13224545445

Committed 09 Feb 2025 09:56AM UTC coverage: 76.363% (+0.4%) from 76.002%

Build # 13224545445

Build Type

Pull #283

github

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web-flow

Commit Message

Merge 256916b10 into 46f79e579

Pull Request Pull Request #283: Issue 200 async Read

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94.26

/src/ACadSharp/IO/DWG/DwgStreamReaders/DwgFileHeaderReader.cs

﻿using ACadSharp.Exceptions;
using CSUtilities.Converters;
using CSUtilities.IO;
using CSUtilities.Text;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace ACadSharp.IO.DWG
{
        internal class DwgFileHeaderReader : DwgSectionIO
        {
                private const int _metaDataSize = 0x7A;

                private const int _encriptedDataSize = 0x6C;

                public override string SectionName { get { return string.Empty; } }

                private StreamIO _fileStream;

                public DwgFileHeaderReader(Stream stream) : base(ACadVersion.Unknown)
                {
                        this._fileStream = new StreamIO(stream);
                }

                public DwgFileHeader Read()
                {
                        this._fileStream.Position = 0;

                        //0x00        6        “ACXXXX” version string
                        byte[] buffer = new byte[6];
                        this._fileStream.Stream.Read(buffer, 0, buffer.Length);
                        this.updateFileVersion(buffer);

                        DwgFileHeader fileHeader = DwgFileHeader.CreateFileHeader(this._version);

                        //Get the stream reader
                        IDwgStreamReader sreader = DwgStreamReaderBase.GetStreamHandler(this._version, _fileStream.Stream);

                        //Read the file header
                        switch (fileHeader.AcadVersion)
                        {
                                case ACadVersion.Unknown:
                                        throw new CadNotSupportedException();
                                case ACadVersion.MC0_0:
                                case ACadVersion.AC1_2:
                                case ACadVersion.AC1_4:
                                case ACadVersion.AC1_50:
                                case ACadVersion.AC2_10:
                                case ACadVersion.AC1002:
                                case ACadVersion.AC1003:
                                case ACadVersion.AC1004:
                                case ACadVersion.AC1006:
                                case ACadVersion.AC1009:
                                        throw new CadNotSupportedException(this._version);
                                case ACadVersion.AC1012:
                                case ACadVersion.AC1014:
                                case ACadVersion.AC1015:
                                        this.readFileHeaderAC15(fileHeader as DwgFileHeaderAC15, sreader);
                                        break;
                                case ACadVersion.AC1018:
                                        this.readFileHeaderAC18(fileHeader as DwgFileHeaderAC18, sreader);
                                        break;
                                case ACadVersion.AC1021:
                                        this.readFileHeaderAC21(fileHeader as DwgFileHeaderAC21, sreader);
                                        break;
                                case ACadVersion.AC1024:
                                case ACadVersion.AC1027:
                                case ACadVersion.AC1032:
                                        //Check if it works...
                                        this.readFileHeaderAC18(fileHeader as DwgFileHeaderAC18, sreader);
                                        break;
                        }

                        return fileHeader;
                }

                public async Task<DwgFileHeader> ReadAsync(CancellationToken cancellationToken = default)
                {
                        this._fileStream.Position = 0;

                        //0x00        6        “ACXXXX” version string
                        byte[] buffer = new byte[6];
                        await this._fileStream.Stream.ReadAsync(buffer, 0, buffer.Length, cancellationToken);
                        this.updateFileVersion(buffer);

                        DwgFileHeader fileHeader = DwgFileHeader.CreateFileHeader(this._version);

                        //Read the file header
                        switch (fileHeader.AcadVersion)
                        {
                                case ACadVersion.Unknown:
                                        throw new CadNotSupportedException();
                                case ACadVersion.MC0_0:
                                case ACadVersion.AC1_2:
                                case ACadVersion.AC1_4:
                                case ACadVersion.AC1_50:
                                case ACadVersion.AC2_10:
                                case ACadVersion.AC1002:
                                case ACadVersion.AC1003:
                                case ACadVersion.AC1004:
                                case ACadVersion.AC1006:
                                case ACadVersion.AC1009:
                                        throw new CadNotSupportedException(this._version);
                                case ACadVersion.AC1012:
                                case ACadVersion.AC1014:
                                case ACadVersion.AC1015:
                                        IDwgStreamReader sreader = DwgStreamReaderBase.GetStreamHandler(_version, await getHeaderAC15Stream(cancellationToken));
                                        this.readFileHeaderAC15(fileHeader as DwgFileHeaderAC15, sreader);
                                        break;
                                case ACadVersion.AC1018:
                                        await this.readFileHeaderAC18Async(fileHeader as DwgFileHeaderAC18, cancellationToken);
                                        break;
                                case ACadVersion.AC1021:
                                        await this.readFileHeaderAC21Async(fileHeader as DwgFileHeaderAC21, cancellationToken);
                                        break;
                                case ACadVersion.AC1024:
                                case ACadVersion.AC1027:
                                case ACadVersion.AC1032:
                                        await this.readFileHeaderAC18Async(fileHeader as DwgFileHeaderAC18, cancellationToken);
                                        break;
                        }

                        return fileHeader;
                }

                private void readFileHeaderAC15(DwgFileHeaderAC15 fileheader, IDwgStreamReader sreader)
                {
                        //The next 7 starting at offset 0x06 are to be six bytes of 0 
                        //(in R14, 5 0’s and the ACADMAINTVER variable) and a byte of 1.
                        sreader.ReadBytes(7);
                        //At 0x0D is a seeker (4 byte long absolute address) for the beginning sentinel of the image data.
                        fileheader.PreviewAddress = sreader.ReadInt();

                        //Undocumented Bytes at 0x11 and 0x12
                        sreader.ReadBytes(2);

                        //Bytes at 0x13 and 0x14 are a raw short indicating the value of the code page for this drawing file.
                        fileheader.DrawingCodePage = CadUtils.GetCodePage(sreader.ReadShort());

                        //At 0x15 is a long that tells how many sets of recno/seeker/length records follow.
                        int nRecords = (int)sreader.ReadRawLong();
                        for (int i = 0; i < nRecords; ++i)
                        {
                                //Record number (raw byte) | Seeker (raw long) | Size (raw long)
                                DwgSectionLocatorRecord record = new DwgSectionLocatorRecord();
                                record.Number = sreader.ReadByte();
                                record.Seeker = sreader.ReadRawLong();
                                record.Size = sreader.ReadRawLong();

                                fileheader.Records.Add(record.Number.Value, record);
                        }

                        //RS : CRC for BOF to this point.
                        sreader.ReadCRC();

                        var sn = sreader.ReadSentinel();
                        if (!CheckSentinel(sn, DwgFileHeaderAC15.EndSentinel))
                        {
                                this.notify($"Invalid section sentinel found in FileHeader", NotificationType.Warning);
                        }
                }

                private void readFileHeaderAC18(DwgFileHeaderAC18 fileheader, IDwgStreamReader sreader)
                {
                        this.readFileMetaData(fileheader, sreader);

                        //0x80        0x6C        Encrypted Data
                        //Metadata:
                        //The encrypted data at 0x80 can be decrypted by exclusive or’ing the 0x6c bytes of data 
                        //from the file with the following magic number sequence:

