private void CheckStamp()
{
// Can't read as uint32 directly since a integer format is non-encoded
// in a standard format.
var stamp = BitConverter.ToUInt32(vcdReader.ReadBytes(4), 0);
if ((stamp & 0xFFFFFF) != MagicStamp)
{
throw new FormatException("not a VCDIFF input");
}
if (stamp >> 24 > SupportedVersion)
{
throw new FormatException("VCDIFF input version > 0 is not supported");
}
}
private Window ReadWindow()
{
var window = new Window();
// Get window indicator
window.Source = (WindowFields)vcdReader.ReadByte();
if (window.Source.Contains(WindowFields.NotSupported))
{
throw new FormatException("unrecognized window indicator bits set");
}
if (window.Source.Contains(WindowFields.Source | WindowFields.Target))
{
window.SourceSegmentLength = vcdReader.ReadInteger();
window.SourceSegmentOffset = vcdReader.ReadInteger();
}
// Copy offset and copy length may not overflow
if (window.SourceSegmentOffset.CheckOverflow(window.SourceSegmentLength))
{
throw new FormatException("decoder copy window overflows a file offset");
}
// Check copy window bounds
if (window.Source.Contains(WindowFields.Target) &&
window.SourceSegmentOffset + window.SourceSegmentLength > lastWindowOffset)
{
throw new FormatException("VCD_TARGET window out of bounds");
}
// Start of Delta Encoding Data
vcdReader.ReadInteger(); // Length of the delta encoding (following data)
// Get the length of target window
window.TargetWindowLength = vcdReader.ReadInteger();
lastWindowLength = window.TargetWindowLength;
// Set the maximum decoder position, beyond which we should not
// decode any data. This may not exceed the size of a UInt32.
if (window.SourceSegmentLength.CheckOverflow(window.TargetWindowLength))
{
throw new FormatException("decoder target window overflows a UInt32");
}
// Check for malicious files
if (window.TargetWindowLength > HardMaxWindowSize)
{
throw new FormatException("Hard window size exceeded");
}
// Get compressed / delta fields
window.CompressedFields = (WindowCompressedFields)vcdReader.ReadByte();
if (window.CompressedFields.Contains(WindowCompressedFields.Invalid))
{
throw new FormatException("unrecognized delta indicator bits set");
}
// Compressed fields is only used with secondary compression
if (window.CompressedFields != WindowCompressedFields.None &&
header.SecondaryCompressor == SecondaryCompressor.None)
{
throw new FormatException("invalid delta indicator bits set");
}
// Read section lengths
var dataLength = vcdReader.ReadInteger();
var instructionsLength = vcdReader.ReadInteger();
var addressesLength = vcdReader.ReadInteger();
// Read checksum if so (it's in big-endian-non-integer)
if (window.Source.Contains(WindowFields.Adler32))
{
var checksum = vcdReader.ReadBytes(4);
window.Checksum = (uint)((checksum[0] << 24) | (checksum[1] << 16) |
(checksum[2] << 8) | checksum[3]);
}
// Read sections
var data = new MemoryStream(vcdReader.ReadBytes(dataLength));
var instructions = new MemoryStream(vcdReader.ReadBytes(instructionsLength));
var addresses = new MemoryStream(vcdReader.ReadBytes(addressesLength));
window.Data = new VcdReader(data);
window.Instructions = new VcdReader(instructions);
window.Addresses = new VcdReader(addresses);
return(window);
}
