package it.cavallium.dbengine.database.disk;

import static it.cavallium.dbengine.client.UninterruptibleScheduler.uninterruptibleScheduler;
import static it.cavallium.dbengine.database.LLUtils.MARKER_LUCENE;
import static it.cavallium.dbengine.database.LLUtils.toDocument;
import static it.cavallium.dbengine.database.LLUtils.toFields;
import static it.cavallium.dbengine.lucene.searcher.LLSearchTransformer.NO_TRANSFORMATION;

import io.micrometer.core.instrument.Counter;
import io.micrometer.core.instrument.MeterRegistry;
import io.micrometer.core.instrument.Timer;
import io.net5.buffer.api.Resource;
import io.net5.buffer.api.Send;
import it.cavallium.dbengine.client.DirectIOOptions;
import it.cavallium.dbengine.client.IndicizerAnalyzers;
import it.cavallium.dbengine.client.IndicizerSimilarities;
import it.cavallium.dbengine.client.LuceneOptions;
import it.cavallium.dbengine.client.NRTCachingOptions;
import it.cavallium.dbengine.client.query.QueryParser;
import it.cavallium.dbengine.client.query.current.data.Query;
import it.cavallium.dbengine.client.query.current.data.QueryParams;
import it.cavallium.dbengine.database.LLIndexRequest;
import it.cavallium.dbengine.database.LLLuceneIndex;
import it.cavallium.dbengine.database.LLSearchResultShard;
import it.cavallium.dbengine.database.LLSnapshot;
import it.cavallium.dbengine.database.LLSoftUpdateDocument;
import it.cavallium.dbengine.database.LLTerm;
import it.cavallium.dbengine.database.LLUpdateDocument;
import it.cavallium.dbengine.database.LLUpdateFields;
import it.cavallium.dbengine.database.LLUtils;
import it.cavallium.dbengine.lucene.AlwaysDirectIOFSDirectory;
import it.cavallium.dbengine.lucene.LuceneHacks;
import it.cavallium.dbengine.lucene.LuceneUtils;
import it.cavallium.dbengine.lucene.collector.Buckets;
import it.cavallium.dbengine.lucene.searcher.AdaptiveLocalSearcher;
import it.cavallium.dbengine.lucene.searcher.BucketParams;
import it.cavallium.dbengine.lucene.searcher.DecimalBucketMultiSearcher;
import it.cavallium.dbengine.lucene.searcher.LLSearchTransformer;
import it.cavallium.dbengine.lucene.searcher.LocalQueryParams;
import it.cavallium.dbengine.lucene.searcher.LocalSearcher;
import java.io.IOException;
import java.nio.file.Path;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Objects;
import java.util.Set;
import java.util.concurrent.Callable;
import java.util.concurrent.Phaser;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.locks.ReentrantLock;
import org.apache.commons.lang3.time.StopWatch;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.apache.lucene.analysis.miscellaneous.PerFieldAnalyzerWrapper;
import org.apache.lucene.index.ConcurrentMergeScheduler;
import org.apache.lucene.index.IndexWriter;
import org.apache.lucene.index.IndexWriterConfig;
import org.apache.lucene.index.KeepOnlyLastCommitDeletionPolicy;
import org.apache.lucene.index.MergeScheduler;
import org.apache.lucene.index.SerialMergeScheduler;
import org.apache.lucene.index.SimpleMergedSegmentWarmer;
import org.apache.lucene.index.SnapshotDeletionPolicy;
import org.apache.lucene.index.TieredMergePolicy;
import org.apache.lucene.misc.store.DirectIODirectory;
import org.apache.lucene.search.similarities.Similarity;
import org.apache.lucene.store.ByteBuffersDirectory;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.FSDirectory;
import org.apache.lucene.store.MMapDirectory;
import org.apache.lucene.store.NIOFSDirectory;
import org.apache.lucene.store.NRTCachingDirectory;
import org.apache.lucene.util.Constants;
import org.apache.lucene.util.InfoStream;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;
import reactor.core.scheduler.Scheduler;
import reactor.core.scheduler.Schedulers;
import reactor.util.function.Tuple2;

