// 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)
{
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.Assert(false, "check why we are here!");
throw new InvalidDataException(SR.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;
}
