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package org.xbib.metrics.common;

import org.xbib.metrics.api.Clock;
import org.xbib.metrics.api.Metered;

import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.LongAdder;

/**
 * A meter metric which measures mean throughput and one-, five-, and fifteen-minute
 * exponentially-weighted moving average throughputs.
 *
 * @see ExpWeightedMovingAverage
 */
public class Meter implements Metered {
    private static final long TICK_INTERVAL = TimeUnit.SECONDS.toNanos(5);

    private final ScheduledExecutorService executorService;

    private final ExpWeightedMovingAverage m1Rate;

    private final ExpWeightedMovingAverage m5Rate;

    private final ExpWeightedMovingAverage m15Rate;

    private final LongAdder count;

    private final AtomicLong lastTick;

    private final Clock clock;

    private long startedAt;

    private ScheduledFuture future;

    /**
     * Creates a new {@link Meter}.
     * @param executorService the executor service
     */
    public Meter(ScheduledExecutorService executorService) {
        this(executorService, UserTimeClock.defaultClock());
    }

    /**
     * Creates a new {@link Meter}.
     *
     * @param executorService the executor service
     * @param clock the clock to use for the meter ticks
     */
    public Meter(ScheduledExecutorService executorService, Clock clock) {
        this.executorService = executorService;
        this.clock = clock;
        this.lastTick = new AtomicLong(startedAt);
        m1Rate = ExpWeightedMovingAverage.oneMinuteEWMA();
        m5Rate = ExpWeightedMovingAverage.fiveMinuteEWMA();
        m15Rate = ExpWeightedMovingAverage.fifteenMinuteEWMA();
        count = new LongAdder();
    }

    public void start(long intervalSeconds) {
        this.startedAt = this.clock.getTick();
        this.future = executorService
                .scheduleAtFixedRate(this::tickIfNecessary, intervalSeconds, intervalSeconds, TimeUnit.SECONDS);
    }

    public void stop() {
        future.cancel(false);
    }

    public void shutdown() {
        stop();
        executorService.shutdownNow();
    }

    /**
     * Mark the occurrence of an event.
     */
    public void mark() {
        mark(1);
    }

    /**
     * Mark the occurrence of a given number of events.
     *
     * @param n the number of events
     */
    public void mark(long n) {
        tickIfNecessary();
        count.add(n);
        m1Rate.update(n);
        m5Rate.update(n);
        m15Rate.update(n);
    }

    private void tickIfNecessary() {
        final long oldTick = lastTick.get();
        final long newTick = clock.getTick();
        final long age = newTick - oldTick;
        if (age > TICK_INTERVAL) {
            final long newIntervalStartTick = newTick - age % TICK_INTERVAL;
            if (lastTick.compareAndSet(oldTick, newIntervalStartTick)) {
                final long requiredTicks = age / TICK_INTERVAL;
                for (long i = 0; i < requiredTicks; i++) {
                    m1Rate.tick();
                    m5Rate.tick();
                    m15Rate.tick();
                }
            }
        }
    }

    @Override
    public long getCount() {
        return count.sum();
    }

    @Override
    public double getFifteenMinuteRate() {
        tickIfNecessary();
        return m15Rate.getRate(TimeUnit.SECONDS);
    }

    @Override
    public double getFiveMinuteRate() {
        tickIfNecessary();
        return m5Rate.getRate(TimeUnit.SECONDS);
    }

    @Override
    public double getMeanRate() {
        if (getCount() == 0) {
            return 0.0;
        } else {
            final double elapsed = clock.getTick() - startedAt;
            return getCount() / elapsed * TimeUnit.SECONDS.toNanos(1);
        }
    }

    @Override
    public double getOneMinuteRate() {
        tickIfNecessary();
        return m1Rate.getRate(TimeUnit.SECONDS);
    }

    public long elapsed() {
        return clock.getTick() - startedAt;
    }
}