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607using System;
using System.Collections.Generic;
using System.IO;
using System.Threading;
using System.Threading.Tasks;
using Xtremegaida.DataStructures;

namespace MediaContainers;

public sealed class EBMLWriter : IDisposable, IAsyncDisposable
{
	private readonly DataBufferCache cache;
	private readonly IDataQueueWriter writer;
	private readonly Stream stream;
	private readonly bool keepWriterOpen;
	private readonly DataBuffer writeBuffer = new(null, new byte[4096]);
	private int writeBusy;
	private long startOffset;
	private bool rewindOnDispose;
	private volatile bool disposed;
	private Level level;

	const int maxMasterElementBufferSize = (1 << (24 - 3)) - 1;

	private class Level : IDisposable
	{
		public readonly Level Parent;
		public readonly EBMLElementDefiniton Definition;
		public readonly DataQueue Buffer;
		public readonly IDataQueueWriter ParentWriter;
		public readonly long RelativeDataOffset;
		public readonly bool CanSeek;
		public bool NeedSizeWritten;
		public int? DataSizeWidth;
		public Task ForwardTask;

		public Level(Level parent, EBMLElementDefiniton def, DataQueue buffer, IDataQueueWriter parentWriter, bool canSeek)
		{
			Parent = parent;
			Definition = def;
			Buffer = buffer;
			ParentWriter = parentWriter;
			CanSeek = canSeek;
			RelativeDataOffset = parentWriter.TotalBytesWritten;
			buffer.OnBlockWrite += OnBlockWriteEvent;
		}

		public Task ForwardBuffer(CancellationToken cancellationToken = default)
		{
			lock (this)
			{
				if (ForwardTask == null) { ForwardTask = ForwardAsync(); }
				if (cancellationToken.CanBeCanceled && !ForwardTask.IsCompleted) { return ForwardTask.WaitAsync(cancellationToken); }
				return ForwardTask;
			}
		}

		private void OnBlockWriteEvent()
		{
			ForwardBuffer();
		}

		private async Task ForwardAsync()
		{
			EBMLVInt sizeVInt;
			if (Buffer.IsWriteClosed)
			{
				sizeVInt = new EBMLVInt((ulong)Buffer.UnreadLength);
			}
			else
			{
				NeedSizeWritten = true;
				if (!CanSeek && Definition.AllowUnknownSize) { NeedSizeWritten = false; }
				sizeVInt = EBMLVInt.CreateUnknown(NeedSizeWritten ? 8 : 1);
			}
			DataSizeWidth = sizeVInt.WidthBytes;
			await sizeVInt.Write(ParentWriter);
			await Buffer.ForwardTo(ParentWriter);
		}

		public void Dispose()
		{
			Buffer.Dispose();
		}
	}

	public bool StreamingMode { get; set; }
	public bool WindowsCompatibilityMode { get; set; } = true;
	public EBMLElementDefiniton CurrentContainer => level?.Definition;
	public long CurrentContainerSize => currentLevelWriter.TotalBytesWritten + writeBuffer.UnreadSize;
	public bool CanSeek => !StreamingMode && (stream?.CanSeek ?? false);

	private IDataQueueWriter currentLevelWriter => level?.Buffer ?? writer;

	public EBMLWriter(IDataQueueWriter writer, bool keepWriterOpen = false, DataBufferCache cache = null)
	{
		this.writer = writer;
		this.keepWriterOpen = keepWriterOpen;
		this.cache = cache ?? DataBufferCache.DefaultCache;
	}

	public EBMLWriter(Stream stream, bool keepStreamOpen = false, DataBufferCache cache = null, bool rewindOnDispose = false)
	   : this(new DataQueueStreamWriter(stream, keepStreamOpen), false, cache)
	{
		this.stream = stream;
		if (stream.CanSeek && rewindOnDispose)
		{
			startOffset = stream.Position;
			this.rewindOnDispose = true;
		}
	}

