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   /*
    * Copyright 2012 The Netty Project
    *
    * The Netty Project licenses this file to you under the Apache License,
    * version 2.0 (the "License"); you may not use this file except in compliance
    * with the License. You may obtain a copy of the License at:
    *
    *   http://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
   * License for the specific language governing permissions and limitations
   * under the License.
   */
  
  /*
   * 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/licenses/publicdomain
   */
  
  package org.jboss.netty.util.internal;
  
  import java.util.Random;
  
  /**
   * A random number generator isolated to the current thread.  Like the
   * global {@link Random} generator used by the {@link
   * 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 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
   */
  public final 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 the first and only allowed call
       * to setSeed (inside Random constructor) to succeed.  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.
       */
      private 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.
      @SuppressWarnings("unused")
      private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7;
  
      /**
       * The actual ThreadLocal
       */
      private static final ThreadLocal<ThreadLocalRandom> localRandom =
          new ThreadLocal<ThreadLocalRandom>() {
              @Override
              protected ThreadLocalRandom initialValue() {
                  return new ThreadLocalRandom();
              }
      };
  
      /**
       * 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
       */
     @Override
     public void setSeed(long seed) {
         // We rely on the fact that the superclass no-arg constructor
         // invokes setSeed exactly once to initialize.
         if (initialized) {
             throw new UnsupportedOperationException();
         }
         initialized = true;
         rnd = (seed ^ multiplier) & mask;
     }
 
     @Override
     protected int next(int bits) {
         rnd = rnd * multiplier + addend & mask;
         return (int) (rnd >>> 48 - bits);
     }
 
     private static final long serialVersionUID = -5851777807851030925L;
 }