/**
 * 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.storm.windowing;

import java.util.concurrent.atomic.AtomicLong;
import org.apache.storm.streams.Pair;

/**
 * An eviction policy that tracks count based on watermark ts and evicts events up to the watermark based on a threshold count.
 *
 * @param <T> the type of event tracked by this policy.
 */
public class WatermarkCountEvictionPolicy<T> implements EvictionPolicy<T, Pair<Long, Long>> {
    protected final int threshold;
    protected final AtomicLong currentCount;
    private EvictionContext context;

    private volatile long processed;

    public WatermarkCountEvictionPolicy(int count) {
        threshold = count;
        currentCount = new AtomicLong();
    }

    public Action evict(Event<T> event) {
        if (getContext() == null) {
            //It is possible to get asked about eviction before we have a context, due to WindowManager.compactWindow.
            //In this case we should hold on to all the events. When the first watermark is received, the context will be set,
            //and the events will be reevaluated for eviction
            return Action.STOP;
        }

        Action action;
        if (event.getTimestamp() <= getContext().getReferenceTime() && processed < currentCount.get()) {
            action = doEvict(event);
            if (action == Action.PROCESS) {
                ++processed;
            }
        } else {
            action = Action.KEEP;
        }
        return action;
    }

    private Action doEvict(Event<T> event) {
        /*
         * atomically decrement the count if its greater than threshold and
         * return if the event should be evicted
         */
        while (true) {
            long curVal = currentCount.get();
            if (curVal > threshold) {
                if (currentCount.compareAndSet(curVal, curVal - 1)) {
                    return Action.EXPIRE;
                }
            } else {
                break;
            }
        }
        return Action.PROCESS;
    }

    @Override
    public void track(Event<T> event) {
        // NOOP
    }

    @Override
    public EvictionContext getContext() {
        return context;
    }

    @Override
    public void setContext(EvictionContext context) {
        this.context = context;
        if (context.getCurrentCount() != null) {
            currentCount.set(context.getCurrentCount());
        } else {
            currentCount.set(processed + context.getSlidingCount());
        }
        processed = 0;
    }

    @Override
    public void reset() {
        processed = 0;
    }

    @Override
    public Pair<Long, Long> getState() {
        return Pair.of(currentCount.get(), processed);
    }

    @Override
    public void restoreState(Pair<Long, Long> state) {
        currentCount.set(state.getFirst());
        processed = state.getSecond();
    }

    @Override
    public String toString() {
        return "WatermarkCountEvictionPolicy{" +
               "} " + super.toString();
    }
}
