CavalliumDBEngine/src/main/java/it/cavallium/dbengine/database/LLUtils.java

package it.cavallium.dbengine.database;

import static io.netty5.buffer.api.StandardAllocationTypes.OFF_HEAP;
import static org.apache.commons.lang3.ArrayUtils.EMPTY_BYTE_ARRAY;

import com.google.common.primitives.Ints;
import com.google.common.primitives.Longs;
import io.netty5.buffer.api.AllocatorControl;
import io.netty5.buffer.api.Buffer;
import io.netty5.buffer.api.BufferAllocator;
import io.netty5.buffer.api.CompositeBuffer;
import io.netty5.buffer.api.Drop;
import io.netty5.buffer.api.ReadableComponent;
import io.netty5.buffer.api.Resource;
import io.netty5.buffer.api.Send;
import io.netty5.buffer.api.WritableComponent;
import io.netty5.buffer.api.internal.Statics;
import io.netty5.util.IllegalReferenceCountException;
import it.cavallium.dbengine.client.Hits;
import it.cavallium.dbengine.database.disk.LLIndexSearcher;
import it.cavallium.dbengine.database.disk.LLIndexSearchers;
import it.cavallium.dbengine.database.serialization.SerializationException;
import it.cavallium.dbengine.database.serialization.SerializationFunction;
import it.cavallium.dbengine.lucene.RandomSortField;
import it.cavallium.dbengine.utils.SimpleResource;
import java.io.Closeable;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.nio.ByteBuffer;
import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Objects;
import java.util.Optional;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.function.Function;
import java.util.function.ToIntFunction;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.Marker;
import org.apache.logging.log4j.MarkerManager;
import org.apache.lucene.document.Document;
import org.apache.lucene.document.DoublePoint;
import org.apache.lucene.document.Field;
import org.apache.lucene.document.Field.Store;
import org.apache.lucene.document.FloatPoint;
import org.apache.lucene.document.IntPoint;
import org.apache.lucene.document.LongPoint;
import org.apache.lucene.document.NumericDocValuesField;
import org.apache.lucene.document.SortedNumericDocValuesField;
import org.apache.lucene.document.StoredField;
import org.apache.lucene.document.StringField;
import org.apache.lucene.document.TextField;
import org.apache.lucene.index.Term;
import org.apache.lucene.search.IndexSearcher;
import org.apache.lucene.search.ScoreDoc;
import org.apache.lucene.search.ScoreMode;
import org.apache.lucene.search.Sort;
import org.apache.lucene.search.SortField;
import org.apache.lucene.search.SortedNumericSortField;
import org.apache.lucene.util.BytesRef;
import org.apache.lucene.util.BytesRefBuilder;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;
import org.rocksdb.AbstractImmutableNativeReference;
import org.rocksdb.ReadOptions;
import org.rocksdb.RocksDB;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Hooks;
import reactor.core.publisher.Mono;
import reactor.core.scheduler.Schedulers;

@SuppressWarnings("unused")
public class LLUtils {

	private static final Logger logger = LogManager.getLogger(LLUtils.class);
	public static final Marker MARKER_ROCKSDB = MarkerManager.getMarker("ROCKSDB");
	public static final Marker MARKER_LUCENE = MarkerManager.getMarker("LUCENE");

	public static final int INITIAL_DIRECT_READ_BYTE_BUF_SIZE_BYTES = 4096;
	public static final ByteBuffer EMPTY_BYTE_BUFFER = ByteBuffer.allocateDirect(0).asReadOnlyBuffer();
	private static final AllocatorControl NO_OP_ALLOCATION_CONTROL = (AllocatorControl) BufferAllocator.offHeapUnpooled();
	private static final byte[] RESPONSE_TRUE = new byte[]{1};
	private static final byte[] RESPONSE_FALSE = new byte[]{0};
	private static final byte[] RESPONSE_TRUE_BUF = new byte[]{1};
	private static final byte[] RESPONSE_FALSE_BUF = new byte[]{0};
	public static final byte[][] LEXICONOGRAPHIC_ITERATION_SEEKS = new byte[256][1];
	public static final AtomicBoolean hookRegistered = new AtomicBoolean();
	public static final boolean MANUAL_READAHEAD = false;
	public static final boolean ALLOW_STATIC_OPTIONS = false;

