/*

 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.

 *

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/*

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 *

 * Written by Doug Lea 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/

 */

// package java.util.concurrent;

import java.io.ObjectStreamField;

import java.math.BigInteger;

// import java.util.Random;

import java.util.Spliterator;

import java.util.concurrent.atomic.AtomicInteger;

import java.util.concurrent.atomic.AtomicLong;

/**

 * A random number generator isolated to the current thread.  Like the

 * global {@link java.util.Random} generator used by the {@link

 * java.lang.Math} class, a {@code ThreadLocalRandom} is initialized

 * with an internally generated seed that may not otherwise be

 * modified. When applicable, use of {@code ThreadLocalRandom} rather

 * than shared {@code Random} objects in concurrent programs will

 * typically encounter much less overhead and contention.  Use of

 * {@code ThreadLocalRandom} is particularly appropriate when multiple

 * tasks (for example, each a {@link ForkJoinTask}) use random numbers

 * in parallel in thread pools.

 *

 * <p>Usages of this class should typically be of the form:

 * {@code ThreadLocalRandom.current().nextX(...)} (where

 * {@code X} is {@code Int}, {@code Long}, etc).

 * When all usages are of this form, it is never possible to

 * accidently share a {@code ThreadLocalRandom} across multiple threads.

 *

 * <p>This class also provides additional commonly used bounded random

 * generation methods.

 *

 * <p>Instances of {@code ThreadLocalRandom} are not cryptographically

 * secure.  Consider instead using {@link java.security.SecureRandom}

 * in security-sensitive applications. Additionally,

 * default-constructed instances do not use a cryptographically random

 * seed unless the {@linkplain System#getProperty system property}

 * {@code java.util.secureRandomSeed} is set to {@code true}.

 *

 * @since 1.7

 * @author Doug Lea

 */

public class ThreadLocalRandom extends Random {

    /*

     * 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.

     *

     * Even though this class subclasses java.util.Random, it uses the

     * same basic algorithm as java.util.SplittableRandom.  (See its

     * internal documentation for explanations, which are not repeated

     * here.)  Because ThreadLocalRandoms are not splittable

     * though, we use only a single 64bit gamma.

     *

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

     * field access methods use Unsafe to bypass access control rules.

     * To conform to the requirements of the Random superclass

     * constructor, the common static ThreadLocalRandom maintains an

     * "initialized" field for the sake of rejecting user calls to

     * setSeed while still allowing a call from constructor.  Note

     * that serialization is completely unnecessary because there is

     * only a static singleton.  But we generate a serial form

     * containing "rnd" and "initialized" fields to ensure

     * compatibility across versions.

     *

     * Implementations of non-core methods are mostly the same as in

     * SplittableRandom, that were in part derived from a previous

     * version of this class.

     *

     * 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.

     */

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

    private static final AtomicInteger probeGenerator =

        new AtomicInteger();

    /**

     * The next seed for default constructors.

     */

    private static final AtomicLong seeder = new AtomicLong(RngSupport.initialSeed());

    /**

     * The seed increment

     */

    private static final long GAMMA = 0x9e3779b97f4a7c15L;

    /**

     * The increment for generating probe values

     */

    private static final int PROBE_INCREMENT = 0x9e3779b9;

    /**

     * The increment of seeder per new instance

     */

    private static final long SEEDER_INCREMENT = 0xbb67ae8584caa73bL;

    // Constants from SplittableRandom

    private static final double DOUBLE_UNIT = 0x1.0p-53;  // 1.0  / (1L << 53)

    private static final float  FLOAT_UNIT  = 0x1.0p-24f; // 1.0f / (1 << 24)

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

    private static final ThreadLocal<Double> nextLocalGaussian =

        new ThreadLocal<Double>();

    private static long mix64(long z) {

        z = (z ^ (z >>> 33)) * 0xff51afd7ed558ccdL;

        z = (z ^ (z >>> 33)) * 0xc4ceb9fe1a85ec53L;

        return z ^ (z >>> 33);

    }

    private static int mix32(long z) {

        z = (z ^ (z >>> 33)) * 0xff51afd7ed558ccdL;

        return (int)(((z ^ (z >>> 33)) * 0xc4ceb9fe1a85ec53L) >>> 32);

    }

    /**

     * Field used only during singleton initialization.

     * True when constructor completes.

     */

    boolean initialized;

    /** 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.addAndGet(PROBE_INCREMENT);

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

        long seed = mix64(seeder.getAndAdd(SEEDER_INCREMENT));

        Thread t = Thread.currentThread();

        UNSAFE.putLong(t, SEED, seed);

        UNSAFE.putInt(t, PROBE, probe);

    }

    /**

     * Returns the current thread's {@code ThreadLocalRandom}.

     *

     * @return the current thread's {@code ThreadLocalRandom}

     */

    public static ThreadLocalRandom current() {

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

            localInit();

        return instance;

    }

    /**

     * Throws {@code UnsupportedOperationException}.  Setting seeds in

     * this generator is not supported.

     *

     * @throws UnsupportedOperationException always

     */

    public void setSeed(long seed) {

        // only allow call from super() constructor

        if (initialized)

            throw new UnsupportedOperationException();

    }

    final long nextSeed() {

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

        UNSAFE.putLong(t = Thread.currentThread(), SEED,

                       r = UNSAFE.getLong(t, SEED) + GAMMA);

        return r;

    }

    // We must define this (to override the definition inherited from

    // class Random), but we don't use it from within other methods.

    protected int next(int bits) {

        return (int)(mix64(nextSeed()) >>> (64 - bits));

    }

    /**

     * Returns a pseudorandom {@code int} value.

     *

     * @return a pseudorandom {@code int} value

     */

    public int nextInt() {

        return mix32(nextSeed());

    }

    /**

     * Returns a pseudorandom {@code long} value.

     *

     * @return a pseudorandom {@code long} value

     */

    public long nextLong() {

        return mix64(nextSeed());

    }

    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;

    }

    static final BigInteger thePeriod = BigInteger.valueOf(1).shiftLeft(64);  // Period is 2**64

    public BigInteger period() { return thePeriod; }

    // 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 {

            localInit();

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

                r = 1; // avoid zero

        }

        UNSAFE.putInt(t, SECONDARY, r);

        return r;

    }

    // Serialization support

    private static final long serialVersionUID = -5851777807851030925L;

    /**

     * @serialField rnd long

     *              seed for random computations

     * @serialField initialized boolean

     *              always true

     */

    private static final ObjectStreamField[] serialPersistentFields = {

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

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

    };

    /**

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

     * @param s the stream

     * @throws java.io.IOException if an I/O error occurs

     */

    private void writeObject(java.io.ObjectOutputStream s)

        throws java.io.IOException {

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

        fields.put("rnd", UNSAFE.getLong(Thread.currentThread(), SEED));

        fields.put("initialized", true);

        s.writeFields();

    }

    /**

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

     * @return 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);

        }

    }

}