	public static int CalculateWidth(long value)
	{
		if (value == 0) { return 0; }
		for (int size = 1; size < 8; size++)
		{
			var shift = 64 - (size << 3);
			if (((value << shift) >> shift) == value) { return size; }
		}
		return 8;
	}

	public static int CalculateWidth(ulong value)
	{
		if (value == 0) { return 0; }
		for (int size = 1; size < 8; size++)
		{
			var mask = (ulong)(-1L << (size << 3));
			if ((mask & value) == 0) { return size; }
		}
		return 8;
	}

	public ValueTask FlushInternalBuffer(CancellationToken cancellationToken = default)
	{
		if (writeBuffer.UnreadSize == 0) { return ValueTask.CompletedTask; }
		var memory = new ReadOnlyMemory<byte>(writeBuffer.Buffer, writeBuffer.ReadOffset, writeBuffer.WriteOffset - writeBuffer.ReadOffset);
		writeBuffer.ReadOffset = 0; writeBuffer.WriteOffset = 0;
		return currentLevelWriter.WriteAsync(memory, cancellationToken);
	}

	public async ValueTask WriteSignedInteger(EBMLElementDefiniton def, long value, CancellationToken cancellationToken = default)
	{
		if (disposed) { throw new ObjectDisposedException(nameof(EBMLWriter)); }
		if (def == null) { throw new ArgumentNullException(nameof(def)); }
		if (def.Type != EBMLElementType.SignedInteger) { throw new ArgumentException("Element type mismatch", nameof(def)); }
		try
		{
			if (Interlocked.Increment(ref writeBusy) != 1) { throw new InvalidOperationException("Concurrent writes not allowed."); }
			var size = (value == 0 && WindowsCompatibilityMode) ? 1 : CalculateWidth(value);
			var sizeVInt = new EBMLVInt((ulong)size);
			if (writeBuffer.SpaceLeft < (def.Id.WidthBytes + sizeVInt.WidthBytes + size)) { await FlushInternalBuffer(cancellationToken); }
			def.Id.Write(writeBuffer);
			sizeVInt.Write(writeBuffer);
			if (size == 0) { return; }
			for (int i = (size - 1) << 3; i >= 0; i -= 8)
			{
				writeBuffer.Buffer[writeBuffer.WriteOffset++] = (byte)((value >> i) & 0xff);
			}
		}
		finally { Interlocked.Decrement(ref writeBusy); }
	}

	public async ValueTask WriteUnsignedInteger(EBMLElementDefiniton def, ulong value, CancellationToken cancellationToken = default)
	{
		if (disposed) { throw new ObjectDisposedException(nameof(EBMLWriter)); }
		if (def == null) { throw new ArgumentNullException(nameof(def)); }
		if (def.Type != EBMLElementType.UnsignedInteger) { throw new ArgumentException("Element type mismatch", nameof(def)); }

		try
		{
			if (Interlocked.Increment(ref writeBusy) != 1) { throw new InvalidOperationException("Concurrent writes not allowed."); }
			var size = (value == 0 && WindowsCompatibilityMode) ? 1 : CalculateWidth(value);
			var sizeVInt = new EBMLVInt((ulong)size);
			if (writeBuffer.SpaceLeft < (def.Id.WidthBytes + sizeVInt.WidthBytes + size)) { await FlushInternalBuffer(cancellationToken); }
			def.Id.Write(writeBuffer);
			sizeVInt.Write(writeBuffer);
			if (size == 0) { return; }
			for (int i = (size - 1) << 3; i >= 0; i -= 8)
			{
				writeBuffer.Buffer[writeBuffer.WriteOffset++] = (byte)((value >> i) & 0xff);
			}
		}
		finally { Interlocked.Decrement(ref writeBusy); }
	}

	public async ValueTask WriteFloat(EBMLElementDefiniton def, float value, CancellationToken cancellationToken = default)
	{
		if (disposed) { throw new ObjectDisposedException(nameof(EBMLWriter)); }
		if (def == null) { throw new ArgumentNullException(nameof(def)); }
		if (def.Type != EBMLElementType.Float) { throw new ArgumentException("Element type mismatch", nameof(def)); }