                        //29 23 BE 84 E1 6C D6 AE 52 90 49 F1 F1 BB E9 EB
                        //B3 A6 DB 3C 87 0C 3E 99 24 5E 0D 1C 06 B7 47 DE
                        //B3 12 4D C8 43 BB 8B A6 1F 03 5A 7D 09 38 25 1F
                        //5D D4 CB FC 96 F5 45 3B 13 0D 89 0A 1C DB AE 32
                        //20 9A 50 EE 40 78 36 FD 12 49 32 F6 9E 7D 49 DC
                        //AD 4F 14 F2 44 40 66 D0 6B C4 30 B7

                        StreamIO headerStream = new StreamIO(new CRC32StreamHandler(sreader.ReadBytes(0x6C), 0U)); //108
                        headerStream.Encoding = TextEncoding.GetListedEncoding(CodePage.Windows1252);

                        #region Read header encrypted data

                        //0x00        12        “AcFssFcAJMB” file ID string
                        string fileId = headerStream.ReadString(12);
                        if (fileId != "AcFssFcAJMB\0")
                        {
                                this.notify($"File validation failed, id should be : AcFssFcAJMB\0, but is : {fileId}", NotificationType.Warning);
                        }

                        //0x0C        4        0x00(long)
                        headerStream.ReadInt<LittleEndianConverter>();
                        //0x10        4        0x6c(long)
                        headerStream.ReadInt<LittleEndianConverter>();
                        //0x14        4        0x04(long)
                        headerStream.ReadInt<LittleEndianConverter>();
                        //0x18        4        Root tree node gap
                        fileheader.RootTreeNodeGap = headerStream.ReadInt<LittleEndianConverter>();
                        //0x1C        4        Lowermost left tree node gap
                        fileheader.LeftGap = headerStream.ReadInt<LittleEndianConverter>();
                        //0x20        4        Lowermost right tree node gap
                        fileheader.RigthGap = headerStream.ReadInt<LittleEndianConverter>();
                        //0x24        4        Unknown long(ODA writes 1)        
                        headerStream.ReadInt<LittleEndianConverter>();
                        //0x28        4        Last section page Id
                        fileheader.LastPageId = headerStream.ReadInt<LittleEndianConverter>();

                        //0x2C        8        Last section page end address
                        fileheader.LastSectionAddr = headerStream.ReadULong<LittleEndianConverter>();
                        //0x34        8        Second header data address pointing to the repeated header data at the end of the file
                        fileheader.SecondHeaderAddr = headerStream.ReadULong<LittleEndianConverter>();

                        //0x3C        4        Gap amount
                        fileheader.GapAmount = headerStream.ReadUInt<LittleEndianConverter>();
                        //0x40        4        Section page amount
                        fileheader.SectionAmount = headerStream.ReadUInt<LittleEndianConverter>();
                        //0x44        4        0x20(long)
                        headerStream.ReadInt<LittleEndianConverter>();
                        //0x48        4        0x80(long)
                        headerStream.ReadInt<LittleEndianConverter>();
                        //0x4C        4        0x40(long)
                        headerStream.ReadInt<LittleEndianConverter>();
                        //0x50        4        Section Page Map Id
                        fileheader.SectionPageMapId = headerStream.ReadUInt<LittleEndianConverter>();
                        //0x54        8        Section Page Map address(add 0x100 to this value)
                        fileheader.PageMapAddress = headerStream.ReadULong<LittleEndianConverter>() + 256UL;
                        //0x5C        4        Section Map Id
                        fileheader.SectionMapId = headerStream.ReadUInt<LittleEndianConverter>();
                        //0x60        4        Section page array size
                        fileheader.SectionArrayPageSize = headerStream.ReadUInt<LittleEndianConverter>();
                        //0x64        4        Gap array size
                        fileheader.GapArraySize = headerStream.ReadUInt<LittleEndianConverter>();
                        //0x68        4        CRC32(long).See paragraph 2.14.2 for the 32 - bit CRC calculation, 
                        //                        the seed is zero. Note that the CRC 
                        //                        calculation is done including the 4 CRC bytes that are 
                        //                        initially zero! So the CRC calculation takes into account 
                        //                        all of the 0x6c bytes of the data in this table.
                        fileheader.CRCSeed = headerStream.ReadUInt();
                        #endregion

                        #region Read page map of the file
                        sreader.Position = (long)fileheader.PageMapAddress;

                        //Get the page size
                        this.getPageHeaderData(sreader, out _, out long decompressedSize, out _, out _, out _);
                        //Get the descompressed stream to read the records
                        StreamIO decompressed = new StreamIO(DwgLZ77AC18Decompressor.Decompress(sreader.Stream, decompressedSize));

                        //Section size
                        int total = 0x100;
                        while (decompressed.Position < decompressed.Length)
                        {
                                DwgSectionLocatorRecord record = new DwgSectionLocatorRecord();
                                //0x00        4        Section page number, starts at 1, page numbers are unique per file.
                                record.Number = decompressed.ReadInt();
                                //0x04        4        Section size
                                record.Size = decompressed.ReadInt();

                                if (record.Number >= 0)
                                {
                                        record.Seeker = total;
                                        fileheader.Records.Add(record.Number.Value, record);
                                }
                                else
                                {
                                        //If the section number is negative, this represents a gap in the sections (unused data). 
                                        //For a negative section number, the following data will be present after the section size:

                                        //0x00        4        Parent
                                        decompressed.ReadInt();
                                        //0x04        4        Left
                                        decompressed.ReadInt();
                                        //0x08        4        Right
                                        decompressed.ReadInt();
                                        //0x0C        4        0x00
                                        decompressed.ReadInt();
                                }

                                total += (int)record.Size;
                        }
                        #endregion

                        #region Read the data section map
                        //Set the positon of the map
                        sreader.Position = fileheader.Records[(int)fileheader.SectionMapId].Seeker;
                        //84244
                        //Get the page size
                        this.getPageHeaderData(sreader, out _, out decompressedSize, out _, out _, out _);
                        //1556
                        StreamIO decompressedStream = new StreamIO(DwgLZ77AC18Decompressor.Decompress(sreader.Stream, decompressedSize));
                        decompressedStream.Encoding = TextEncoding.GetListedEncoding(CodePage.Windows1252);

                        //0x00        4        Number of section descriptions(NumDescriptions)
                        int ndescriptions = decompressedStream.ReadInt<LittleEndianConverter>();
                        //0x04        4        0x02 (long)
                        decompressedStream.ReadInt<LittleEndianConverter>();
                        //0x08        4        0x00007400 (long)
                        decompressedStream.ReadInt<LittleEndianConverter>();
                        //0x0C        4        0x00 (long)
                        decompressedStream.ReadInt<LittleEndianConverter>();
                        //0x10        4        Unknown (long), ODA writes NumDescriptions here.
                        decompressedStream.ReadInt<LittleEndianConverter>();