	public static final boolean FORCE_DISABLE_CHECKSUM_VERIFICATION
			= Boolean.parseBoolean(System.getProperty("it.cavallium.dbengine.checksum.disable.force", "false"));

	public static final boolean DEBUG_ALL_DROPS
			= Boolean.parseBoolean(System.getProperty("it.cavallium.dbengine.drops.log", "false"));
	public static final boolean DEBUG_ALL_DISCARDS
			= Boolean.parseBoolean(System.getProperty("it.cavallium.dbengine.discards.log", "false"));

	static {
		for (int i1 = 0; i1 < 256; i1++) {
			var b = LEXICONOGRAPHIC_ITERATION_SEEKS[i1];
			b[0] = (byte) i1;
		}
		initHooks();
	}

	public static void initHooks() {
		if (hookRegistered.compareAndSet(false, true)) {
			Hooks.onNextDropped(LLUtils::onNextDropped);
		}
	}

	public static boolean responseToBoolean(byte[] response) {
		return response[0] == 1;
	}

	public static boolean responseToBoolean(Send<Buffer> responseToReceive) {
		try (var response = responseToReceive.receive()) {
			assert response.readableBytes() == 1;
			return response.getByte(response.readerOffset()) == 1;
		}
	}

	public static boolean responseToBoolean(Buffer response) {
		try (response) {
			assert response.readableBytes() == 1;
			return response.getByte(response.readerOffset()) == 1;
		}
	}

	public static byte[] booleanToResponse(boolean bool) {
		return bool ? RESPONSE_TRUE : RESPONSE_FALSE;
	}

	public static Buffer booleanToResponseByteBuffer(BufferAllocator alloc, boolean bool) {
		return alloc.allocate(1).writeByte(bool ? (byte) 1 : 0);
	}

	@Nullable
	public static Sort toSort(@Nullable LLSort sort) {
		if (sort == null) {
			return null;
		}
		if (sort.getType() == LLSortType.LONG) {
			return new Sort(new SortedNumericSortField(sort.getFieldName(), SortField.Type.LONG, sort.isReverse()));
		} else if (sort.getType() == LLSortType.RANDOM) {
			return new Sort(new RandomSortField());
		} else if (sort.getType() == LLSortType.SCORE) {
			return new Sort(SortField.FIELD_SCORE);
		} else if (sort.getType() == LLSortType.DOC) {
			return new Sort(SortField.FIELD_DOC);
		}
		return null;
	}

	public static ScoreMode toScoreMode(LLScoreMode scoreMode) {
		return switch (scoreMode) {
			case COMPLETE -> ScoreMode.COMPLETE;
			case TOP_SCORES -> ScoreMode.TOP_SCORES;
			case COMPLETE_NO_SCORES -> ScoreMode.COMPLETE_NO_SCORES;
			case NO_SCORES -> ScoreMode.TOP_DOCS;
		};
	}

	public static Term toTerm(LLTerm term) {
		var valueRef = new FakeBytesRefBuilder(term);
		return new Term(term.getKey(), valueRef);
	}

	public static Document toDocument(LLUpdateDocument document) {
		return toDocument(document.items());
	}

	public static Document toDocument(List<LLItem> document) {
		Document d = new Document();
		for (LLItem item : document) {
			if (item != null) {
				d.add(LLUtils.toField(item));
			}
		}
		return d;
	}

	public static Field[] toFields(List<LLItem> fields) {
		Field[] d = new Field[fields.size()];
		for (int i = 0; i < fields.size(); i++) {
			d[i] = LLUtils.toField(fields.get(i));
		}
		return d;
	}

	public static Collection<Document> toDocuments(Collection<LLUpdateDocument> document) {
		List<Document> d = new ArrayList<>(document.size());
		for (LLUpdateDocument doc : document) {
			d.add(LLUtils.toDocument(doc));
		}
		return d;
	}

	public static Collection<Document> toDocumentsFromEntries(Collection<Entry<LLTerm, LLUpdateDocument>> documentsList) {
		ArrayList<Document> results = new ArrayList<>(documentsList.size());
		for (Entry<LLTerm, LLUpdateDocument> entry : documentsList) {
			results.add(LLUtils.toDocument(entry.getValue()));
		}
		return results;
	}

