org.apache.solr.util.stats.UniformSample Maven / Gradle / Ivy
/*
* 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.
*/
/*
* Forked from https://github.com/codahale/metrics
*/
package org.apache.solr.util.stats;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.AtomicLongArray;
/**
* A random sample of a stream of {@code long}s. Uses Vitter's Algorithm R to produce a
* statistically representative sample.
*
* @see Random Sampling with a Reservoir
*/
public class UniformSample implements Sample {
private static final int BITS_PER_LONG = 63;
private final AtomicLong count = new AtomicLong();
private final AtomicLongArray values;
//TODO: Maybe replace with a Mersenne twister for better distribution
private static final Random random = new Random();
/**
* Creates a new {@link UniformSample}.
*
* @param reservoirSize the number of samples to keep in the sampling reservoir
*/
public UniformSample(int reservoirSize) {
this.values = new AtomicLongArray(reservoirSize);
clear();
}
@Override
public void clear() {
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 = random.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 copy = new ArrayList(s);
for (int i = 0; i < s; i++) {
copy.add(values.get(i));
}
return new Snapshot(copy);
}
}