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/**
 * Copyright 2013 Netflix, 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.netflix.servo.monitor;

import com.netflix.servo.annotations.DataSourceType;
import com.netflix.servo.util.Clock;
import com.netflix.servo.util.ClockWithOffset;

import java.util.concurrent.atomic.AtomicLong;

/**
 * Gauge that keeps track of the maximum value seen since the last reset. Updates should be
 * non-negative, the reset value is 0.
 */
public class MaxGauge extends AbstractMonitor
        implements Gauge {
    private final StepLong max;

    /** Creates a new instance of the gauge. */
    public MaxGauge(MonitorConfig config) {
        this(config, ClockWithOffset.INSTANCE);
    }

    /** Creates a new instance of the gauge using a specific clock. Useful for unit testing. */
    MaxGauge(MonitorConfig config, Clock clock) {
        super(config.withAdditionalTag(DataSourceType.GAUGE));
        max = new StepLong(0L, clock);
    }

    /**
     * Update the max for the given index if the provided value is larger than the current max.
     */
    private void updateMax(int idx, long v) {
        AtomicLong current = max.getCurrent(idx);
        long m = current.get();
        while (v > m) {
            if (current.compareAndSet(m, v)) {
                break;
            }
            m = current.get();
        }
    }

    /**
     * Update the max if the provided value is larger than the current max.
     */
    public void update(long v) {
        for (int i = 0; i < Pollers.NUM_POLLERS; ++i) {
            updateMax(i, v);
        }
    }

    /** {@inheritDoc} */
    @Override
    public Long getValue(int nth) {
        return max.poll(nth).getValue();
    }

    /**
     * Returns the current max value since the last reset.
     */
    public long getCurrentValue(int nth) {
        return max.getCurrent(nth).get();
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null || !(obj instanceof MaxGauge)) {
            return false;
        }
        MaxGauge m = (MaxGauge) obj;
        return config.equals(m.getConfig()) && getValue(0).equals(m.getValue(0));
    }

    /** {@inheritDoc} */
    @Override
    public int hashCode() {
        int result = getConfig().hashCode();
        result = 31 * result + getValue(0).hashCode();
        return result;
    }

    /** {@inheritDoc} */
    @Override
    public String toString() {
        return "MaxGauge{config=" + config + ", max=" + max + '}';
    }
}