/*
    * 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
    *
    *      https://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.commons.lang3.concurrent;
  
  import static org.junit.jupiter.api.Assertions.assertFalse;
  import static org.junit.jupiter.api.Assertions.assertNotNull;
  import static org.junit.jupiter.api.Assertions.assertTimeout;
  
  import java.lang.management.ManagementFactory;
  import java.lang.management.ThreadMXBean;
  import java.time.Duration;
  import java.util.ArrayList;
  import java.util.List;
  import java.util.concurrent.CountDownLatch;
  import java.util.concurrent.ExecutorService;
  import java.util.concurrent.Executors;
  import java.util.concurrent.Future;
  import java.util.concurrent.atomic.AtomicLong;
  
  import org.junit.jupiter.api.Test;
  
  /**
   * AtomicSafeInitializer.get() spins in a while-loop without Thread.yield() or LockSupport.parkNanos() when the CAS fails (another thread is initializing).
   *
   * <p>
   * Concurrent callers who lose the CAS busy-wait for the duration of initialize(), burning CPU proportional to init latency * thread count. A slow initializer
   * combined with many concurrent callers use more CPU than it can.
   * </p>
   *
   * <p>
   * This test measures CPU time spent in spinning threads during a 100 ms init. Pre-patch: spinning threads consume significant CPU. Post-patch: spinning threads
   * yield, keeping CPU near zero while waiting.
   * </p>
   */
  class AtomicSafeInitializerInitTest {
  
      /** Slow initializer: sleeps 100 ms to widen the spin window. */
      private static final int INIT_MS = 100;
      private static final int SPINNER_THREADS = 8;
  
      private static long threadCpuTimeNanos() {
          final ThreadMXBean mx = ManagementFactory.getThreadMXBean();
          return mx.isCurrentThreadCpuTimeSupported() ? mx.getCurrentThreadCpuTime() : 0;
      }
  
      @Test
      void testSpinningThreadsYieldDuringSlowInit() throws Exception {
          final CountDownLatch startLatch = new CountDownLatch(1);
          final AtomicLong totalCpuNanos = new AtomicLong();
          final AtomicSafeInitializer<String> initializer = AtomicSafeInitializer.<String>builder().setInitializer(() -> {
              Thread.sleep(INIT_MS);
              return "done";
          }).get();
          final ExecutorService exec = Executors.newFixedThreadPool(SPINNER_THREADS + 1);
          try {
              final List<Future<?>> futures = new ArrayList<>();
              for (int i = 0; i < SPINNER_THREADS; i++) {
                  futures.add(exec.submit(() -> {
                      try {
                          startLatch.await();
                          final long cpuBeforeNanos = threadCpuTimeNanos();
                          initializer.get();
                          totalCpuNanos.addAndGet(threadCpuTimeNanos() - cpuBeforeNanos);
                      } catch (final Exception e) {
                          Thread.currentThread().interrupt();
                      }
                  }));
              }
              startLatch.countDown();
              for (final Future<?> f : futures) {
                  f.get();
              }
          } finally {
              exec.shutdown();
          }
          assertNotNull(initializer.get());
          // Post-patch: CPU consumed by spinner threads during 100 ms init must be
          // significantly less than INIT_MS per thread. We allow 50 ms total CPU
          // across all spinner threads (vs. ~800 ms if spinning at 100%).
          // This threshold is conservative, a yielding implementation uses ~0 ms.
          final long cpuMs = totalCpuNanos.get() / 1_000_000;
          // Re-express as a blocking assertion: if cpuMs > threshold, fail.
          assertTimeout(Duration.ofMillis(INIT_MS * SPINNER_THREADS / 4), () -> assertFalse(cpuMs > INIT_MS * SPINNER_THREADS / 4,
                  () -> "Spinner threads consumed " + cpuMs + " ms CPU during " + INIT_MS + " ms init — missing Thread.yield() in get() spin loop"));
     }
 }