/*
 * 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.util.Random;
import java.util.Spliterator;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.util.function.DoubleConsumer;
import java.util.function.IntConsumer;
import java.util.function.LongConsumer;
import java.util.stream.DoubleStream;
import java.util.stream.IntStream;
import java.util.stream.LongStream;
import java.util.stream.StreamSupport;

/**
 * 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.
     */

    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
    }

    /**
     * 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();
        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;
    }

    // We must define this, but never use it.
    protected int next(int bits) {
        return (int)(mix64(nextSeed()) >>> (64 - bits));
    }

    /**
     * The form of nextLong used by LongStream Spliterators.  If
     * origin is greater than bound, acts as unbounded form of
     * nextLong, else as bounded form.
     *
     * @param origin the least value, unless greater than bound
     * @param bound the upper bound (exclusive), must not equal origin
     * @return a pseudorandom value
     */
    final long internalNextLong(long origin, long bound) {
        long r = mix64(nextSeed());
        if (origin < bound) {
            long n = bound - origin, m = n - 1;
            if ((n & m) == 0L)  // power of two
                r = (r & m) + origin;
            else if (n > 0L) {  // reject over-represented candidates
                for (long u = r >>> 1;            // ensure nonnegative
                     u + m - (r = u % n) < 0L;    // rejection check
                     u = mix64(nextSeed()) >>> 1) // retry
                    ;
                r += origin;
            }
            else {              // range not representable as long
                while (r < origin || r >= bound)
                    r = mix64(nextSeed());
            }
        }
        return r;
    }

    /**
     * The form of nextInt used by IntStream Spliterators.
     * Exactly the same as long version, except for types.
     *
     * @param origin the least value, unless greater than bound
     * @param bound the upper bound (exclusive), must not equal origin
     * @return a pseudorandom value
     */
    final int internalNextInt(int origin, int bound) {
        int r = mix32(nextSeed());
        if (origin < bound) {
            int n = bound - origin, m = n - 1;
            if ((n & m) == 0)
                r = (r & m) + origin;
            else if (n > 0) {
                for (int u = r >>> 1;
                     u + m - (r = u % n) < 0;
                     u = mix32(nextSeed()) >>> 1)
                    ;
                r += origin;
            }
            else {
                while (r < origin || r >= bound)
                    r = mix32(nextSeed());
            }
        }
        return r;
    }

    /**
     * The form of nextDouble used by DoubleStream Spliterators.
     *
     * @param origin the least value, unless greater than bound
     * @param bound the upper bound (exclusive), must not equal origin
     * @return a pseudorandom value
     */
    final double internalNextDouble(double origin, double bound) {
        double r = (nextLong() >>> 11) * DOUBLE_UNIT;
        if (origin < bound) {
            r = r * (bound - origin) + origin;
            if (r >= bound) // correct for rounding
                r = Double.longBitsToDouble(Double.doubleToLongBits(bound) - 1);
        }
        return r;
    }

    /**
     * Returns a pseudorandom {@code int} value.
     *
     * @return a pseudorandom {@code int} value
     */
    public int nextInt() {
        return mix32(nextSeed());
    }

    /**
     * Returns a pseudorandom {@code int} value between zero (inclusive)
     * and the specified bound (exclusive).
     *
     * @param bound the upper bound (exclusive).  Must be positive.
     * @return a pseudorandom {@code int} value between zero
     *         (inclusive) and the bound (exclusive)
     * @throws IllegalArgumentException if {@code bound} is not positive
     */
    public int nextInt(int bound) {
        if (bound <= 0)
            throw new IllegalArgumentException(BAD_BOUND);
        int r = mix32(nextSeed());
        int m = bound - 1;
        if ((bound & m) == 0) // power of two
            r &= m;
        else { // reject over-represented candidates
            for (int u = r >>> 1;
                 u + m - (r = u % bound) < 0;
                 u = mix32(nextSeed()) >>> 1)
                ;
        }
        return r;
    }

