// 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[CodeOrder[_loopCounter]] = (byte) bits;
++_loopCounter;
}
for (int i = _codeLengthCodeCount; i < CodeOrder.Length; i++)
{
_codeLengthTreeCodeLength[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)
{
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
{
if (!_input.EnsureBitsAvailable(7))
{
// it doesn't matter if we require more bits here
_state = InflaterState.ReadingTreeCodesAfter;
return false;
}
int repeatCount;
if (_lengthCode == 16)
{
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)
{
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
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.Assert(false, "check why we are here!");
throw new InvalidDataException("SR.UnknownState");
}
var literalTreeCodeLength = new byte[HuffmanTree.MaxLiteralTreeElements];
var distanceTreeCodeLength = new byte[HuffmanTree.MaxDistTreeElements];
// Create literal and distance tables
Array.Copy(_codeList, literalTreeCodeLength, _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;
}
private bool DecodeBlock(out bool endOfBlockCodeSeen)
{
endOfBlockCodeSeen = false;
int freeBytes = _output.FreeBytes; // it is a little bit faster than frequently accessing the property
while (freeBytes > 258)
{
// 258 means we can safely do decoding since maximum repeat length is 258
int symbol;
switch (_state)
{
case InflaterState.DecodeTop:
// decode an element from the literal tree
//
symbol = _literalLengthTree.GetNextSymbol(_input);
if (symbol < 0)
{
// running out of input
return false;
}
if (symbol < 256)
{
// literal
_output.Write((byte) symbol);
--freeBytes;
}
else if (symbol == 256)
{
// end of block
endOfBlockCodeSeen = true;
// Reset state
_state = InflaterState.ReadingBFinal;
return true; // ***********
}
else
{
// length/distance pair
symbol -= 257; // length code started at 257
if (symbol < 8)
{
symbol += 3; // match length = 3,4,5,6,7,8,9,10
_extraBits = 0;
}
else if (symbol == 28)
{
// extra bits for code 285 is 0
symbol = 258; // code 285 means length 258
_extraBits = 0;
}
else
{
_extraBits = ExtraLengthBits[symbol];
Debug.Assert(_extraBits != 0, "We handle other cases seperately!");
}
_length = symbol;
goto case InflaterState.HaveInitialLength;
}
break;
case InflaterState.HaveInitialLength:
if (_extraBits > 0)
{
_state = InflaterState.HaveInitialLength;
int bits = _input.GetBits(_extraBits);
if (bits < 0)
{
return false;
}
_length = LengthBase[_length] + bits;
}
_state = InflaterState.HaveFullLength;
goto case InflaterState.HaveFullLength;
case InflaterState.HaveFullLength:
if (blockType == BlockType.Dynamic)
{
_distanceCode = _distanceTree.GetNextSymbol(_input);
}
else
{
// get distance code directly for static block
_distanceCode = _input.GetBits(5);
if (_distanceCode >= 0)
{
_distanceCode = StaticDistanceTreeTable[_distanceCode];
}
}
if (_distanceCode < 0)
{
// running out input
return false;
}
_state = InflaterState.HaveDistCode;
goto case InflaterState.HaveDistCode;
case InflaterState.HaveDistCode:
// To avoid a table lookup we note that for distanceCode >= 2,
// extra_bits = (distanceCode-2) >> 1
int offset;
if (_distanceCode > 3)
{
_extraBits = (_distanceCode - 2) >> 1;
int bits = _input.GetBits(_extraBits);
if (bits < 0)
{
return false;
}
offset = DistanceBasePosition[_distanceCode] + bits;
}
else
{
offset = _distanceCode + 1;
}
Debug.Assert(freeBytes >= 258, "following operation is not safe!");
_output.WriteLengthDistance(_length, offset);
freeBytes -= _length;
_state = InflaterState.DecodeTop;
break;
default:
Debug.Assert(false, "check why we are here!");
throw new InvalidDataException("SR.UnknownState");
}
}
return true;
}