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Elastic Inferred Spans extension for OpenTelemetry Java

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/*
 * Licensed to Elasticsearch B.V. under one or more contributor
 * license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright
 * ownership. Elasticsearch B.V. 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 co.elastic.otel.profiler;

public class ThreadMatcher {

  private final ThreadGroup systemThreadGroup;
  private Thread[] threads = new Thread[16];

  public ThreadMatcher() {
    ThreadGroup threadGroup = Thread.currentThread().getThreadGroup();
    while (threadGroup.getParent() != null) {
      threadGroup = threadGroup.getParent();
    }
    systemThreadGroup = threadGroup;
  }

  public  void forEachThread(
      NonCapturingPredicate predicate,
      S1 state1,
      NonCapturingConsumer consumer,
      S2 state2) {
    int count = systemThreadGroup.activeCount();
    do {
      int expectedArrayLength = count + (count / 2) + 1;
      if (threads.length < expectedArrayLength) {
        threads = new Thread[expectedArrayLength]; // slightly grow the array size
      }
      count = systemThreadGroup.enumerate(threads, true);
      // return value of enumerate() must be strictly less than the array size according to javadoc
    } while (count >= threads.length);

    for (int i = 0; i < count; i++) {
      Thread thread = threads[i];
      if (predicate.test(thread, state1)) {
        consumer.accept(thread, state2);
      }
      threads[i] = null;
    }
  }

  interface NonCapturingPredicate {
    boolean test(T t, S state);
  }

  interface NonCapturingConsumer {
    void accept(T t, S state);
  }
}