public class LLLocalLuceneIndex implements LLLuceneIndex {

	protected static final Logger logger = LogManager.getLogger(LLLocalLuceneIndex.class);
	private final LocalSearcher localSearcher;
	private final DecimalBucketMultiSearcher decimalBucketMultiSearcher = new DecimalBucketMultiSearcher();
	/**
	 * Global lucene index scheduler.
	 * There is only a single thread globally to not overwhelm the disk with
	 * concurrent commits or concurrent refreshes.
	 */
	private static final ReentrantLock shutdownLock = new ReentrantLock();
	private static final Scheduler luceneHeavyTasksScheduler = uninterruptibleScheduler(Schedulers.single(Schedulers.boundedElastic()));

	private final Counter startedDocIndexings;
	private final Counter endeddDocIndexings;
	private final Timer docIndexingTime;
	private final Timer snapshotTime;
	private final Timer flushTime;
	private final Timer commitTime;
	private final Timer mergeTime;
	private final Timer refreshTime;

	private final String luceneIndexName;
	private final IndexWriter indexWriter;
	private final SnapshotsManager snapshotsManager;
	private final IndexSearcherManager searcherManager;
	private final PerFieldAnalyzerWrapper luceneAnalyzer;
	private final Similarity luceneSimilarity;
	private final Directory directory;
	private final boolean lowMemory;

	private final Phaser activeTasks = new Phaser(1);
	private final AtomicBoolean closeRequested = new AtomicBoolean();

	public LLLocalLuceneIndex(LLTempLMDBEnv env,
			@Nullable Path luceneBasePath,
			MeterRegistry meterRegistry,
			@Nullable String clusterName,
			@Nullable String shardName,
			IndicizerAnalyzers indicizerAnalyzers,
			IndicizerSimilarities indicizerSimilarities,
			LuceneOptions luceneOptions,
			@Nullable LuceneHacks luceneHacks) throws IOException {
		if (clusterName == null && shardName == null) {
			throw new IllegalArgumentException("Clustern name and/or shard name must be set");
		}
		String logName = Objects.requireNonNullElse(clusterName, shardName);
		String luceneIndexName = Objects.requireNonNullElse(shardName, clusterName);

		Path directoryPath;
		if (luceneOptions.inMemory() != (luceneBasePath == null)) {
			throw new IllegalArgumentException();
		} else if (luceneBasePath != null) {
			directoryPath = luceneBasePath.resolve(shardName + ".lucene.db");
		} else {
			directoryPath = null;
		}
		if (luceneIndexName.length() == 0) {
			throw new IOException("Empty lucene database name");
		}
		if (!MMapDirectory.UNMAP_SUPPORTED) {
			logger.error("Unmap is unsupported, lucene will run slower: {}", MMapDirectory.UNMAP_NOT_SUPPORTED_REASON);
		} else {
			logger.debug("Lucene MMap is supported");
		}
		boolean lowMemory = luceneOptions.lowMemory();
		if (luceneOptions.inMemory()) {
			this.directory = new ByteBuffersDirectory();
		} else {
			Directory directory;
			{
				Directory forcedDirectFsDirectory = null;
				if (luceneOptions.directIOOptions().isPresent()) {
					DirectIOOptions directIOOptions = luceneOptions.directIOOptions().get();
						if (directIOOptions.alwaysForceDirectIO()) {
							try {
								forcedDirectFsDirectory = new AlwaysDirectIOFSDirectory(directoryPath);
							} catch (UnsupportedOperationException ex) {
								logger.warn("Failed to open FSDirectory with DIRECT flag", ex);
							}
						}
				}
				if (forcedDirectFsDirectory != null) {
					directory = forcedDirectFsDirectory;
				} else {
					FSDirectory fsDirectory;
					if (luceneOptions.allowMemoryMapping()) {
						fsDirectory = FSDirectory.open(directoryPath);
					} else {
						fsDirectory = new NIOFSDirectory(directoryPath);
					}
					if (Constants.LINUX || Constants.MAC_OS_X) {
						try {
							int mergeBufferSize;
							long minBytesDirect;
							if (luceneOptions.directIOOptions().isPresent()) {
								var directIOOptions = luceneOptions.directIOOptions().get();
								mergeBufferSize = directIOOptions.mergeBufferSize();
								minBytesDirect = directIOOptions.minBytesDirect();
							} else {
								mergeBufferSize = DirectIODirectory.DEFAULT_MERGE_BUFFER_SIZE;
								minBytesDirect = DirectIODirectory.DEFAULT_MIN_BYTES_DIRECT;
							}
							directory = new DirectIODirectory(fsDirectory, mergeBufferSize, minBytesDirect);
						} catch (UnsupportedOperationException ex) {
							logger.warn("Failed to open FSDirectory with DIRECT flag", ex);
							directory = fsDirectory;
						}
					} else {
						directory = fsDirectory;
					}
				}
			}