    /**
     * Returns a pseudorandom {@code int} value between the specified
     * origin (inclusive) and the specified bound (exclusive).
     *
     * @param origin the least value returned
     * @param bound the upper bound (exclusive)
     * @return a pseudorandom {@code int} value between the origin
     *         (inclusive) and the bound (exclusive)
     * @throws IllegalArgumentException if {@code origin} is greater than
     *         or equal to {@code bound}
     */
    public int nextInt(int origin, int bound) {
        if (origin >= bound)
            throw new IllegalArgumentException(BAD_RANGE);
        return internalNextInt(origin, bound);
    }

    /**
     * Returns a pseudorandom {@code long} value.
     *
     * @return a pseudorandom {@code long} value
     */
    public long nextLong() {
        return mix64(nextSeed());
    }

    /**
     * Returns a pseudorandom {@code long} value between zero (inclusive)
     * and the specified bound (exclusive).
     *
     * @param bound the upper bound (exclusive).  Must be positive.
     * @return a pseudorandom {@code long} value between zero
     *         (inclusive) and the bound (exclusive)
     * @throws IllegalArgumentException if {@code bound} is not positive
     */
    public long nextLong(long bound) {
        if (bound <= 0)
            throw new IllegalArgumentException(BAD_BOUND);
        long r = mix64(nextSeed());
        long m = bound - 1;
        if ((bound & m) == 0L) // power of two
            r &= m;
        else { // reject over-represented candidates
            for (long u = r >>> 1;
                 u + m - (r = u % bound) < 0L;
                 u = mix64(nextSeed()) >>> 1)
                ;
        }
        return r;
    }

    /**
     * Returns a pseudorandom {@code long} value between the specified
     * origin (inclusive) and the specified bound (exclusive).
     *
     * @param origin the least value returned
     * @param bound the upper bound (exclusive)
     * @return a pseudorandom {@code long} value between the origin
     *         (inclusive) and the bound (exclusive)
     * @throws IllegalArgumentException if {@code origin} is greater than
     *         or equal to {@code bound}
     */
    public long nextLong(long origin, long bound) {
        if (origin >= bound)
            throw new IllegalArgumentException(BAD_RANGE);
        return internalNextLong(origin, bound);
    }

    /**
     * Returns a pseudorandom {@code double} value between zero
     * (inclusive) and one (exclusive).
     *
     * @return a pseudorandom {@code double} value between zero
     *         (inclusive) and one (exclusive)
     */
    public double nextDouble() {
        return (mix64(nextSeed()) >>> 11) * DOUBLE_UNIT;
    }

    /**
     * Returns a pseudorandom {@code double} value between 0.0
     * (inclusive) and the specified bound (exclusive).
     *
     * @param bound the upper bound (exclusive).  Must be positive.
     * @return a pseudorandom {@code double} value between zero
     *         (inclusive) and the bound (exclusive)
     * @throws IllegalArgumentException if {@code bound} is not positive
     */
    public double nextDouble(double bound) {
        if (!(bound > 0.0))
            throw new IllegalArgumentException(BAD_BOUND);
        double result = (mix64(nextSeed()) >>> 11) * DOUBLE_UNIT * bound;
        return (result < bound) ? result : // correct for rounding
            Double.longBitsToDouble(Double.doubleToLongBits(bound) - 1);
    }

    /**
     * Returns a pseudorandom {@code double} value between the specified
     * origin (inclusive) and bound (exclusive).
     *
     * @param origin the least value returned
     * @param bound the upper bound (exclusive)
     * @return a pseudorandom {@code double} value between the origin
     *         (inclusive) and the bound (exclusive)
     * @throws IllegalArgumentException if {@code origin} is greater than
     *         or equal to {@code bound}
     */
    public double nextDouble(double origin, double bound) {
        if (!(origin < bound))
            throw new IllegalArgumentException(BAD_RANGE);
        return internalNextDouble(origin, bound);
    }

    /**
     * Returns a pseudorandom {@code boolean} value.
     *
     * @return a pseudorandom {@code boolean} value
     */
    public boolean nextBoolean() {
        return mix32(nextSeed()) < 0;
    }

    /**
     * Returns a pseudorandom {@code float} value between zero
     * (inclusive) and one (exclusive).
     *
     * @return a pseudorandom {@code float} value between zero
     *         (inclusive) and one (exclusive)
     */
    public float nextFloat() {
        return (mix32(nextSeed()) >>> 8) * FLOAT_UNIT;
    }

