code: /*
   * 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/
   */
  
  // This is based on 1.16 version
  
  package org.jruby.ext.thread_safe.jsr166y;
  
  import java.util.Random;
  
  /**
   * 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.
   *
   * @since 1.7
   * @author Doug Lea
   */
  public class ThreadLocalRandom extends Random {
      // same constants as Random, but must be redeclared because private
      private static final long multiplier = 0x5DEECE66DL;
      private static final long addend = 0xBL;
      private static final long mask = (1L << 48) - 1;
  
      /**
       * The random seed. We can't use super.seed.
       */
      private long rnd;
  
      /**
       * Initialization flag to permit calls to setSeed to succeed only
       * while executing the Random constructor.  We can't allow others
       * since it would cause setting seed in one part of a program to
       * unintentionally impact other usages by the thread.
       */
      boolean initialized;
  
      // Padding to help avoid memory contention among seed updates in
      // different TLRs in the common case that they are located near
      // each other.
      private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7;
  
      /**
       * The actual ThreadLocal
       */
      private static final ThreadLocal<ThreadLocalRandom> localRandom =
          new ThreadLocal<ThreadLocalRandom>() {
              protected ThreadLocalRandom initialValue() {
                  return new ThreadLocalRandom();
              }
      };
  
  
      /**
       * Constructor called only by localRandom.initialValue.
       */
      ThreadLocalRandom() {
          super();
          initialized = true;
      }
  
      /**
       * Returns the current thread's {@code ThreadLocalRandom}.
       *
       * @return the current thread's {@code ThreadLocalRandom}
       */
      public static ThreadLocalRandom current() {
          return localRandom.get();
      }
  
      /**
       * Throws {@code UnsupportedOperationException}.  Setting seeds in
       * this generator is not supported.
       *
       * @throws UnsupportedOperationException always
       */
      public void setSeed(long seed) {
          if (initialized)
              throw new UnsupportedOperationException();
          rnd = (seed ^ multiplier) & mask;
      }
  
      protected int next(int bits) {
          rnd = (rnd * multiplier + addend) & mask;
          return (int) (rnd >>> (48-bits));
      }
  
      /**
       * Returns a pseudorandom, uniformly distributed value between the
       * given least value (inclusive) and bound (exclusive).
       *
       * @param least the least value returned
       * @param bound the upper bound (exclusive)
       * @throws IllegalArgumentException if least greater than or equal
       * to bound
       * @return the next value
       */
      public int nextInt(int least, int bound) {
          if (least >= bound)
              throw new IllegalArgumentException();
          return nextInt(bound - least) + least;
      }
  
      /**
       * Returns a pseudorandom, uniformly distributed value
       * between 0 (inclusive) and the specified value (exclusive).
       *
       * @param n the bound on the random number to be returned.  Must be
       *        positive.
       * @return the next value
       * @throws IllegalArgumentException if n is not positive
       */
      public long nextLong(long n) {
          if (n <= 0)
              throw new IllegalArgumentException("n must be positive");
          // Divide n by two until small enough for nextInt. On each
          // iteration (at most 31 of them but usually much less),
          // randomly choose both whether to include high bit in result
          // (offset) and whether to continue with the lower vs upper
          // half (which makes a difference only if odd).
          long offset = 0;
          while (n >= Integer.MAX_VALUE) {
              int bits = next(2);
              long half = n >>> 1;
              long nextn = ((bits & 2) == 0) ? half : n - half;
              if ((bits & 1) == 0)
                  offset += n - nextn;
              n = nextn;
          }
          return offset + nextInt((int) n);
      }
  
      /**
       * Returns a pseudorandom, uniformly distributed value between the
       * given least value (inclusive) and bound (exclusive).
       *
       * @param least the least value returned
       * @param bound the upper bound (exclusive)
       * @return the next value
       * @throws IllegalArgumentException if least greater than or equal
       * to bound
       */
      public long nextLong(long least, long bound) {
          if (least >= bound)
              throw new IllegalArgumentException();
          return nextLong(bound - least) + least;
      }
  
      /**
       * Returns a pseudorandom, uniformly distributed {@code double} value
       * between 0 (inclusive) and the specified value (exclusive).
       *
       * @param n the bound on the random number to be returned.  Must be
       *        positive.
       * @return the next value
       * @throws IllegalArgumentException if n is not positive
       */
      public double nextDouble(double n) {
          if (n <= 0)
              throw new IllegalArgumentException("n must be positive");
          return nextDouble() * n;
      }
  
      /**
       * Returns a pseudorandom, uniformly distributed value between the
       * given least value (inclusive) and bound (exclusive).
       *
       * @param least the least value returned
       * @param bound the upper bound (exclusive)
       * @return the next value
       * @throws IllegalArgumentException if least greater than or equal
       * to bound
       */
      public double nextDouble(double least, double bound) {
          if (least >= bound)
              throw new IllegalArgumentException();
          return nextDouble() * (bound - least) + least;
      }
  
      private static final long serialVersionUID = -5851777807851030925L;
  }
  