			if (luceneOptions.nrtCachingOptions().isPresent()) {
				NRTCachingOptions nrtCachingOptions = luceneOptions.nrtCachingOptions().get();
				directory = new NRTCachingDirectory(directory, nrtCachingOptions.maxMergeSizeMB(),
						nrtCachingOptions.maxCachedMB());
			}

			this.directory = directory;
		}

		this.luceneIndexName = luceneIndexName;
		var snapshotter = new SnapshotDeletionPolicy(new KeepOnlyLastCommitDeletionPolicy());
		this.lowMemory = lowMemory;
		this.luceneAnalyzer = LuceneUtils.toPerFieldAnalyzerWrapper(indicizerAnalyzers);
		this.luceneSimilarity = LuceneUtils.toPerFieldSimilarityWrapper(indicizerSimilarities);

		var useLMDB = luceneOptions.allowNonVolatileCollection();
		var maxInMemoryResultEntries = luceneOptions.maxInMemoryResultEntries();
		if (luceneHacks != null && luceneHacks.customLocalSearcher() != null) {
			localSearcher = luceneHacks.customLocalSearcher().get();
		} else {
			localSearcher = new AdaptiveLocalSearcher(env, useLMDB, maxInMemoryResultEntries);
		}

		var indexWriterConfig = new IndexWriterConfig(luceneAnalyzer);
		indexWriterConfig.setOpenMode(IndexWriterConfig.OpenMode.CREATE_OR_APPEND);
		indexWriterConfig.setIndexDeletionPolicy(snapshotter);
		indexWriterConfig.setCommitOnClose(true);
		var mergePolicy = new TieredMergePolicy();
		indexWriterConfig.setMergePolicy(mergePolicy);
		indexWriterConfig.setMergedSegmentWarmer(new SimpleMergedSegmentWarmer(InfoStream.getDefault()));
		int writerSchedulerMaxThreadCount;
		MergeScheduler mergeScheduler;
		if (lowMemory) {
			mergeScheduler = new SerialMergeScheduler();
			writerSchedulerMaxThreadCount = 1;
		} else {
			var concurrentMergeScheduler = new ConcurrentMergeScheduler();
			// false means SSD, true means HDD
			concurrentMergeScheduler.setDefaultMaxMergesAndThreads(false);
			if (luceneOptions.inMemory()) {
				concurrentMergeScheduler.disableAutoIOThrottle();
			} else {
				concurrentMergeScheduler.enableAutoIOThrottle();
			}
			writerSchedulerMaxThreadCount = concurrentMergeScheduler.getMaxThreadCount();
			mergeScheduler = concurrentMergeScheduler;
		}
		logger.trace("WriterSchedulerMaxThreadCount: {}", writerSchedulerMaxThreadCount);
		indexWriterConfig.setMergeScheduler(mergeScheduler);
		if (luceneOptions.indexWriterBufferSize() == -1) {
			//todo: allow to configure maxbuffereddocs fallback
			indexWriterConfig.setMaxBufferedDocs(80000);
			// disable ram buffer size after enabling maxBufferedDocs
			indexWriterConfig.setRAMBufferSizeMB(-1);
		} else {
			indexWriterConfig.setRAMBufferSizeMB(luceneOptions.indexWriterBufferSize() / 1024D / 1024D);
		}
		indexWriterConfig.setReaderPooling(false);
		indexWriterConfig.setSimilarity(getLuceneSimilarity());
		this.indexWriter = new IndexWriter(directory, indexWriterConfig);
		this.snapshotsManager = new SnapshotsManager(indexWriter, snapshotter);
		this.searcherManager = new CachedIndexSearcherManager(indexWriter,
				snapshotsManager,
				getLuceneSimilarity(),
				luceneOptions.applyAllDeletes(),
				luceneOptions.writeAllDeletes(),
				luceneOptions.queryRefreshDebounceTime()
		);