                        for (int i = 0; i < ndescriptions; ++i)
                        {
                                DwgSectionDescriptor descriptor = new DwgSectionDescriptor();
                                //0x00        8        Size of section(OdUInt64)
                                descriptor.CompressedSize = decompressedStream.ReadULong();
                                /*0x08        4        Page count(PageCount). Note that there can be more pages than PageCount,
                                                        as PageCount is just the number of pages written to file.
                                                        If a page contains zeroes only, that page is not written to file.
                                                        These “zero pages” can be detected by checking if the page’s start 
                                                        offset is bigger than it should be based on the sum of previously read pages 
                                                        decompressed size(including zero pages).After reading all pages, if the total 
                                                        decompressed size of the pages is not equal to the section’s size, add more zero 
                                                        pages to the section until this condition is met.
                                */
                                descriptor.PageCount = decompressedStream.ReadInt<LittleEndianConverter>();
                                //0x0C        4        Max Decompressed Size of a section page of this type(normally 0x7400)
                                descriptor.DecompressedSize = (ulong)decompressedStream.ReadInt<LittleEndianConverter>();
                                //0x10        4        Unknown(long)
                                decompressedStream.ReadInt<LittleEndianConverter>();
                                //0x14        4        Compressed(1 = no, 2 = yes, normally 2)
                                descriptor.CompressedCode = decompressedStream.ReadInt<LittleEndianConverter>();
                                //0x18        4        Section Id(starts at 0). The first section(empty section) is numbered 0, consecutive sections are numbered descending from(the number of sections – 1) down to 1.
                                descriptor.SectionId = decompressedStream.ReadInt<LittleEndianConverter>();
                                //0x1C        4        Encrypted(0 = no, 1 = yes, 2 = unknown)
                                descriptor.Encrypted = decompressedStream.ReadInt<LittleEndianConverter>();
                                //0x20        64        Section Name(string)
                                descriptor.Name = decompressedStream.ReadString(64).Split('\0')[0];

                                //Following this, the following (local) section page map data will be present
                                for (int j = 0; j < descriptor.PageCount; ++j)
                                {
                                        DwgLocalSectionMap localmap = new DwgLocalSectionMap();
                                        //0x00        4        Page number(index into SectionPageMap), starts at 1
                                        localmap.PageNumber = decompressedStream.ReadInt<LittleEndianConverter>();
                                        //0x04        4        Data size for this page(compressed size).
                                        localmap.CompressedSize = (ulong)decompressedStream.ReadInt<LittleEndianConverter>();
                                        //0x08        8        Start offset for this page(OdUInt64).If this start offset is smaller than the sum of the decompressed size of all previous pages, then this page is to be preceded by zero pages until this condition is met.
                                        localmap.Offset = decompressedStream.ReadULong();

                                        //same decompressed size and seeker (temporal values)
                                        localmap.DecompressedSize = descriptor.DecompressedSize;
                                        localmap.Seeker = fileheader.Records[localmap.PageNumber].Seeker;

                                        //Maximum section page size appears to be 0x7400 bytes in the normal case.
                                        //If a logical section of the file (the database objects, for example) exceeds this size, then it is broken up into pages of size 0x7400.

                                        descriptor.LocalSections.Add(localmap);
                                }

                                //Get the final size for the local section
                                uint sizeLeft = (uint)(descriptor.CompressedSize % descriptor.DecompressedSize);
                                if (sizeLeft > 0U && descriptor.LocalSections.Count > 0)
                                        descriptor.LocalSections[descriptor.LocalSections.Count - 1].DecompressedSize = sizeLeft;

                                fileheader.Descriptors.Add(descriptor.Name, descriptor);
                        }
                        #endregion
                }

                private async Task readFileHeaderAC18Async(DwgFileHeaderAC18 fileheader, CancellationToken cancellationToken = default)
                {
                        this.readFileMetaData(fileheader, await this.getStreamChunkAsync(_metaDataSize, cancellationToken));

                        this.readEncriptedData(fileheader, await this.getStreamChunkAsync(_encriptedDataSize, cancellationToken));

                        _fileStream.Position = (long)fileheader.PageMapAddress;

                        //Get the page size
                        this.getPageHeaderData(await this.getStreamChunkAsync(0x14, cancellationToken)
                                , out _, out long decompressedSize, out long compressedSize, out _, out _);

                        //Get the descompressed stream to read the records
                        IDwgStreamReader compressed = await this.getStreamChunkAsync((int)compressedSize, cancellationToken);

                        this.readRecords(fileheader, compressed.Stream, decompressedSize);

                        this._fileStream.Position = fileheader.Records[(int)fileheader.SectionMapId].Seeker;
                        //Get the page size
                        this.getPageHeaderData(await this.getStreamChunkAsync(0x14, cancellationToken)
                                , out _, out decompressedSize, out compressedSize, out _, out _);

                        compressed = await this.getStreamChunkAsync((int)compressedSize, cancellationToken);

                        this.readDescriptors(fileheader, compressed.Stream, decompressedSize);
                }

                private void readRecords(DwgFileHeaderAC18 fileheader, Stream compressed, long decompressedSize)
                {
                        StreamIO decompressed = new StreamIO(DwgLZ77AC18Decompressor.Decompress(compressed, decompressedSize));

                        //Section size
                        int total = 0x100;
                        while (decompressed.Position < decompressed.Length)
                        {
                                DwgSectionLocatorRecord record = new DwgSectionLocatorRecord();
                                //0x00        4        Section page number, starts at 1, page numbers are unique per file.
                                record.Number = decompressed.ReadInt();
                                //0x04        4        Section size
                                record.Size = decompressed.ReadInt();

                                if (record.Number >= 0)
                                {
                                        record.Seeker = total;
                                        fileheader.Records.Add(record.Number.Value, record);
                                }
                                else
                                {
                                        //If the section number is negative, this represents a gap in the sections (unused data). 
                                        //For a negative section number, the following data will be present after the section size:

                                        //0x00        4        Parent
                                        decompressed.ReadInt();
                                        //0x04        4        Left
                                        decompressed.ReadInt();
                                        //0x08        4        Right
                                        decompressed.ReadInt();
                                        //0x0C        4        0x00
                                        decompressed.ReadInt();
                                }

                                total += (int)record.Size;
                        }
                }

                private void readDescriptors(DwgFileHeaderAC18 fileheader, Stream compressed, long decompressedSize)
                {
                        StreamIO decompressed = new StreamIO(DwgLZ77AC18Decompressor.Decompress(compressed, decompressedSize));
                        decompressed.Encoding = TextEncoding.GetListedEncoding(CodePage.Windows1252);

                        //0x00        4        Number of section descriptions(NumDescriptions)
                        int ndescriptions = decompressed.ReadInt<LittleEndianConverter>();
                        //0x04        4        0x02 (long)
                        decompressed.ReadInt<LittleEndianConverter>();
                        //0x08        4        0x00007400 (long)
                        decompressed.ReadInt<LittleEndianConverter>();
                        //0x0C        4        0x00 (long)
                        decompressed.ReadInt<LittleEndianConverter>();
                        //0x10        4        Unknown (long), ODA writes NumDescriptions here.
                        decompressed.ReadInt<LittleEndianConverter>();