	public static Iterable<Term> toTerms(Iterable<LLTerm> terms) {
		List<Term> d = new ArrayList<>();
		for (LLTerm term : terms) {
			d.add(LLUtils.toTerm(term));
		}
		return d;
	}

	private static Field toField(LLItem item) {
		return switch (item.getType()) {
			case IntPoint -> new IntPoint(item.getName(), item.intData());
			case DoublePoint -> new DoublePoint(item.getName(), item.doubleData());
			case IntPointND -> new IntPoint(item.getName(), item.intArrayData());
			case LongPoint -> new LongPoint(item.getName(), item.longData());
			case LongPointND -> new LongPoint(item.getName(), item.longArrayData());
			case FloatPointND -> new FloatPoint(item.getName(), item.floatArrayData());
			case DoublePointND -> new DoublePoint(item.getName(), item.doubleArrayData());
			case LongStoredField -> new StoredField(item.getName(), item.longData());
			case BytesStoredField -> new StoredField(item.getName(), (BytesRef) item.getData());
			case FloatPoint -> new FloatPoint(item.getName(), item.floatData());
			case TextField -> new TextField(item.getName(), item.stringValue(), Store.NO);
			case TextFieldStored -> new TextField(item.getName(), item.stringValue(), Store.YES);
			case SortedNumericDocValuesField -> new SortedNumericDocValuesField(item.getName(), item.longData());
			case NumericDocValuesField -> new NumericDocValuesField(item.getName(), item.longData());
			case StringField -> {
				if (item.getData() instanceof BytesRef bytesRef) {
					yield new StringField(item.getName(), bytesRef, Store.NO);
				} else {
					yield new StringField(item.getName(), item.stringValue(), Store.NO);
				}
			}
			case StringFieldStored -> {
				if (item.getData() instanceof BytesRef bytesRef) {
					yield new StringField(item.getName(), bytesRef, Store.YES);
				} else {
					yield new StringField(item.getName(), item.stringValue(), Store.YES);
				}
			}
		};
	}

	private static int[] getIntArray(byte[] data) {
		var count = data.length / Integer.BYTES;
		var items = new int[count];
		for (int i = 0; i < items.length; i++) {
			items[i] = Ints.fromBytes(data[i * Integer.BYTES],
					data[i * Integer.BYTES + 1],
					data[i * Integer.BYTES + 2],
					data[i * Integer.BYTES + 3]
			);
		}
		return items;
	}

	private static long[] getLongArray(byte[] data) {
		var count = data.length / Long.BYTES;
		var items = new long[count];
		for (int i = 0; i < items.length; i++) {
			items[i] = Longs.fromBytes(data[i * Long.BYTES],
					data[i * Long.BYTES + 1],
					data[i * Long.BYTES + 2],
					data[i * Long.BYTES + 3],
					data[i * Long.BYTES + 4],
					data[i * Long.BYTES + 5],
					data[i * Long.BYTES + 6],
					data[i * Long.BYTES + 7]
			);
		}
		return items;
	}

	public static it.cavallium.dbengine.database.LLKeyScore toKeyScore(LLKeyScore hit) {
		return new it.cavallium.dbengine.database.LLKeyScore(hit.docId(), hit.shardId(), hit.score(), hit.key());
	}

	public static String toStringSafe(@Nullable Buffer key) {
		try {
			if (key == null || key.isAccessible()) {
				return toString(key);
			} else {
				return "(released)";
			}
		} catch (IllegalReferenceCountException ex) {
			return "(released)";
		}
	}

	public static String toStringSafe(byte @Nullable[] key) {
		try {
			if (key == null) {
				return toString(key);
			} else {
				return "(released)";
			}
		} catch (IllegalReferenceCountException ex) {
			return "(released)";
		}
	}

	public static String toStringSafe(@Nullable LLRange range) {
		try {
			if (range == null || range.isAccessible()) {
				return toString(range);
			} else {
				return "(released)";
			}
		} catch (IllegalReferenceCountException ex) {
			return "(released)";
		}
	}

	public static String toString(@Nullable LLRange range) {
		if (range == null) {
			return "null";
		} else if (range.isAll()) {
			return "ξ";
		} else if (range.hasMin() && range.hasMax()) {
			return "[" + toStringSafe(range.getMinUnsafe()) + "," + toStringSafe(range.getMaxUnsafe()) + ")";
		} else if (range.hasMin()) {
			return "[" + toStringSafe(range.getMinUnsafe()) + ",*)";
		} else if (range.hasMax()) {
			return "[*," + toStringSafe(range.getMaxUnsafe()) + ")";
		} else {
			return "∅";
		}
	}

