Function lodepng_huffman_code_lengths

src/lodepng.cpp:714–831

}

unsigned lodepng_huffman_code_lengths(unsigned* lengths, const unsigned* frequencies,
                                      size_t numcodes, unsigned maxbitlen)
{
  unsigned i, j;
  size_t sum = 0, numpresent = 0;
  unsigned error = 0;
  Coin* coins; /*the coins of the currently calculated row*/
  Coin* prev_row; /*the previous row of coins*/
  unsigned numcoins;
  unsigned coinmem;

  if(numcodes == 0) return 80; /*error: a tree of 0 symbols is not supposed to be made*/

  for(i = 0; i < numcodes; i++)
  {
    if(frequencies[i] > 0)
    {
      numpresent++;
      sum += frequencies[i];
    }
  }

  for(i = 0; i < numcodes; i++) lengths[i] = 0;

  /*ensure at least two present symbols. There should be at least one symbol
  according to RFC 1951 section 3.2.7. To decoders incorrectly require two. To
  make these work as well ensure there are at least two symbols. The
  Package-Merge code below also doesn't work correctly if there's only one
  symbol, it'd give it the theoritical 0 bits but in practice zlib wants 1 bit*/
  if(numpresent == 0)
  {
    lengths[0] = lengths[1] = 1; /*note that for RFC 1951 section 3.2.7, only lengths[0] = 1 is needed*/
  }
  else if(numpresent == 1)
  {
    for(i = 0; i < numcodes; i++)
    {
      if(frequencies[i])
      {
        lengths[i] = 1;
        lengths[i == 0 ? 1 : 0] = 1;
        break;
      }
    }
  }
  else
  {
    /*Package-Merge algorithm represented by coin collector's problem
    For every symbol, maxbitlen coins will be created*/

    coinmem = numpresent * 2; /*max amount of coins needed with the current algo*/
    coins = (Coin*)lodepng_malloc(sizeof(Coin) * coinmem);
    prev_row = (Coin*)lodepng_malloc(sizeof(Coin) * coinmem);
    if(!coins || !prev_row)
    {
      lodepng_free(coins);
      lodepng_free(prev_row);
      return 83; /*alloc fail*/