                        for (int i = 0; i < ndescriptions; ++i)
                        {
                                DwgSectionDescriptor descriptor = new DwgSectionDescriptor();
                                //0x00        8        Size of section(OdUInt64)
                                descriptor.CompressedSize = decompressed.ReadULong();
                                /*0x08        4        Page count(PageCount). Note that there can be more pages than PageCount,
                                                        as PageCount is just the number of pages written to file.
                                                        If a page contains zeroes only, that page is not written to file.
                                                        These “zero pages” can be detected by checking if the page’s start 
                                                        offset is bigger than it should be based on the sum of previously read pages 
                                                        decompressed size(including zero pages).After reading all pages, if the total 
                                                        decompressed size of the pages is not equal to the section’s size, add more zero 
                                                        pages to the section until this condition is met.
                                */
                                descriptor.PageCount = decompressed.ReadInt<LittleEndianConverter>();
                                //0x0C        4        Max Decompressed Size of a section page of this type(normally 0x7400)
                                descriptor.DecompressedSize = (ulong)decompressed.ReadInt<LittleEndianConverter>();
                                //0x10        4        Unknown(long)
                                decompressed.ReadInt<LittleEndianConverter>();
                                //0x14        4        Compressed(1 = no, 2 = yes, normally 2)
                                descriptor.CompressedCode = decompressed.ReadInt<LittleEndianConverter>();
                                //0x18        4        Section Id(starts at 0). The first section(empty section) is numbered 0, consecutive sections are numbered descending from(the number of sections – 1) down to 1.
                                descriptor.SectionId = decompressed.ReadInt<LittleEndianConverter>();
                                //0x1C        4        Encrypted(0 = no, 1 = yes, 2 = unknown)
                                descriptor.Encrypted = decompressed.ReadInt<LittleEndianConverter>();
                                //0x20        64        Section Name(string)
                                descriptor.Name = decompressed.ReadString(64).Split('\0')[0];

                                //Following this, the following (local) section page map data will be present
                                for (int j = 0; j < descriptor.PageCount; ++j)
                                {
                                        DwgLocalSectionMap localmap = new DwgLocalSectionMap();
                                        //0x00        4        Page number(index into SectionPageMap), starts at 1
                                        localmap.PageNumber = decompressed.ReadInt<LittleEndianConverter>();
                                        //0x04        4        Data size for this page(compressed size).
                                        localmap.CompressedSize = (ulong)decompressed.ReadInt<LittleEndianConverter>();
                                        //0x08        8        Start offset for this page(OdUInt64).If this start offset is smaller than the sum of the decompressed size of all previous pages, then this page is to be preceded by zero pages until this condition is met.
                                        localmap.Offset = decompressed.ReadULong();

                                        //same decompressed size and seeker (temporal values)
                                        localmap.DecompressedSize = descriptor.DecompressedSize;
                                        localmap.Seeker = fileheader.Records[localmap.PageNumber].Seeker;

                                        //Maximum section page size appears to be 0x7400 bytes in the normal case.
                                        //If a logical section of the file (the database objects, for example) exceeds this size, then it is broken up into pages of size 0x7400.

                                        descriptor.LocalSections.Add(localmap);
                                }

                                //Get the final size for the local section
                                uint sizeLeft = (uint)(descriptor.CompressedSize % descriptor.DecompressedSize);
                                if (sizeLeft > 0U && descriptor.LocalSections.Count > 0)
                                        descriptor.LocalSections[descriptor.LocalSections.Count - 1].DecompressedSize = sizeLeft;

                                fileheader.Descriptors.Add(descriptor.Name, descriptor);
                        }
                }

                private void readFileHeaderAC21(DwgFileHeaderAC21 fileheader, IDwgStreamReader sreader)
                {
                        this.readFileMetaData(fileheader, sreader);

                        //The last 0x28 bytes of this section consists of check data, 
                        //containing 5 Int64 values representing CRC’s and related numbers 
                        //(starting from 0x3D8 until the end). The first 0x3D8 bytes 
                        //should be decoded using Reed-Solomon (255, 239) decoding, with a factor of 3.
                        byte[] compressedData = sreader.ReadBytes(0x400);
                        byte[] decodedData = new byte[3 * 239]; //factor * blockSize
                        this.reedSolomonDecoding(compressedData, decodedData, 3, 239);

                        //0x00        8        CRC
                        long crc = LittleEndianConverter.Instance.ToInt64(decodedData, 0);
                        //0x08        8        Unknown key
                        long unknownKey = LittleEndianConverter.Instance.ToInt64(decodedData, 8);
                        //0x10        8        Compressed Data CRC
                        long compressedDataCRC = LittleEndianConverter.Instance.ToInt64(decodedData, 16);
                        //0x18        4        ComprLen
                        int comprLen = LittleEndianConverter.Instance.ToInt32(decodedData, 24);
                        //0x1C        4        Length2
                        int length2 = LittleEndianConverter.Instance.ToInt32(decodedData, 28);

                        //The decompressed size is a fixed 0x110.
                        byte[] buffer = new byte[0x110];
                        //If ComprLen is negative, then Data is not compressed (and data length is ComprLen).
                        if (comprLen < 0)
                        {
                                //buffer = decodedData
                                throw new NotImplementedException();
                        }
                        //If ComprLen is positive, the ComprLen bytes of data are compressed
                        else
                        {
                                DwgLZ77AC21Decompressor.Decompress(decodedData, 32U, (uint)comprLen, buffer);
                        }

                        //Get the descompressed stream to read the records
                        StreamIO decompressed = new StreamIO(buffer);

                        //Read the compressed data
                        fileheader.CompressedMetadata = new Dwg21CompressedMetadata()
                        {
                                //0x00        8        Header size (normally 0x70)
                                HeaderSize = decompressed.ReadULong(),
                                //0x08        8        File size
                                FileSize = decompressed.ReadULong(),
                                //0x10        8        PagesMapCrcCompressed
                                PagesMapCrcCompressed = decompressed.ReadULong(),
                                //0x18        8        PagesMapCorrectionFactor
                                PagesMapCorrectionFactor = decompressed.ReadULong(),
                                //0x20        8        PagesMapCrcSeed
                                PagesMapCrcSeed = decompressed.ReadULong(),
                                //0x28        8        Pages map2offset(relative to data page map 1, add 0x480 to get stream position)
                                Map2Offset = decompressed.ReadULong(),
                                //0x30        8        Pages map2Id
                                Map2Id = decompressed.ReadULong(),
                                //0x38        8        PagesMapOffset(relative to data page map 1, add 0x480 to get stream position)
                                PagesMapOffset = decompressed.ReadULong(),
                                //0x40        8        PagesMapId
                                PagesMapId = decompressed.ReadULong(),
                                //0x48        8        Header2offset(relative to page map 1 address, add 0x480 to get stream position)
                                Header2offset = decompressed.ReadULong(),
                                //0x50        8        PagesMapSizeCompressed
                                PagesMapSizeCompressed = decompressed.ReadULong(),
                                //0x58        8        PagesMapSizeUncompressed
                                PagesMapSizeUncompressed = decompressed.ReadULong(),
                                //0x60        8        PagesAmount
                                PagesAmount = decompressed.ReadULong(),
                                //0x68        8        PagesMaxId
                                PagesMaxId = decompressed.ReadULong(),
                                //0x70        8        Unknown(normally 0x20, 32)
                                Unknow0x20 = decompressed.ReadULong(),
                                //0x78        8        Unknown(normally 0x40, 64)
                                Unknow0x40 = decompressed.ReadULong(),
                                //0x80        8        PagesMapCrcUncompressed
                                PagesMapCrcUncompressed = decompressed.ReadULong(),
                                //0x88        8        Unknown(normally 0xf800, 63488)
                                Unknown0xF800 = decompressed.ReadULong(),
                                //0x90        8        Unknown(normally 4)
                                Unknown4 = decompressed.ReadULong(),
                                //0x98        8        Unknown(normally 1)
                                Unknown1 = decompressed.ReadULong(),
                                //0xA0        8        SectionsAmount(number of sections + 1)
                                SectionsAmount = decompressed.ReadULong(),
                                //0xA8        8        SectionsMapCrcUncompressed
                                SectionsMapCrcUncompressed = decompressed.ReadULong(),
                                //0xB0        8        SectionsMapSizeCompressed
                                SectionsMapSizeCompressed = decompressed.ReadULong(),
                                //0xB8        8        SectionsMap2Id
                                SectionsMap2Id = decompressed.ReadULong(),
                                //0xC0        8        SectionsMapId
                                SectionsMapId = decompressed.ReadULong(),
                                //0xC8        8        SectionsMapSizeUncompressed
                                SectionsMapSizeUncompressed = decompressed.ReadULong(),
                                //0xD0        8        SectionsMapCrcCompressed
                                SectionsMapCrcCompressed = decompressed.ReadULong(),
                                //0xD8        8        SectionsMapCorrectionFactor
                                SectionsMapCorrectionFactor = decompressed.ReadULong(),
                                //0xE0        8        SectionsMapCrcSeed
                                SectionsMapCrcSeed = decompressed.ReadULong(),
                                //0xE8        8        StreamVersion(normally 0x60100)
                                StreamVersion = decompressed.ReadULong(),
                                //0xF0        8        CrcSeed
                                CrcSeed = decompressed.ReadULong(),
                                //0xF8        8        CrcSeedEncoded
                                CrcSeedEncoded = decompressed.ReadULong(),
                                //0x100        8        RandomSeed
                                RandomSeed = decompressed.ReadULong(),
                                //0x108        8        Header CRC64
                                HeaderCRC64 = decompressed.ReadULong()
                        };