	public static String toString(@Nullable Buffer key) {
		if (key == null) {
			return "null";
		} else {
			int startIndex = key.readerOffset();
			int iMax = key.readableBytes() - 1;
			int iLimit = 128;
			if (iMax <= -1) {
				return "[]";
			} else {
				StringBuilder arraySB = new StringBuilder();
				StringBuilder asciiSB = new StringBuilder();
				boolean isAscii = true;
				arraySB.append('[');
				int i = 0;

				while (true) {
					var byteVal = key.getUnsignedByte(startIndex + i);
					arraySB.append(byteVal);
					if (isAscii) {
						if (byteVal >= 32 && byteVal < 127) {
							asciiSB.append((char) byteVal);
						} else if (byteVal == 0) {
							asciiSB.append('␀');
						} else {
							isAscii = false;
							asciiSB = null;
						}
					}
					if (i == iLimit) {
						arraySB.append("…");
					}
					if (i == iMax || i == iLimit) {
						if (isAscii) {
							return asciiSB.insert(0, "\"").append("\"").toString();
						} else {
							return arraySB.append(']').toString();
						}
					}

					arraySB.append(", ");
					++i;
				}
			}
		}
	}

	public static String toString(byte @Nullable[] key) {
		if (key == null) {
			return "null";
		} else {
			int startIndex = 0;
			int iMax = key.length - 1;
			int iLimit = 128;
			if (iMax <= -1) {
				return "[]";
			} else {
				StringBuilder arraySB = new StringBuilder();
				StringBuilder asciiSB = new StringBuilder();
				boolean isAscii = true;
				arraySB.append('[');
				int i = 0;

				while (true) {
					var byteVal = (int) key[startIndex + i];
					arraySB.append(byteVal);
					if (isAscii) {
						if (byteVal >= 32 && byteVal < 127) {
							asciiSB.append((char) byteVal);
						} else if (byteVal == 0) {
							asciiSB.append('␀');
						} else {
							isAscii = false;
							asciiSB = null;
						}
					}
					if (i == iLimit) {
						arraySB.append("…");
					}
					if (i == iMax || i == iLimit) {
						if (isAscii) {
							return asciiSB.insert(0, "\"").append("\"").toString();
						} else {
							return arraySB.append(']').toString();
						}
					}

					arraySB.append(", ");
					++i;
				}
			}
		}
	}

	public static boolean equals(Buffer a, Buffer b) {
		if (a == null && b == null) {
			return true;
		} else if (a != null && b != null) {
			var aCur = a.openCursor();
			var bCur = b.openCursor();
			if (aCur.bytesLeft() != bCur.bytesLeft()) {
				return false;
			}
			while (aCur.readByte() && bCur.readByte()) {
				if (aCur.getByte() != bCur.getByte()) {
					return false;
				}
			}
			return true;
		} else {
			return false;
		}
	}


	/**
	 * Returns {@code true} if and only if the two specified buffers are identical to each other for {@code length} bytes
	 * starting at {@code aStartIndex} index for the {@code a} buffer and {@code bStartIndex} index for the {@code b}
	 * buffer. A more compact way to express this is:
	 * <p>
	 * {@code a[aStartIndex : aStartIndex + length] == b[bStartIndex : bStartIndex + length]}
	 */
	public static boolean equals(Buffer a, int aStartIndex, Buffer b, int bStartIndex, int length) {
		var aCur = a.openCursor(aStartIndex, length);
		var bCur = b.openCursor(bStartIndex, length);
		if (aCur.bytesLeft() != bCur.bytesLeft()) {
			return false;
		}
		while (aCur.readByte() && bCur.readByte()) {
			if (aCur.getByte() != bCur.getByte()) {
				return false;
			}
		}
		return true;
	}

	public static byte[] toArray(@Nullable Buffer key) {
		if (key == null) {
			return EMPTY_BYTE_ARRAY;
		}
		byte[] array = new byte[key.readableBytes()];
		key.copyInto(key.readerOffset(), array, 0, key.readableBytes());
		return array;
	}

