package org.infinispan.commons.util.concurrent.jdk7backported;

/*
 * 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/
 */

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;
}
