/**
 * 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.curator.x.async.migrations;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Throwables;
import org.apache.curator.framework.api.transaction.CuratorOp;
import org.apache.curator.framework.imps.ExtractingCuratorOp;
import org.apache.curator.framework.recipes.locks.InterProcessLock;
import org.apache.curator.framework.recipes.locks.InterProcessSemaphoreMutex;
import org.apache.curator.utils.ZKPaths;
import org.apache.curator.x.async.AsyncCuratorFramework;
import org.apache.zookeeper.CreateMode;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.time.Duration;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CompletionStage;
import java.util.concurrent.Executor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.stream.Collectors;

import static org.apache.curator.x.async.AsyncWrappers.*;

/**
 * Manages migrations
 */
public class MigrationManager
{
    private final AsyncCuratorFramework client;
    private final String lockPath;
    private final String metaDataPath;
    private final Executor executor;
    private final Duration lockMax;

    private static final String META_DATA_NODE_NAME = "meta-";

    /**
     * @param client the curator client
     * @param lockPath base path for locks used by the manager
     * @param metaDataPath base path to store the meta data
     * @param executor the executor to use
     * @param lockMax max time to wait for locks
     */
    public MigrationManager(AsyncCuratorFramework client, String lockPath, String metaDataPath, Executor executor, Duration lockMax)
    {
        this.client = Objects.requireNonNull(client, "client cannot be null");
        this.lockPath = Objects.requireNonNull(lockPath, "lockPath cannot be null");
        this.metaDataPath = Objects.requireNonNull(metaDataPath, "metaDataPath cannot be null");
        this.executor = Objects.requireNonNull(executor, "executor cannot be null");
        this.lockMax = Objects.requireNonNull(lockMax, "lockMax cannot be null");
    }

    /**
     * Process the given migration set
     *
     * @param set the set
     * @return completion stage. If there is a migration-specific error, the stage will be completed
     * exceptionally with {@link org.apache.curator.x.async.migrations.MigrationException}.
     */
    public CompletionStage<Void> migrate(MigrationSet set)
    {
        InterProcessLock lock = new InterProcessSemaphoreMutex(client.unwrap(), ZKPaths.makePath(lockPath, set.id()));
        CompletionStage<Void> lockStage = lockAsync(lock, lockMax.toMillis(), TimeUnit.MILLISECONDS, executor);
        return lockStage.thenCompose(__ -> runMigrationInLock(lock, set));
    }

    /**
     * Can be overridden to change how the comparison to previous migrations is done. The default
     * version ensures that the meta data from previous migrations matches the current migration
     * set exactly (by order and version). If there is a mismatch, <code>MigrationException</code> is thrown.
     *
     * @param set the migration set being applied
     * @param operationHashesInOrder previous operation hashes (may be empty)
     * @return the list of actual migrations to perform. The filter can return any value here or an empty list.
     * @throws MigrationException errors
     */
    protected List<Migration> filter(MigrationSet set, List<byte[]> operationHashesInOrder) throws MigrationException
    {
        if ( operationHashesInOrder.size() > set.migrations().size() )
        {
            throw new MigrationException(set.id(), String.format("More metadata than migrations. Migration ID: %s", set.id()));
        }

        int compareSize = Math.min(set.migrations().size(), operationHashesInOrder.size());
        for ( int i = 0; i < compareSize; ++i )
        {
            byte[] setHash = hash(set.migrations().get(i).operations());
            if ( !Arrays.equals(setHash, operationHashesInOrder.get(i)) )
            {
                throw new MigrationException(set.id(), String.format("Metadata mismatch. Migration ID: %s", set.id()));
            }
        }
        return set.migrations().subList(operationHashesInOrder.size(), set.migrations().size());
    }

    private byte[] hash(List<CuratorOp> operations)
    {
        MessageDigest digest;
        try
        {
            digest = MessageDigest.getInstance("SHA-256");
        }
        catch ( NoSuchAlgorithmException e )
        {
            throw new RuntimeException(e);
        }
        operations.forEach(op -> {
            if ( op instanceof ExtractingCuratorOp )
            {
                ((ExtractingCuratorOp)op).addToDigest(digest);
            }
            else
            {
                digest.update(op.toString().getBytes());
            }
        });
        return digest.digest();
    }

    private CompletionStage<Void> runMigrationInLock(InterProcessLock lock, MigrationSet set)
    {
        String thisMetaDataPath = ZKPaths.makePath(metaDataPath, set.id());
        return childrenWithData(client, thisMetaDataPath)
            .thenCompose(metaData -> applyMetaData(set, metaData, thisMetaDataPath))
            .handle((v, e) -> {
                release(lock, true);
                if ( e != null )
                {
                    Throwables.propagate(e);
                }
                return v;
            }
        );
    }

    private CompletionStage<Void> applyMetaData(MigrationSet set, Map<String, byte[]> metaData, String thisMetaDataPath)
    {
        List<byte[]> sortedMetaData = metaData.keySet()
            .stream()
            .sorted(Comparator.naturalOrder())
            .map(metaData::get)
            .collect(Collectors.toList());

        List<Migration> toBeApplied;
        try
        {
            toBeApplied = filter(set, sortedMetaData);
        }
        catch ( MigrationException e )
        {
            CompletableFuture<Void> future = new CompletableFuture<>();
            future.completeExceptionally(e);
            return future;
        }

        if ( toBeApplied.size() == 0 )
        {
            return CompletableFuture.completedFuture(null);
        }

        return asyncEnsureContainers(client, thisMetaDataPath)
            .thenCompose(__ -> applyMetaDataAfterEnsure(toBeApplied, thisMetaDataPath));
    }

    @VisibleForTesting
    volatile AtomicInteger debugCount = null;

    private CompletionStage<Void> applyMetaDataAfterEnsure(List<Migration> toBeApplied, String thisMetaDataPath)
    {
        if ( debugCount != null )
        {
            debugCount.incrementAndGet();
        }

        List<CuratorOp> operations = new ArrayList<>();
        String metaDataBasePath = ZKPaths.makePath(thisMetaDataPath, META_DATA_NODE_NAME);
        toBeApplied.forEach(migration -> {
            List<CuratorOp> thisMigrationOperations = migration.operations();
            operations.addAll(thisMigrationOperations);
            operations.add(client.transactionOp().create().withMode(CreateMode.PERSISTENT_SEQUENTIAL).forPath(metaDataBasePath, hash(thisMigrationOperations)));
        });
        return client.transaction().forOperations(operations).thenApply(__ -> null);
    }
}
