public ushort ComputeCrc(ushort initialValue = 0)
{
if (_crcStartValues.Count == 0)
{
throw new InvalidOperationException($"You must call {nameof(StartCrcCheck)}() first.");
}
var startOffset = _crcStartValues.Pop();
var endOffset = bitOffset == 0
? Offset
: Offset + 1;
return(BitReaderExtensions.ComputeCRC(Data, startOffset, endOffset - startOffset, initialValue));
}
private void UpdateCrc(byte value)
{
CurrentCrcValue = BitReaderExtensions.ComputeCRC(value, CurrentCrcValue);
}
internal void WriteSecondHeader(BitWriter writer, DwgHeaderVariables headerVariables, int pointer)
{
// write to memory the backtrack to fill in size and crc
using (var ms = new MemoryStream())
{
var tempWriter = new BitWriter(ms);
tempWriter.WriteBytes(SecondHeaderStartSentinel);
var sizeOffset = tempWriter.Position;
tempWriter.Write_RL(0); // size, filled in later
tempWriter.Write_BL(pointer);
tempWriter.WriteStringAscii(Version.VersionString(), nullTerminated: false);
tempWriter.WriteBytes(new byte[] { 0, 0, 0, 0, 0, 0 }); // 6 zero bytes
tempWriter.WriteBits(0x00000000, 4); // 4 zero bits
tempWriter.WriteBytes(new byte[] { 0, 0 }); // 2 unknown bytes
tempWriter.WriteBytes(SecondHeaderMidSentinel);
tempWriter.WriteByte(5); // record locator count
HeaderVariablesLocator.Write(tempWriter, writingSecondHeader: true);
ClassSectionLocator.Write(tempWriter, writingSecondHeader: true);
ObjectMapLocator.Write(tempWriter, writingSecondHeader: true);
UnknownSection_R13C3AndLaterLocator.Write(tempWriter, writingSecondHeader: true);
UnknownSection_PaddingLocator.Write(tempWriter, writingSecondHeader: true);
tempWriter.Write_BS(14);
headerVariables.NextAvailableHandle.WriteSecondHeader(tempWriter, 0);
headerVariables.BlockControlObjectHandle.WriteSecondHeader(tempWriter, 1);
headerVariables.LayerControlObjectHandle.WriteSecondHeader(tempWriter, 2);
headerVariables.StyleObjectControlHandle.WriteSecondHeader(tempWriter, 3);
headerVariables.LineTypeObjectControlHandle.WriteSecondHeader(tempWriter, 4);
headerVariables.ViewControlObjectHandle.WriteSecondHeader(tempWriter, 5);
headerVariables.UcsControlObjectHandle.WriteSecondHeader(tempWriter, 6);
headerVariables.ViewPortControlObjectHandle.WriteSecondHeader(tempWriter, 7);
headerVariables.AppIdControlObjectHandle.WriteSecondHeader(tempWriter, 8);
headerVariables.DimStyleControlObjectHandle.WriteSecondHeader(tempWriter, 9);
headerVariables.ViewPortEntityHeaderControlObjectHandle.WriteSecondHeader(tempWriter, 10);
headerVariables.NamedObjectsDictionaryHandle.WriteSecondHeader(tempWriter, 11);
headerVariables.MLineStyleDictionaryHandle.WriteSecondHeader(tempWriter, 12);
headerVariables.GroupDictionaryHandle.WriteSecondHeader(tempWriter, 13);
tempWriter.WriteByte(0); // unknown
tempWriter.AlignByte();
var crcOffset = tempWriter.Position;
tempWriter.WriteShort(0); // CRC, filled in later
if (Version == DwgVersionId.R14)
{
// unknown garbage bytes
tempWriter.WriteBytes(new byte[] { 0, 0, 0, 0, 0, 0, 0, 0 });
}
tempWriter.WriteBytes(SecondHeaderEndSentinel);
// get bytes of header
var secondHeaderBytes = tempWriter.AsBytes();
// fill in size
var sizeBytes = BitConverter.GetBytes(secondHeaderBytes.Length - SecondHeaderStartSentinel.Length - SecondHeaderEndSentinel.Length);
if (!BitConverter.IsLittleEndian)
{
Array.Reverse(sizeBytes);
}
Array.Copy(sizeBytes, 0, secondHeaderBytes, sizeOffset, sizeBytes.Length);
// fill in crc
var crc = BitReaderExtensions.ComputeCRC(secondHeaderBytes, sizeOffset, crcOffset - sizeOffset, DwgHeaderVariables.InitialCrcValue);
var crcBytes = BitConverter.GetBytes(crc);
if (!BitConverter.IsLittleEndian)
{
Array.Reverse(crcBytes);
}
Array.Copy(crcBytes, 0, secondHeaderBytes, crcOffset, crcBytes.Length);
// write to the real writer
writer.WriteBytes(secondHeaderBytes);
}
}