statefun-flink/statefun-flink-common/src/main/java/org/apache/flink/statefun/flink/common/protopath/ProtobufDynamicMessageLens.java

/*
 * 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.flink.statefun.flink.common.protopath;

import com.google.protobuf.Message;
import java.util.List;
import java.util.function.Function;

final class ProtobufDynamicMessageLens implements Function<Message, Object> {
  private final PathFragmentDescriptor[] path;
  private final PathFragmentDescriptor value;

  ProtobufDynamicMessageLens(List<PathFragmentDescriptor> path) {
    this.path = path.subList(0, path.size() - 1).toArray(new PathFragmentDescriptor[0]);
    this.value = path.get(path.size() - 1);
  }

  @Override
  public Object apply(Message message) {
    message = traverseToTheLastMessage(message);
    return value.value(message);
  }

  /**
   * Traverse the path from root to the last nested message. At each traversed depth follow the next
   * FiledDescriptor specified in descriptorPath for that depth. The returned message would be the
   * last Message which contains the desired value. For example the path defined by: {@code .a.b.c}
   * would result with {@code b} returned.
   */
  private Message traverseToTheLastMessage(Message root) {
    for (PathFragmentDescriptor p : path) {
      root = (Message) p.value(root);
    }
    return root;
  }
}