// Format of the dynamic block header:
// 5 Bits: HLIT, # of Literal/Length codes - 257 (257 - 286)
// 5 Bits: HDIST, # of Distance codes - 1 (1 - 32)
// 4 Bits: HCLEN, # of Code Length codes - 4 (4 - 19)
//
// (HCLEN + 4) x 3 bits: code lengths for the code length
// alphabet given just above, in the order: 16, 17, 18,
// 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15
//
// These code lengths are interpreted as 3-bit integers
// (0-7); as above, a code length of 0 means the
// corresponding symbol (literal/length or distance code
// length) is not used.
//
// HLIT + 257 code lengths for the literal/length alphabet,
// encoded using the code length Huffman code
//
// HDIST + 1 code lengths for the distance alphabet,
// encoded using the code length Huffman code
//
// The code length repeat codes can cross from HLIT + 257 to the
// HDIST + 1 code lengths. In other words, all code lengths form
// a single sequence of HLIT + HDIST + 258 values.
private bool DecodeDynamicBlockHeader()
{
switch (_state)
{
case InflaterState.ReadingNumLitCodes:
_literalLengthCodeCount = _input.GetBits(5);
if (_literalLengthCodeCount < 0)
{
return(false);
}
_literalLengthCodeCount += 257;
_state = InflaterState.ReadingNumDistCodes;
goto case InflaterState.ReadingNumDistCodes;
case InflaterState.ReadingNumDistCodes:
_distanceCodeCount = _input.GetBits(5);
if (_distanceCodeCount < 0)
{
return(false);
}
_distanceCodeCount += 1;
_state = InflaterState.ReadingNumCodeLengthCodes;
goto case InflaterState.ReadingNumCodeLengthCodes;
case InflaterState.ReadingNumCodeLengthCodes:
_codeLengthCodeCount = _input.GetBits(4);
if (_codeLengthCodeCount < 0)
{
return(false);
}
_codeLengthCodeCount += 4;
_loopCounter = 0;
_state = InflaterState.ReadingCodeLengthCodes;
goto case InflaterState.ReadingCodeLengthCodes;
case InflaterState.ReadingCodeLengthCodes:
while (_loopCounter < _codeLengthCodeCount)
{
int bits = _input.GetBits(3);
if (bits < 0)
{
return(false);
}
_codeLengthTreeCodeLength[s_codeOrder[_loopCounter]] = (byte)bits;
++_loopCounter;
}
for (int i = _codeLengthCodeCount; i < s_codeOrder.Length; i++)
{
_codeLengthTreeCodeLength[s_codeOrder[i]] = 0;
}
// create huffman tree for code length
_codeLengthTree = new HuffmanTree(_codeLengthTreeCodeLength);
_codeArraySize = _literalLengthCodeCount + _distanceCodeCount;
_loopCounter = 0; // reset loop count
_state = InflaterState.ReadingTreeCodesBefore;
goto case InflaterState.ReadingTreeCodesBefore;
case InflaterState.ReadingTreeCodesBefore:
case InflaterState.ReadingTreeCodesAfter:
while (_loopCounter < _codeArraySize)
{
if (_state == InflaterState.ReadingTreeCodesBefore)
{
Debug.Assert(_codeLengthTree != null);
if ((_lengthCode = _codeLengthTree.GetNextSymbol(_input)) < 0)
{
return(false);
}
}
// The alphabet for code lengths is as follows:
// 0 - 15: Represent code lengths of 0 - 15
// 16: Copy the previous code length 3 - 6 times.
// The next 2 bits indicate repeat length
// (0 = 3, ... , 3 = 6)
// Example: Codes 8, 16 (+2 bits 11),
// 16 (+2 bits 10) will expand to
// 12 code lengths of 8 (1 + 6 + 5)
// 17: Repeat a code length of 0 for 3 - 10 times.
// (3 bits of length)
// 18: Repeat a code length of 0 for 11 - 138 times
// (7 bits of length)
if (_lengthCode <= 15)
{
_codeList[_loopCounter++] = (byte)_lengthCode;
}
else
{
int repeatCount;
if (_lengthCode == 16)
{
if (!_input.EnsureBitsAvailable(2))
{
_state = InflaterState.ReadingTreeCodesAfter;
return(false);
}
if (_loopCounter == 0)
{
// can't have "prev code" on first code
throw new InvalidDataException();
}
byte previousCode = _codeList[_loopCounter - 1];
repeatCount = _input.GetBits(2) + 3;
if (_loopCounter + repeatCount > _codeArraySize)
{
throw new InvalidDataException();
}
for (int j = 0; j < repeatCount; j++)
{
_codeList[_loopCounter++] = previousCode;
}
}
else if (_lengthCode == 17)
{
if (!_input.EnsureBitsAvailable(3))
{
_state = InflaterState.ReadingTreeCodesAfter;
return(false);
}
repeatCount = _input.GetBits(3) + 3;
if (_loopCounter + repeatCount > _codeArraySize)
{
throw new InvalidDataException();
}
for (int j = 0; j < repeatCount; j++)
{
_codeList[_loopCounter++] = 0;
}
}
else
{
// code == 18
if (!_input.EnsureBitsAvailable(7))
{
_state = InflaterState.ReadingTreeCodesAfter;
return(false);
}
repeatCount = _input.GetBits(7) + 11;
if (_loopCounter + repeatCount > _codeArraySize)
{
throw new InvalidDataException();
}
for (int j = 0; j < repeatCount; j++)
{
_codeList[_loopCounter++] = 0;
}
}
}
_state = InflaterState.ReadingTreeCodesBefore; // we want to read the next code.