		this.startedDocIndexings = meterRegistry.counter("index.write.doc.started.counter", "index.name", logName);
		this.endeddDocIndexings = meterRegistry.counter("index.write.doc.ended.counter", "index.name", logName);
		this.docIndexingTime = Timer.builder("index.write.doc.timer").publishPercentiles(0.2, 0.5, 0.95).publishPercentileHistogram().tag("index.name", logName).register(meterRegistry);
		this.snapshotTime = Timer.builder("index.write.snapshot.timer").publishPercentiles(0.2, 0.5, 0.95).publishPercentileHistogram().tag("index.name", logName).register(meterRegistry);
		this.flushTime = Timer.builder("index.write.flush.timer").publishPercentiles(0.2, 0.5, 0.95).publishPercentileHistogram().tag("index.name", logName).register(meterRegistry);
		this.commitTime = Timer.builder("index.write.commit.timer").publishPercentiles(0.2, 0.5, 0.95).publishPercentileHistogram().tag("index.name", logName).register(meterRegistry);
		this.mergeTime = Timer.builder("index.write.merge.timer").publishPercentiles(0.2, 0.5, 0.95).publishPercentileHistogram().tag("index.name", logName).register(meterRegistry);
		this.refreshTime = Timer.builder("index.search.refresh.timer").publishPercentiles(0.2, 0.5, 0.95).publishPercentileHistogram().tag("index.name", logName).register(meterRegistry);

		// Start scheduled tasks
		var commitMillis = luceneOptions.commitDebounceTime().toMillis();
		luceneHeavyTasksScheduler.schedulePeriodically(this::scheduledCommit, commitMillis, commitMillis,
				TimeUnit.MILLISECONDS);
	}

	private Similarity getLuceneSimilarity() {
		return luceneSimilarity;
	}

	@Override
	public String getLuceneIndexName() {
		return luceneIndexName;
	}

	@Override
	public Mono<LLSnapshot> takeSnapshot() {
		return snapshotsManager.takeSnapshot().elapsed().map(elapsed -> {
			snapshotTime.record(elapsed.getT1(), TimeUnit.MILLISECONDS);
			return elapsed.getT2();
		}).transform(this::ensureOpen);
	}

	private <V> Mono<V> ensureOpen(Mono<V> mono) {
		return Mono.<Void>fromCallable(() -> {
			if (closeRequested.get()) {
				throw new IllegalStateException("Lucene index is closed");
			} else {
				return null;
			}
		}).then(mono).doFirst(activeTasks::register).doFinally(s -> activeTasks.arriveAndDeregister());
	}

	private <V> Mono<V> runSafe(Callable<V> callable) {
		return Mono
				.fromCallable(callable)
				.subscribeOn(uninterruptibleScheduler(Schedulers.boundedElastic()))
				.publishOn(Schedulers.parallel());
	}

	@Override
	public Mono<Void> releaseSnapshot(LLSnapshot snapshot) {
		return snapshotsManager
				.releaseSnapshot(snapshot)
				.elapsed()
				.doOnNext(elapsed -> snapshotTime.record(elapsed.getT1(), TimeUnit.MILLISECONDS))
				.then();
	}

	@Override
	public Mono<Void> addDocument(LLTerm key, LLUpdateDocument doc) {
		return this.<Void>runSafe(() -> docIndexingTime.recordCallable(() -> {
			startedDocIndexings.increment();
			try {
				indexWriter.addDocument(toDocument(doc));
			} finally {
				endeddDocIndexings.increment();
			}
			return null;
		})).transform(this::ensureOpen);
	}

	@Override
	public Mono<Void> addDocuments(Flux<Entry<LLTerm, LLUpdateDocument>> documents) {
		return documents.collectList().flatMap(documentsList -> this.<Void>runSafe(() -> {
			var count = documentsList.size();
			StopWatch stopWatch = StopWatch.createStarted();
			try {
				startedDocIndexings.increment(count);
				try {
					indexWriter.addDocuments(LLUtils.toDocumentsFromEntries(documentsList));
				} finally {
					endeddDocIndexings.increment(count);
				}
			} finally {
				docIndexingTime.record(stopWatch.getTime(TimeUnit.MILLISECONDS) / Math.max(count, 1), TimeUnit.MILLISECONDS);
			}
			return null;
		})).transform(this::ensureOpen);
	}


	@Override
	public Mono<Void> deleteDocument(LLTerm id) {
		return this.<Void>runSafe(() -> docIndexingTime.recordCallable(() -> {
			startedDocIndexings.increment();
			try {
				indexWriter.deleteDocuments(LLUtils.toTerm(id));
			} finally {
				endeddDocIndexings.increment();
			}
			return null;
		})).transform(this::ensureOpen);
	}

