/// <summary>
/// Reads and decodes a window, returning whether or not there was
/// any more data to read.
/// </summary>
/// <returns>
/// Whether or not the delta stream had reached the end of its data.
/// </returns>
bool DecodeWindow()
{
int windowIndicator = delta.ReadByte();
// Have we finished?
if (windowIndicator == -1)
{
return(false);
}
// The stream to load source data from for this window, if any
Stream sourceStream;
// Where to reposition the source stream to after reading from it, if anywhere
int sourceStreamPostReadSeek = -1;
// xdelta3 uses an undocumented extra bit which indicates that there are an extra
// 4 bytes at the end of the encoding for the window
bool hasAdler32Checksum = ((windowIndicator & 4) == 4);
// Get rid of the checksum bit for the rest
windowIndicator &= 0xfb;
// Work out what the source data is, and detect invalid window indicators
switch (windowIndicator & 3)
{
// No source data used in this window
case 0:
sourceStream = null;
break;
// Source data comes from the original stream
case 1:
if (original == null)
{
throw new VcdiffFormatException
("Source stream requested by delta but not provided by caller.");
}
sourceStream = original;
break;
case 2:
sourceStream = output;
sourceStreamPostReadSeek = (int)output.Position;
break;
case 3:
throw new VcdiffFormatException
("Invalid window indicator - bits 0 and 1 both set.");
default:
throw new VcdiffFormatException("Invalid window indicator - bits 3-7 not all zero.");
}
// Read the source data, if any
byte[] sourceData = null;
int sourceLength = 0;
if (sourceStream != null)
{
sourceLength = IOHelper.ReadBigEndian7BitEncodedInt(delta);
int sourcePosition = IOHelper.ReadBigEndian7BitEncodedInt(delta);
sourceStream.Position = sourcePosition;
sourceData = IOHelper.CheckedReadBytes(sourceStream, sourceLength);
// Reposition the source stream if appropriate
if (sourceStreamPostReadSeek != -1)
{
sourceStream.Position = sourceStreamPostReadSeek;
}
}
// Read how long the delta encoding is - then ignore it
IOHelper.ReadBigEndian7BitEncodedInt(delta);
// Read how long the target window is
int targetLength = IOHelper.ReadBigEndian7BitEncodedInt(delta);
byte[] targetData = new byte[targetLength];
MemoryStream targetDataStream = new MemoryStream(targetData, true);
// Read the indicator and the lengths of the different data sections
byte deltaIndicator = IOHelper.CheckedReadByte(delta);
if (deltaIndicator != 0)
{
throw new VcdiffFormatException("VcdiffDecoder is unable to handle compressed delta sections.");
}
int addRunDataLength = IOHelper.ReadBigEndian7BitEncodedInt(delta);
int instructionsLength = IOHelper.ReadBigEndian7BitEncodedInt(delta);
int addressesLength = IOHelper.ReadBigEndian7BitEncodedInt(delta);
// If we've been given a checksum, we have to read it and we might as well
// use it to check the data.
int checksumInFile = 0;
if (hasAdler32Checksum)
{
byte[] checksumBytes = IOHelper.CheckedReadBytes(delta, 4);
checksumInFile = (checksumBytes[0] << 24) |
(checksumBytes[1] << 16) |
(checksumBytes[2] << 8) |
checksumBytes[3];
}
// Read all the data for this window
byte[] addRunData = IOHelper.CheckedReadBytes(delta, addRunDataLength);
byte[] instructions = IOHelper.CheckedReadBytes(delta, instructionsLength);
byte[] addresses = IOHelper.CheckedReadBytes(delta, addressesLength);
int addRunDataIndex = 0;
MemoryStream instructionStream = new MemoryStream(instructions, false);
cache.Reset(addresses);
while (true)
{
int instructionIndex = instructionStream.ReadByte();
if (instructionIndex == -1)
{
break;
}
for (int i = 0; i < 2; i++)
{
Instruction instruction = codeTable[instructionIndex, i];
int size = instruction.Size;
if (size == 0 && instruction.Type != InstructionType.NoOp)
{
size = IOHelper.ReadBigEndian7BitEncodedInt(instructionStream);
}
switch (instruction.Type)
{
case InstructionType.NoOp:
break;
case InstructionType.Add:
targetDataStream.Write(addRunData, addRunDataIndex, size);
addRunDataIndex += size;
break;
case InstructionType.Copy:
int addr = cache.DecodeAddress((int)targetDataStream.Position + sourceLength, instruction.Mode);
if (sourceData != null && addr < sourceData.Length)
{
targetDataStream.Write(sourceData, addr, size);
}
else // Data is in target data
{
// Get rid of the offset
addr -= sourceLength;
// Can we just ignore overlap issues?
if (addr + size < targetDataStream.Position)
{
targetDataStream.Write(targetData, addr, size);
}
else
{
for (int j = 0; j < size; j++)
{
targetDataStream.WriteByte(targetData[addr++]);
}
}
}
break;
case InstructionType.Run:
byte data = addRunData[addRunDataIndex++];
for (int j = 0; j < size; j++)
{
targetDataStream.WriteByte(data);
}
break;
default:
throw new VcdiffFormatException("Invalid instruction type found.");
}
}
}
output.Write(targetData, 0, targetLength);
if (hasAdler32Checksum)
{
int actualChecksum = Adler32.ComputeChecksum(1, targetData);
if (actualChecksum != checksumInFile)
{
throw new VcdiffFormatException("Invalid checksum after decoding window");
}
}
return(true);
}