}
break;
default:
Debug.Fail("check why we are here!");
throw new InvalidDataException("UnknownState");
}
byte[] literalTreeCodeLength = new byte[HuffmanTree.MaxLiteralTreeElements];
byte[] distanceTreeCodeLength = new byte[HuffmanTree.MaxDistTreeElements];
// Create literal and distance tables
Array.Copy(_codeList, 0, literalTreeCodeLength, 0, _literalLengthCodeCount);
Array.Copy(_codeList, _literalLengthCodeCount, distanceTreeCodeLength, 0, _distanceCodeCount);
// Make sure there is an end-of-block code, otherwise how could we ever end?
if (literalTreeCodeLength[HuffmanTree.EndOfBlockCode] == 0)
{
throw new InvalidDataException();
}
_literalLengthTree = new HuffmanTree(literalTreeCodeLength);
_distanceTree = new HuffmanTree(distanceTreeCodeLength);
_state = InflaterState.DecodeTop;
return(true);
}
//Each block of compressed data begins with 3 header bits
// containing the following data:
// first bit BFINAL
// next 2 bits BTYPE
// Note that the header bits do not necessarily begin on a byte
// boundary, since a block does not necessarily occupy an integral
// number of bytes.
// BFINAL is set if and only if this is the last block of the data
// set.
// BTYPE specifies how the data are compressed, as follows:
// 00 - no compression
// 01 - compressed with fixed Huffman codes
// 10 - compressed with dynamic Huffman codes
// 11 - reserved (error)
// The only difference between the two compressed cases is how the
// Huffman codes for the literal/length and distance alphabets are
// defined.
//
// This function returns true for success (end of block or output window is full,)
// false if we are short of input
//
private bool Decode()
{
bool eob = false;
bool result = false;
if (Finished())
{
return(true);
}
if (_hasFormatReader)
{
Debug.Assert(_formatReader != null);
if (_state == InflaterState.ReadingHeader)
{
if (!_formatReader.ReadHeader(_input))
{
return(false);
}
_state = InflaterState.ReadingBFinal;
}
else if (_state == InflaterState.StartReadingFooter || _state == InflaterState.ReadingFooter)
{
if (!_formatReader.ReadFooter(_input))
{
return(false);
}
_state = InflaterState.VerifyingFooter;
return(true);
}
}
if (_state == InflaterState.ReadingBFinal)
{
// reading bfinal bit
// Need 1 bit
if (!_input.EnsureBitsAvailable(1))
{
return(false);
}
_bfinal = _input.GetBits(1);
_state = InflaterState.ReadingBType;
}
if (_state == InflaterState.ReadingBType)
{
// Need 2 bits
if (!_input.EnsureBitsAvailable(2))
{
_state = InflaterState.ReadingBType;
return(false);
}
_blockType = (BlockType)_input.GetBits(2);
if (_blockType == BlockType.Dynamic)
{
_state = InflaterState.ReadingNumLitCodes;
}
else if (_blockType == BlockType.Static)
{
_literalLengthTree = HuffmanTree.StaticLiteralLengthTree;
_distanceTree = HuffmanTree.StaticDistanceTree;
_state = InflaterState.DecodeTop;
}
else if (_blockType == BlockType.Uncompressed)
{
_state = InflaterState.UncompressedAligning;
}
else
{
throw new InvalidDataException("UnknownBlockType");
}
}
if (_blockType == BlockType.Dynamic)
{
if (_state < InflaterState.DecodeTop)
{
// we are reading the header
result = DecodeDynamicBlockHeader();
}
else
{
result = DecodeBlock(out eob); // this can returns true when output is full
}
}
else if (_blockType == BlockType.Static)
{
result = DecodeBlock(out eob);
}
else if (_blockType == BlockType.Uncompressed)
{
result = DecodeUncompressedBlock(out eob);
}
else
{
throw new InvalidDataException("UnknownBlockType");
}
//
// If we reached the end of the block and the block we were decoding had
// bfinal=1 (final block)
//
if (eob && (_bfinal != 0))
{
if (_hasFormatReader)
{
_state = InflaterState.StartReadingFooter;
}
else
{
_state = InflaterState.Done;
}
}
return(result);
}