                        //Prepare the page data stream to read
                        byte[] arr = this.getPageBuffer(
                                fileheader.CompressedMetadata.PagesMapOffset,
                                fileheader.CompressedMetadata.PagesMapSizeCompressed,
                                fileheader.CompressedMetadata.PagesMapSizeUncompressed,
                                fileheader.CompressedMetadata.PagesMapCorrectionFactor,
                                0xEF, sreader.Stream);

                        //Read the page data
                        StreamIO pageDataStream = new StreamIO(arr);

                        long offset = 0;
                        while (pageDataStream.Position < pageDataStream.Length)
                        {
                                long size = pageDataStream.ReadLong();
                                long id = Math.Abs(pageDataStream.ReadLong());
                                fileheader.Records.Add((int)id, new DwgSectionLocatorRecord((int)id, (int)offset, (int)size));

                                //Add the size to the current offset
                                offset += size;
                        }

                        //Prepare the section map data stream to read
                        arr = this.getPageBuffer(
                                (ulong)fileheader.Records[(int)fileheader.CompressedMetadata.SectionsMapId].Seeker,
                                fileheader.CompressedMetadata.SectionsMapSizeCompressed,
                                fileheader.CompressedMetadata.SectionsMapSizeUncompressed,
                                fileheader.CompressedMetadata.SectionsMapCorrectionFactor,
                                239, sreader.Stream);

                        //Section map stream
                        StreamIO sectionMapStream = new StreamIO(arr);

                        while (sectionMapStream.Position < sectionMapStream.Length)
                        {
                                DwgSectionDescriptor section = new DwgSectionDescriptor();
                                //0x00        8        Data size
                                section.CompressedSize = sectionMapStream.ReadULong<LittleEndianConverter>();
                                //0x08        8        Max size
                                section.DecompressedSize = sectionMapStream.ReadULong<LittleEndianConverter>();
                                //0x10        8        Encryption
                                section.Encrypted = (int)sectionMapStream.ReadULong<LittleEndianConverter>();
                                //0x18        8        HashCode
                                section.HashCode = sectionMapStream.ReadULong<LittleEndianConverter>();
                                //0x20        8        SectionNameLength
                                int sectionNameLength = (int)sectionMapStream.ReadLong<LittleEndianConverter>();
                                //0x28        8        Unknown
                                sectionMapStream.ReadULong<LittleEndianConverter>();
                                //0x30        8        Encoding
                                section.Encoding = sectionMapStream.ReadULong<LittleEndianConverter>();
                                //0x38        8        NumPages.This is the number of pages present 
                                //                        in the file for the section, but this does not include 
                                //                        pages that contain zeroes only.A page that contains zeroes 
                                //                        only is not written to file.If a page’s data offset is 
                                //                        smaller than the sum of the decompressed size of all previous 
                                //                        pages, then it is to be preceded by a zero page with a size 
                                //                        that is equal to the difference between these two numbers.
                                section.PageCount = (int)sectionMapStream.ReadULong<LittleEndianConverter>();

                                //Read the name
                                if (sectionNameLength > 0)
                                {
                                        section.Name = sectionMapStream.ReadString(sectionNameLength, Encoding.Unicode);
                                        //Remove the empty characters
                                        section.Name = section.Name.Replace("\0", "");
                                }

                                ulong currentOffset = 0;
                                for (int i = 0; i < section.PageCount; ++i)
                                {
                                        DwgLocalSectionMap page = new DwgLocalSectionMap();
                                        //8        Page data offset.If a page’s data offset is 
                                        //        smaller than the sum of the decompressed size
                                        //        of all previous pages, then it is to be preceded 
                                        //        by a zero page with a size that is equal to the 
                                        //        difference between these two numbers.
                                        page.Offset = sectionMapStream.ReadULong<LittleEndianConverter>();
                                        //8        Page Size
                                        page.Size = sectionMapStream.ReadLong<LittleEndianConverter>();
                                        //8        Page Id
                                        page.PageNumber = (int)sectionMapStream.ReadLong<LittleEndianConverter>();
                                        //8        Page Uncompressed Size
                                        page.DecompressedSize = sectionMapStream.ReadULong<LittleEndianConverter>();
                                        //8        Page Compressed Size
                                        page.CompressedSize = sectionMapStream.ReadULong<LittleEndianConverter>();
                                        //8        Page Checksum
                                        page.Checksum = sectionMapStream.ReadULong<LittleEndianConverter>();
                                        //8        Page CRC
                                        page.CRC = sectionMapStream.ReadULong<LittleEndianConverter>();

                                        //Add the page to the section
                                        section.LocalSections.Add(page);
                                        //Move the offset
                                        currentOffset = page.Offset + page.DecompressedSize;
                                }
                                if (sectionNameLength > 0)
                                        fileheader.Descriptors.Add(section.Name, section);
                        }
                }

                private async Task readFileHeaderAC21Async(DwgFileHeaderAC21 fileheader, CancellationToken cancellationToken = default)
                {
                        this.readFileMetaData(fileheader, await this.getStreamChunkAsync(_metaDataSize, cancellationToken));

                        //The last 0x28 bytes of this section consists of check data, 
                        //containing 5 Int64 values representing CRC’s and related numbers 
                        //(starting from 0x3D8 until the end). The first 0x3D8 bytes 
                        //should be decoded using Reed-Solomon (255, 239) decoding, with a factor of 3.
                        byte[] compressedData = await _fileStream.ReadBytesAsync(0x400);
                        byte[] decodedData = new byte[3 * 239]; //factor * blockSize
                        this.reedSolomonDecoding(compressedData, decodedData, 3, 239);

