.arrow.ffi.NativeUtil.NULL; +import static org.apache.arrow.util.Preconditions.checkState; + +import java.nio.ByteBuffer; +import java.nio.ByteOrder; +import java.nio.charset.StandardCharsets; +import java.util.ArrayList; +import java.util.HashMap; +import java.util.List; +import java.util.Map; + +import org.apache.arrow.memory.ArrowBuf; +import org.apache.arrow.memory.BufferAllocator; +import org.apache.arrow.memory.util.MemoryUtil; + +/** + * Encode and decode metadata. + */ +final class Metadata { + + private Metadata() { + } + + static ArrowBuf encode(BufferAllocator allocator, Map<String, String> metadata) { + if (metadata == null || metadata.size() == 0) { + return null; + } + + List<byte[]> buffers = new ArrayList<>(metadata.size() * 2); + int totalSize = 4 + metadata.size() * 8; // number of key/value pairs + buffer length fields + for (Map.Entry<String, String> entry : metadata.entrySet()) { + byte[] keyBuffer = entry.getKey().getBytes(StandardCharsets.UTF_8); + byte[] valueBuffer = entry.getValue().getBytes(StandardCharsets.UTF_8); + totalSize += keyBuffer.length; + totalSize += valueBuffer.length; + buffers.add(keyBuffer); + buffers.add(valueBuffer); + } + + ArrowBuf result = allocator.buffer(totalSize); + ByteBuffer writer = MemoryUtil.directBuffer(result.memoryAddress(), totalSize).order(ByteOrder.nativeOrder()); + writer.putInt(metadata.size()); + for (byte[] buffer : buffers) { + writer.putInt(buffer.length); + writer.put(buffer); + } + return result.slice(0, totalSize); + } + + static Map<String, String> decode(long bufferAddress) { + if (bufferAddress == NULL) { + return null; + } + + ByteBuffer reader = MemoryUtil.directBuffer(bufferAddress, Integer.MAX_VALUE).order(ByteOrder.nativeOrder()); + + int size = reader.getInt(); + checkState(size >= 0, "Metadata size must not be negative"); + if (size == 0) { + return null; + } + + Map<String, String> result = new HashMap<>(size); + for (int i = 0; i < size; i++) { + String key = readString(reader); + String value = readString(reader); + result.put(key, value); + } + return result; + } + + private static String readString(ByteBuffer reader) { + int length = reader.getInt(); + checkState(length >= 0, "Metadata item length must not be negative"); + String result = ""; + if (length > 0) { + byte[] dst = new byte[length]; + reader.get(dst); + result = new String(dst, StandardCharsets.UTF_8); Review comment: Metadata in Arrow Java uses String keys and values and I think that java strings must represent text and have an encoding. Looking at the arrow specification the keys and values of custom_metadata are defined in flatbuffers as strings. Flatbuffers [docs](https://google.github.io/flatbuffers/md__schemas.html) says about strings: > string, which may only hold UTF-8 or 7-bit ASCII. For other text encodings or general binary data use vectors ([byte] or [ubyte]) instead. So we're good with UTF8 with exports from any arrow llibrary. I understand that a custom exporter can be a different story but would highly recommend to change the C data interface spec here to match the arrow specification because there is no reliable implementation. -- 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]