    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;
    }

    // stream methods, coded in a way intended to better isolate for
    // maintenance purposes the small differences across forms.
    /**
     * Returns a stream producing the given {@code streamSize} number of
     * pseudorandom {@code int} values.
     *
     * @param streamSize the number of values to generate
     * @return a stream of pseudorandom {@code int} values
     * @throws IllegalArgumentException if {@code streamSize} is
     *         less than zero
     * @since 1.8
     */
    public IntStream ints(long streamSize) {
        if (streamSize < 0L)
            throw new IllegalArgumentException(BAD_SIZE);
        return StreamSupport.intStream
            (new RandomIntsSpliterator
             (0L, streamSize, Integer.MAX_VALUE, 0),
             false);
    }

    /**
     * Returns an effectively unlimited stream of pseudorandom {@code int}
     * values.
     *
     * @implNote This method is implemented to be equivalent to {@code
     * ints(Long.MAX_VALUE)}.
     *
     * @return a stream of pseudorandom {@code int} values
     * @since 1.8
     */
    public IntStream ints() {
        return StreamSupport.intStream
            (new RandomIntsSpliterator
             (0L, Long.MAX_VALUE, Integer.MAX_VALUE, 0),
             false);
    }

    /**
     * Returns a stream producing the given {@code streamSize} number
     * of pseudorandom {@code int} values, each conforming to the given
     * origin (inclusive) and bound (exclusive).
     *
     * @param streamSize the number of values to generate
     * @param randomNumberOrigin the origin (inclusive) of each random value
     * @param randomNumberBound the bound (exclusive) of each random value
     * @return a stream of pseudorandom {@code int} values,
     *         each with the given origin (inclusive) and bound (exclusive)
     * @throws IllegalArgumentException if {@code streamSize} is
     *         less than zero, or {@code randomNumberOrigin}
     *         is greater than or equal to {@code randomNumberBound}
     * @since 1.8
     */
    public IntStream ints(long streamSize, int randomNumberOrigin,
                          int randomNumberBound) {
        if (streamSize < 0L)
            throw new IllegalArgumentException(BAD_SIZE);
        if (randomNumberOrigin >= randomNumberBound)
            throw new IllegalArgumentException(BAD_RANGE);
        return StreamSupport.intStream
            (new RandomIntsSpliterator
             (0L, streamSize, randomNumberOrigin, randomNumberBound),
             false);
    }

    /**
     * Returns an effectively unlimited stream of pseudorandom {@code
     * int} values, each conforming to the given origin (inclusive) and bound
     * (exclusive).
     *
     * @implNote This method is implemented to be equivalent to {@code
     * ints(Long.MAX_VALUE, randomNumberOrigin, randomNumberBound)}.
     *
     * @param randomNumberOrigin the origin (inclusive) of each random value
     * @param randomNumberBound the bound (exclusive) of each random value
     * @return a stream of pseudorandom {@code int} values,
     *         each with the given origin (inclusive) and bound (exclusive)
     * @throws IllegalArgumentException if {@code randomNumberOrigin}
     *         is greater than or equal to {@code randomNumberBound}
     * @since 1.8
     */
    public IntStream ints(int randomNumberOrigin, int randomNumberBound) {
        if (randomNumberOrigin >= randomNumberBound)
            throw new IllegalArgumentException(BAD_RANGE);
        return StreamSupport.intStream
            (new RandomIntsSpliterator
             (0L, Long.MAX_VALUE, randomNumberOrigin, randomNumberBound),
             false);
    }

    /**
     * Returns a stream producing the given {@code streamSize} number of
     * pseudorandom {@code long} values.
     *
     * @param streamSize the number of values to generate
     * @return a stream of pseudorandom {@code long} values
     * @throws IllegalArgumentException if {@code streamSize} is
     *         less than zero
     * @since 1.8
     */
    public LongStream longs(long streamSize) {
        if (streamSize < 0L)
            throw new IllegalArgumentException(BAD_SIZE);
        return StreamSupport.longStream
            (new RandomLongsSpliterator
             (0L, streamSize, Long.MAX_VALUE, 0L),
             false);
    }