	@Override
	public Mono<Void> update(LLTerm id, LLIndexRequest request) {
		return this
				.<Void>runSafe(() -> docIndexingTime.recordCallable(() -> {
					startedDocIndexings.increment();
					try {
						switch (request) {
							case LLUpdateDocument updateDocument ->
									indexWriter.updateDocument(LLUtils.toTerm(id), toDocument(updateDocument));
							case LLSoftUpdateDocument softUpdateDocument ->
									indexWriter.softUpdateDocument(LLUtils.toTerm(id), toDocument(softUpdateDocument.items()),
											toFields(softUpdateDocument.softDeleteItems()));
							case LLUpdateFields updateFields -> indexWriter.updateDocValues(LLUtils.toTerm(id),
									toFields(updateFields.items()));
							case null, default -> throw new UnsupportedOperationException("Unexpected request type: " + request);
						}
					} finally {
						endeddDocIndexings.increment();
					}
					return null;
				}))
				.transform(this::ensureOpen);
	}

	@Override
	public Mono<Void> updateDocuments(Mono<Map<LLTerm, LLUpdateDocument>> documents) {
		return documents.flatMap(this::updateDocuments).then();
	}

	private Mono<Void> updateDocuments(Map<LLTerm, LLUpdateDocument> documentsMap) {
		return this.<Void>runSafe(() -> {
			for (Entry<LLTerm, LLUpdateDocument> entry : documentsMap.entrySet()) {
				LLTerm key = entry.getKey();
				LLUpdateDocument value = entry.getValue();
				startedDocIndexings.increment();
				try {
					docIndexingTime.recordCallable(() -> {
						indexWriter.updateDocument(LLUtils.toTerm(key), toDocument(value));
						return null;
					});
				} finally {
					endeddDocIndexings.increment();
				}
			}
			return null;
		}).transform(this::ensureOpen);
	}

	@Override
	public Mono<Void> deleteAll() {
		return this.<Void>runSafe(() -> {
			shutdownLock.lock();
			try {
				indexWriter.deleteAll();
				indexWriter.forceMergeDeletes(true);
				indexWriter.commit();
			} finally {
				shutdownLock.unlock();
			}
			return null;
		}).subscribeOn(luceneHeavyTasksScheduler).transform(this::ensureOpen);
	}

	@Override
	public Mono<LLSearchResultShard> moreLikeThis(@Nullable LLSnapshot snapshot,
			QueryParams queryParams,
			String keyFieldName,
			Flux<Tuple2<String, Set<String>>> mltDocumentFieldsFlux) {
		LocalQueryParams localQueryParams = LuceneUtils.toLocalQueryParams(queryParams, luceneAnalyzer);
		var searcher = this.searcherManager.retrieveSearcher(snapshot);
		var transformer = new MoreLikeThisTransformer(mltDocumentFieldsFlux);

		return localSearcher
				.collect(searcher, localQueryParams, keyFieldName, transformer)
				.map(result -> new LLSearchResultShard(result.results(), result.totalHitsCount(), result::close))
				.doOnDiscard(Send.class, Send::close)
				.doOnDiscard(Resource.class, Resource::close);
	}

	@Override
	public Mono<LLSearchResultShard> search(@Nullable LLSnapshot snapshot, QueryParams queryParams,
			String keyFieldName) {
		LocalQueryParams localQueryParams = LuceneUtils.toLocalQueryParams(queryParams, luceneAnalyzer);
		var searcher = searcherManager.retrieveSearcher(snapshot);

		return localSearcher
				.collect(searcher, localQueryParams, keyFieldName, NO_TRANSFORMATION)
				.map(result -> new LLSearchResultShard(result.results(), result.totalHitsCount(), result::close))
				.doOnDiscard(Send.class, Send::close)
				.doOnDiscard(Resource.class, Resource::close);
	}