	public static List<byte[]> toArray(List<Buffer> input) {
		List<byte[]> result = new ArrayList<>(input.size());
		for (Buffer byteBuf : input) {
			result.add(toArray(byteBuf));
		}
		return result;
	}

	public static int hashCode(Buffer buf) {
		if (buf == null) {
			return 0;
		}

		int result = 1;
		var cur = buf.openCursor();
		while (cur.readByte()) {
			var element = cur.getByte();
			result = 31 * result + element;
		}

		return result;
	}

	/**
	 * @return null if size is equal to RocksDB.NOT_FOUND
	 */
	@Nullable
	public static Buffer readNullableDirectNioBuffer(BufferAllocator alloc, ToIntFunction<ByteBuffer> reader) {
		if (alloc.getAllocationType() != OFF_HEAP) {
			throw new UnsupportedOperationException("Allocator type is not direct: " + alloc);
		}
		var directBuffer = alloc.allocate(INITIAL_DIRECT_READ_BYTE_BUF_SIZE_BYTES);
		try {
			assert directBuffer.readerOffset() == 0;
			assert directBuffer.writerOffset() == 0;
			var directBufferWriter = ((WritableComponent) directBuffer).writableBuffer();
			assert directBufferWriter.position() == 0;
			assert directBufferWriter.capacity() >= directBuffer.capacity();
			assert directBufferWriter.isDirect();
			int trueSize = reader.applyAsInt(directBufferWriter);
			if (trueSize == RocksDB.NOT_FOUND) {
				directBuffer.close();
				return null;
			}
			int readSize = directBufferWriter.limit();
			if (trueSize < readSize) {
				throw new IllegalStateException();
			} else if (trueSize == readSize) {
				return directBuffer.writerOffset(directBufferWriter.limit());
			} else {
				assert directBuffer.readerOffset() == 0;
				directBuffer.ensureWritable(trueSize);
				assert directBuffer.writerOffset() == 0;
				directBufferWriter = ((WritableComponent) directBuffer).writableBuffer();
				assert directBufferWriter.position() == 0;
				assert directBufferWriter.isDirect();
				reader.applyAsInt(directBufferWriter.position(0));
				return directBuffer.writerOffset(trueSize);
			}
		} catch (Throwable t) {
			directBuffer.close();
			throw t;
		}
	}

	public static void ensureBlocking() {
		if (Schedulers.isInNonBlockingThread()) {
			throw new UnsupportedOperationException("Called collect in a nonblocking thread");
		}
	}

	// todo: remove this ugly method
	/**
	 * cleanup resource
	 * @param cleanupOnSuccess if true the resource will be cleaned up if the function is successful
	 */
	public static <U, T extends Resource<T>> Mono<U> usingSendResource(Mono<Send<T>> resourceSupplier,
			Function<T, Mono<U>> resourceClosure,
			boolean cleanupOnSuccess) {
		return Mono.usingWhen(resourceSupplier.map(Send::receive), resourceClosure, r -> {
					if (cleanupOnSuccess) {
						return Mono.fromRunnable(() -> r.close());
					} else {
						return Mono.empty();
					}
				}, (r, ex) -> Mono.fromRunnable(() -> {
					if (r.isAccessible()) {
						r.close();
					}
				}), r -> Mono.fromRunnable(() -> {
					if (r.isAccessible()) {
						r.close();
					}
				}));
	}

	public static boolean isSet(ScoreDoc[] scoreDocs) {
		for (ScoreDoc scoreDoc : scoreDocs) {
			if (scoreDoc == null) {
				return false;
			}
		}
		return true;
	}

	public static Send<Buffer> empty(BufferAllocator allocator) {
		try {
			return allocator.allocate(0).send();
		} catch (Exception ex) {
			try (var empty = CompositeBuffer.compose(allocator)) {
				assert empty.readableBytes() == 0;
				assert empty.capacity() == 0;
				return empty.send();
			}
		}
	}

	public static Send<Buffer> copy(BufferAllocator allocator, Buffer buf) {
		if (CompositeBuffer.isComposite(buf) && buf.capacity() == 0) {
			return empty(allocator);
		} else {
			return buf.copy().send();
		}
	}

	public static boolean isBoundedRange(LLRange rangeShared) {
		return rangeShared.hasMin() && rangeShared.hasMax();
	}

