/*

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * 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.security.AccessControlContext;

import java.util.Random;

import java.util.Spliterator;

import java.util.concurrent.atomic.AtomicInteger;

import java.util.concurrent.atomic.AtomicLong;

import jdk.internal.misc.Unsafe;

import jdk.internal.misc.VM;

/**

 * 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

 * accidentally 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 more opportunistically, we also define here

     * other package-private utilities that access Thread class

     * fields.

     *

     * 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.

     *

     */

    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

    }

    /**

     * 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

        Thread t = Thread.currentThread();

        U.putLong(t, SEED, seed);

        U.putInt(t, PROBE, probe);

    }

    /**

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

     *

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

     */

    public static ThreadLocalRandom current() {

        if (U.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

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

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

        return r;

    }

    /**

     * Generates a pseudorandom number with the indicated number of

     * low-order bits.  Because this class has no subclasses, this

     * method cannot be invoked or overridden.

     *

     * @param  bits random bits

     * @return the next pseudorandom value from this random number

     *         generator's sequence

     */

    protected int next(int bits) {

        return nextInt() >>> (32 - 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() {

    }

    // 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 U.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;

        U.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 = U.getInt(t, SECONDARY)) != 0) {

            r ^= r << 13;   // xorshift

            r ^= r >>> 17;

            r ^= r << 5;

        }

        else if ((r = mix32(seeder.getAndAdd(SEEDER_INCREMENT))) == 0)

            r = 1; // avoid zero

        U.putInt(t, SECONDARY, r);

        return r;

    }

    // Support for other package-private ThreadLocal access

    /**

     * Erases ThreadLocals by nulling out Thread maps.

     */

    static final void eraseThreadLocals(Thread thread) {

        U.putReference(thread, THREADLOCALS, null);

        U.putReference(thread, INHERITABLETHREADLOCALS, null);

    }

    static final void setInheritedAccessControlContext(Thread thread,

                                                       AccessControlContext acc) {

        U.putReferenceRelease(thread, INHERITEDACCESSCONTROLCONTEXT, acc);

    }

    // 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", U.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();

    }

    // Static initialization

    /**

     * 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;

    // IllegalArgumentException messages

    static final String BAD_BOUND = "bound must be positive";

    static final String BAD_RANGE = "bound must be greater than origin";

    static final String BAD_SIZE  = "size must be non-negative";

    // Unsafe mechanics

    private static final Unsafe U = Unsafe.getUnsafe();

    private static final long SEED

        = U.objectFieldOffset(Thread.class, "threadLocalRandomSeed");

    private static final long PROBE

        = U.objectFieldOffset(Thread.class, "threadLocalRandomProbe");

    private static final long SECONDARY

        = U.objectFieldOffset(Thread.class, "threadLocalRandomSecondarySeed");

    private static final long THREADLOCALS

        = U.objectFieldOffset(Thread.class, "threadLocals");

    private static final long INHERITABLETHREADLOCALS

        = U.objectFieldOffset(Thread.class, "inheritableThreadLocals");

    private static final long INHERITEDACCESSCONTROLCONTEXT

        = U.objectFieldOffset(Thread.class, "inheritedAccessControlContext");

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

    private static final AtomicInteger probeGenerator = new AtomicInteger();

    /** The common ThreadLocalRandom */

    static final ThreadLocalRandom instance = new ThreadLocalRandom();

    /**

     * The next seed for default constructors.

     */

    private static final AtomicLong seeder

    // at end of <clinit> to survive static initialization circularity

    static {

        String sec = VM.getSavedProperty("java.util.secureRandomSeed");

        if (Boolean.parseBoolean(sec)) {

            byte[] seedBytes = java.security.SecureRandom.getSeed(8);

            long s = (long)seedBytes[0] & 0xffL;

            for (int i = 1; i < 8; ++i)

                s = (s << 8) | ((long)seedBytes[i] & 0xffL);

            seeder.set(s);

        }

    }

}