		try
		{
			if (Interlocked.Increment(ref writeBusy) != 1) { throw new InvalidOperationException("Concurrent writes not allowed."); }
			var sizeVInt = new EBMLVInt(4);
			if (writeBuffer.SpaceLeft < (def.Id.WidthBytes + sizeVInt.WidthBytes + 4)) { await FlushInternalBuffer(cancellationToken); }
			def.Id.Write(writeBuffer);
			sizeVInt.Write(writeBuffer);
			System.Buffers.Binary.BinaryPrimitives.WriteSingleBigEndian(writeBuffer.WriteSpan, value);
			writeBuffer.WriteOffset += 4;
		}
		finally { Interlocked.Decrement(ref writeBusy); }
	}

	public async ValueTask WriteFloat(EBMLElementDefiniton def, double value, CancellationToken cancellationToken = default)
	{
		if (disposed) { throw new ObjectDisposedException(nameof(EBMLWriter)); }
		if (def == null) { throw new ArgumentNullException(nameof(def)); }
		if (def.Type != EBMLElementType.Float) { throw new ArgumentException("Element type mismatch", nameof(def)); }

		try
		{
			if (Interlocked.Increment(ref writeBusy) != 1) { throw new InvalidOperationException("Concurrent writes not allowed."); }
			var sizeVInt = new EBMLVInt(8);
			if (writeBuffer.SpaceLeft < (def.Id.WidthBytes + sizeVInt.WidthBytes + 8)) { await FlushInternalBuffer(cancellationToken); }
			def.Id.Write(writeBuffer);
			sizeVInt.Write(writeBuffer);
			System.Buffers.Binary.BinaryPrimitives.WriteDoubleBigEndian(writeBuffer.WriteSpan, value);
			writeBuffer.WriteOffset += 8;
		}
		finally { Interlocked.Decrement(ref writeBusy); }
	}

	public async ValueTask WriteDate(EBMLElementDefiniton def, DateTime date, CancellationToken cancellationToken = default)
	{
		if (disposed) { throw new ObjectDisposedException(nameof(EBMLWriter)); }
		if (def == null) { throw new ArgumentNullException(nameof(def)); }
		if (def.Type != EBMLElementType.Date) { throw new ArgumentException("Element type mismatch", nameof(def)); }

		try
		{
			if (Interlocked.Increment(ref writeBusy) != 1) { throw new InvalidOperationException("Concurrent writes not allowed."); }
			var sizeVInt = new EBMLVInt(8);
			if (writeBuffer.SpaceLeft < (def.Id.WidthBytes + sizeVInt.WidthBytes + 8)) { await FlushInternalBuffer(cancellationToken); }
			def.Id.Write(writeBuffer);
			sizeVInt.Write(writeBuffer);
			var value = (date.Ticks - EBMLDateElement.Epoch.Ticks) * 100;
			System.Buffers.Binary.BinaryPrimitives.WriteInt64BigEndian(writeBuffer.WriteSpan, value);
			writeBuffer.WriteOffset += 8;
		}
		finally { Interlocked.Decrement(ref writeBusy); }
	}

	public async ValueTask WriteString(EBMLElementDefiniton def, string str, CancellationToken cancellationToken = default)
	{
		if (disposed) { throw new ObjectDisposedException(nameof(EBMLWriter)); }
		if (def == null) { throw new ArgumentNullException(nameof(def)); }
		if (def.Type != EBMLElementType.String && def.Type != EBMLElementType.UTF8) { throw new ArgumentException("Element type mismatch", nameof(def)); }