    /**
     * Returns an effectively unlimited stream of pseudorandom {@code long}
     * values.
     *
     * @implNote This method is implemented to be equivalent to {@code
     * longs(Long.MAX_VALUE)}.
     *
     * @return a stream of pseudorandom {@code long} values
     * @since 1.8
     */
    public LongStream longs() {
        return StreamSupport.longStream
            (new RandomLongsSpliterator
             (0L, Long.MAX_VALUE, Long.MAX_VALUE, 0L),
             false);
    }

    /**
     * Returns a stream producing the given {@code streamSize} number of
     * pseudorandom {@code long}, each conforming to the given origin
     * (inclusive) and bound (exclusive).
     *
     * @param streamSize the number of values to generate
     * @param randomNumberOrigin the origin (inclusive) of each random value
     * @param randomNumberBound the bound (exclusive) of each random value
     * @return a stream of pseudorandom {@code long} values,
     *         each with the given origin (inclusive) and bound (exclusive)
     * @throws IllegalArgumentException if {@code streamSize} is
     *         less than zero, or {@code randomNumberOrigin}
     *         is greater than or equal to {@code randomNumberBound}
     * @since 1.8
     */
    public LongStream longs(long streamSize, long randomNumberOrigin,
                            long randomNumberBound) {
        if (streamSize < 0L)
            throw new IllegalArgumentException(BAD_SIZE);
        if (randomNumberOrigin >= randomNumberBound)
            throw new IllegalArgumentException(BAD_RANGE);
        return StreamSupport.longStream
            (new RandomLongsSpliterator
             (0L, streamSize, randomNumberOrigin, randomNumberBound),
             false);
    }

    /**
     * Returns an effectively unlimited stream of pseudorandom {@code
     * long} values, each conforming to the given origin (inclusive) and bound
     * (exclusive).
     *
     * @implNote This method is implemented to be equivalent to {@code
     * longs(Long.MAX_VALUE, randomNumberOrigin, randomNumberBound)}.
     *
     * @param randomNumberOrigin the origin (inclusive) of each random value
     * @param randomNumberBound the bound (exclusive) of each random value
     * @return a stream of pseudorandom {@code long} values,
     *         each with the given origin (inclusive) and bound (exclusive)
     * @throws IllegalArgumentException if {@code randomNumberOrigin}
     *         is greater than or equal to {@code randomNumberBound}
     * @since 1.8
     */
    public LongStream longs(long randomNumberOrigin, long randomNumberBound) {
        if (randomNumberOrigin >= randomNumberBound)
            throw new IllegalArgumentException(BAD_RANGE);
        return StreamSupport.longStream
            (new RandomLongsSpliterator
             (0L, Long.MAX_VALUE, randomNumberOrigin, randomNumberBound),
             false);
    }

    /**
     * Returns a stream producing the given {@code streamSize} number of
     * pseudorandom {@code double} values, each between zero
     * (inclusive) and one (exclusive).
     *
     * @param streamSize the number of values to generate
     * @return a stream of {@code double} values
     * @throws IllegalArgumentException if {@code streamSize} is
     *         less than zero
     * @since 1.8
     */
    public DoubleStream doubles(long streamSize) {
        if (streamSize < 0L)
            throw new IllegalArgumentException(BAD_SIZE);
        return StreamSupport.doubleStream
            (new RandomDoublesSpliterator
             (0L, streamSize, Double.MAX_VALUE, 0.0),
             false);
    }

    /**
     * Returns an effectively unlimited stream of pseudorandom {@code
     * double} values, each between zero (inclusive) and one
     * (exclusive).
     *
     * @implNote This method is implemented to be equivalent to {@code
     * doubles(Long.MAX_VALUE)}.
     *
     * @return a stream of pseudorandom {@code double} values
     * @since 1.8
     */
    public DoubleStream doubles() {
        return StreamSupport.doubleStream
            (new RandomDoublesSpliterator
             (0L, Long.MAX_VALUE, Double.MAX_VALUE, 0.0),
             false);
    }

