Difference between revisions of "RNC ProPack"

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(RNC description, version 2 from VAG, version 1 from ScummVM)
(No difference)

Revision as of 06:27, 9 March 2011

RNC is a general data compression library (like zlib), name probably stands for Rob Northern Compression (got it from here).

There are several known variants in the wild, version 1 was employed to compress data files in various games, like Beneath a Steel Sky or Dungeon Keeper, version 2 seems to be used in ETV codec.

Header

All numbers are stored as big-endian.

 3 bytes   signature "RNC"
 1 byte    version
 4 bytes   unpacked data length
 4 bytes   packed data length
 2 bytes   CRC
 2 bytes   tail CRC
 2 bytes   unknown

Version 0

This is uncompressed data.

Version 1

Decoder source is in ScummVM repository.

Bits are packed into bytes and those are interspersed with byte values (like in KMVC for example).

Data is packed with LZ77 scheme into blocks, each block has three Huffman tables for literal and copy lengths and offsets.

Huffman tables:

 5 bits - number of entries (0-16)
 4x<number> bits - code lengths

for codes reconstruction and tree reading pleas refer to decoder code.

Block starts with three tables description and 16-bit number of (literal, copy) pairs.

 while (!end) {
   read literal length table
   read offset table
   read copy length table
   blocks = get_bits(16);
 
   while (blocks--) {
     read_length = get_value(literal_table);
     while (read_length--)
       *dst++ = get_byte();
     if (blocks) {
       offset = get_value(offset_table);
       length = get_value(copy_len_table);
       memmove(dst, dst - offset, length);
       dst += length;
     }
   }
 }

Version 2

This seems to be designed to be faster on decoding so it does not use Huffman tables (but it's still LZ77 scheme with the same bitreader).

 get_bits(2); // unused
 while (!end) {
   if (!get_bit()) {
     *dst++ = get_byte();
   } else {
     if (!get_bit()) {
       length = 4 + get_bit();
       if (get_bit()) {
         length = (length - 1) * 2 + get_bit();
         if (length == 9) {
           length = (get_bits(4) + 3) * 4;
           for (i = 0; i < length; i++)
               *dst++ = get_byte();
         }
       }
       offset = get_offset();
     } else {
       if (get_bit()) {
         if (get_bit()) {
           length = get_byte() + 8;
           if (length == 8) {
              get_bit();
              continue; //dunno why
           }
         } else {
           length = 3;
         }
         offset = get_offset();
       } else {
         offset = get_byte() + 1;
       }
     }
     memmove(dst, dst - offset, length);
     dst += length;
   }
 }

Offset is coded like that:

 value = 0;
 if (get_bit()) {
   value = get_bit();
   if (get_bit()) {
     value = value * 2 + 4 + get_bit();
     if (!get_bit())
       value = value * 2 + get_bit();
   } else if (value == 0)
     value = get_bit() + 2;
 }
 offset = (value << 8) + get_byte() + 1;