		try
		{
			if (Interlocked.Increment(ref writeBusy) != 1) { throw new InvalidOperationException("Concurrent writes not allowed."); }
			if (str == null) { str = string.Empty; }
			var estimatedMaxSize = def.Id.WidthBytes + 8 + (def.Type != EBMLElementType.String ? str.Length : (str.Length * 4));
			if (writeBuffer.SpaceLeft < estimatedMaxSize) { await FlushInternalBuffer(cancellationToken); }
			if (writeBuffer.SpaceLeft >= estimatedMaxSize)
			{
				var originalOffset = writeBuffer.WriteOffset;
				writeBuffer.WriteOffset += def.Id.WidthBytes + 1;
				var size = (def.Type == EBMLElementType.UTF8 ? System.Text.Encoding.UTF8 : System.Text.Encoding.ASCII).GetBytes(str, writeBuffer.WriteSpan);
				var sizeVInt = new EBMLVInt((ulong)size);
				if (sizeVInt.WidthBytes > 1) { writeBuffer.WriteSpan.Slice(0, size).CopyTo(writeBuffer.WriteSpan.Slice(sizeVInt.WidthBytes - 1)); }
				writeBuffer.WriteOffset = originalOffset;
				def.Id.Write(writeBuffer);
				sizeVInt.Write(writeBuffer);
				writeBuffer.WriteOffset += size;
			}
			else
			{
				if (writeBuffer.UnreadSize > 0) { await FlushInternalBuffer(cancellationToken); }
				var writer = currentLevelWriter;
				using (var tmp = cache.Pop(str.Length * 4))
				{
					var size = (def.Type == EBMLElementType.UTF8 ? System.Text.Encoding.UTF8 : System.Text.Encoding.ASCII).GetBytes(str, tmp.Buffer);
					var sizeVInt = new EBMLVInt((ulong)size);
					await def.Id.Write(writer, cancellationToken);
					await sizeVInt.Write(writer, cancellationToken);
					await writer.WriteAsync(new ReadOnlyMemory<byte>(tmp.Buffer, 0, size), cancellationToken);
				}
			}
		}
		finally { Interlocked.Decrement(ref writeBusy); }
	}

	public async ValueTask WriteVoid(int bytes, CancellationToken cancellationToken = default)
	{
		if (disposed) { throw new ObjectDisposedException(nameof(EBMLWriter)); }

		try
		{
			if (Interlocked.Increment(ref writeBusy) != 1) { throw new InvalidOperationException("Concurrent writes not allowed."); }
			if (bytes < 0) { bytes = 0; }
			var sizeVInt = new EBMLVInt((ulong)bytes);
			if (writeBuffer.SpaceLeft < (EBMLElementDefiniton.Void.Id.WidthBytes + sizeVInt.WidthBytes + 16)) { await FlushInternalBuffer(cancellationToken); }
			EBMLElementDefiniton.Void.Id.Write(writeBuffer);
			sizeVInt.Write(writeBuffer);
			while (bytes > 0)
			{
				if (writeBuffer.SpaceLeft < 16) { await FlushInternalBuffer(cancellationToken); }
				var canWrite = writeBuffer.SpaceLeft;
				if (canWrite > bytes) { canWrite = bytes; }
				writeBuffer.WriteSpan.Slice(0, canWrite).Clear();
				writeBuffer.WriteOffset += canWrite;
				bytes -= canWrite;
			}
		}
		finally { Interlocked.Decrement(ref writeBusy); }
	}

	public async ValueTask WriteBinary(EBMLElementDefiniton def, ReadOnlyMemory<byte> bytes, CancellationToken cancellationToken = default)
	{
		if (disposed) { throw new ObjectDisposedException(nameof(EBMLWriter)); }
		if (def == null) { throw new ArgumentNullException(nameof(def)); }
		if (def.Type != EBMLElementType.Binary && def.Type != EBMLElementType.Unknown) { throw new ArgumentException("Element type mismatch", nameof(def)); }

		try
		{
			if (Interlocked.Increment(ref writeBusy) != 1) { throw new InvalidOperationException("Concurrent writes not allowed."); }
			var sizeVInt = new EBMLVInt((ulong)bytes.Length);
			var estimatedMaxSize = def.Id.WidthBytes + sizeVInt.WidthBytes + bytes.Length;
			if (writeBuffer.SpaceLeft < estimatedMaxSize) { await FlushInternalBuffer(cancellationToken); }
			def.Id.Write(writeBuffer);
			sizeVInt.Write(writeBuffer);
			if (bytes.Length == 0) { return; }