	@Override
	public Mono<Buckets> computeBuckets(@Nullable LLSnapshot snapshot,
			@NotNull List<Query> queries,
			@Nullable Query normalizationQuery,
			BucketParams bucketParams) {
		List<org.apache.lucene.search.Query> localQueries = new ArrayList<>(queries.size());
		for (Query query : queries) {
			localQueries.add(QueryParser.toQuery(query, luceneAnalyzer));
		}
		var localNormalizationQuery = QueryParser.toQuery(normalizationQuery, luceneAnalyzer);
		var searchers = searcherManager
				.retrieveSearcher(snapshot)
				.map(indexSearcher -> LLIndexSearchers.unsharded(indexSearcher).send());

		return decimalBucketMultiSearcher
				.collectMulti(searchers, bucketParams, localQueries, localNormalizationQuery)
				.doOnDiscard(Send.class, Send::close)
				.doOnDiscard(Resource.class, Resource::close);
	}

	public Mono<Send<LLIndexSearcher>> retrieveSearcher(@Nullable LLSnapshot snapshot) {
		return searcherManager
				.retrieveSearcher(snapshot)
				.doOnDiscard(Send.class, Send::close)
				.doOnDiscard(Resource.class, Resource::close);
	}

	@Override
	public Mono<Void> close() {
		return Mono
				.<Void>fromCallable(() -> {
					logger.info("Waiting IndexWriter tasks...");
					activeTasks.arriveAndAwaitAdvance();
					logger.info("IndexWriter tasks ended");
					return null;
				})
				.subscribeOn(luceneHeavyTasksScheduler)
				.then(searcherManager.close())
				.then(Mono.<Void>fromCallable(() -> {
					shutdownLock.lock();
					try {
						logger.info("Closing IndexWriter...");
						indexWriter.close();
						directory.close();
						logger.info("IndexWriter closed");
					} finally {
						shutdownLock.unlock();
					}
					return null;
				}).subscribeOn(luceneHeavyTasksScheduler))

				// Avoid closing multiple times
				.transformDeferred(mono -> {
					if (this.closeRequested.compareAndSet(false, true)) {
						logger.trace("Set closeRequested to true. Further update/write calls will result in an error");
						return mono;
					} else {
						logger.debug("Tried to close more than once");
						return Mono.empty();
					}
				});
	}

	@Override
	public Mono<Void> flush() {
		return Mono
				.<Void>fromCallable(() -> {
					if (activeTasks.isTerminated()) return null;
					shutdownLock.lock();
					try {
						flushTime.recordCallable(() -> {
							indexWriter.flush();
							return null;
						});
					} finally {
						shutdownLock.unlock();
					}
					return null;
				})
				.subscribeOn(luceneHeavyTasksScheduler)
				.transform(this::ensureOpen);
	}

	@Override
	public Mono<Void> refresh(boolean force) {
		return Mono
				.<Void>fromCallable(() -> {
					activeTasks.register();
					try {
						if (activeTasks.isTerminated()) return null;
						shutdownLock.lock();
						try {
							refreshTime.recordCallable(() -> {
								if (force) {
									searcherManager.maybeRefreshBlocking();
								} else {
									searcherManager.maybeRefresh();
								}
								return null;
							});
						} finally {
							shutdownLock.unlock();
						}
					} finally {
						activeTasks.arriveAndDeregister();
					}
					return null;
				})
				.subscribeOn(luceneHeavyTasksScheduler);
	}

	private void scheduledCommit() {
		shutdownLock.lock();
		try {
			commitTime.recordCallable(() -> {
				indexWriter.commit();
				return null;
			});
		} catch (Exception ex) {
			logger.error(MARKER_LUCENE, "Failed to execute a scheduled commit", ex);
		} finally {
			shutdownLock.unlock();
		}
	}

	private void scheduledMerge() { // Do not use. Merges are done automatically by merge policies
		shutdownLock.lock();
		try {
			mergeTime.recordCallable(() -> {
				indexWriter.maybeMerge();
				return null;
			});
		} catch (Exception ex) {
			logger.error(MARKER_LUCENE, "Failed to execute a scheduled merge", ex);
		} finally {
			shutdownLock.unlock();
		}
	}

	@Override
	public boolean isLowMemoryMode() {
		return lowMemory;
	}

	private class MoreLikeThisTransformer implements LLSearchTransformer {

		private final Flux<Tuple2<String, Set<String>>> mltDocumentFieldsFlux;

		public MoreLikeThisTransformer(Flux<Tuple2<String, Set<String>>> mltDocumentFieldsFlux) {
			this.mltDocumentFieldsFlux = mltDocumentFieldsFlux;
		}

		@Override
		public Mono<LocalQueryParams> transform(Mono<TransformerInput> inputMono) {
			return inputMono.flatMap(input -> LuceneUtils.getMoreLikeThisQuery(input.indexSearchers(), input.queryParams(),
					luceneAnalyzer, luceneSimilarity, mltDocumentFieldsFlux));
		}
	}
}