/**
 * 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 backtype.storm.windowing;

import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import backtype.storm.generated.GlobalStreamId;
import backtype.storm.topology.FailedException;

/**
 * Tracks tuples across input streams and periodically emits watermark events.
 * Watermark event timestamp is the minimum of the latest tuple timestamps
 * across all the input streams (minus the lag). Once a watermark event is emitted
 * any tuple coming with an earlier timestamp can be considered as late events.
 */
public class WaterMarkEventGenerator<T> implements Runnable {
    private static final Logger LOG = LoggerFactory.getLogger(WaterMarkEventGenerator.class);
    private final WindowManager<T> windowManager;
    private final int eventTsLag;
    private final Set<GlobalStreamId> inputStreams;
    private final Map<GlobalStreamId, Long> streamToTs;
    private final ScheduledExecutorService executorService;
    private final int interval;
    private ScheduledFuture<?> executorFuture;
    private long lastWaterMarkTs = 0;

    public WaterMarkEventGenerator(WindowManager<T> windowManager, int interval,
                                   int eventTsLag, Set<GlobalStreamId> inputStreams) {
        this.windowManager = windowManager;
        streamToTs = new ConcurrentHashMap<>();
        executorService = Executors.newSingleThreadScheduledExecutor();
        this.interval = interval;
        this.eventTsLag = eventTsLag;
        this.inputStreams = inputStreams;
    }

    /**
     * Tracks the timestamp of the event in the stream, returns
     * true if the event can be considered for processing or
     * false if its a late event.
     */
    public boolean track(GlobalStreamId stream, long ts) {
        Long currentVal = streamToTs.get(stream);
        if (currentVal == null || ts > currentVal) {
            streamToTs.put(stream, ts);
        }
        checkFailures();
        return ts >= lastWaterMarkTs;
    }

    @Override
    public void run() {
        try {
            long waterMarkTs = computeWaterMarkTs();
            if (waterMarkTs > lastWaterMarkTs) {
                this.windowManager.add(new WaterMarkEvent<T>(waterMarkTs));
                lastWaterMarkTs = waterMarkTs;
            }
        } catch (Throwable th) {
            LOG.error("Failed while processing watermark event ", th);
            throw th;
        }
    }

    /**
     * Computes the min ts across all streams.
     */
    private long computeWaterMarkTs() {
        long ts = 0;
        // only if some data has arrived on each input stream
        if (streamToTs.size() >= inputStreams.size()) {
            ts = Long.MAX_VALUE;
            for (Map.Entry<GlobalStreamId, Long> entry : streamToTs.entrySet()) {
                ts = Math.min(ts, entry.getValue());
            }
        }
        return ts - eventTsLag;
    }

    private void checkFailures() {
        if (executorFuture != null && executorFuture.isDone()) {
            try {
                executorFuture.get();
            } catch (InterruptedException ex) {
                LOG.error("Got exception ", ex);
                throw new FailedException(ex);
            } catch (ExecutionException ex) {
                LOG.error("Got exception ", ex);
                throw new FailedException(ex.getCause());
            }
        }
    }

    public void start() {
        this.executorFuture = executorService.scheduleAtFixedRate(this, interval, interval, TimeUnit.MILLISECONDS);
    }
}