private int ReadHuffmanCode(int alphabetSize, int[] codeLengths, Span <HuffmanCode> table)
{
bool simpleCode = this.bitReader.ReadBit();
for (int i = 0; i < alphabetSize; i++)
{
codeLengths[i] = 0;
}
if (simpleCode)
{
// (i) Simple Code Length Code.
// This variant is used in the special case when only 1 or 2 Huffman code lengths are non-zero,
// and are in the range of[0, 255]. All other Huffman code lengths are implicitly zeros.
// Read symbols, codes & code lengths directly.
uint numSymbols = this.bitReader.ReadValue(1) + 1;
uint firstSymbolLenCode = this.bitReader.ReadValue(1);
// The first code is either 1 bit or 8 bit code.
uint symbol = this.bitReader.ReadValue(firstSymbolLenCode == 0 ? 1 : 8);
codeLengths[symbol] = 1;
// The second code (if present), is always 8 bit long.
if (numSymbols == 2)
{
symbol = this.bitReader.ReadValue(8);
codeLengths[symbol] = 1;
}
}
else
{
// (ii) Normal Code Length Code:
// The code lengths of a Huffman code are read as follows: num_code_lengths specifies the number of code lengths;
// the rest of the code lengths (according to the order in kCodeLengthCodeOrder) are zeros.
int[] codeLengthCodeLengths = new int[NumCodeLengthCodes];
uint numCodes = this.bitReader.ReadValue(4) + 4;
if (numCodes > NumCodeLengthCodes)
{
WebpThrowHelper.ThrowImageFormatException("Bitstream error, numCodes has an invalid value");
}
for (int i = 0; i < numCodes; i++)
{
codeLengthCodeLengths[CodeLengthCodeOrder[i]] = (int)this.bitReader.ReadValue(3);
}
this.ReadHuffmanCodeLengths(table, codeLengthCodeLengths, alphabetSize, codeLengths);
}
int size = HuffmanUtils.BuildHuffmanTable(table, HuffmanUtils.HuffmanTableBits, codeLengths, alphabetSize);
return(size);
}
private void ReadHuffmanCodeLengths(Span <HuffmanCode> table, int[] codeLengthCodeLengths, int numSymbols, int[] codeLengths)
{
int maxSymbol;
int symbol = 0;
int prevCodeLen = WebpConstants.DefaultCodeLength;
int size = HuffmanUtils.BuildHuffmanTable(table, WebpConstants.LengthTableBits, codeLengthCodeLengths, NumCodeLengthCodes);
if (size == 0)
{
WebpThrowHelper.ThrowImageFormatException("Error building huffman table");
}
if (this.bitReader.ReadBit())
{
int lengthNBits = 2 + (2 * (int)this.bitReader.ReadValue(3));
maxSymbol = 2 + (int)this.bitReader.ReadValue(lengthNBits);
}
else
{
maxSymbol = numSymbols;
}
while (symbol < numSymbols)
{
if (maxSymbol-- == 0)
{
break;
}
this.bitReader.FillBitWindow();
ulong prefetchBits = this.bitReader.PrefetchBits();
int idx = (int)(prefetchBits & 127);
HuffmanCode huffmanCode = table[idx];
this.bitReader.AdvanceBitPosition(huffmanCode.BitsUsed);
uint codeLen = huffmanCode.Value;
if (codeLen < WebpConstants.CodeLengthLiterals)
{
codeLengths[symbol++] = (int)codeLen;
if (codeLen != 0)
{
prevCodeLen = (int)codeLen;
}
}
else
{
bool usePrev = codeLen == WebpConstants.CodeLengthRepeatCode;
uint slot = codeLen - WebpConstants.CodeLengthLiterals;
int extraBits = WebpConstants.CodeLengthExtraBits[slot];
int repeatOffset = WebpConstants.CodeLengthRepeatOffsets[slot];
int repeat = (int)(this.bitReader.ReadValue(extraBits) + repeatOffset);
if (symbol + repeat > numSymbols)
{
return;
}
int length = usePrev ? prevCodeLen : 0;
while (repeat-- > 0)
{
codeLengths[symbol++] = length;
}
}
}
}