			var canWrite = writeBuffer.SpaceLeft;
			if (canWrite > bytes.Length) { canWrite = bytes.Length; }
			bytes.Slice(0, canWrite).Span.CopyTo(writeBuffer.WriteSpan);
			writeBuffer.WriteOffset += canWrite;
			if (bytes.Length > canWrite)
			{
				await FlushInternalBuffer(cancellationToken);
				await currentLevelWriter.WriteAsync(bytes.Slice(canWrite), cancellationToken);
			}
		}
		finally { Interlocked.Decrement(ref writeBusy); }
	}

	public async ValueTask WriteBinary(EBMLElementDefiniton def, ReadOnlyMemory<byte> header, ReadOnlyMemory<byte> bytes, CancellationToken cancellationToken = default)
	{
		if (disposed) { throw new ObjectDisposedException(nameof(EBMLWriter)); }
		if (def == null) { throw new ArgumentNullException(nameof(def)); }
		if (def.Type != EBMLElementType.Binary && def.Type != EBMLElementType.Unknown) { throw new ArgumentException("Element type mismatch", nameof(def)); }

		try
		{
			if (Interlocked.Increment(ref writeBusy) != 1) { throw new InvalidOperationException("Concurrent writes not allowed."); }
			var sizeVInt = new EBMLVInt((ulong)(header.Length + bytes.Length));
			var estimatedMaxSize = def.Id.WidthBytes + sizeVInt.WidthBytes + header.Length + bytes.Length;
			if (writeBuffer.SpaceLeft < estimatedMaxSize) { await FlushInternalBuffer(cancellationToken); }
			def.Id.Write(writeBuffer);
			sizeVInt.Write(writeBuffer);
			if (header.Length == 0 && bytes.Length == 0) { return; }

			var canWrite = writeBuffer.SpaceLeft;
			if (canWrite > header.Length) { canWrite = header.Length; }
			header.Slice(0, canWrite).Span.CopyTo(writeBuffer.WriteSpan);
			writeBuffer.WriteOffset += canWrite;
			if (header.Length > canWrite)
			{
				await FlushInternalBuffer(cancellationToken);
				await currentLevelWriter.WriteAsync(header.Slice(canWrite), cancellationToken);
			}

			if (writeBuffer.UnreadSize == 0) { canWrite = 0; }
			else
			{
				canWrite = writeBuffer.SpaceLeft;
				if (canWrite > bytes.Length) { canWrite = bytes.Length; }
				bytes.Slice(0, canWrite).Span.CopyTo(writeBuffer.WriteSpan);
				writeBuffer.WriteOffset += canWrite;
			}
			if (bytes.Length > canWrite)
			{
				await FlushInternalBuffer(cancellationToken);
				await currentLevelWriter.WriteAsync(bytes.Slice(canWrite), cancellationToken);
			}
		}
		finally { Interlocked.Decrement(ref writeBusy); }
	}

	public async ValueTask WriteBinary(EBMLElementDefiniton def, Stream bytes, CancellationToken cancellationToken = default)
	{
		if (disposed) { throw new ObjectDisposedException(nameof(EBMLWriter)); }
		if (def == null) { throw new ArgumentNullException(nameof(def)); }
		if (bytes == null) { throw new ArgumentNullException(nameof(bytes)); }
		if (def.Type != EBMLElementType.Binary && def.Type != EBMLElementType.Unknown) { throw new ArgumentException("Element type mismatch", nameof(def)); }

