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Metrics is a Java library which gives you unparalleled insight into what your code does in production. Metrics provides a powerful toolkit of ways to measure the behavior of critical components in your production environment.

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package com.codahale.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 Random Sampling with a Reservoir
 */
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 copy = new ArrayList(s);
        for (int i = 0; i < s; i++) {
            copy.add(values.get(i));
        }
        return new Snapshot(copy);
    }
}