Method MakeDecodeTables

Libraries/unrar/unpack.cpp:227–365

LengthTable contains the length in bits for every element of alphabet. Dec is the structure to decode Huffman code/ Size is size of length table and DecodeNum field in Dec structure,

// Dec is the structure to decode Huffman code/
// Size is size of length table and DecodeNum field in Dec structure,
void Unpack::MakeDecodeTables(byte *LengthTable,DecodeTable *Dec,uint Size)
{
  // Size of alphabet and DecodePos array.
  Dec->MaxNum=Size;

  // Calculate how many entries for every bit length in LengthTable we have.
  uint LengthCount[16];
  memset(LengthCount,0,sizeof(LengthCount));
  for (size_t I=0;I<Size;I++)
    LengthCount[LengthTable[I] & 0xf]++;

  // We must not calculate the number of zero length codes.
  LengthCount[0]=0;

  // Set the entire DecodeNum to zero.
  memset(Dec->DecodeNum,0,Size*sizeof(*Dec->DecodeNum));

  // Initialize not really used entry for zero length code.
  Dec->DecodePos[0]=0;

  // Start code for bit length 1 is 0.
  Dec->DecodeLen[0]=0;

  // Right aligned upper limit code for current bit length.
  uint UpperLimit=0;

  for (size_t I=1;I<16;I++)
  {
    // Adjust the upper limit code.
    UpperLimit+=LengthCount[I];

    // Left aligned upper limit code.
    uint LeftAligned=UpperLimit<<(16-I);

    // Prepare the upper limit code for next bit length.
    UpperLimit*=2;

    // Store the left aligned upper limit code.
    Dec->DecodeLen[I]=(uint)LeftAligned;

    // Every item of this array contains the sum of all preceding items.
    // So it contains the start position in code list for every bit length. 
    Dec->DecodePos[I]=Dec->DecodePos[I-1]+LengthCount[I-1];
  }

  // Prepare the copy of DecodePos. We'll modify this copy below,
  // so we cannot use the original DecodePos.
  uint CopyDecodePos[ASIZE(Dec->DecodePos)];
  memcpy(CopyDecodePos,Dec->DecodePos,sizeof(CopyDecodePos));

  // For every bit length in the bit length table and so for every item
  // of alphabet.
  for (uint I=0;I<Size;I++)
  {
    // Get the current bit length.
    byte CurBitLength=LengthTable[I] & 0xf;

    if (CurBitLength!=0)