/*
 * Written by Martin Buchholz with assistance from members of JCP
 * JSR-166 Expert Group and released to the public domain, as
 * explained at http://creativecommons.org/publicdomain/zero/1.0/
 */

/*
 * @test
 * @summary Stress test looks for lost unparks
 */

import static java.util.concurrent.TimeUnit.SECONDS;
import java.lang.management.ManagementFactory;
import java.lang.management.ThreadInfo;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.AtomicReferenceArray;
import java.util.concurrent.locks.LockSupport;

public final class ParkLoops {
    public static void main(String[] args) throws Exception {
        final int nThreads = 4; // must be power of two
        final int iters = 2_000_000;
        final int timeout = 3500;  // in seconds
        final ExecutorService pool = Executors.newCachedThreadPool();
        final AtomicReferenceArray<Thread> threads
            = new AtomicReferenceArray<>(nThreads);
        final CountDownLatch done = new CountDownLatch(nThreads);
        final Runnable parker = new Runnable() { public void run() {
            final SimpleRandom rng = new SimpleRandom();
            final Thread current = Thread.currentThread();
            for (int k = iters, j; k > 0; k--) {
                do {
                    j = rng.next() & (nThreads - 1);
                } while (!threads.compareAndSet(j, null, current));
                do {                    // handle spurious wakeups
                    LockSupport.park();
                } while (threads.get(j) == current);
            }
            done.countDown();
        }};
        final Runnable unparker = new Runnable() { public void run() {
            final SimpleRandom rng = new SimpleRandom();
            for (int n = 0; (n++ & 0xff) != 0 || done.getCount() > 0;) {
                int j = rng.next() & (nThreads - 1);
                Thread parker = threads.get(j);
                if (parker != null &&
                    threads.compareAndSet(j, parker, null)) {
                    LockSupport.unpark(parker);
                }
            }
        }};
        for (int i = 0; i < nThreads; i++) {
            pool.submit(parker);
            pool.submit(unparker);
        }
        try {
          if (!done.await(timeout, SECONDS)) {
            dumpAllStacks();
            throw new AssertionError("lost unpark");
          }
        } finally {
          pool.shutdown();
          pool.awaitTermination(10L, SECONDS);
        }
    }

    static void dumpAllStacks() {
        ThreadInfo[] threadInfos =
            ManagementFactory.getThreadMXBean().dumpAllThreads(true, true);
        for (ThreadInfo threadInfo : threadInfos) {
            System.err.print(threadInfo);
        }
    }

    /**
     * An actually useful random number generator, but unsynchronized.
     * Basically same as java.util.Random.
     */
    public static class SimpleRandom {
        private static final long multiplier = 0x5DEECE66DL;
        private static final long addend = 0xBL;
        private static final long mask = (1L << 48) - 1;
        static final AtomicLong seq = new AtomicLong(1);
        private long seed = System.nanoTime() + seq.getAndIncrement();

        public int next() {
            long nextseed = (seed * multiplier + addend) & mask;
            seed = nextseed;
            return ((int)(nextseed >>> 17)) & 0x7FFFFFFF;
        }
    }
}