                        //0x00        8        CRC
                        long crc = LittleEndianConverter.Instance.ToInt64(decodedData, 0);
                        //0x08        8        Unknown key
                        long unknownKey = LittleEndianConverter.Instance.ToInt64(decodedData, 8);
                        //0x10        8        Compressed Data CRC
                        long compressedDataCRC = LittleEndianConverter.Instance.ToInt64(decodedData, 16);
                        //0x18        4        ComprLen
                        int comprLen = LittleEndianConverter.Instance.ToInt32(decodedData, 24);
                        //0x1C        4        Length2
                        int length2 = LittleEndianConverter.Instance.ToInt32(decodedData, 28);

                        //The decompressed size is a fixed 0x110.
                        byte[] buffer = new byte[0x110];
                        //If ComprLen is negative, then Data is not compressed (and data length is ComprLen).
                        if (comprLen < 0)
                        {
                                //buffer = decodedData
                                throw new NotImplementedException();
                        }
                        //If ComprLen is positive, the ComprLen bytes of data are compressed
                        else
                        {
                                DwgLZ77AC21Decompressor.Decompress(decodedData, 32U, (uint)comprLen, buffer);
                        }

                        //Get the descompressed stream to read the records
                        StreamIO decompressed = new StreamIO(buffer);

                        //Read the compressed data
                        fileheader.CompressedMetadata = new Dwg21CompressedMetadata()
                        {
                                //0x00        8        Header size (normally 0x70)
                                HeaderSize = decompressed.ReadULong(),
                                //0x08        8        File size
                                FileSize = decompressed.ReadULong(),
                                //0x10        8        PagesMapCrcCompressed
                                PagesMapCrcCompressed = decompressed.ReadULong(),
                                //0x18        8        PagesMapCorrectionFactor
                                PagesMapCorrectionFactor = decompressed.ReadULong(),
                                //0x20        8        PagesMapCrcSeed
                                PagesMapCrcSeed = decompressed.ReadULong(),
                                //0x28        8        Pages map2offset(relative to data page map 1, add 0x480 to get stream position)
                                Map2Offset = decompressed.ReadULong(),
                                //0x30        8        Pages map2Id
                                Map2Id = decompressed.ReadULong(),
                                //0x38        8        PagesMapOffset(relative to data page map 1, add 0x480 to get stream position)
                                PagesMapOffset = decompressed.ReadULong(),
                                //0x40        8        PagesMapId
                                PagesMapId = decompressed.ReadULong(),
                                //0x48        8        Header2offset(relative to page map 1 address, add 0x480 to get stream position)
                                Header2offset = decompressed.ReadULong(),
                                //0x50        8        PagesMapSizeCompressed
                                PagesMapSizeCompressed = decompressed.ReadULong(),
                                //0x58        8        PagesMapSizeUncompressed
                                PagesMapSizeUncompressed = decompressed.ReadULong(),
                                //0x60        8        PagesAmount
                                PagesAmount = decompressed.ReadULong(),
                                //0x68        8        PagesMaxId
                                PagesMaxId = decompressed.ReadULong(),
                                //0x70        8        Unknown(normally 0x20, 32)
                                Unknow0x20 = decompressed.ReadULong(),
                                //0x78        8        Unknown(normally 0x40, 64)
                                Unknow0x40 = decompressed.ReadULong(),
                                //0x80        8        PagesMapCrcUncompressed
                                PagesMapCrcUncompressed = decompressed.ReadULong(),
                                //0x88        8        Unknown(normally 0xf800, 63488)
                                Unknown0xF800 = decompressed.ReadULong(),
                                //0x90        8        Unknown(normally 4)
                                Unknown4 = decompressed.ReadULong(),
                                //0x98        8        Unknown(normally 1)
                                Unknown1 = decompressed.ReadULong(),
                                //0xA0        8        SectionsAmount(number of sections + 1)
                                SectionsAmount = decompressed.ReadULong(),
                                //0xA8        8        SectionsMapCrcUncompressed
                                SectionsMapCrcUncompressed = decompressed.ReadULong(),
                                //0xB0        8        SectionsMapSizeCompressed
                                SectionsMapSizeCompressed = decompressed.ReadULong(),
                                //0xB8        8        SectionsMap2Id
                                SectionsMap2Id = decompressed.ReadULong(),
                                //0xC0        8        SectionsMapId
                                SectionsMapId = decompressed.ReadULong(),
                                //0xC8        8        SectionsMapSizeUncompressed
                                SectionsMapSizeUncompressed = decompressed.ReadULong(),
                                //0xD0        8        SectionsMapCrcCompressed
                                SectionsMapCrcCompressed = decompressed.ReadULong(),
                                //0xD8        8        SectionsMapCorrectionFactor
                                SectionsMapCorrectionFactor = decompressed.ReadULong(),
                                //0xE0        8        SectionsMapCrcSeed
                                SectionsMapCrcSeed = decompressed.ReadULong(),
                                //0xE8        8        StreamVersion(normally 0x60100)
                                StreamVersion = decompressed.ReadULong(),
                                //0xF0        8        CrcSeed
                                CrcSeed = decompressed.ReadULong(),
                                //0xF8        8        CrcSeedEncoded
                                CrcSeedEncoded = decompressed.ReadULong(),
                                //0x100        8        RandomSeed
                                RandomSeed = decompressed.ReadULong(),
                                //0x108        8        Header CRC64
                                HeaderCRC64 = decompressed.ReadULong()
                        };

                        //Prepare the page data stream to read
                        byte[] arr = await this.getPageBufferAsync(
                                fileheader.CompressedMetadata.PagesMapOffset,
                                fileheader.CompressedMetadata.PagesMapSizeCompressed,
                                fileheader.CompressedMetadata.PagesMapSizeUncompressed,
                                fileheader.CompressedMetadata.PagesMapCorrectionFactor,
                                0xEF, _fileStream.Stream);

                        //Read the page data
                        StreamIO pageDataStream = new StreamIO(arr);

                        long offset = 0;
                        while (pageDataStream.Position < pageDataStream.Length)
                        {
                                long size = pageDataStream.ReadLong();
                                long id = Math.Abs(pageDataStream.ReadLong());
                                fileheader.Records.Add((int)id, new DwgSectionLocatorRecord((int)id, (int)offset, (int)size));

                                //Add the size to the current offset
                                offset += size;
                        }

                        //Prepare the section map data stream to read
                        arr = await this.getPageBufferAsync(
                                (ulong)fileheader.Records[(int)fileheader.CompressedMetadata.SectionsMapId].Seeker,
                                fileheader.CompressedMetadata.SectionsMapSizeCompressed,
                                fileheader.CompressedMetadata.SectionsMapSizeUncompressed,
                                fileheader.CompressedMetadata.SectionsMapCorrectionFactor,
                                239, _fileStream.Stream);

                        //Section map stream
                        StreamIO sectionMapStream = new StreamIO(arr);