    /**
     * Returns a stream producing the given {@code streamSize} number of
     * pseudorandom {@code double} values, each conforming to the given origin
     * (inclusive) and bound (exclusive).
     *
     * @param streamSize the number of values to generate
     * @param randomNumberOrigin the origin (inclusive) of each random value
     * @param randomNumberBound the bound (exclusive) of each random value
     * @return a stream of pseudorandom {@code double} values,
     *         each with the given origin (inclusive) and bound (exclusive)
     * @throws IllegalArgumentException if {@code streamSize} is
     *         less than zero
     * @throws IllegalArgumentException if {@code randomNumberOrigin}
     *         is greater than or equal to {@code randomNumberBound}
     * @since 1.8
     */
    public DoubleStream doubles(long streamSize, double randomNumberOrigin,
                                double randomNumberBound) {
        if (streamSize < 0L)
            throw new IllegalArgumentException(BAD_SIZE);
        if (!(randomNumberOrigin < randomNumberBound))
            throw new IllegalArgumentException(BAD_RANGE);
        return StreamSupport.doubleStream
            (new RandomDoublesSpliterator
             (0L, streamSize, randomNumberOrigin, randomNumberBound),
             false);
    }

    /**
     * Returns an effectively unlimited stream of pseudorandom {@code
     * double} values, each conforming to the given origin (inclusive) and bound
     * (exclusive).
     *
     * @implNote This method is implemented to be equivalent to {@code
     * doubles(Long.MAX_VALUE, randomNumberOrigin, randomNumberBound)}.
     *
     * @param randomNumberOrigin the origin (inclusive) of each random value
     * @param randomNumberBound the bound (exclusive) of each random value
     * @return a stream of pseudorandom {@code double} values,
     *         each with the given origin (inclusive) and bound (exclusive)
     * @throws IllegalArgumentException if {@code randomNumberOrigin}
     *         is greater than or equal to {@code randomNumberBound}
     * @since 1.8
     */
    public DoubleStream doubles(double randomNumberOrigin, double randomNumberBound) {
        if (!(randomNumberOrigin < randomNumberBound))
            throw new IllegalArgumentException(BAD_RANGE);
        return StreamSupport.doubleStream
            (new RandomDoublesSpliterator
             (0L, Long.MAX_VALUE, randomNumberOrigin, randomNumberBound),
             false);
    }

    /**
     * Spliterator for int streams.  We multiplex the four int
     * versions into one class by treating a bound less than origin as
     * unbounded, and also by treating "infinite" as equivalent to
     * Long.MAX_VALUE. For splits, it uses the standard divide-by-two
     * approach. The long and double versions of this class are
     * identical except for types.
     */
    private static final class RandomIntsSpliterator
            implements Spliterator.OfInt {
        long index;
        final long fence;
        final int origin;
        final int bound;
        RandomIntsSpliterator(long index, long fence,
                              int origin, int bound) {
            this.index = index; this.fence = fence;
            this.origin = origin; this.bound = bound;
        }

        public RandomIntsSpliterator trySplit() {
            long i = index, m = (i + fence) >>> 1;
            return (m <= i) ? null :
                new RandomIntsSpliterator(i, index = m, origin, bound);
        }

        public long estimateSize() {
            return fence - index;
        }

        public int characteristics() {
            return (Spliterator.SIZED | Spliterator.SUBSIZED |
                    Spliterator.NONNULL | Spliterator.IMMUTABLE);
        }

        public boolean tryAdvance(IntConsumer consumer) {
            if (consumer == null) throw new NullPointerException();
            long i = index, f = fence;
            if (i < f) {
                consumer.accept(ThreadLocalRandom.current().internalNextInt(origin, bound));
                index = i + 1;
                return true;
            }
            return false;
        }

        public void forEachRemaining(IntConsumer consumer) {
            if (consumer == null) throw new NullPointerException();
            long i = index, f = fence;
            if (i < f) {
                index = f;
                int o = origin, b = bound;
                ThreadLocalRandom rng = ThreadLocalRandom.current();
                do {
                    consumer.accept(rng.internalNextInt(o, b));
                } while (++i < f);
            }
        }
    }

