content: package com.taobao.arthas.core.util.metrics;
  
  import java.util.concurrent.atomic.AtomicLong;
  import java.util.concurrent.atomic.AtomicLongArray;
  import java.util.concurrent.ThreadLocalRandom;
  
  /**
   * <pre>
   * 统计平均速率，比如统计5秒内的平均速率。
   * 5秒的数据是：234, 345,124,366,235，
   * 则速率是 (234+345+124+366+235)/5 = 260
   *
   * </pre>
   *
   * @author hengyunabc 2015年12月18日 下午3:40:19
   *
   */
  public class RateCounter {
      private static final int BITS_PER_LONG = 63;
      public static final int DEFAULT_SIZE = 5;
  
      private final AtomicLong count = new AtomicLong();
      private final AtomicLongArray values;
  
      public RateCounter() {
          this(DEFAULT_SIZE);
      }
  
      public RateCounter(int size) {
          this.values = new AtomicLongArray(size);
          for (int i = 0; i < values.length(); i++) {
              values.set(i, 0);
          }
          count.set(0);
      }
  
      public int size() {
          final long c = count.get();
          if (c > values.length()) {
              return values.length();
          }
          return (int) c;
      }
  
      public void update(long value) {
          final long c = count.incrementAndGet();
          if (c <= values.length()) {
              values.set((int) c - 1, value);
          } else {
              final long r = nextLong(c);
              if (r < values.length()) {
                  values.set((int) r, value);
              }
          }
      }
  
      public double rate() {
          long c = count.get();
          int countLength = 0;
          long sum = 0;
          if (c > values.length()) {
              countLength = values.length();
          } else {
              countLength = (int) c;
          }
  
          for (int i = 0; i < countLength; ++i) {
              sum += values.get(i);
          }
  
          return sum / (double) countLength;
      }
  
      /**
       * Get a pseudo-random long uniformly between 0 and n-1. Stolen from
       * {@link java.util.Random#nextInt()}.
       *
       * @param n
       *            the bound
       * @return a value select randomly from the range {@code [0..n)}.
       */
      private static long nextLong(long n) {
          long bits, val;
          do {
              bits = ThreadLocalRandom.current().nextLong() & (~(1L << BITS_PER_LONG));
              val = bits % n;
          } while (bits - val + (n - 1) < 0L);
          return val;
      }
  
  }
  