	/**
	 * Generate a ReadOptions, with some parameters modified to help with bulk iterations
	 * @param readOptions the read options to start with, it will be modified
	 * @param canFillCache true to fill the cache. If closedRange is false, this field will be ignored
	 * @param boundedRange true if the range is bounded from both sides
	 * @param smallRange true if the range is small
	 * @return the passed instance of ReadOptions, or a new one if the passed readOptions is null
	 */
	public static ReadOptions generateCustomReadOptions(@Nullable ReadOptions readOptions,
			boolean canFillCache,
			boolean boundedRange,
			boolean smallRange) {
		if (readOptions == null) {
			//noinspection resource
			readOptions = new ReadOptions();
		}
		if (boundedRange || smallRange) {
			readOptions.setFillCache(canFillCache);
		} else {
			if (readOptions.readaheadSize() <= 0) {
				readOptions.setReadaheadSize(4 * 1024 * 1024); // 4MiB
			}
			readOptions.setFillCache(false);
			readOptions.setVerifyChecksums(false);
		}

		if (FORCE_DISABLE_CHECKSUM_VERIFICATION) {
			readOptions.setVerifyChecksums(false);
		}

		return readOptions;
	}

	public static Mono<Void> finalizeResource(Resource<?> resource) {
		return Mono.fromRunnable(() -> LLUtils.finalizeResourceNow(resource));
	}

	public static Mono<Void> finalizeResource(SimpleResource resource) {
		return Mono.fromRunnable(resource::close);
	}

	public static void finalizeResourceNow(Resource<?> resource) {
		if (resource.isAccessible()) {
			resource.close();
		}
	}

	public static void finalizeResourceNow(SimpleResource resource) {
		resource.close();
	}

	public static <V> Flux<V> handleDiscard(Flux<V> flux) {
		return flux.doOnDiscard(Object.class, LLUtils::onDiscard);
	}

	public static <V> Mono<V> handleDiscard(Mono<V> flux) {
		return flux.doOnDiscard(Object.class, LLUtils::onDiscard);
	}

	/**
	 * Obtain the resource, then run the closure.
	 * If the closure publisher returns a single element, then the resource is kept open,
	 * otherwise it is closed.
	 */
	public static <T extends AutoCloseable, U> Mono<U> singleOrClose(Mono<T> resourceMono,
			Function<T, Mono<U>> closure) {
		return Mono.usingWhen(resourceMono,
				resource -> closure.apply(resource).doOnSuccess(s -> {
					if (s == null) {
						try {
							resource.close();
						} catch (Exception e) {
							throw new RuntimeException(e);
						}
					}
				}),
				resource -> Mono.empty(),
				(resource, ex) -> Mono.fromCallable(() -> {
					resource.close();
					return null;
				}),
				resource -> Mono.fromCallable(() -> {
					resource.close();
					return null;
				})
		);
	}

	@Deprecated
	public record DirectBuffer(@NotNull Buffer buffer, @NotNull ByteBuffer byteBuffer) {}

	@NotNull
	public static ByteBuffer newDirect(int size) {
		return ByteBuffer.allocateDirect(size);
	}

	private static Drop<Buffer> drop() {
		// We cannot reliably drop unsafe memory. We have to rely on the cleaner to do that.
		return Statics.NO_OP_DROP;
	}

	public static boolean isReadOnlyDirect(Buffer inputBuffer) {
		return inputBuffer.isDirect() && inputBuffer instanceof ReadableComponent;
	}

	public static ByteBuffer getReadOnlyDirect(Buffer inputBuffer) {
		assert isReadOnlyDirect(inputBuffer);
		return ((ReadableComponent) inputBuffer).readableBuffer();
	}

	public static Buffer fromByteArray(BufferAllocator alloc, byte[] array) {
		Buffer result = alloc.allocate(array.length);
		result.writeBytes(array);
		return result;
	}

	@NotNull
	public static Buffer readDirectNioBuffer(BufferAllocator alloc, ToIntFunction<ByteBuffer> reader) {
		var nullable = readNullableDirectNioBuffer(alloc, reader);
		if (nullable == null) {
			throw new IllegalStateException("A non-nullable buffer read operation tried to return a \"not found\" element");
		}
		return nullable;
	}

	public static Buffer compositeBuffer(BufferAllocator alloc, Send<Buffer> buffer) {
		return buffer.receive();
	}

