/*
 * Copyright 2015 Open Networking Laboratory
 *
 * 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 org.onosproject.store.intent.impl;

import org.apache.felix.scr.annotations.Activate;
import org.apache.felix.scr.annotations.Component;
import org.apache.felix.scr.annotations.Deactivate;
import org.apache.felix.scr.annotations.Reference;
import org.apache.felix.scr.annotations.ReferenceCardinality;
import org.apache.felix.scr.annotations.Service;
import org.onosproject.cluster.ClusterEvent;
import org.onosproject.cluster.ClusterEventListener;
import org.onosproject.cluster.ClusterService;
import org.onosproject.cluster.ControllerNode;
import org.onosproject.cluster.Leadership;
import org.onosproject.cluster.LeadershipEvent;
import org.onosproject.cluster.LeadershipEventListener;
import org.onosproject.cluster.LeadershipService;
import org.onosproject.cluster.NodeId;
import org.onosproject.net.intent.Key;
import org.onosproject.net.intent.PartitionService;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.List;
import java.util.Objects;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.stream.Collectors;

/**
 * Manages the assignment of intent keyspace partitions to instances.
 */
@Component(immediate = true)
@Service
public class PartitionManager implements PartitionService {

    private static final Logger log = LoggerFactory.getLogger(PartitionManager.class);

    @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
    protected LeadershipService leadershipService;

    @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
    protected ClusterService clusterService;

    protected final AtomicBoolean rebalanceScheduled = new AtomicBoolean(false);

    static final int NUM_PARTITIONS = 14;
    private static final int BACKOFF_TIME = 2;
    private static final int CHECK_PARTITION_BALANCE_PERIOD_SEC = 10;
    private static final int RETRY_AFTER_DELAY_SEC = 5;

    private static final String ELECTION_PREFIX = "intent-partition-";

    private LeadershipEventListener leaderListener = new InternalLeadershipListener();
    private ClusterEventListener clusterListener = new InternalClusterEventListener();

    private ScheduledExecutorService executor = Executors
            .newScheduledThreadPool(1);

    @Activate
    public void activate() {
        leadershipService.addListener(leaderListener);
        clusterService.addListener(clusterListener);

        for (int i = 0; i < NUM_PARTITIONS; i++) {
            leadershipService.runForLeadership(getPartitionPath(i));
        }

        executor.scheduleAtFixedRate(() -> scheduleRebalance(0), 0,
                                     CHECK_PARTITION_BALANCE_PERIOD_SEC, TimeUnit.SECONDS);
    }

    @Deactivate
    public void deactivate() {
        executor.shutdownNow();

        leadershipService.removeListener(leaderListener);
        clusterService.removeListener(clusterListener);
    }

    /**
     * Sets the specified executor to be used for scheduling background tasks.
     *
     * @param executor scheduled executor service for background tasks
     * @return this PartitionManager
     */
    public PartitionManager withScheduledExecutor(ScheduledExecutorService executor) {
        this.executor = executor;
        return this;
    }

    private String getPartitionPath(int i) {
        return ELECTION_PREFIX + i;
    }

    private String getPartitionPath(PartitionId id) {
        return getPartitionPath(id.value());
    }

    private PartitionId getPartitionForKey(Key intentKey) {
        int partition = Math.abs((int) intentKey.hash()) % NUM_PARTITIONS;
        //TODO investigate Guava consistent hash method
        // ... does it add significant computational complexity? is it worth it?
        //int partition = consistentHash(intentKey.hash(), NUM_PARTITIONS);
        PartitionId id = new PartitionId(partition);
        return id;
    }

    @Override
    public boolean isMine(Key intentKey) {
        return Objects.equals(leadershipService.getLeader(getPartitionPath(getPartitionForKey(intentKey))),
                              clusterService.getLocalNode().id());
    }

    @Override
    public NodeId getLeader(Key intentKey) {
        return leadershipService.getLeader(getPartitionPath(getPartitionForKey(intentKey)));
    }

    protected void doRebalance() {
        rebalanceScheduled.set(false);
        try {
            rebalance();
        } catch (Exception e) {
            log.warn("Exception caught during rebalance task. Will retry in " + RETRY_AFTER_DELAY_SEC + " seconds", e);
            scheduleRebalance(RETRY_AFTER_DELAY_SEC);
        }
    }

    /**
     * Determine whether we have more than our fair share of partitions, and if
     * so, relinquish leadership of some of them for a little while to let
     * other instances take over.
     */
    private void rebalance() {
        int activeNodes = (int) clusterService.getNodes()
                .stream()
                .filter(node -> ControllerNode.State.ACTIVE == clusterService.getState(node.id()))
                .count();

        int myShare = (int) Math.ceil((double) NUM_PARTITIONS / activeNodes);

        List<Leadership> myPartitions = leadershipService.getLeaderBoard().values()
                .stream()
                .filter(l -> clusterService.getLocalNode().id().equals(l.leader()))
                .filter(l -> l.topic().startsWith(ELECTION_PREFIX))
                .collect(Collectors.toList());

        int relinquish = myPartitions.size() - myShare;

        if (relinquish <= 0) {
            return;
        }

        for (int i = 0; i < relinquish; i++) {
            String topic = myPartitions.get(i).topic();
            leadershipService.withdraw(topic);

            executor.schedule(() -> recontest(topic),
                              BACKOFF_TIME, TimeUnit.SECONDS);
        }
    }

    private void scheduleRebalance(int afterDelaySec) {
        if (rebalanceScheduled.compareAndSet(false, true)) {
            executor.schedule(this::doRebalance, afterDelaySec, TimeUnit.SECONDS);
        }
    }

    /**
     * Try and recontest for leadership of a partition.
     *
     * @param path topic name to recontest
     */
    private void recontest(String path) {
        leadershipService.runForLeadership(path);
    }

    private final class InternalLeadershipListener implements LeadershipEventListener {

        @Override
        public void event(LeadershipEvent event) {
            Leadership leadership = event.subject();

            if (Objects.equals(leadership.leader(), clusterService.getLocalNode().id()) &&
                    leadership.topic().startsWith(ELECTION_PREFIX)) {

                // See if we need to let some partitions go
                scheduleRebalance(0);
            }
        }
    }

    private final class InternalClusterEventListener implements
            ClusterEventListener {

        @Override
        public void event(ClusterEvent event) {
            scheduleRebalance(0);
        }
    }
}