RNC ProPack - Revision history

VAG at 17:23, 12 March 2011

2011-03-12T17:23:47Z

← Older revision		Revision as of 10:23, 12 March 2011
Line 1:		Line 1:
	RNC is a general data compression library (like [[zlib]]), name ~~probably stands for~~ Rob ~~Northern Compression (got it from [http://code.google.com/p/corsix-th/wiki/RNC here])~~.		RNC Pro-Pack is a general data compression library (like [[zlib]]), RNC is acronym for author's company name, Rob Northen Computing.

	Packer DOS binary, along with assembly decoding modules (barely useable nowdays) can be freely downloaded [http://aminet.net/package/util/pack/RNC_ProPack here].		Packer DOS binary, along with assembly decoding modules (barely useable nowdays) can be freely downloaded [http://aminet.net/package/util/pack/RNC_ProPack here].

VAG: fix header, add info about encryption, fix broken algo

2011-03-09T18:56:23Z

fix header, add info about encryption, fix broken algo

← Older revision		Revision as of 11:56, 9 March 2011
Line 1:		Line 1:
	RNC is a general data compression library (like [[zlib]]), name probably stands for Rob Northern Compression (got it from [http://code.google.com/p/corsix-th/wiki/RNC here]).		RNC is a general data compression library (like [[zlib]]), name probably stands for Rob Northern Compression (got it from [http://code.google.com/p/corsix-th/wiki/RNC here]).

			Packer DOS binary, along with assembly decoding modules (barely useable nowdays) can be freely downloaded [http://aminet.net/package/util/pack/RNC_ProPack 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.		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 ===		== Header ==

	All numbers are stored as big-endian.		All numbers are stored as big-endian.
Line 11:		Line 13:
	4 bytes unpacked data length		4 bytes unpacked data length
	4 bytes packed data length		4 bytes packed data length
	2 bytes CRC		2 bytes CRC-16 of packed data
	2 bytes ~~tail~~ CRC		2 bytes CRC-16 of unpacked data
	2 bytes ~~unknown~~		2 bytes overlap size (used for inplace unpacking)

			== Compression ==

			As an extra protection measure against curious hackers compressed data can be encrypted with secret 16-bit key (supplied to decompression routine by programmer.) In that case all byte literals decoded as follows:

			byte ^= key; key = _rotr(key, 1);

	=== Version 0 ===		=== Version 0 ===
Line 72:		Line 80:
	for (i = 0; i < length; i++)		for (i = 0; i < length; i++)
	*dst++ = get_byte();		*dst++ = get_byte();
			continue;
	}		}
	}		}
Line 109:		Line 118:
	}		}
	offset = (value << 8) + get_byte() + 1;		offset = (value << 8) + get_byte() + 1;


	[[Category:General Compression Methods]]		[[Category:General Compression Methods]]

Kostya: RNC description, version 2 from VAG, version 1 from ScummVM

2011-03-09T10:27:40Z

RNC description, version 2 from VAG, version 1 from ScummVM

New page

RNC is a general data compression library (like [[zlib]]), name probably stands for Rob Northern Compression (got it from [http://code.google.com/p/corsix-th/wiki/RNC 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 [https://github.com/scummvm/scummvm/blob/master/engines/sky/rnc_deco.cpp 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;

[[Category:General Compression Methods]]