                        while (sectionMapStream.Position < sectionMapStream.Length)
                        {
                                DwgSectionDescriptor section = new DwgSectionDescriptor();
                                //0x00        8        Data size
                                section.CompressedSize = sectionMapStream.ReadULong<LittleEndianConverter>();
                                //0x08        8        Max size
                                section.DecompressedSize = sectionMapStream.ReadULong<LittleEndianConverter>();
                                //0x10        8        Encryption
                                section.Encrypted = (int)sectionMapStream.ReadULong<LittleEndianConverter>();
                                //0x18        8        HashCode
                                section.HashCode = sectionMapStream.ReadULong<LittleEndianConverter>();
                                //0x20        8        SectionNameLength
                                int sectionNameLength = (int)sectionMapStream.ReadLong<LittleEndianConverter>();
                                //0x28        8        Unknown
                                sectionMapStream.ReadULong<LittleEndianConverter>();
                                //0x30        8        Encoding
                                section.Encoding = sectionMapStream.ReadULong<LittleEndianConverter>();
                                //0x38        8        NumPages.This is the number of pages present 
                                //                        in the file for the section, but this does not include 
                                //                        pages that contain zeroes only.A page that contains zeroes 
                                //                        only is not written to file.If a page’s data offset is 
                                //                        smaller than the sum of the decompressed size of all previous 
                                //                        pages, then it is to be preceded by a zero page with a size 
                                //                        that is equal to the difference between these two numbers.
                                section.PageCount = (int)sectionMapStream.ReadULong<LittleEndianConverter>();

                                //Read the name
                                if (sectionNameLength > 0)
                                {
                                        section.Name = sectionMapStream.ReadString(sectionNameLength, Encoding.Unicode);
                                        //Remove the empty characters
                                        section.Name = section.Name.Replace("\0", "");
                                }

                                ulong currentOffset = 0;
                                for (int i = 0; i < section.PageCount; ++i)
                                {
                                        DwgLocalSectionMap page = new DwgLocalSectionMap();
                                        //8        Page data offset.If a page’s data offset is 
                                        //        smaller than the sum of the decompressed size
                                        //        of all previous pages, then it is to be preceded 
                                        //        by a zero page with a size that is equal to the 
                                        //        difference between these two numbers.
                                        page.Offset = sectionMapStream.ReadULong<LittleEndianConverter>();
                                        //8        Page Size
                                        page.Size = sectionMapStream.ReadLong<LittleEndianConverter>();
                                        //8        Page Id
                                        page.PageNumber = (int)sectionMapStream.ReadLong<LittleEndianConverter>();
                                        //8        Page Uncompressed Size
                                        page.DecompressedSize = sectionMapStream.ReadULong<LittleEndianConverter>();
                                        //8        Page Compressed Size
                                        page.CompressedSize = sectionMapStream.ReadULong<LittleEndianConverter>();
                                        //8        Page Checksum
                                        page.Checksum = sectionMapStream.ReadULong<LittleEndianConverter>();
                                        //8        Page CRC
                                        page.CRC = sectionMapStream.ReadULong<LittleEndianConverter>();

                                        //Add the page to the section
                                        section.LocalSections.Add(page);
                                        //Move the offset
                                        currentOffset = page.Offset + page.DecompressedSize;
                                }
                                if (sectionNameLength > 0)
                                        fileheader.Descriptors.Add(section.Name, section);
                        }
                }

                private void readFileMetaData(DwgFileHeaderAC18 fileheader, IDwgStreamReader sreader)
                {
                        //5 bytes of 0x00 
                        sreader.Advance(5);

                        //0x0B        1        Maintenance release version
                        fileheader.AcadMaintenanceVersion = sreader.ReadByte();
                        //0x0C        1        Byte 0x00, 0x01, or 0x03
                        sreader.Advance(1);
                        //0x0D        4        Preview address(long), points to the image page + page header size(0x20).
                        fileheader.PreviewAddress = sreader.ReadRawLong();
                        //0x11        1        Dwg version (Acad version that writes the file)
                        fileheader.DwgVersion = sreader.ReadByte();
                        //0x12        1        Application maintenance release version(Acad maintenance version that writes the file)
                        fileheader.AppReleaseVersion = sreader.ReadByte();

                        //0x13        2        Codepage
                        fileheader.DrawingCodePage = CadUtils.GetCodePage(sreader.ReadShort());

                        //Advance empty bytes 
                        //0x15        3        3 0x00 bytes
                        sreader.Advance(3);

                        //0x18        4        SecurityType (long), see R2004 meta data, the definition is the same, paragraph 4.1.
                        fileheader.SecurityType = sreader.ReadRawLong();
                        //0x1C        4        Unknown long
                        sreader.ReadRawLong();
                        //0x20        4        Summary info Address in stream
                        fileheader.SummaryInfoAddr = sreader.ReadRawLong();
                        //0x24        4        VBA Project Addr(0 if not present)
                        fileheader.VbaProjectAddr = sreader.ReadRawLong();

                        //0x28        4        0x00000080
                        sreader.ReadRawLong();

                        //0x2C        4        App info Address in stream
                        sreader.ReadRawLong();

                        //Get to offset 0x80
                        //0x30        0x80        0x00 bytes
                        sreader.Advance(80);
                }

                private void readEncriptedData(DwgFileHeaderAC18 fileHeader, IDwgStreamReader sreader)
                {
                        //0x80        0x6C        Encrypted Data
                        //Metadata:
                        //The encrypted data at 0x80 can be decrypted by exclusive or’ing the 0x6c bytes of data 
                        //from the file with the following magic number sequence:

                        //29 23 BE 84 E1 6C D6 AE 52 90 49 F1 F1 BB E9 EB
                        //B3 A6 DB 3C 87 0C 3E 99 24 5E 0D 1C 06 B7 47 DE
                        //B3 12 4D C8 43 BB 8B A6 1F 03 5A 7D 09 38 25 1F
                        //5D D4 CB FC 96 F5 45 3B 13 0D 89 0A 1C DB AE 32
                        //20 9A 50 EE 40 78 36 FD 12 49 32 F6 9E 7D 49 DC
                        //AD 4F 14 F2 44 40 66 D0 6B C4 30 B7

                        StreamIO headerStream = new StreamIO(new CRC32StreamHandler(sreader.ReadBytes(0x6C), 0U)); //108
                        headerStream.Encoding = TextEncoding.GetListedEncoding(CodePage.Windows1252);

                        //0x00        12        “AcFssFcAJMB” file ID string
                        string fileId = headerStream.ReadString(12);
                        if (fileId != "AcFssFcAJMB\0")
                        {
                                this.notify($"File validation failed, id should be : AcFssFcAJMB\0, but is : {fileId}", NotificationType.Warning);
                        }

                        //0x0C        4        0x00(long)
                        headerStream.ReadInt<LittleEndianConverter>();
                        //0x10        4        0x6c(long)
                        headerStream.ReadInt<LittleEndianConverter>();
                        //0x14        4        0x04(long)
                        headerStream.ReadInt<LittleEndianConverter>();
                        //0x18        4        Root tree node gap
                        fileHeader.RootTreeNodeGap = headerStream.ReadInt<LittleEndianConverter>();
                        //0x1C        4        Lowermost left tree node gap
                        fileHeader.LeftGap = headerStream.ReadInt<LittleEndianConverter>();
                        //0x20        4        Lowermost right tree node gap
                        fileHeader.RigthGap = headerStream.ReadInt<LittleEndianConverter>();
                        //0x24        4        Unknown long(ODA writes 1)        
                        headerStream.ReadInt<LittleEndianConverter>();
                        //0x28        4        Last section page Id
                        fileHeader.LastPageId = headerStream.ReadInt<LittleEndianConverter>();

