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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 com.twitter.distributedlog.util;

import com.google.common.annotations.VisibleForTesting;

import java.util.concurrent.atomic.AtomicInteger;
import org.apache.bookkeeper.feature.Feature;
import org.apache.bookkeeper.stats.OpStatsLogger;
import org.apache.bookkeeper.stats.StatsLogger;
import org.apache.bookkeeper.stats.Counter;
import org.apache.bookkeeper.stats.Gauge;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Simple counter based {@link PermitLimiter}.
 *
 * Metrics
 * 
 *  `permits`: gauge. how many permits are acquired right now?
 * 
 `permits`/*: opstats. the characteristics about number of permits already acquired on each acquires.
 * 
 `acquireFailure`: counter. how many acquires failed? failure means it already reached maximum permits
 * when trying to acquire.
 * 
 */
public class SimplePermitLimiter implements PermitLimiter {

    final Counter acquireFailureCounter;
    final OpStatsLogger permitsMetric;
    final AtomicInteger permits;
    final int permitsMax;
    final boolean darkmode;
    final Feature disableWriteLimitFeature;

    public SimplePermitLimiter(boolean darkmode, int permitsMax, StatsLogger statsLogger,
                               boolean singleton, Feature disableWriteLimitFeature) {
        this.permits = new AtomicInteger(0);
        this.permitsMax = permitsMax;
        this.darkmode = darkmode;
        this.disableWriteLimitFeature = disableWriteLimitFeature;

        // stats
        if (singleton) {
            statsLogger.registerGauge("num_permits", new Gauge() {
                @Override
                public Number getDefaultValue() {
                    return 0;
                }
                @Override
                public Number getSample() {
                    return permits.get();
                }
            });
        }
        acquireFailureCounter = statsLogger.getCounter("acquireFailure");
        permitsMetric = statsLogger.getOpStatsLogger("permits");
    }

    public boolean isDarkmode() {
        return darkmode || disableWriteLimitFeature.isAvailable();
    }

    @Override
    public boolean acquire() {
        permitsMetric.registerSuccessfulEvent(permits.get());
        if (permits.incrementAndGet() <= permitsMax || isDarkmode()) {
            return true;
        } else {
            acquireFailureCounter.inc();
            permits.decrementAndGet();
            return false;
        }
    }

    @Override
    public void release(int permitsToRelease) {
        permits.addAndGet(-permitsToRelease);
    }

    @VisibleForTesting
    public int getPermits() {
        return permits.get();
    }
}