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/*
 * 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.
 */

package org.apache.jackrabbit.oak.stats;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;

public final class SimpleStats implements TimerStats, MeterStats, CounterStats, HistogramStats {
    public enum Type {COUNTER, METER, TIMER, HISTOGRAM}
    private final AtomicLong statsHolder;
    private long counter;

    /*
        Using 2 different variables for managing the sum in meter calls
        1. Primitive variant is used for just increment
        2. AtomicLong variant is used for increment by 'n'

        This is done to ensure that more frequent mark() is fast (used for Session reads)
        and overhead of AtomicLong is used only for less critical flows

        Once we move to JDK 8 we can probably use LongAdder from that has lesser
        impact on performance
     */
    private long meterSum;
    private final AtomicLong meterSumRef = new AtomicLong();

    private final Type type;

    public SimpleStats(AtomicLong statsHolder, Type type) {
        this.statsHolder = statsHolder;
        this.type = type;
    }

    @Override
    public long getCount() {
        switch(type){
            case HISTOGRAM:
            case TIMER:
                //For timer and histogram we need to manage explicit
                //invocation count
                return counter;
            case COUNTER:
                return statsHolder.get();
            case METER:
                //For Meter it can happen that backing statsHolder gets
                //reset each second. So need to manage that sum separately
                return meterSum + meterSumRef.get();
        }
        throw new IllegalStateException();
    }

    @Override
    public void inc() {
        statsHolder.getAndIncrement();
    }

    @Override
    public void dec() {
        statsHolder.getAndDecrement();
    }

    @Override
    public void inc(long n) {
        statsHolder.getAndAdd(n);
    }

    @Override
    public void dec(long n) {
        statsHolder.getAndAdd(-n);
    }

    @Override
    public void mark() {
        inc();
        meterSum++;
    }

    @Override
    public void mark(long n) {
        meterSumRef.getAndAdd(n);
        statsHolder.getAndAdd(n);
    }

    @Override
    public void update(long duration, TimeUnit unit) {
        counter++;
        statsHolder.getAndAdd(unit.toMillis(duration));
    }

    @Override
    public Context time() {
        return new SimpleContext(this);
    }

    @Override
    public void update(long value) {
        counter++;
        statsHolder.getAndAdd(value);
    }

    private static final class SimpleContext implements Context {
        private final TimerStats timer;
        private final long startTime;

        private SimpleContext(TimerStats timer) {
            this.timer = timer;
            this.startTime = System.nanoTime();
        }

        @Override
        public long stop() {
            final long elapsed = System.nanoTime() - startTime;
            timer.update(elapsed, TimeUnit.NANOSECONDS);
            return elapsed;
        }

        @Override
        public void close() {
            stop();
        }
    }
}