    /**
     * Spliterator for long streams.
     */
    private static final class RandomLongsSpliterator
            implements Spliterator.OfLong {
        long index;
        final long fence;
        final long origin;
        final long bound;
        RandomLongsSpliterator(long index, long fence,
                               long origin, long bound) {
            this.index = index; this.fence = fence;
            this.origin = origin; this.bound = bound;
        }

        public RandomLongsSpliterator trySplit() {
            long i = index, m = (i + fence) >>> 1;
            return (m <= i) ? null :
                new RandomLongsSpliterator(i, index = m, origin, bound);
        }

        public long estimateSize() {
            return fence - index;
        }

        public int characteristics() {
            return (Spliterator.SIZED | Spliterator.SUBSIZED |
                    Spliterator.NONNULL | Spliterator.IMMUTABLE);
        }

        public boolean tryAdvance(LongConsumer consumer) {
            if (consumer == null) throw new NullPointerException();
            long i = index, f = fence;
            if (i < f) {
                consumer.accept(ThreadLocalRandom.current().internalNextLong(origin, bound));
                index = i + 1;
                return true;
            }
            return false;
        }

        public void forEachRemaining(LongConsumer consumer) {
            if (consumer == null) throw new NullPointerException();
            long i = index, f = fence;
            if (i < f) {
                index = f;
                long o = origin, b = bound;
                ThreadLocalRandom rng = ThreadLocalRandom.current();
                do {
                    consumer.accept(rng.internalNextLong(o, b));
                } while (++i < f);
            }
        }

    }

    /**
     * Spliterator for double streams.
     */
    private static final class RandomDoublesSpliterator
            implements Spliterator.OfDouble {
        long index;
        final long fence;
        final double origin;
        final double bound;
        RandomDoublesSpliterator(long index, long fence,
                                 double origin, double bound) {
            this.index = index; this.fence = fence;
            this.origin = origin; this.bound = bound;
        }

        public RandomDoublesSpliterator trySplit() {
            long i = index, m = (i + fence) >>> 1;
            return (m <= i) ? null :
                new RandomDoublesSpliterator(i, index = m, origin, bound);
        }

        public long estimateSize() {
            return fence - index;
        }

        public int characteristics() {
            return (Spliterator.SIZED | Spliterator.SUBSIZED |
                    Spliterator.NONNULL | Spliterator.IMMUTABLE);
        }

        public boolean tryAdvance(DoubleConsumer consumer) {
            if (consumer == null) throw new NullPointerException();
            long i = index, f = fence;
            if (i < f) {
                consumer.accept(ThreadLocalRandom.current().internalNextDouble(origin, bound));
                index = i + 1;
                return true;
            }
            return false;
        }

        public void forEachRemaining(DoubleConsumer consumer) {
            if (consumer == null) throw new NullPointerException();
            long i = index, f = fence;
            if (i < f) {
                index = f;
                double o = origin, b = bound;
                ThreadLocalRandom rng = ThreadLocalRandom.current();
                do {
                    consumer.accept(rng.internalNextDouble(o, b));
                } while (++i < f);
            }
        }
    }


    // 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;
    }

    // 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;

    // 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)

    // 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 sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
    private static final long SEED;
    private static final long PROBE;
    private static final long SECONDARY;
    static {
        try {
            SEED = U.objectFieldOffset
                (Thread.class.getDeclaredField("threadLocalRandomSeed"));
            PROBE = U.objectFieldOffset
                (Thread.class.getDeclaredField("threadLocalRandomProbe"));
            SECONDARY = U.objectFieldOffset
                (Thread.class.getDeclaredField("threadLocalRandomSecondarySeed"));
        } catch (ReflectiveOperationException e) {
            throw new Error(e);
        }
    }

    /** Rarely-used holder for the second of a pair of Gaussians */
    private static final ThreadLocal<Double> nextLocalGaussian =
        new ThreadLocal<>();

    /** 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
        = new AtomicLong(mix64(System.currentTimeMillis()) ^
                         mix64(System.nanoTime()));

    // at end of <clinit> to survive static initialization circularity
    static {
        if (java.security.AccessController.doPrivileged(
            new java.security.PrivilegedAction<Boolean>() {
                public Boolean run() {
                    return Boolean.getBoolean("java.util.secureRandomSeed");
                }})) {
            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);
        }
    }
}
