Copyright 2018 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.
/**
 * Copyright 2018 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.concurrency.limits.limit;

import com.netflix.concurrency.limits.Limit;
import com.netflix.concurrency.limits.internal.Preconditions;
import com.netflix.concurrency.limits.limit.window.AverageSampleWindowFactory;
import com.netflix.concurrency.limits.limit.window.SampleWindow;
import com.netflix.concurrency.limits.limit.window.SampleWindowFactory;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Consumer;

public class WindowedLimit implements Limit {
    private static final long DEFAULT_MIN_WINDOW_TIME = TimeUnit.SECONDS.toNanos(1);
    private static final long DEFAULT_MAX_WINDOW_TIME = TimeUnit.SECONDS.toNanos(1);
    private static final long DEFAULT_MIN_RTT_THRESHOLD = TimeUnit.MICROSECONDS.toNanos(100);

    
Minimum observed samples to filter out sample windows with not enough significant samples
/**
     * Minimum observed samples to filter out sample windows with not enough significant samples
     */
    private static final int DEFAULT_WINDOW_SIZE = 10;

    public static Builder newBuilder() {
        return new Builder();
    }

    public static class Builder {
        private long maxWindowTime = DEFAULT_MAX_WINDOW_TIME;
        private long minWindowTime = DEFAULT_MIN_WINDOW_TIME;
        private int windowSize = DEFAULT_WINDOW_SIZE;
        private long minRttThreshold = DEFAULT_MIN_RTT_THRESHOLD;
        private SampleWindowFactory sampleWindowFactory = AverageSampleWindowFactory.create();

        
Minimum window duration for sampling a new minRtt
/**
         * Minimum window duration for sampling a new minRtt
         */
        public Builder minWindowTime(long minWindowTime, TimeUnit units) {
            Preconditions.checkArgument(units.toMillis(minWindowTime) >= 100, "minWindowTime must be >= 100 ms");
            this.minWindowTime = units.toNanos(minWindowTime);
            return this;
        }

        
Maximum window duration for sampling a new minRtt
/**
         * Maximum window duration for sampling a new minRtt
         */
        public Builder maxWindowTime(long maxWindowTime, TimeUnit units) {
            Preconditions.checkArgument(units.toMillis(maxWindowTime) >= 100, "maxWindowTime must be >= 100 ms");
            this.maxWindowTime = units.toNanos(maxWindowTime);
            return this;
        }

        
Minimum sampling window size for finding a new minimum rtt
/**
         * Minimum sampling window size for finding a new minimum rtt
         */
        public Builder windowSize(int windowSize) {
            Preconditions.checkArgument(windowSize >= 10, "Window size must be >= 10");
            this.windowSize = windowSize;
            return this;
        }

        public Builder minRttThreshold(long threshold, TimeUnit units) {
            this.minRttThreshold = units.toNanos(threshold);
            return this;
        }

        public Builder sampleWindowFactory(SampleWindowFactory sampleWindowFactory) {
            this.sampleWindowFactory = sampleWindowFactory;
            return this;
        }

        public WindowedLimit build(Limit delegate) {
            return new WindowedLimit(this, delegate);
        }
    }

    private final Limit delegate;

    
End time for the sampling window at which point the limit should be updated
/**
     * End time for the sampling window at which point the limit should be updated
     */
    private volatile long nextUpdateTime = 0;

    private final long minWindowTime;

    private final long maxWindowTime;

    private final int windowSize;

    private final long minRttThreshold;

    private final Object lock = new Object();

    private final SampleWindowFactory sampleWindowFactory;

    
Object tracking stats for the current sample window
/**
     * Object tracking stats for the current sample window
     */
    private final AtomicReference<SampleWindow> sample;

    private WindowedLimit(Builder builder, Limit delegate) {
        this.delegate = delegate;
        this.minWindowTime = builder.minWindowTime;
        this.maxWindowTime = builder.maxWindowTime;
        this.windowSize = builder.windowSize;
        this.minRttThreshold = builder.minRttThreshold;
        this.sampleWindowFactory = builder.sampleWindowFactory;
        this.sample = new AtomicReference<>(sampleWindowFactory.newInstance());
    }

    @Override
    public void notifyOnChange(Consumer<Integer> consumer) {
        delegate.notifyOnChange(consumer);
    }

    @Override
    public void onSample(long startTime, long rtt, int inflight, boolean didDrop) {
        long endTime = startTime + rtt;

        if (rtt < minRttThreshold) {
            return;
        }

        sample.updateAndGet(current -> current.addSample(rtt, inflight, didDrop));

        if (endTime > nextUpdateTime) {
            synchronized (lock) {
                // Double check under the lock
                if (endTime > nextUpdateTime) {
                    SampleWindow current = sample.getAndSet(sampleWindowFactory.newInstance());
                    nextUpdateTime = endTime + Math.min(Math.max(current.getCandidateRttNanos() * 2, minWindowTime), maxWindowTime);

                    if (isWindowReady(current)) {
                        delegate.onSample(startTime, current.getTrackedRttNanos(), current.getMaxInFlight(), current.didDrop());
                    }
                }
            }
        }
    }

    private boolean isWindowReady(SampleWindow sample) {
        return sample.getCandidateRttNanos() < Long.MAX_VALUE && sample.getSampleCount() >= windowSize;
    }

    @Override
    public int getLimit() {
        return delegate.getLimit();
    }
}