		try
		{
			if (Interlocked.Increment(ref writeBusy) != 1) { throw new InvalidOperationException("Concurrent writes not allowed."); }
			if (writeBuffer.UnreadSize > 0) { await FlushInternalBuffer(cancellationToken); }
			var writer = currentLevelWriter;
			await def.Id.Write(writer, cancellationToken);
			if (bytes.CanSeek)
			{
				var size = new EBMLVInt((ulong)(bytes.Length - bytes.Position));
				await size.Write(writer, cancellationToken);
				if (writer is IDataQueue dq) { await bytes.CopyToAsync(dq.WriteStream, cancellationToken); }
				else
				{
					using (var target = new DataQueueWriteStream(writer, true))
					{
						await bytes.CopyToAsync(target, cancellationToken);
					}
				}
			}
			else
			{
				using (var mem = new MemoryStream())
				{
					await bytes.CopyToAsync(mem, cancellationToken);
					await new EBMLVInt((ulong)mem.Length).Write(writer, cancellationToken);
					mem.Position = 0;
					using (var target = new DataQueueWriteStream(writer, true)) { await mem.CopyToAsync(target, cancellationToken); }
				}
			}
		}
		finally { Interlocked.Decrement(ref writeBusy); }
	}

	public async ValueTask WriteElement(EBMLElement element, CancellationToken cancellationToken = default)
	{
		if (disposed) { throw new ObjectDisposedException(nameof(EBMLWriter)); }
		if (element == null) { throw new ArgumentNullException(nameof(element)); }
		switch (element.ElementType)
		{
			case EBMLElementType.SignedInteger:
				await WriteSignedInteger(element.Definition, element.IntValue, cancellationToken);
				break;
			case EBMLElementType.UnsignedInteger:
				await WriteUnsignedInteger(element.Definition, element.UIntValue, cancellationToken);
				break;
			case EBMLElementType.Float:
				if (element.DataSize.Value == 8) { await WriteFloat(element.Definition, element.DoubleValue, cancellationToken); }
				else { await WriteFloat(element.Definition, element.FloatValue, cancellationToken); }
				break;
			case EBMLElementType.Date:
				await WriteDate(element.Definition, element.DateValue ?? EBMLDateElement.Epoch, cancellationToken);
				break;
			case EBMLElementType.String:
			case EBMLElementType.UTF8:
				await WriteString(element.Definition, element.StringValue, cancellationToken);
				break;
			case EBMLElementType.Binary:
			case EBMLElementType.Unknown:
				if (element is EBMLVoidElement)
				{
					await WriteVoid((int)element.DataSize.Value, cancellationToken);
				}
				else
				{
					var bin = (EBMLBinaryElement)element;
					if (bin.Reader == null) { await WriteBinary(element.Definition, bin.Value, cancellationToken); }
					else { await WriteBinary(element.Definition, new DataQueueReadStream(bin.Reader), cancellationToken); }
				}
				break;
			case EBMLElementType.Master:
				var master = (EBMLMasterElement)element;
				await BeginMasterElement(master.Definition, cancellationToken);
				foreach (var child in master.Children) { await WriteElement(child, cancellationToken); }
				await EndMasterElement(cancellationToken);
				break;
		}
	}

	public async ValueTask BeginMasterElement(EBMLElementDefiniton def, CancellationToken cancellationToken = default)
	{
		if (disposed) { throw new ObjectDisposedException(nameof(EBMLWriter)); }
		if (def == null) { throw new ArgumentNullException(nameof(def)); }
		if (def.Type != EBMLElementType.Master) { throw new ArgumentException("Element type mismatch", nameof(def)); }

		try
		{
			if (Interlocked.Increment(ref writeBusy) != 1) { throw new InvalidOperationException("Concurrent writes not allowed."); }
			if (writeBuffer.SpaceLeft < def.Id.WidthBytes) { await FlushInternalBuffer(cancellationToken); }
			def.Id.Write(writeBuffer);
			await FlushInternalBuffer(cancellationToken);
			level = new Level(level, def, new DataQueue(maxMasterElementBufferSize, cache), currentLevelWriter, CanSeek);
		}
		finally { Interlocked.Decrement(ref writeBusy); }
	}

