com.facebook.stats.DecayCounter.java Source code

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Here is the source code for com.facebook.stats.DecayCounter.java

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/*
 * Copyright (C) 2012 Facebook, Inc.
 *
 * Licensed 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.facebook.stats;

import org.joda.time.DateTime;
import org.joda.time.Duration;
import org.joda.time.ReadableDateTime;

import java.util.concurrent.atomic.AtomicLong;

/**
 * special purpose counter that decays exponentially fashion.
 * <p/>
 * Note: all value is decayed as though it was added at the 'start' value.
 * This is best used to decay the counter value once it has been fixed
 */
public class DecayCounter implements EventCounter {
    private final AtomicLong count = new AtomicLong(0);
    private final ReadableDateTime start;
    private final ReadableDateTime decayStart;
    private final ReadableDateTime end;
    private final float decayRatePerSecond;

    /**
     * @param start              start of the counter range
     * @param decayStart         time from which to compute decay
     * @param end                end of the counter range
     * @param decayRatePerSecond ex, 0.05 => 5% decay per second
     */
    public DecayCounter(ReadableDateTime start, ReadableDateTime decayStart, ReadableDateTime end,
            float decayRatePerSecond) {
        this.start = start;
        this.decayStart = decayStart;
        this.end = end;
        this.decayRatePerSecond = decayRatePerSecond;
    }

    public DecayCounter(ReadableDateTime start, ReadableDateTime end, float decayRatePerSecond) {
        this(start, start, end, decayRatePerSecond);
    }

    @Override
    public void add(long delta) {
        count.addAndGet(delta);
    }

    /**
     * @return counter value after computing exponential decay of the counter
     *         value
     */
    @Override
    public long getValue() {
        DateTime now = getNow();

        // don't start decay unless it's at least 1s after the decayStart
        if (now.isAfter(decayStart.toDateTime().plusSeconds(1))) {
            Duration elapsed = new Duration(decayStart, now);
            long millis = elapsed.getMillis();

            // compute total decay for millis / 1000 seconds
            double thisDecay = Math.pow(1.0 - decayRatePerSecond, (double) (millis / (double) 1000));

            return (long) (count.get() * thisDecay);
        } else {
            return count.get();
        }
    }

    @Override
    public ReadableDateTime getStart() {
        return start;
    }

    @Override
    public ReadableDateTime getEnd() {
        return end;
    }

    @Override
    public Duration getLength() {
        return new Duration(start, end);
    }

    /**
     * creates a merged counter with our decayed value + the other counter's
     * value.  This will be a DecayCounter with a decayStart of getNow()
     * and a range that spans the extend of both
     *
     * @param counter : any EventCounter
     * @return
     */
    @Override
    public EventCounter merge(EventCounter counter) {
        ReadableDateTime mergedStart = start.isBefore(counter.getStart()) ? start : counter.getStart();
        ReadableDateTime mergedEnd = end.isAfter(counter.getEnd()) ? end : counter.getEnd();
        DateTime now = getNow();
        DecayCounter mergedCounter = new DecayCounter(mergedStart, now.isAfter(decayStart) ? now : decayStart,
                mergedEnd, decayRatePerSecond);

        mergedCounter.add(getValue());
        mergedCounter.add(counter.getValue());

        return mergedCounter;
    }

    DateTime getNow() {
        return new DateTime();
    }
}