Changes
From Final Fantasy Inside
m
Minor formalisation
(O = Offset, L = Length)
So you get a 12-bit offset and a 4-bit length, but both of these values need modifying to work on directly. The length is easy to work with: just add 3 to it. Why? Well, This is because if a piece of repeated data was less than 3 bytes long, you wouldn't bother repeating it - it'd take up no more space to actually just put literal data in. So all references are at least 3 in length. So a length of 0 means 3 bytes repeated, 1 means 4 bytes repeated, so on.
Since we have 4 bits available, that gives us a final length ranging from 3-18 bytes long. (That also means the absolute maximum compression we can ever get using LZSS is a touch under 9:1, since the best possible is to replace 18 bytes of data with two bytes of reference, and then you have to add control bytes as well).
Offset needs a bit work doing on it, depending on how you're actually holding your data. If all you have is an input buffer and an output buffer, what you really need is an output position in your buffer to start reading data from. In other words, if you've already written 10,000 bytes to your output, you want to know where to retrieve the repeated data from - it could fall anywhere in the past 4K of data (i.e. from 5904 through to 9999 bytes).
real_offset = tail - ((tail - 18 - raw_offset) mod 4096)
Here, 'tail' is your current output position (eg. 10,000), 'raw_offset' is the 12-bit data value you've retrieved from the compressed reference, and 'real_offset' is the position in your output buffer you can begin reading from. This is a bit complex because it's not exactly the way LZSS traditionally does (de) compression; it uses a 4K circular buffer; if you do that, the offset is more or less usable directly.
Once you've got to the start position for your reference, you just copy the appropriate length of data over to your output, and you've dealt with that piece of data.
= 357
So our final offset is 357. We go to position 357 in our output data, read in 5 bytes (remember the length?), then write those 5 bytes out to our output. Now we're ready to read in the next bit of data (another compressed reference), and do the procedure again...
==== Complications ====
Unfortunately, that doesn't quite cover everything - there's two more things to be aware of when decompressing data that *will* ruin you when using FF7 files, since they do use these features.
First, if you end up with an negative offset, i.e. reading data from 'before the beginning of the file', write out nulls (zero bytes). That's because the compression buffer is, by default, initialized to zeros; so it's possible, if the start of the file contains a run of zeros, that the file may reference a block you haven't written... EG: If For example, if you're at position 50 in your output, it's possible you may get an offset indicating to go back 60 bytes to offset -10! . If you have to read 5 bytes from there, it's simple: you just write out 5 nulls. However, you *could* have to read 15 bytes from there. In that case, you write out 10 nulls (the part of the data 'before' the file start), then the 5 bytes from the beginning of the file.
Secondly, you can have a repeated run. This is almost the opposite problem: when you go off the end of your output. Say you're at offset 100 in your output, and you have to go to offset 95 to read in a reference. That's OK ... This is okay, but what if the reference length is >5? In that case, you loop the output. So if you had to write out 15 bytes, you'd write out the five bytes that *were* available ... and , then write them out again ... , then again, to make up the 15 bytes you needed.
The FF7 files use both of these 'tricks', so you can't ignore them!.
