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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.flink.runtime.metrics;

import org.apache.flink.annotation.VisibleForTesting;
import org.apache.flink.metrics.Gauge;
import org.apache.flink.metrics.View;
import org.apache.flink.util.clock.Clock;
import org.apache.flink.util.clock.SystemClock;

import java.util.ArrayList;
import java.util.Collection;

/**
 * {@link TimerGauge} measures how much time is spent in a given state, with entry into that state
 * being signaled by {@link #markStart()}. Measuring is stopped by {@link #markEnd()}. This class in
 * particularly takes care of the case, when {@link #update()} is called when some measurement
 * started but has not yet finished. For example even if next {@link #markEnd()} call is expected to
 * happen in a couple of hours, the returned value will account for this ongoing measurement.
 */
public class TimerGauge implements Gauge, View {

    private static final int DEFAULT_TIME_SPAN_IN_SECONDS = 60;

    private final Clock clock;

    private final Collection startStopListeners = new ArrayList<>();

    /** The time-span over which the average is calculated. */
    private final int timeSpanInSeconds;
    /** Circular array containing the history of values. */
    private final long[] values;
    /** The index in the array for the current time. */
    private int idx = 0;

    private boolean fullWindow = false;

    private long currentValue;
    private long currentCount;
    private long currentMeasurementStartTS;
    /**
     * This differ from {@link #currentMeasurementStartTS} that {@link #currentUpdateTS} is bumped
     * on every {@link #update()} call, while {@link #currentMeasurementStartTS} always marks the
     * last {@link #markStart()} call.
     */
    private long currentUpdateTS;

    private long previousMaxSingleMeasurement;
    private long currentMaxSingleMeasurement;

    private long accumulatedCount;

    public TimerGauge() {
        this(DEFAULT_TIME_SPAN_IN_SECONDS);
    }

    public TimerGauge(int timeSpanInSeconds) {
        this(SystemClock.getInstance(), timeSpanInSeconds);
    }

    public TimerGauge(Clock clock) {
        this(clock, DEFAULT_TIME_SPAN_IN_SECONDS);
    }

    public TimerGauge(Clock clock, int timeSpanInSeconds) {
        this.clock = clock;
        this.timeSpanInSeconds =
                Math.max(
                        timeSpanInSeconds - (timeSpanInSeconds % UPDATE_INTERVAL_SECONDS),
                        UPDATE_INTERVAL_SECONDS);
        this.values = new long[this.timeSpanInSeconds / UPDATE_INTERVAL_SECONDS];
    }

    public synchronized void registerListener(StartStopListener listener) {
        if (currentMeasurementStartTS != 0) {
            listener.markStart();
        }
        startStopListeners.add(listener);
    }

    public synchronized void unregisterListener(StartStopListener listener) {
        if (currentMeasurementStartTS != 0) {
            listener.markEnd();
        }
        startStopListeners.remove(listener);
    }

    public synchronized void markStart() {
        if (currentMeasurementStartTS != 0) {
            return;
        }
        currentUpdateTS = clock.absoluteTimeMillis();
        currentMeasurementStartTS = currentUpdateTS;
        for (StartStopListener startStopListener : startStopListeners) {
            startStopListener.markStart();
        }
    }

    public synchronized void markEnd() {
        if (currentMeasurementStartTS == 0) {
            return;
        }
        long now = clock.absoluteTimeMillis();
        long currentMeasurement = now - currentMeasurementStartTS;
        long currentIncrement = now - currentUpdateTS;
        currentCount += currentIncrement;
        accumulatedCount += currentIncrement;
        currentMaxSingleMeasurement = Math.max(currentMaxSingleMeasurement, currentMeasurement);
        currentUpdateTS = 0;
        currentMeasurementStartTS = 0;
        for (StartStopListener startStopListener : startStopListeners) {
            startStopListener.markEnd();
        }
    }

    @Override
    public synchronized void update() {
        if (currentMeasurementStartTS != 0) {
            long now = clock.absoluteTimeMillis();
            // we adding to the current count only the time elapsed since last markStart or update
            // call
            currentCount += now - currentUpdateTS;
            accumulatedCount += now - currentUpdateTS;
            currentUpdateTS = now;
            // on the other hand, max measurement has to be always checked against last markStart
            // call
            currentMaxSingleMeasurement =
                    Math.max(currentMaxSingleMeasurement, now - currentMeasurementStartTS);
        }
        updateCurrentValue();
        previousMaxSingleMeasurement = currentMaxSingleMeasurement;
        currentCount = 0;
        currentMaxSingleMeasurement = 0;
    }

    private void updateCurrentValue() {
        if (idx == values.length - 1) {
            fullWindow = true;
        }
        values[idx] = currentCount;
        idx = (idx + 1) % values.length;

        int maxIndex = fullWindow ? values.length : idx;
        long totalTime = 0;
        for (int i = 0; i < maxIndex; i++) {
            totalTime += values[i];
        }

        currentValue =
                Math.max(Math.min(totalTime / (UPDATE_INTERVAL_SECONDS * maxIndex), 1000), 0);
    }

    @Override
    public synchronized Long getValue() {
        return currentValue;
    }

    /**
     * @return the longest marked period as measured by the given * TimerGauge. For example the
     *     longest consecutive back pressured period.
     */
    public synchronized long getMaxSingleMeasurement() {
        return previousMaxSingleMeasurement;
    }

    /** @return the accumulated period by the given * TimerGauge. */
    public synchronized long getAccumulatedCount() {
        return accumulatedCount;
    }

    @VisibleForTesting
    public synchronized long getCount() {
        return currentCount;
    }

    @VisibleForTesting
    public synchronized boolean isMeasuring() {
        return currentMeasurementStartTS != 0;
    }

    /**
     * Listens for {@link TimerGauge#markStart()} and {@link TimerGauge#markEnd()} events.
     *
     * Beware! As it is right now, {@link StartStopListener} is notified under the {@link
     * TimerGauge}'s lock, so those callbacks should be very short, without long call stacks that
     * acquire more locks. Otherwise, a potential for deadlocks can be introduced.
     */
    public interface StartStopListener {
        void markStart();

        void markEnd();
    }
}