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package org.glassfish.jersey.server.internal.monitoring;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.TimeUnit;
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.
*
* @author Stepan Vavra (stepan.vavra at oracle.com)
* @author Dropwizard Team
* @see <a href="http://www.cs.umd.edu/~samir/498/vitter.pdf">Random Sampling with a Reservoir</a>
* @see <a href="https://github.com/dropwizard/metrics">https://github.com/dropwizard/metrics</a>
*/
class UniformTimeReservoir implements TimeReservoir<Long> {
private final long startTime;
private final TimeUnit startTimeUnit;
private static final int DEFAULT_SIZE = 1024;
private static final int BITS_PER_LONG = 63;
private final AtomicLong count = new AtomicLong();
private final AtomicLongArray values;
/**
* Creates a new {@code UniformTimeReservoir} instance of 1024 elements, which offers a 99.9% confidence level
* with a 5% margin of error assuming a normal distribution.
*
* @param startTime The start time
* @param startTimeUnit The start time unit
*/
public UniformTimeReservoir(final long startTime, final TimeUnit startTimeUnit) {
this(DEFAULT_SIZE, startTime, startTimeUnit);
}
/**
* Creates a new {@code UniformTimeReservoir} instance.
*
* @param size the number of samples to keep in the sampling reservoir
* @param startTime The start time
* @param startTimeUnit The start time unit
*/
public UniformTimeReservoir(int size, final long startTime, final TimeUnit startTimeUnit) {
this.startTime = startTime;
this.startTimeUnit = startTimeUnit;
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 time, final TimeUnit timeUnit) {
final long c = count.get();
if (c > values.length()) {
return values.length();
}
return (int) c;
}
@Override
public void update(Long value, final long time, final TimeUnit timeUnit) {
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;
long val;
do {
bits = ThreadLocalRandom.current().nextLong() & (~(1L << BITS_PER_LONG));
val = bits % n;
} while (bits - val + (n - 1) < 0L);
return val;
}
@Override
public UniformTimeSnapshot getSnapshot(final long time, final TimeUnit timeUnit) {
final int s = size(time, timeUnit);
final List<Long> copy = new ArrayList<>(s);
for (int i = 0; i < s; i++) {
copy.add(values.get(i));
}
return new UniformTimeValuesSnapshot(copy,
startTimeUnit.convert(time, timeUnit) - startTime,
startTimeUnit) {
/**
* Method size must be overridden because it returns the values size by default which is {@link
* UniformTimeReservoir#DEFAULT_SIZE}.
*
* @return total number of data entries in the reservoir.
*/
@Override
public long size() {
return count.get();
}
};
}
@Override
public long interval(final TimeUnit timeUnit) {
// Uniform Interval returns 0 for infinity
return 0;
}
}