                        //0x2C        8        Last section page end address
                        fileHeader.LastSectionAddr = headerStream.ReadULong<LittleEndianConverter>();
                        //0x34        8        Second header data address pointing to the repeated header data at the end of the file
                        fileHeader.SecondHeaderAddr = headerStream.ReadULong<LittleEndianConverter>();

                        //0x3C        4        Gap amount
                        fileHeader.GapAmount = headerStream.ReadUInt<LittleEndianConverter>();
                        //0x40        4        Section page amount
                        fileHeader.SectionAmount = headerStream.ReadUInt<LittleEndianConverter>();
                        //0x44        4        0x20(long)
                        headerStream.ReadInt<LittleEndianConverter>();
                        //0x48        4        0x80(long)
                        headerStream.ReadInt<LittleEndianConverter>();
                        //0x4C        4        0x40(long)
                        headerStream.ReadInt<LittleEndianConverter>();
                        //0x50        4        Section Page Map Id
                        fileHeader.SectionPageMapId = headerStream.ReadUInt<LittleEndianConverter>();
                        //0x54        8        Section Page Map address(add 0x100 to this value)
                        fileHeader.PageMapAddress = headerStream.ReadULong<LittleEndianConverter>() + 256UL;
                        //0x5C        4        Section Map Id
                        fileHeader.SectionMapId = headerStream.ReadUInt<LittleEndianConverter>();
                        //0x60        4        Section page array size
                        fileHeader.SectionArrayPageSize = headerStream.ReadUInt<LittleEndianConverter>();
                        //0x64        4        Gap array size
                        fileHeader.GapArraySize = headerStream.ReadUInt<LittleEndianConverter>();
                        //0x68        4        CRC32(long).See paragraph 2.14.2 for the 32 - bit CRC calculation, 
                        //                        the seed is zero. Note that the CRC 
                        //                        calculation is done including the 4 CRC bytes that are 
                        //                        initially zero! So the CRC calculation takes into account 
                        //                        all of the 0x6c bytes of the data in this table.
                        fileHeader.CRCSeed = headerStream.ReadUInt();
                }

                private async Task<byte[]> getPageBufferAsync(ulong pageOffset, ulong compressedSize, ulong uncompressedSize, ulong correctionFactor, int blockSize, Stream stream)
                {
                        //Avoid shifted bits
                        ulong v = compressedSize + 7L;
                        ulong v1 = v & 0b11111111_11111111_11111111_11111000L;

                        uint totalSize = (uint)(v1 * correctionFactor);

                        int factor = (int)(totalSize + blockSize - 1L) / blockSize;
                        int lenght = factor * byte.MaxValue;

                        byte[] buffer = new byte[lenght];

                        //Relative to data page map 1, add 0x480 to get stream position
                        stream.Position = (long)(0x480 + pageOffset);
                        await stream.ReadAsync(buffer, 0, lenght);

                        byte[] compressedData = new byte[(int)totalSize];
                        this.reedSolomonDecoding(buffer, compressedData, factor, blockSize);

                        byte[] decompressedData = new byte[uncompressedSize];

                        DwgLZ77AC21Decompressor.Decompress(compressedData, 0U, (uint)compressedSize, decompressedData);

                        return decompressedData;
                }

                private byte[] getPageBuffer(ulong pageOffset, ulong compressedSize, ulong uncompressedSize, ulong correctionFactor, int blockSize, Stream stream)
                {
                        //Avoid shifted bits
                        ulong v = compressedSize + 7L;
                        ulong v1 = v & 0b11111111_11111111_11111111_11111000L;

                        uint totalSize = (uint)(v1 * correctionFactor);

                        int factor = (int)(totalSize + blockSize - 1L) / blockSize;
                        int lenght = factor * byte.MaxValue;

                        byte[] buffer = new byte[lenght];

                        //Relative to data page map 1, add 0x480 to get stream position
                        stream.Position = (long)(0x480 + pageOffset);
                        stream.Read(buffer, 0, lenght);

                        byte[] compressedData = new byte[(int)totalSize];
                        this.reedSolomonDecoding(buffer, compressedData, factor, blockSize);

                        byte[] decompressedData = new byte[uncompressedSize];

                        DwgLZ77AC21Decompressor.Decompress(compressedData, 0U, (uint)compressedSize, decompressedData);

                        return decompressedData;
                }

                private void reedSolomonDecoding(byte[] encoded, byte[] buffer, int factor, int blockSize)
                {
                        int index = 0;
                        int n = 0;
                        int length = buffer.Length;
                        for (int i = 0; i < factor; ++i)
                        {
                                int cindex = n;
                                if (n < encoded.Length)
                                {
                                        int size = Math.Min(length, blockSize);
                                        length -= size;
                                        int offset = index + size;
                                        while (index < offset)
                                        {
                                                buffer[index] = encoded[cindex];
                                                ++index;
                                                cindex += factor;
                                        }
                                }
                                ++n;
                        }
                }

                private void getPageHeaderData(IDwgStreamReader sreader,
                        out long sectionType,
                        out long decompressedSize,
                        out long compressedSize,
                        out long compressionType,
                        out long checksum
                        )
                {
                        //0x00        4        Section page type:
                        //Section page map: 0x41630e3b
                        //Section map: 0x4163003b
                        sectionType = sreader.ReadRawLong();
                        //0x04        4        Decompressed size of the data that follows
                        decompressedSize = sreader.ReadRawLong();
                        //0x08        4        Compressed size of the data that follows(CompDataSize)
                        compressedSize = sreader.ReadRawLong();

                        //0x0C        4        Compression type(0x02)
                        compressionType = sreader.ReadRawLong();
                        //0x10        4        Section page checksum
                        checksum = sreader.ReadRawLong();
                }

                private async Task<Stream> getHeaderAC15Stream(CancellationToken cancellationToken = default)
                {
                        bool match = false;
                        List<byte> buffer = new List<byte>();
                        buffer.AddRange(await this._fileStream.ReadBytesAsync(16, cancellationToken));

                        do
                        {
                                byte[] sn = buffer.Skip(Math.Max(0, buffer.Count() - 16)).ToArray();
                                if (CheckSentinel(sn, DwgFileHeaderAC15.EndSentinel))
                                {
                                        match = true;
                                }
                                else
                                {
                                        buffer.Add(await this._fileStream.ReadByteAsync(cancellationToken));
                                }
                        }
                        while (!match);

                        return new MemoryStream(buffer.ToArray());
                }

                private async Task<IDwgStreamReader> getStreamChunkAsync(int length, CancellationToken cancellationToken = default)
                {
                        MemoryStream ms = new MemoryStream(await this._fileStream.ReadBytesAsync(length, cancellationToken));
                        return DwgStreamReaderBase.GetStreamHandler(this._version, ms);
                }

                private void updateFileVersion(byte[] buffer)
                {
                        ACadVersion version = CadUtils.GetVersionFromName(Encoding.ASCII.GetString(buffer));
                        this.setVersion(version);
                }
        }
}

DomCR / ACadSharp / 13224545445

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