content: /*
   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements.  See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership.  The ASF 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.
   */
  package com.alibaba.nacos.client.naming.utils;
  
  import java.util.Random;
  
  /**
   * <p><code>JVMRandom</code> is a wrapper that supports all possible Random methods via the {@link
   * java.lang.Math#random()} method and its system-wide {@link Random} object.</p>
   * <p>
   * It does this to allow for a Random class in which the seed is shared between all members of the class - a better name
   * would have been SharedSeedRandom.
   * <p>
   * <b>N.B.</b> the current implementation overrides the methods {@link Random#nextInt(int)} and {@link
   * Random#nextLong()} to produce positive numbers ranging from 0 (inclusive) to MAX_VALUE (exclusive).
   *
   * @author unknown
   * @version $Id: JVMRandom.java 911986 2010-02-19 21:19:05Z niallp $
   * @since 2.0
   */
  public final class JvmRandom extends Random {
  
      /**
       * Required for serialization support.
       *
       * @see java.io.Serializable
       */
      private static final long serialVersionUID = 1L;
  
      private static final Random SHARED_RANDOM = new Random();
  
      /**
       * Ensures that only the parent constructor can call reseed.
       */
      private boolean constructed = false;
  
      /**
       * Constructs a new instance.
       */
      public JvmRandom() {
          this.constructed = true;
      }
  
      /**
       * Unsupported in 2.0.
       *
       * @param seed ignored
       * @throws UnsupportedOperationException
       */
      @Override
      public synchronized void setSeed(long seed) {
          if (this.constructed) {
              throw new UnsupportedOperationException();
          }
      }
  
      /**
       * Unsupported in 2.0.
       *
       * @return Nothing, this method always throws an UnsupportedOperationException.
       * @throws UnsupportedOperationException
       */
      @Override
      public synchronized double nextGaussian() {
          throw new UnsupportedOperationException();
      }
  
      /**
       * Unsupported in 2.0.
       *
       * @param byteArray ignored
       * @throws UnsupportedOperationException
       */
      @Override
      public void nextBytes(byte[] byteArray) {
          throw new UnsupportedOperationException();
      }
  
      /**
       * <p>Returns the next pseudorandom, uniformly distributed int value from the Math.random() sequence.</p> Identical
       * to <code>nextInt(Integer.MAX_VALUE)</code> <p> <b>N.B. All values are >= 0.<b> </p>
       *
       * @return the random int
       */
      @Override
      public int nextInt() {
          return nextInt(Integer.MAX_VALUE);
      }
  
      /**
       * <p>Returns a pseudorandom, uniformly distributed int value between <code>0</code> (inclusive) and the specified
       * value (exclusive), from the Math.random() sequence.</p>
       *
       * @param n the specified exclusive max-value
       * @return the random int
       * @throws IllegalArgumentException when <code>n &lt;= 0</code>
       */
      @Override
      public int nextInt(int n) {
          return SHARED_RANDOM.nextInt(n);
      }
  
      /**
       * <p>Returns the next pseudorandom, uniformly distributed long value from the Math.random() sequence.</p> Identical
       * to <code>nextLong(Long.MAX_VALUE)</code> <p> <b>N.B. All values are >= 0.<b> </p>
       *
       * @return the random long
       */
      @Override
      public long nextLong() {
          return nextLong(Long.MAX_VALUE);
      }
  
      /**
       * <p>Returns a pseudorandom, uniformly distributed long value between <code>0</code> (inclusive) and the specified
       * value (exclusive), from the Math.random() sequence.</p>
       *
       * @param n the specified exclusive max-value
       * @return the random long
       * @throws IllegalArgumentException when <code>n &lt;= 0</code>
       */
      public static long nextLong(long n) {
          if (n <= 0) {
              throw new IllegalArgumentException(
                  "Upper bound for nextInt must be positive"
              );
          }
          // Code adapted from Harmony Random#nextInt(int)
          // n is power of 2
          if ((n & -n) == n) {
              // dropping lower order bits improves behaviour for low values of n
              return next63bits() >> 63
                  - bitsRequired(n - 1);
          }
          // Not a power of two
          long val;
          long bits;
          // reject some values to improve distribution
          do {
              bits = next63bits();
              val = bits % n;
          } while (bits - val + (n - 1) < 0);
          return val;
      }
  
      /**
       * <p>Returns the next pseudorandom, uniformly distributed boolean value from the Math.random() sequence.</p>
       *
       * @return the random boolean
       */
      @Override
      public boolean nextBoolean() {
          return SHARED_RANDOM.nextBoolean();
      }
  
      /**
       * <p>Returns the next pseudorandom, uniformly distributed float value between <code>0.0</code> and <code>1.0</code>
       * from the Math.random() sequence.</p>
       *
       * @return the random float
       */
      @Override
      public float nextFloat() {
          return SHARED_RANDOM.nextFloat();
      }
  
      /**
       * <p>Synonymous to the Math.random() call.</p>
       *
       * @return the random double
       */
      @Override
      public double nextDouble() {
          return SHARED_RANDOM.nextDouble();
      }
  
      /**
       * Get the next unsigned random long
       *
       * @return unsigned random long
       */
      private static long next63bits() {
          // drop the sign bit to leave 63 random bits
          return SHARED_RANDOM.nextLong() & 0x7fffffffffffffffL;
      }
  
      /**
       * Count the number of bits required to represent a long number.
       *
       * @param num long number
       * @return number of bits required
       */
      private static int bitsRequired(long num) {
          // Derived from Hacker's Delight, Figure 5-9
          long y = num;
          int n = 0;
          while (true) {
              // 64 = number of bits in a long
              if (num < 0) {
                  return 64 - n;
              }
              if (y == 0) {
                  return n;
              }
              n++;
              num = num << 1;
              y = y >> 1;
          }
      }
  }
  