import java.io.ObjectStreamField;

import java.util.concurrent.atomic.AtomicInteger;

import java.util.concurrent.atomic.AtomicLong;

    /*

     * This class implements the java.util.Random API (and subclasses

     * Random) using a single static instance that accesses random

     * number state held in class Thread (primarily, field

     * threadLocalRandomSeed). In doing so, it also provides a home

     * for managing package-private utilities that rely on exactly the

     * same state as needed to maintain the ThreadLocalRandom

     * instances. We leverage the need for an initialization flag

     * field to also use it as a "probe" -- a self-adjusting thread

     * hash used for contention avoidance, as well as a secondary

     * simpler (xorShift) random seed that is conservatively used to

     * avoid otherwise surprising users by hijacking the

     * ThreadLocalRandom sequence.  The dual use is a marriage of

     * convenience, but is a simple and efficient way of reducing

     * application-level overhead and footprint of most concurrent

     * programs.

     *

     * Because this class is in a different package than class Thread,

     * field access methods must use Unsafe to bypass access control

     * rules.  The base functionality of Random methods is

     * conveniently isolated in method next(bits), that just reads and

     * writes the Thread field rather than its own field. However, to

     * conform to the requirements of the Random constructor, during

     * construction, the common static ThreadLocalRandom must maintain

     * initialization and value fields, mainly for the sake of

     * disabling user calls to setSeed while still allowing a call

     * from constructor.  For serialization compatibility, these

     * fields are left with the same declarations as used in the

     * previous ThreadLocal-based version of this class, that used

     * them differently. Note that serialization is completely

     * unnecessary because there is only a static singleton. But these

     * mechanics still ensure compatibility across versions.

     *

     * Per-instance initialization is similar to that in the no-arg

     * Random constructor, but we avoid correlation among not only

     * initial seeds of those created in different threads, but also

     * those created using class Random itself; while at the same time

     * not changing any statistical properties.  So we use the same

     * underlying multiplicative sequence, but start the sequence far

     * away from the base version, and then merge (xor) current time

     * and per-thread probe bits to generate initial values.

     *

     * The nextLocalGaussian ThreadLocal supports the very rarely used

     * nextGaussian method by providing a holder for the second of a

     * pair of them. As is true for the base class version of this

     * method, this time/space tradeoff is probably never worthwhile,

     * but we provide identical statistical properties.

     */

    private static final int PROBE_INCREMENT = 0x61c88647;

    /** Generates the basis for per-thread initial seed values */

    private static final AtomicLong seedGenerator =

        new AtomicLong(1269533684904616924L);

    /** Generates per-thread initialization/probe field */

    private static final AtomicInteger probeGenerator =

        new AtomicInteger(0xe80f8647);

    /** Rarely-used holder for the second of a pair of Gaussians */

    private static final ThreadLocal<Double> nextLocalGaussian =

        new ThreadLocal<Double>();

    /*

     * Field used only during singleton initialization

    boolean initialized; // true when constructor completes

    /** Constructor used only for static singleton */

    private ThreadLocalRandom() {

        initialized = true; // false during super() call

    }

    /** The common ThreadLocalRandom */

    static final ThreadLocalRandom instance = new ThreadLocalRandom();

     * Initialize Thread fields for the current thread.  Called only

     * when Thread.threadLocalRandomProbe is zero, indicating that a

     * thread local seed value needs to be generated. Note that even

     * though the initialization is purely thread-local, we need to

     * rely on (static) atomic generators to initialize the values.

    static final void localInit() {

        int p = probeGenerator.getAndAdd(PROBE_INCREMENT);

        int probe = (p == 0) ? 1 : p; // skip 0

        long current, next;

        do { // same sequence as j.u.Random but different initial value

            current = seedGenerator.get();

            next = current * 181783497276652981L;

        } while (!seedGenerator.compareAndSet(current, next));

        long r = next ^ ((long)probe << 32) ^ System.nanoTime();

        Thread t = Thread.currentThread();

        UNSAFE.putLong(t, SEED, r);

        UNSAFE.putInt(t, PROBE, probe);

        if (UNSAFE.getInt(Thread.currentThread(), PROBE) == 0)

            localInit();

        return instance;

        if (initialized) // allow call from super() constructor

        Thread t; long r; // read and update per-thread seed

        UNSAFE.putLong

            (t = Thread.currentThread(), SEED,

             r = (UNSAFE.getLong(t, SEED) * multiplier + addend) & mask);

        return (int) (r >>> (48-bits));

    public double nextGaussian() {

        // Use nextLocalGaussian instead of nextGaussian field

        Double d = nextLocalGaussian.get();

        if (d != null) {

            nextLocalGaussian.set(null);

            return d.doubleValue();