	public async ValueTask EndMasterElement(CancellationToken cancellationToken = default)
	{
		if (level == null) { throw new InvalidOperationException(); }
		Level endedLevel = null;
		try
		{
			if (Interlocked.Increment(ref writeBusy) != 1) { throw new InvalidOperationException("Concurrent writes not allowed."); }
			await FlushInternalBuffer(cancellationToken);
			endedLevel = level;
			level = level.Parent;
			endedLevel.Buffer.CloseWrite();
			await endedLevel.ForwardBuffer(cancellationToken);
			if (endedLevel.NeedSizeWritten)
			{
				if (CanSeek)
				{
					await FlushAsync(cancellationToken);
					var blockSize = endedLevel.Buffer.TotalBytesWritten;
					stream.Seek(-blockSize - 8, SeekOrigin.Current);
					if ((blockSize & ~((1L << 56) - 1)) != 0) { throw new InvalidOperationException("Block size too large"); }
					System.Buffers.Binary.BinaryPrimitives.WriteInt64BigEndian(writeBuffer.WriteSpan, blockSize | (1L << 56));
					await stream.WriteAsync(writeBuffer.Buffer, writeBuffer.WriteOffset, 8, cancellationToken);
					stream.Seek(blockSize, SeekOrigin.Current);
				}
				else if (!endedLevel.Definition.AllowUnknownSize)
				{
					throw new InvalidOperationException("Block size unknown without seekable stream");
				}
			}
		}
		finally
		{
			Interlocked.Decrement(ref writeBusy);
			endedLevel?.Dispose();
		}
	}

	public struct RelativeOffset
	{
		private readonly EBMLElementDefiniton relativeTo;
		private readonly Level parent;
		private readonly long relOffset;
		private readonly bool inited;

		public bool Marked => inited;

		public RelativeOffset(EBMLWriter writer, EBMLElementDefiniton relativeTo)
		{
			this.relativeTo = relativeTo;
			parent = writer.level;
			relOffset = writer.currentLevelWriter.TotalBytesWritten + writer.writeBuffer.UnreadSize;
			inited = true;
		}

		public long? Resolve()
		{
			if (!inited) { return null; }
			var offset = relOffset;
			var current = parent;
			while (current != null && current.Definition != relativeTo)
			{
				if (current.DataSizeWidth == null)
				{
					current.ForwardBuffer();
					if (current.DataSizeWidth == null) { return null; }
				}
				offset += current.DataSizeWidth.Value + current.RelativeDataOffset;
				current = current.Parent;
			}
			if (current?.Definition != relativeTo)
			{
				throw new ArgumentException("Element not found in current hierarchy.", "relativeTo");
			}
			return offset;
		}
	}

	public RelativeOffset MarkRelativeOffset(EBMLElementDefiniton relativeTo = null)
	{
		return new RelativeOffset(this, relativeTo);
	}

	public async ValueTask FlushAsync(CancellationToken cancellationToken = default)
	{
		if (level != null)
		{
			var current = level; do { _ = current.ForwardBuffer(); } while ((current = current.Parent) != null);
			current = level; do { await current.Buffer.WaitForEmpty(cancellationToken); } while ((current = current.Parent) != null);
		}
		if (stream != null) { await stream.FlushAsync(cancellationToken); }
	}

	public async ValueTask<EBMLWriter> SeekWriter(long offset, SeekOrigin origin, CancellationToken cancellationToken = default)
	{
		if (!CanSeek) { throw new InvalidOperationException(); }
		await FlushAsync(cancellationToken);
		var writer = new EBMLWriter(stream, true, cache, true);
		stream.Seek(offset, origin);
		return writer;
	}

	public async ValueTask DisposeAsync()
	{
		if (disposed) { return; }
		disposed = true;
		while (level != null) { await EndMasterElement(); }
		await FlushInternalBuffer();
		if (rewindOnDispose) { stream.Seek(startOffset, SeekOrigin.Begin); }
		if (!keepWriterOpen)
		{
			writer.CloseWrite();
			if (writer is IAsyncDisposable a) { await a.DisposeAsync(); }
			else if (writer is IDisposable b) { b.Dispose(); }
		}
	}

	public void Dispose()
	{
		var task = DisposeAsync();
		if (!task.IsCompletedSuccessfully) { task.AsTask().Wait(); }
	}
}