RNC ProPack: Difference between revisions
(RNC description, version 2 from VAG, version 1 from ScummVM) |
(No difference)
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Revision as of 03:27, 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;