	@NotNull
	public static Buffer compositeBuffer(BufferAllocator alloc,
			@NotNull Send<Buffer> buffer1,
			@NotNull Send<Buffer> buffer2) {
		var b1 = buffer1.receive();
		try (var b2 = buffer2.receive()) {
			if (b1.writerOffset() < b1.capacity() || b2.writerOffset() < b2.capacity()) {
				b1.ensureWritable(b2.readableBytes(), b2.readableBytes(), true);
				b2.copyInto(b2.readerOffset(), b1, b1.writerOffset(), b2.readableBytes());
				b1.writerOffset(b1.writerOffset() + b2.readableBytes());
				return b1;
			} else {
				return alloc.compose(List.of(b1.send(), b2.send()));
			}
		}
	}

	@NotNull
	public static Buffer compositeBuffer(BufferAllocator alloc,
			@NotNull Send<Buffer> buffer1,
			@NotNull Send<Buffer> buffer2,
			@NotNull Send<Buffer> buffer3) {
		var b1 = buffer1.receive();
		try (var b2 = buffer2.receive()) {
			try (var b3 = buffer3.receive()) {
				if (b1.writerOffset() < b1.capacity()
						|| b2.writerOffset() < b2.capacity()
						|| b3.writerOffset() < b3.capacity()) {
					b1.ensureWritable(b2.readableBytes(), b2.readableBytes(), true);
					b2.copyInto(b2.readerOffset(), b1, b1.writerOffset(), b2.readableBytes());
					b1.writerOffset(b1.writerOffset() + b2.readableBytes());

					b1.ensureWritable(b3.readableBytes(), b3.readableBytes(), true);
					b3.copyInto(b3.readerOffset(), b1, b1.writerOffset(), b3.readableBytes());
					b1.writerOffset(b1.writerOffset() + b3.readableBytes());
					return b1;
				} else {
					return alloc.compose(List.of(b1.send(), b2.send(), b3.send()));
				}
			}
		}
	}

	public static <T> Mono<T> resolveDelta(Mono<Delta<T>> prev, UpdateReturnMode updateReturnMode) {
		return prev.handle((delta, sink) -> {
			switch (updateReturnMode) {
				case GET_NEW_VALUE -> {
					var current = delta.current();
					if (current != null) {
						sink.next(current);
					} else {
						sink.complete();
					}
				}
				case GET_OLD_VALUE -> {
					var previous = delta.previous();
					if (previous != null) {
						sink.next(previous);
					} else {
						sink.complete();
					}
				}
				case NOTHING -> sink.complete();
				default -> sink.error(new IllegalStateException());
			}
		});
	}

	public static Mono<Buffer> resolveLLDelta(Mono<LLDelta> prev, UpdateReturnMode updateReturnMode) {
		return prev.mapNotNull(delta -> {
			final Buffer previous = delta.previousUnsafe();
			final Buffer current = delta.currentUnsafe();
			return switch (updateReturnMode) {
				case GET_NEW_VALUE -> {
					if (previous != null && previous.isAccessible()) {
						previous.close();
					}
					yield current;
				}
				case GET_OLD_VALUE -> {
					if (current != null && current.isAccessible()) {
						current.close();
					}
					yield previous;
				}
				case NOTHING -> {
					if (previous != null && previous.isAccessible()) {
						previous.close();
					}
					if (current != null && current.isAccessible()) {
						current.close();
					}
					yield null;
				}
			};
		});
	}

	public static <T, U> Mono<Delta<U>> mapDelta(Mono<Delta<T>> mono,
			SerializationFunction<@NotNull T, @Nullable U> mapper) {
		return mono.handle((delta, sink) -> {
			try {
				T prev = delta.previous();
				T curr = delta.current();
				U newPrev;
				U newCurr;
				if (prev != null) {
					newPrev = mapper.apply(prev);
				} else {
					newPrev = null;
				}
				if (curr != null) {
					newCurr = mapper.apply(curr);
				} else {
					newCurr = null;
				}
				sink.next(new Delta<>(newPrev, newCurr));
			} catch (SerializationException ex) {
				sink.error(ex);
			}
		});
	}

