Jul -- General Game/ROM Hacking: LZ77 stuff

GuyPerfect
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Posted on 02-19-09 05:59:41 PM

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Originally posted by Treeki
Is there an easier way to do this?

One of the best pieces of programming advice you'll ever encounter: Avoid magic wands. Don't reach for someone else's code unless you understand how to write it yourself; it'll save you trouble down the road, AND you'll be able to do it yourself!

LZ77 has a simple procedure:

* Is the next piece of data raw or encoded?

   * If it is raw...

      * Read from the input buffer 1 byte to the end of the output buffer

   * If it is encoded...

      * Read from the input buffer the Distance and Length values

      * Go back Distance bytes into the output buffer and read Length bytes

        from there onto the end of the output buffer

* Go back to the first step

Usually, the structure of an LZ77 stream contains a header byte, which describes the next 8 chunks of data as being either 0 - Raw or 1 - Encoded. When 8 chunks are read, there's another header byte and so-on. Find out how the Distance/Length pairs are stored (usually 2 bytes; 12-4 bits) and you should be pretty good on your way to writing a compressor.

As a bare minimum, a compressed LZ77 stream needs to include the original data size prior to compression so the decompressor knows when to stop.

Also note that Length can be greater than Distance. It is a common mistake to forget this, and it often results in compressed data being larger than it needs to be.

Ninji

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Posted on 02-19-09 06:47:42 PM

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Thanks for the help. I couldn't figure out how to make it actually compress the file (or rather, too lazy to

) but I managed to make my file into a LZ77 format file, which although isn't much compression (the file is bigger than the original) works fine in the game.

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GUESS WHAT MEGA-MARIO? I'm not ROM hacking any more. So leave me alone and stop emailing me okay?! · Hacking Tools: Nitro Explorer · NARC Explorer

Hectamatatortron
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Posted on 02-20-09 03:57:02 AM

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Is the intent of this project to compress data in the LZ77 format that is decompressed by the GBA and NDS's SWIs?

I have some Java code I can help you port for that purpose as a part of a GBA ROM editor I've written.

Unfortunately, I don't know enough about how it works to write it on my own (that is, if I wanted to optimize the code, I wouldn't know where to start; really, though, it's probably as optimal as it gets). I do know the format of LZ77 compression, however. Not that it matters; I just ported a friend's code from C# to Java as his tool that the code was for was also for hacking GBA ROMs.

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Posted on 02-20-09 04:26:19 PM

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Originally posted by Treeki
I tried translating the VB function for it from the Elitemap code to Python, but it just stays stuck in an infinite loop, and the code is too complex for me to understand. I'm not sure if it even worked in the first place.

Just for future reference, no, the code probably doesn't/didn't/barely worked in the first place if that's where you're getting it from

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