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.metrics;
  
  import java.util.ArrayList;
  import java.util.List;
  import java.util.concurrent.atomic.AtomicLong;
  import java.util.concurrent.atomic.AtomicLongArray;
  
  /**
   * A random sampling reservoir of a stream of {@code long}s. Uses Vitter's Algorithm R to produce a
   * statistically representative sample.
   *
   * @see <a href="http://www.cs.umd.edu/~samir/498/vitter.pdf">Random Sampling with a Reservoir</a>
   */
  public class UniformReservoir implements Reservoir {
      private static final int DEFAULT_SIZE = 1028;
      private static final int BITS_PER_LONG = 63;
      private final AtomicLong count = new AtomicLong();
      private final AtomicLongArray values;
  
      /**
       * Creates a new {@link UniformReservoir} of 1028 elements, which offers a 99.9% confidence level
       * with a 5% margin of error assuming a normal distribution.
       */
      public UniformReservoir() {
          this(DEFAULT_SIZE);
      }
  
      /**
       * Creates a new {@link UniformReservoir}.
       *
       * @param size the number of samples to keep in the sampling reservoir
       */
      public UniformReservoir(int size) {
          this.values = new AtomicLongArray(size);
          for (int i = 0; i < values.length(); i++) {
              values.set(i, 0);
          }
          count.set(0);
      }
  
      @Override
      public int size() {
          final long c = count.get();
          if (c > values.length()) {
              return values.length();
          }
          return (int) c;
      }
  
      @Override
      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);
              }
          }
      }
  
      /**
       * 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;
      }
  
      @Override
      public Snapshot getSnapshot() {
          final int s = size();
          final List<Long> copy = new ArrayList<Long>(s);
          for (int i = 0; i < s; i++) {
              copy.add(values.get(i));
          }
          return new UniformSnapshot(copy);
      }
  }
  