        }

        double v1, v2, s;

        do {

            v1 = 2 * nextDouble() - 1; // between -1 and 1

            v2 = 2 * nextDouble() - 1; // between -1 and 1

            s = v1 * v1 + v2 * v2;

        } while (s >= 1 || s == 0);

        double multiplier = StrictMath.sqrt(-2 * StrictMath.log(s)/s);

        nextLocalGaussian.set(new Double(v2 * multiplier));

        return v1 * multiplier;

    }

    // Within-package utilities

    /*

     * Descriptions of the usages of the methods below can be found in

     * the classes that use them. Briefly, a thread's "probe" value is

     * a non-zero hash code that (probably) does not collide with

     * other existing threads with respect to any power of two

     * collision space. When it does collide, it is pseudo-randomly

     * adjusted (using a Marsaglia XorShift). The nextSecondarySeed

     * method is used in the same contexts as ThreadLocalRandom, but

     * only for transient usages such as random adaptive spin/block

     * sequences for which a cheap RNG suffices and for which it could

     * in principle disrupt user-visible statistical properties of the

     * main ThreadLocalRandom if we were to use it.

     *

     * Note: Because of package-protection issues, versions of some

     * these methods also appear in some subpackage classes.

     */

    /**

     * Returns the probe value for the current thread without forcing

     * initialization. Note that invoking ThreadLocalRandom.current()

     * can be used to force initialization on zero return.

     */

    static final int getProbe() {

        return UNSAFE.getInt(Thread.currentThread(), PROBE);

    }

    /**

     * Pseudo-randomly advances and records the given probe value for the

     * given thread.

     */

    static final int advanceProbe(int probe) {

        probe ^= probe << 13;   // xorshift

        probe ^= probe >>> 17;

        probe ^= probe << 5;

        UNSAFE.putInt(Thread.currentThread(), PROBE, probe);

        return probe;

    }

    /**

     * Returns the pseudo-randomly initialized or updated secondary seed.

     */

    static final int nextSecondarySeed() {

        int r;

        Thread t = Thread.currentThread();

        if ((r = UNSAFE.getInt(t, SECONDARY)) != 0) {

            r ^= r << 13;   // xorshift

            r ^= r >>> 17;

            r ^= r << 5;

        }

        else if ((r = (int)UNSAFE.getLong(t, SEED)) == 0)

            r = 1; // avoid zero

        UNSAFE.putInt(t, SECONDARY, r);

        return r;

    }

    // Serialization support, maintains original persistent form.

    /**

     * @serialField rnd long

     * @serialField initialized boolean

     * @serialField pad0 long

     * @serialField pad1 long

     * @serialField pad2 long

     * @serialField pad3 long

     * @serialField pad4 long

     * @serialField pad5 long

     * @serialField pad6 long

     * @serialField pad7 long

     */

    private static final ObjectStreamField[] serialPersistentFields = {

            new ObjectStreamField("rnd", long.class),

            new ObjectStreamField("initialized", boolean.class),

            new ObjectStreamField("pad0", long.class),

            new ObjectStreamField("pad1", long.class),

            new ObjectStreamField("pad2", long.class),

            new ObjectStreamField("pad3", long.class),

            new ObjectStreamField("pad4", long.class),

            new ObjectStreamField("pad5", long.class),

            new ObjectStreamField("pad6", long.class),

            new ObjectStreamField("pad7", long.class) };

    /**

     * Saves the {@code ThreadLocalRandom} to a stream (that is, serializes it).

     */

    private void writeObject(java.io.ObjectOutputStream out)

        throws java.io.IOException {

        java.io.ObjectOutputStream.PutField fields = out.putFields();

        fields.put("rnd", 0L);

        fields.put("initialized", true);

        fields.put("pad0", 0L);

        fields.put("pad1", 0L);

        fields.put("pad2", 0L);

        fields.put("pad3", 0L);

        fields.put("pad4", 0L);

        fields.put("pad5", 0L);

        fields.put("pad6", 0L);

        fields.put("pad7", 0L);

        out.writeFields();

    }

    /**

     * Returns the {@link #current() current} thread's {@code ThreadLocalRandom}.

     */

    private Object readResolve() {

        return current();

    }

    // Unsafe mechanics

    private static final sun.misc.Unsafe UNSAFE;

    private static final long SEED;

    private static final long PROBE;

    private static final long SECONDARY;

    static {

        try {

            UNSAFE = sun.misc.Unsafe.getUnsafe();

            Class<?> tk = Thread.class;

            SEED = UNSAFE.objectFieldOffset

                (tk.getDeclaredField("threadLocalRandomSeed"));

            PROBE = UNSAFE.objectFieldOffset

                (tk.getDeclaredField("threadLocalRandomProbe"));

            SECONDARY = UNSAFE.objectFieldOffset

                (tk.getDeclaredField("threadLocalRandomSecondarySeed"));

        } catch (Exception e) {

            throw new Error(e);

        }

    }