	public static <U> Mono<Delta<U>> mapLLDelta(Mono<LLDelta> mono,
			SerializationFunction<@NotNull Buffer, @Nullable U> mapper) {
		return Mono.usingWhen(mono, delta -> Mono.fromCallable(() -> {
			Buffer prev = delta.previousUnsafe();
			Buffer curr = delta.currentUnsafe();
			U newPrev;
			U newCurr;
			if (prev != null) {
				newPrev = mapper.apply(prev);
			} else {
				newPrev = null;
			}
			if (curr != null) {
				newCurr = mapper.apply(curr);
			} else {
				newCurr = null;
			}
			return new Delta<>(newPrev, newCurr);
		}), LLUtils::finalizeResource);
	}

	public static <R, V> boolean isDeltaChanged(Delta<V> delta) {
		return !Objects.equals(delta.previous(), delta.current());
	}

	public static boolean isDirect(Buffer key) {
		var readableComponents = key.countReadableComponents();
		if (readableComponents == 0) {
			return true;
		} else if (readableComponents == 1) {
			return key.forEachReadable(0, (index, component) -> component.readableBuffer().isDirect()) >= 0;
		} else {
			return false;
		}
	}

	public static String deserializeString(Send<Buffer> bufferSend, int readerOffset, int length, Charset charset) {
		try (var buffer = bufferSend.receive()) {
			byte[] bytes = new byte[Math.min(length, buffer.readableBytes())];
			buffer.copyInto(readerOffset, bytes, 0, length);
			return new String(bytes, charset);
		}
	}

	public static String deserializeString(@NotNull Buffer buffer, int readerOffset, int length, Charset charset) {
		byte[] bytes = new byte[Math.min(length, buffer.readableBytes())];
		buffer.copyInto(readerOffset, bytes, 0, length);
		return new String(bytes, charset);
	}

	public static int utf8MaxBytes(String deserialized) {
		return deserialized.length() * 3;
	}

	private static void onNextDropped(Object next) {
		if (DEBUG_ALL_DROPS) {
			logger.trace("Dropped: {}", () -> next.getClass().getName());
		}
		closeResource(next);
	}

	public static void onDiscard(Object next) {
		if (DEBUG_ALL_DISCARDS) {
			logger.trace("Discarded: {}", () -> next.getClass().getName());
		}
		closeResource(next);
	}

	public static void closeResource(Object next) {
		if (next instanceof Send<?> send) {
			send.close();
		} if (next instanceof SimpleResource simpleResource) {
			simpleResource.close();
		} else if (next instanceof Resource<?> resource && resource.isAccessible()) {
			resource.close();
		} else if (next instanceof Iterable<?> iterable) {
			iterable.forEach(LLUtils::onNextDropped);
		} else if (next instanceof SafeCloseable safeCloseable) {
			safeCloseable.close();
		} else if (next instanceof AbstractImmutableNativeReference rocksObj) {
			if (rocksObj.isOwningHandle()) {
				rocksObj.close();
			}
		} else if (next instanceof LLIndexSearcher searcher) {
			try {
				searcher.close();
			} catch (UncheckedIOException e) {
				logger.error("Failed to close searcher {}", searcher, e);
			}
		} else if (next instanceof LLIndexSearchers searchers) {
			try {
				searchers.close();
			} catch (UncheckedIOException e) {
				logger.error("Failed to close searchers {}", searchers, e);
			}
		} else if (next instanceof Optional<?> optional) {
			optional.ifPresent(LLUtils::onNextDropped);
		} else if (next instanceof Map.Entry<?, ?> entry) {
			var key = entry.getKey();
			if (key != null) {
				onNextDropped(key);
			}
			var value = entry.getValue();
			if (value != null) {
				onNextDropped(value);
			}
		} else if (next instanceof Delta<?> delta) {
			var previous = delta.previous();
			if (previous != null) {
				onNextDropped(previous);
			}
			var current = delta.current();
			if (current != null) {
				onNextDropped(current);
			}
		} else if (next instanceof Map<?, ?> map) {
			map.forEach((key, value) -> {
				if (key != null) {
					onNextDropped(key);
				}
				if (value != null) {
					onNextDropped(value);
				}
			});
		}
	}

	private static class FakeBytesRefBuilder extends BytesRefBuilder {

		private final LLTerm term;

		public FakeBytesRefBuilder(LLTerm term) {
			this.term = term;
		}

		@Override
		public BytesRef toBytesRef() {
			return term.getValueBytesRef();
		}
	}
}