Compression

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Terranigma uses a compression for most of the data.

Format

Compressed data always starts with a 4 byte header:

Offset Length Description
0 1 Chunk Counter ??, always 0 (not used in the game)
1 2 Uncompressed data size (LE)
3 1 First byte of uncompressed data

After that follows the compressed data. It consists of data bytes intersparsed with a control code bitstream, both use MSB first (BE) encoding. Data bytes are always aligned at byte boundary. The control code stream is fit in between the data bytes so that a new cc byte is only placed when a bit needs to be used and the previous cc byte is full.

Terranigma Compression Format
cc cc cc cc 8 bit 5 bit 3 bit 8 bit Description
1 - Literal - Literal Byte
0 1 - Offset Length != 0 - Copy Length+2 bytes from next_output_pos-0x2000+offset to the output
0 Length != 0 Copy Length+1 bytes from next_output_pos-0x2000+offset to the output
0 End of Data (usually in this case Offset is also 0)
0 Length Offset - Copy Length+2 bytes from next_output_pos-0x100+offset to the output

In the table, each cc stands for a control code bit. The second data byte is divided into 5+3 bits for mode 01.

Compression used for the Game

In general there is more than one way to compress a chunk of data with the given format. But the game's developers used a very specific greedy algorithm to compress all the data in the game. It can be reproduced with the following algorithm:

  1. Append 0x100 zero bytes to the (uncompressed) input data. These don't count into input_size and are only there for padding!
  2. Output the header, using 00, input_size, input[0]
  3. input_pos = 1
  4. While there is still input to process, repeat the following:
    1. Find the longest match of data for the current input_pos:
      • Look in the range max(0,input_pos-0x2000)..(input_pos-1) for a match of up to 0x100 bytes.
      • Call the position at which the match is found best_offset and the length of the match best_length.
      • If there are multiple matches of the same length, use the one with the highest offset.
    2. if input_pos + best_length > input_size, then best_length = input_size - input_pos
    3. Encode the match:
      • if 2 <= best_length <= 5 and best_offset is no less than inpus_pos-0x100, then encode the match using mode 00.
      • else if best_length <= 2, then encode the current input byte as literal and set best_length = 1.
      • else if best_length <= 9, then encode the match using mode 01.
      • else encode the match using mode 01L.
    4. inpus_pos += best_length

Tool

Crediar re'ed the decompression from the code and translated it to C code.

You can download a tool with the source to decompress data from any given offset here.

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